The role of salicylic acid, L-ascorbic acid and oxalic

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The role of salicylic acid, L-ascorbic acid and oxalic acid in promoting the oxidation of alkenes with H2 O2 catalysed by [MnIV 2 (O)3 (tmtacn)2 ]2+ † Johannes W. de Boer,a,b Paul L. Alsters,c Auke Meetsma,a Ronald Hage,b,d Wesley R. Brownea and Ben L. Feringa*a Received 30th May 2008, Accepted 12th August 2008 First published as an Advance Article on the web 10th October 2008 DOI: 10.1039/b809177c The role played by the additives salicylic acid, L-ascorbic acid and oxalic acid in promoting the catalytic activity of [MnIV 2 (O)3 (tmtacn)2 ](PF6 )2 {1(PF6 )2 , where tmtacn = N,N¢,N¢¢-trimethyl-1,4,7triazacyclononane} in the epoxidation and cis-dihydroxylation of alkenes with H2 O2 and in suppressing the catalysed decomposition of H2 O2 is examined. Whereas aliphatic and aromatic carboxylic acids effect enhancement of the catalytic activity of 1 through the in situ formation dinuclear carboxylato bridged complexes of the type [MnIII 2 (m-O)(m-RCO2 )2 (tmtacn)2 ]2+ , for L-ascorbic acid and oxalic acid notable differences in reactivity are observed. Although for L-ascorbic acid key differences in the spectroscopic properties of the reaction mixtures are observed compared with carboxylic acids, the involvement of carboxylic acids formed in situ is apparent. For oxalic acid the situation is more complex with two distinct catalyst systems in operation; the first, which engages in epoxidation only, is dominant until the oxalic acid additive is consumed completely at which point carboxylic acids formed in situ take on the role of additives to form a second distinct catalyst system, i.e. that which was observed for alkyl and aromatic carboxylic acids, which yield both cis-diol and epoxide products.

Introduction Manganese complexes based on the ligand tmtacn (where tmtacn = N,N¢,N¢¢-trimethyl-1,4,7-triazacyclononane) have attracted considerable attention due to their catalytic activity, as structural models for dinuclear manganese containing proteins and their often rich magnetic spectroscopic and electrochemical properties.1,2 Although initially developed as functional models for bioinorganic manganese systems, in particular, dinuclear manganese based catalase3–5 enzymes and the water splitting component of photosystem II (PSII),6 the ability of many of these complexes to engage in catalysing a wide range of oxidative transformations with H2 O2 in both aqueous7 and non-aqueous8 media has generated considerable interest in their use for, e.g. textile stain bleaching,9 benzyl alcohol oxidation,8c C–H bond activation,8f sulfoxidation10 and cis-dihydroxylation and epoxidation of alkenes.8,11 The oxidation of alkenes to cis-diol and epoxide products is a central oxidative transformation12 employed in biological systems,5 synthetic organic chemistry and the chemical industry.13–15 Atom-efficient and environmentally friendly catalytic methods employing H2 O2 ,13 most notably in the use of MnII salts16 a Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747, AG, Groningen, The Netherlands. E-mail: [email protected]; Fax: 0031-50-363-4235; Tel: 003150-363-4296 b Rahu Catalytics, Olivier van Noortlaan 120, 3133, AT, Vlaardingen, The Netherlands c Advanced Synthesis, Catalysis and Development, DSM Pharma Products, PO Box 18, 6160, MD, Geleen, The Netherlands d Unilever R & D Vlaardingen, PO box 114, 3130, AC, Vlaardingen, The Netherlands † CCDC reference number 674966. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b809177c

This journal is © The Royal Society of Chemistry 2008

and complexes17 and FeII complexes18 in the catalytic epoxidation of alkenes, have seen increasingly rapid progress. The potential of 1st row transition metals towards cis-dihydroxylation of alkenes has been demonstrated in the work of de Vos and coworkers using heterogenised Mn-tmtacn,19 Que and coworkers with FeII pyridylamine based complexes18 and our own recent reports on the use of [MnIV 2 O3 (tmtacn)2 ]2+ (1, Scheme 1), in the presence of carboxylic acids.11

Scheme 1 cis-Dihydroxylation and epoxidation of alkenes by 1.

Recently, we reported that 1 can engage in the atom efficient cis-dihydroxylation of alkenes with high turnover numbers when combined with electron deficient carboxylic acids (Scheme 1).11 We demonstrated that carboxylic acids, at co-catalytic levels, are effective in suppressing the inherent catalase activity of 111 and allow for the tuning of the catalyst’s selectivity towards either cis-dihydroxylation or epoxidation. Electrochemical and spectroscopic investigations11 demonstrated that control over the outcome of the reaction towards cis-dihydroxylation or epoxidation arises from the in situ formation of carboxylato bridged dinuclear complexes, e.g., complex 2 {[MnIII 2 (m-O)(mCCl3 CO2 )2 (tmtacn)2 ]2+ }, during catalysis (Fig. 1), with the steric nature of the bridging carboxylato ligands being the primary factor controlling the selectivity of the catalyst. Dalton Trans., 2008, 6283–6295 | 6283

Fig. 1 Complexes described in the text–complexes 1,1 2,11a and 3 have been characterized structurally. All complexes are PF6 - salts.

In our earlier reports, the structural and mechanistic features as well as the parameters that govern the activity and selectivity of the catalysis of this exceptionally H2 O2 efficient catalytic system were described. The role of complexes such as 2 in the catalysis and the importance of the formation of the m-carboxylato bridged MnIII 2 complexes (in which the m-oxo bridge is replaced by two labile hydroxido/aqua ligands) was addressed. We demonstrated that the predominant species present under catalytic conditions are MnIII 2 bis-m-carboxylato bridged complexes such as A and B (Scheme 2) and that the reaction of these complexes with H2 O and H2 O2 is rate limiting.

bridged complexes, e.g., 2 (Fig. 1), were employed as the catalyst and the H2 O content of the reaction mixture was sufficiently high. For all of the alkanoic and benzoic acids examined, the formation of the MnIII 2 bis-m-carboxylato bridged complex was manifested by a disappearance of 1 (MnIV 2 ) from both the ESIMS and UV-Vis absorption spectra and the appearance of the corresponding MnIII 2 complex. However, there was one notable exception to this—salicylic acid. The reaction mixture, with the catalyst system 1/salicylic acid, exhibited distinct UV-Vis and ESIMS spectroscopic differences,11a although, it otherwise behaved similarly to other carboxylic acids in terms of the catalytic oxidation of alkenes. Furthermore, during the course of these studies, the question arose as to whether the additives used by others (Fig. 2), i.e. Lascorbic acid (Berkessel et al.20 ) and oxalate buffer (De Vos et al.21 ), played a similar role as other alkanoic and aromatic carboxylic acids (Scheme 2). Making an accurate comparison between the carboxylic acid additives reported by our group, which allows 1 to engage in cis-dihydroxylation and epoxidation and the additives found to be effective for the epoxidation of alkenes by the groups of Berkessel and De Vos, i.e. L-ascorbic acid and oxalic acid, requires that these latter additives should be examined under conditions11 similar with that used for the 1/CCl3 CO2 H promoted reaction.

Fig. 2 Additives used in suppressing the catalase activity and enhancing the reactivity of 1.

Scheme 2 Proposed general mechanism for the oxidation of alkenes catalysed by 1/RCO2 H.11b

In addition, the outcome of the oxidation reactions catalysed by 1/RCO2 H was sensitive to bulk solvent conditions. It was found that when the reaction was carried out with the combination 1/carboxylic acid, a lag period is observed before the formation of MnIII 2 bis-m-carboxylato bridged complexes, e.g., 2 (Fig. 1), and the commencement of cis-dihydroxylation and epoxidation. This lag period was not observed when the MnIII 2 bis-m-carboxylato 6284 | Dalton Trans., 2008, 6283–6295

In this contribution, the oxidation of alkenes catalysed by 1 in combination with a series of hydroxy-substituted benzoic acids, oxalic acid and L-ascorbic acid are examined, both in terms of the time dependence of the conversion of the substrate and product formation and the spectroscopic properties of the reaction mixture. The key question addressed is whether all of these systems behave in essentially the same manner or if there are fundamental mechanistic differences that may present opportunities with regard to providing further insight into controlling the reactivity and selectivity of the system 1/H2 O2 .

Experimental All reagents are of commercial grade (Aldrich, Acros, Fluka) and used as received unless stated otherwise. H2 O2 used: 50% w/w (Acros) or 30% v/v (Merck, medicinal grade) solution in water. H2 18 O (Icon Isotopes): 97 atom% 18 O. The synthesis and characterisation of complexes 1,1a 2,11a 6 and 811b as their PF6 salts were reported previously. This journal is © The Royal Society of Chemistry 2008

[MnIII 2 (l-O)(l-benzoato)2 (tmtacn)2 ](PF6 )2 (3) Complex 3 was prepared by modification of the general procedure reported by Hage et al.22 L-Ascorbic acid (19 mg, 0.105 mmol) in 2 ml of H2 O was added to a solution of 1 (81 mg, 0.10 mmol) and benzoic acid (24 mg, 0.22 mmol) in 20 ml of H2 O with rapid stirring. The red–purple precipitate was isolated by filtration and recrystallized from acetonitrile in an ethylacetate bath. Yield 75% (75 mg, 0.075 mmol). Mass spec. (calc. Mn2 C32 H52 N6 O5 m/z) 3(PF6 )+ 855.4, 32+ 355.2; 1 H NMR (400 MHz, CD3 CN) d 72, 35, 21, 14, 6, 0, -80, -92, -96. Elemental analysis (calc. Mn2 C32 H52 N6 O5 P2 F12 ) C 36.7% (38.4%), H 5.22% (5.24%), N 8.67% (8.40%). CCDC No. 674966: [C32 H52 Mn2 N6 O5 ]2+ ·2(PF6 )- ·2(C4 H8 O2 )0 .5 , Mr = 1088.71, triclinic, P-1, a = 11.4974(5), b = 13.7430(6), c = ˚ , a= 76.826(1)◦ , b = 76.090(1)◦ , g = 69.359(1)◦ , V = 16.3508(7) A ˚ 3 , Z = 2, Dx = 1.560 g cm-3 , F(000) = 1124, m = 2317.49(17) A ˚ , T = 100(1) K, 22101 reflections 7.14 cm-1 , l(Mo Ka) = 0.71073 A measured, GooF = 1.025, wR(F 2 ) = 0.1078 for 11061 unique reflections and 538 parameters and R(F) = 0.0384 for 10023 reflections obeying F o ≥ 4.0 sF o ) criterion of observability. The asymmetric unit consists of five moieties: a cationic dinuclear Mncomplex, two hexafluorophosphate anions and two disordered, half ethylacetate solvent molecules. Neutral atom scattering factors and anomalous dispersion corrections were taken from International Tables for Crystallography.23 All refinement calculations and graphics were performed on a Pentium-III/DebianLinux computer with the program packages SHELXL24 (leastsquare refinements), a locally modified version of the program PLUTO25 (preparation of illustrations) and PLATON 26 package (checking the final results for missed symmetry with the MISSYM option, solvent accessible voids with the SOLV option, calculation of geometric data and the ORTEP26 illustrations). No classic hydrogen bonds, no missed symmetry (MISSYM), but potential solvent-accessible area (total volume after squeezing out of the ˚ 3 /unit cell) present highly disordered solvent molecules: 373.2 A were detected by procedures implemented in PLATON.26 [MnIII 2 (l-O)(l-salicylato)2 (tmtacn)2 ](PF6 )2 (4) 4 was prepared as for 3 except salicylic acid (31 mg, 0.22 mmol) was used in place of benzoic acid. Yield 11% (11 mg, 0.011 mmol). Mass spec. (calc. Mn2 C32 H52 N6 O7 m/z) [4(PF6 )+ ] 887.3, [42+ ] 371.2. [8+ ] 362.2 is observed also. [MnIII 2 (l-O)(l-3-hydroxybenzoato)2 (tmtacn)2 ](PF6 )2 (5) 5 was prepared as for 3 except 3-hydroxybenzoic acid (70 mg, 0.22 mmol) was used in place of benzoic acid. Yield 68% (70 mg, 0.068 mmol). Mass spec. (calc. Mn2 C32 H52 N6 O7 m/z) [5(PF6 )+ ] 887.3, [52+ ] 371.1; 1 H NMR (400 MHz, CD3 CN) d 72, 35, 20, 14, 7, 5, 3, -1, -80, -92, -96. Elemental analysis (calc. Mn2 C32 H52 N6 O7 P2 F12 ) C 37.3% (37.2%), H 5.14% (5.08%), N 8.06% (8.14%).

d 71, 35, 21, 15, 13, 6, 0, -80, -94. Elemental analysis (calc. Mn2 C34 H56 N6 O7 P2 F12 ) C 38.8% (38.5%), H 5.49% (5.32%), N 8.06% (7.92%). Physical measurements 1

H NMR spectra (400.0 MHz) were recorded on a Varian Mercury Plus. Chemical shifts are denoted relative to the solvent residual peak (1 H NMR spectra CD3 CN: 1.94 ppm).27 Elemental analyses were performed with a Foss-Heraeus CHN-O-Rapid or a EuroVector Euro EA elemental analyzer. EPR spectra (X-band, 9.46 GHz) were recorded on a Bruker ECS106 instrument in liquid nitrogen (77 K). Samples for measurement were transferred from the reaction solution (250 mL) to an EPR tube, which was frozen to 77 K immediately. UV-Vis spectra were recorded on a Hewlett Packard 8453 spectrophotometer or a JASCO 570 UVVis-NIR spectrometer using either 2 or 10 mm pathlength quartz cuvettes. Electro-spray ionization mass spectra were recorded on a Triple Quadrupole LC/MS/MS Mass spectrometer (API 3000, Perkin-Elmer Sciex Instruments). A sample (2 ml) was taken from the reaction mixture at the indicated times (vide infra) and was diluted in CH3 CN (1 ml) before injection in the mass spectrometer (via syringe pump). Mass spectra were measured in positive and negative mode and in the range of 100–1500 m/z. Ion-spray voltage: 5200 V, orifice: 15 V, ring: 150 V, Q0: -10 V. Procedures employed for catalysis studies Procedure A. The alkene (10 mmol), 1,2-dichlorobenzene (internal standard, 735 mg, 5.0 mmol), 1 (8.1 mg, 10 mmol) and the respective acid (0.10 mmol) in acetonitrile (10 ml) were cooled to 0 ◦ C. H2 O2 (0.74 ml, 13 mmol) was added via syringe pump over 6 h (0.12 ml/h). The reaction mixture was stirred at 0 ◦ C for 1 h after the addition of H2 O2 was completed, prior to sampling by GC. Procedure B. Catalyst pretreatment with H2 O2 . H2 O2 (30 ml, 0.53 mmol) was added to a mixture of 1,2-dichlorobenzene (735 mg, 5.0 mmol), 1 (8.1 mg, 10 mmol) and the respective carboxylic acid (0.10 mmol) in acetonitrile (7 ml) at room temperature. The mixture was stirred for 20 min, after which the alkene (10 mmol) was added together with acetonitrile (3 ml) and the mixture was cooled to 0 ◦ C. H2 O2 (0.71 ml, 12.5 mmol) was added via syringe pump (0.12 ml h-1 ). The reaction mixture was stirred at 0 ◦ C for 1 h after the addition of H2 O2 was completed, prior to sampling by GC. GC analyses were performed on an Agilent 6890 Gas Chromatograph equipped with a HP-1 dimethyl polysiloxane column (30 m ¥ 0.25 mm ¥ 0.25 mm). Peak identification and calibration were performed using independent samples (either purchased from a commercial supplier or synthesized independently11 ). Conversion and turnover numbers were determined in duplo employing 1,2dichlorobenzene as internal standard. All values are ± 10%. 18

[Mn

III 2

(l-O)(l-2-methoxy benzoato)2 (tmtacn)2 ](PF6 )2 (7)

7 was prepared as for 3 except 2-methoxybenzoic acid acid (33.5 mg, 0.22 mmol) was used in place of benzoic acid. Yield 71% (75 mg, 0.07 mmol). Mass spec. (calc. Mn2 C34 H56 N6 O7 m/z) 7(PF6 )+ 915.4, 72+ 385.3; 1 H NMR (400 MHz, CD3 CN) This journal is © The Royal Society of Chemistry 2008

O-labelling

Samples at t = 60 min where analyzed by GC-MS. GC: HP 6890 equipped with a HP-5 column (30 m ¥ 0.32 mm ¥ 0.25 mm). MS: JMS-600H using chemical ionization (reaction gas: NH3 ). The samples were analyzed by GC (HP-1 column, with FID) also as described above to determine t.o.n. 18 O-labelling studies Dalton Trans., 2008, 6283–6295 | 6285

where performed on cyclooctene on 1/20th scale of standard conditions,11b with the adjustment that a 2% H2 O2 solution was used. Values corrected for H2 O2 /H2 O isotopic composition. For the system 1/oxalic acid, the isotopic labelling with H2 18 O water for the second stage of the reaction (vide infra) was carried out as follows. The reaction was first performed using procedure A except that cis-2-heptene (250 mmol) was used as substrate in place of cyclooctene. H2 O2 (50 w/w%, 4 ¥30 ml, 2.1 mmol) was added every 15 min over 1 h. The reaction mixture was then left overnight and the presence of a MnIII 2 -bis(m-carboxylato) complex was confirmed by UV-Vis spectroscopy. Part of this reaction mixture (500 ml) was taken and subjected to the procedure for 18 O-labelling described above except that 250 mmol of cycloctene were used in place of 500 mmol of cycloctene. Fig. 3 Molecular structure of 3. PF6 - anions omitted for clarity.

Definition of t.o.n. and mass balance. Turnover number (t.o.n.): mol product/mol catalyst. Mass balance [%] = unreacted alkene [%] + (cis-diol and epoxide products [%]). Deviation from a mass balance of 100% indicates loss through further oxidation of the cisdiol formed initially and/or the occurrence of competing oxidation pathways other than cis-dihydroxylation and epoxidation.11a Preparation of suberic acid from cis-1,2-cyclooctanediol. H2 O2 (30 ml, 0.53 mmol) was added to a mixture of 1 (8.1 mg, 10 mmol) and 2,6-dichlorobenzoic acid (57.3 mg, 0.30 mmol) in CH3 CN (7 ml) at room temperature. The mixture was stirred for 20 min at room temperature followed by addition of cis-1,2-cyclooctanediol (0.72 g, 5 mmol) and CH3 CN (3 ml). The mixture was cooled at 0 ◦ C. H2 O2 (50% w/w, 1.13 ml, 20 mmol) was added via syringe pump (0.14 ml h-1 ). The reaction mixture was stirred at 0 ◦ C for 1 h after addition of H2 O2 was completed. Water (10 ml) was added and the mixture was adjusted to pH 12 by addition of 2 M aq. NaOH. The basic aqueous layer was washed with Et2 O (3 ¥ 15 ml) and subsequently acidified to pH 1 with 4 M aq. HCl. The acidic aqueous layer was extracted with Et2 O (5 ¥ 15 ml) and the combined organic extracts were washed with brine (20 ml). After drying over anhydrous Na2 SO4 the solvent was evaporated in vacuo yielding a colourless solid (367 mg, 42%). 1 H NMR spectrum (400 MHz, acetone-d6 ) d 1.31–1.35 (m, 4H), 1.55–1.58 (m, 4H), 2.25 (t, J = 7.3 Hz, 4H). CI-MS m/z 192 [M + NH4 ]+ .

Fig. 4 Equilibrium between [MnIII 2 (m-O)(m-2-hydroxybenzoato)2 (tmtacn)2 ]2+ 4 and [MnIII (salicylato)(tmtacn)]+ 8.

Results The synthesis and characterisation of complexes 1,1 2,11a 611b and 811b are described elsewhere. Complexes 3 (Fig. 3), 5 and 7 were prepared by previously reported methods.11 The dinuclear complex [MnIII 2 (m-O)(m-2-hydroxybenzoato)2 (tmtacn)2 ]2+ 4 could be obtained in low yield by the general method employing Lascorbic acid as reductant or in situ by reduction of 1 with hydrazine in CH3 CN in the presence of 2 equiv. of salicylic acid.11b However, even in dry CH3 CN this complex is unstable and converts readily to the more stable green mononuclear complex [MnIII (salicylato)(tmtacn)]+ (8), Fig. 4.11b Complexes 4 and 8 exhibit distinct UV-Vis absorption spectra (Fig. 5) with 4 showing the characteristic absorptions of a [MnIII 2 (m-O)(mRCO2 )2 (tmtacn)2 ]2+ complex.1 ESI-MS of 4 in CH3 CN confirmed the presence of the dinuclear complex (m/z 371 [4]2+ and 887 [4+ PF6 ]+ ), however, the mononuclear complex [8]+ (m/z 362) was observed also. 6286 | Dalton Trans., 2008, 6283–6295

Fig. 5 UV-Vis spectra in CH3 CN of [MnIII 2 (m-O)(m-hydroxybenzoato)2 (tmtacn)2]2+ 4 (1 mM) and [MnIII (salicylato)(tmtacn)]+ 8 (2 mM).

Oxidation of cyclooctene catalysed by 1/salicylic acid The influence of co-catalytic amounts of salicylic acid, L-ascorbic acid and oxalic acid on the oxidation of cyclooctene with H2 O2 catalysed by 1 was investigated under the conditions employed in our earlier studies;11 i.e. 1/RCO2 H/cyclooctene/H2 O2 in the ratio 1/10/1000/1300 in CH3 CN. The changes in concentration of substrate and cis-diol and epoxide products over the course of the reaction where salicylic acid is present at 1 mol% w.r.t. cyclooctene (Fig. 6 and Table 1, entry 1), show that i) there is an initial lag period before conversion commences and ii) formation of both the cis-diol and epoxide products commences concomitantly, as observed previously for other carboxylic acids.11 The lag period is due primarily to the This journal is © The Royal Society of Chemistry 2008

ratio (Table 1, entry 7), comparable with the other para-substituted benzoic acids. The mononuclear [MnIII (salicylato)(tmtacn)]+ complex 8 (vide infra) (in combination with 1 mol% of salicylic acid) shows essentially the same reactivity as the system 1/salicylic acid (entries 5 and 1, respectively), however the lag period is reduced from 2 h to 45 min, and as a consequence the conversion is somewhat higher (the cis-diol/epoxide ratio is unaffected). Oxidation of cyclooctene and 1-octene catalysed by 1/L-ascorbic acid

Fig. 6 Catalysed oxidation of cyclooctene by 1 (1 mM) in the presence of salicylic acid (10 mM, 1 mol%) in CH3 CN with H2 O2 .

delayed transformation of the {MnIV 2 (m-O)3 } bridged complex 1 to a MnIII 2 bis(m-carboxylato) complexes similar to 2 or B (Fig. 1 and Scheme 2, respectively).11 With increased concentrations of salicylic acid (5 and 10 mol%), it is apparent that epoxidation becomes favoured over cisdihydroxylation (Table 1, entries 1–3). A higher preference for epoxidation at increased carboxylic acid concentration was also observed previously for other carboxylic acids, such as CCl3 CO2 H.11 3-Hydroxybenzoic acid and 5-bromosalicylic acid show similar product distributions (entries 6 and 8). 4Hydroxybenzoic acid gives a substantially higher cis-diol/epoxide

The use of L-ascorbic acid/sodium ascorbate as an effective additive in the epoxidation of two terminal alkenes (methyl acrylate and 1-octene) and the oxidation of both 2-pentanol (to 2-pentanone) and 1-butanol (to butanoic acid) catalysed by Mn(II)/tmtacn has been reported previously by Berkessel and coworkers.20 The oxidation of 1-octene in the presence of 1 mol% of L-ascorbic acid under the conditions employed in the present study resulted in efficient epoxidation of 1-octene with only trace amounts of the cis-diol product being formed (Table 2, entry 2), in agreement with the results of Berkessel et al.20 (entry 1). By contrast, oxidation of cyclooctene resulted in the formation of substantial amounts of the corresponding cis-diol in addition to the epoxide product (entry 3).28 The conversion of cyclooctene to both the cis-diol and epoxide products commenced at the same time and the ratio of both products remained constant over the entire course of the catalysed

Table 1 Oxidation of cyclooctene catalysed by 1/hydroxy- and methoxy-substituted benzoic acidsa T.o.n.

1 2 3 4 5 6 7 8 9 10 a

Carboxylic acid

Conv. (%)

cis-Diol

Epoxide

cis-Diol/epoxide ratio

Mass bal. (%)

Salicylic Salicylicd Salicylice Salicylicb Salicylicc 3-Hydroxybenzoic 4-Hydroxybenzoic 5-Bromosalicylic 2-Methoxybenzoic 4-Methoxybenzoic

64 74 61 75 75 69 46 62 47 26

225 132 102 30 249 260 219 252 244 137

320 510 431 590 371 325 170 296 158 83

0.7 0.25 0.24 0.05 0.7 0.8 1.3 0.9 1.5 1.7

90 90 92 87 87 89 93 92 93 96

Procedure A, 1 mol% carboxylic acid. b 1-Octene as substrate. c Complex 8. d 5 mol%. e 10 mol%.

Table 2 Oxidation of 1-octene and cyclooctene catalysed by 1 (0.1 mol%) at 0 ◦ C in the presence of L-ascorbic acid (1 mol%)a T.o.n. Additive (mol%) 1 2 3 4

1-Octene L-Ascorbic acid/Na ascorbateb L-Ascorbic acid (1) Cyclooctene L-Ascorbic acid (1) Dehydroascorbic acid (1)

Conv. (%) N.d. 89 83 3

cis-Diol

0 36 275 52

Epoxide

Mass bal. (%)

1110 (max. 1333) 672

N.d. 82

384 76

83 110

Conditions: alkene (1000 mM), 1 (1 mM), L-ascorbic acid (10 mM), 1,2-dichlorobenzene (500 mM) in CH3 CN at 0 ◦ C. H2 O2 (50% aq., 1300 mM) was added by syringe pump addition over 6 h, t.o.n. and conversion reported after 7 h (general procedure A). b From ref. 1, conditions: 1octene:H2 O2 :MnII (OAc)2 :tmtacn:L-ascorbic acid:Na ascorbate 1333:3500:1:1.3:0.5:2.1 in CH3 CN/H2 O (6:4).

a


Dalton Trans., 2008, 6283–6295 | 6287

reaction (Fig. 7). In contrast to the system 1/CCl3 CO2 H, a lag period was not observed (vide infra). For the oxidised form of L-ascorbic acid, i.e. dehydroascorbic acid, only very low activity was observed.

Fig. 7 Oxidation of 1-octene (upper) and cyclooctene (lower) catalysed by 1 (1 mM)/L-ascorbic acid (10 mM) in CH3 CN at 0 ◦ C.

Oxidation of cyclooctene and 1-octene catalysed by 1 in the presence of oxalic acid De Vos and coworkers reported the use of oxalate buffers in CH3 CN/H2 O in the epoxidation of several alkenes catalysed by Mn(II)/tmtacn.21 Again, in order to make a direct comparison with the catalytic system 1/CCl3 CO2 H, reactions were performed at 0.1 mol% of 1 and 1 mol% of oxalic acid, Table 3, entry 2). In agreement with the results reported by De Vos et al. (entry 1), under the present conditions effective epoxidation of 1octene was observed. However, with cyclooctene, substantial levels of cis-dihydroxylation are observed in addition to epoxidation (entry 3). Whereas for the majority of carboxylic acids examined, a lag period followed by a rapid conversion of 1 to a MnIII 2 -mbiscarboxylato complex and simultaneous commencement of both cis-dihydroxylation and epoxidation is observed, for the system 1/oxalic acid, oxidation of the alkene commences immediately upon addition of H2 O2 and two distinct phases can be distinguished over the course of the reaction: (i) over the first 3 h (at 298 K)29 conversion of the alkene to the epoxide product is the only significant process observed; while (ii) from 3 h onwards both epoxidation and cis-dihydroxylation proceed with similar levels of conversion. This change in product distribution occurs at a time coincident with a pronounced change in the UV-Vis spectrum of the reaction mixture (Fig. 12c and d, vide infra). Over the first 3 h of the oxidation of cyclooctene catalysed by 1/oxalic acid, 7 t.o.n.s of cis-diol and 272 t.o.n.s of epoxide product are obtained, giving a cis-diol/epoxide ratio of 0.03. Between 3–5 h,30 however, 136 t.o.n.s for the cis-diol and 91 t.o.n.s of epoxide product are obtained, resulting in a cis-diol/epoxide ratio of 1.5 (within this 2 h period). The cis-diol/epoxide selectivity in the later stage of the reaction is similar to that found for alkanoic acids (ca. 2–3:1)11 and that reported for 1/gmha8l (cis-diol/epoxide 1.2). If, after 2.5 h, a second 1 mol% of oxalic acid is added to the reaction mixture, the formation of the cis-diol product (from 3 h onwards) is suppressed in favour of continued epoxide formation (Fig. 8). This suggests that the oxalic acid inhibits formation of the catalytically active species present in the later stage of the reaction. The observation of two distinct stages in the oxidation of cyclooctene catalysed by 1/oxalic acid is intriguing and raises

Table 3 Catalysed oxidation of alkenes by 1 (0.1 mol%) at 0 ◦ C in the presence of oxalic acid (1 mol%)a T.o.n. Additive (mol%) 1 2 3 4 5 6 7

1-Hexene Oxalic acid/Na oxalate (0.2/0.2)b 1-Octene Oxalic acid (1) Cyclooctene Oxalic acid (1) Oxalic acid (1)c Oxalic acid (1)d Oxalic acid (1)e Oxalic acid (1)e , f

Conv. (%)

cis-Diol

Epoxide

Mass bal. (%)

>99

0

660

>98

86

0

724

86

92 97 99 94 99

169 154 14 176 15

599 622 860 564 838

85 81 89 80 87

a

Procedure A. b From ref. [21], conditions: 1-hexene:H2 O2 :oxalate:Mn 666:1000:3:1 in CH3 CN:H2 O (3.5:1). c H2 O2 and H2 O pre-treatment. d An extra 1 mol% of oxalic acid added at t = 3.5 h. e Performed at r.t. f An extra 1 mol% of oxalic acid added at t = 2.5 h.

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study is in agreement with that observed by Gilbert et al.8i This suggests that in both the present study and in the report of Gilbert et al. carboxylic acids other than oxalic acid may play the role of ligand to the catalyst. Attempts to probe 18 O incorporation into the cis-diol and epoxide substrate is hampered by the uncertainty in the 18 O level of the H2 O present in the reaction mixture at this stage (as water is both added with the H2 O2 and is released stoichiometrically from H2 O2 following epoxidation). The absence of significant levels of cis-dihydroxylation during the first stage of the reaction can allow for the approximation that the single incorporation into the cisdiol (55%, 16 O18 O using H2 18 O/H2 16 O2 ) product is representative of the 2nd stage of the reaction.32 Electronic and EPR spectroscopy

Fig. 8 Oxidation of cyclooctene (1 M, squares) in CH3 CN at r.t. catalysed by 1 (1 mM) and oxalic acid (10 mM) yielding both epoxide (circles) and cis-diol (triangles) products (upper) and with an additional batch of oxalic acid (10 mM) added at t = 2.5 h (lower).

the possibility that two distinct catalysts or catalytic pathways are operating in each of these two stages. 18 O labelling of both H2 O2 and H2 O has proven to be a valuable probe in elucidating the source of oxygen in the products of oxidation reactions.11b,18,31 However its use in probing the oxalic acid promoted oxidation of cyclooctene catalyzed by 1 is complicated by the fact that there are two distinct mechanisms operating during the 1st and 2nd half of the reaction. These two distinct catalyst systems require examination of the two stages independently; otherwise only an ‘average’ result will be obtained. Probing the present system through 18 O labelling as performed previously11 is therefore non-trivial. In the present study 18 O labelled water was employed as a probe. For the epoxide product the incorporation of oxygen atoms from water in the first part of the reaction is minor (2%). Indeed it is significantly lower than the level of 18 O incorporation reported by Gilbert et al. (7%) in the epoxidation of cinnamic acid catalysed by Mn-tmtacn/oxalic acid.8i However, it should be noted that the substrate employed in that example (i.e. cinnamic acid) contains a free carboxylic acid group. Indeed the level 18 O incorporation from water in the 2nd stage of the reaction (9%) in the present This journal is © The Royal Society of Chemistry 2008

Hydroxybenzoic acids. For the majority of carboxylic acids examined, with the exception of salicylic acid, the formation of MnIII 2 bis(m-carboxylato) species from 1 at the end of the lag period (Fig. 6) was confirmed earlier11 by the appearance of characteristic features in the UV-Vis absorption spectra of the reaction mixture. For 1/salicylic acid, a considerable increase in absorption in both the UV and visible regions was observed at the end of the lag period also.11a However, the increase was larger and the final spectrum atypical of a MnIII 2 bis(m-carboxylato) complex (Fig. 9). The differences in the absorption spectra of the reaction mixture with 1/salicylic acid compared with those of the other carboxylic acids can be assigned, tentatively, to the presence of a ring-opened dinuclear complex (i.e. B, Scheme 2).11b When 8 ([MnIII (salicylato)(tmtacn)]+ ) is used in place of 1, the UV-Vis spectrum of the reaction mixture after the lag period is identical to that observed for the reaction with 1/salicylic acid. This is in agreement with the observation that the 1/salicylic acid and 8/salicylic acid systems provide essentially the same product distributions (Table 1). ESI-MS of the reaction mixture at several stages of the catalysis did not reveal the appearance of a MnIII 2 bis(carboxylato) dimer (i.e. 4) and instead only the mononuclear complex [MnIII (salicylato)(tmtacn)]+ has been observed. However, it should be noted that even the ESI mass spectrum of the MnIII 2 bis(msalicylato) complex 4 in dry CH3 CN solution shows the signal assigned to the mononuclear complex 8 in addition to the dinuclear MnIII 2 bis(m-carboxylato) complex 4.33 EPR spectra (77 K) of the reaction mixture at all stages of the oxidation of cyclooctene catalysed by 1/salicylic acid showed no signals as was the case for the reaction catalysed by 1/CCl3 CO2 H.11 This precludes the presence of complexes in oxidation states which are EPR active such as a MnII monomer and MnII 2 and MnIII,IV 2 dimer. MnIII 2 dimers such as 2–7 are EPR silent at 77 K. L-Ascorbic acid. The oxidation of cyclooctene catalyzed by 1/ascorbic acid was monitored by UV-Vis spectroscopy. Addition of L-ascorbic acid (10 mM, 1 mol%) to the mixture of 1 (1 mM) and cyclooctene (1.0 M) in CH3 CN, results in a decrease in the absorbance due to 1 (395 and 495 nm) over several minutes. EPR spectroscopy after addition of L-ascorbic acid shows the appearance of a 16-line spectrum (a = 69 G, Fig. 11a(i)). This 16-line species is indicative of a mixed valent MnIII,IV 2 species,34 most probably {MnIII,IV 2 (m-O)3 }, with an a-value identical to that

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Addition of H2 O2 (50 equiv.) resulted in an increase in the absorbance in both the UV and visible regions of the spectrum, which then decreased in intensity over time, and eventually, at low concentrations of H2 O2 (i.e. after consumption of the H2 O2 during the oxidation of cyclooctene), a spectrum typical for a {MnIII 2 (mO)(m-carboxylato)2 } species was observed (Fig. 10a, dotted line).

Fig. 9 (a) UV-Vis spectra of the reaction mixture during the catalytic oxidation of cyclooctene (1 M) in CH3 CN at 0 ◦ C with 1 (1 mM) and salicylic acid (10 mM) (at 10 min intervals between 0–2 h) from ref. 11a, SI. (b) UV-Vis spectra in CH3 CN (1 mM) of complexes 4, 5 and 6.

reported by Hage et al.22 for the one electron reduction of 1 by Co(Cp)2 in CH3 CN (see Scheme 3). Fig. 10 UV-Vis spectra of the reaction mixture of the oxidation of cyclooctene (1 M) by H2 O2 catalysed by 1 (1 mM) and L-ascorbic acid (10 mM) in CH3 CN. L-Ascorbic acid is added in one batch at t = 0, addition of H2 O2 begins at t = 10 min. (a) Immediately after each addition of H2 O2 (53 mM, every 15 min) an intense featureless spectrum is observed (dashed line), while at low H2 O2 concentrations (14 min after addition of each batch of H2 O2 ) a spectrum characteristic of a MnIII 2 bis(carboxylato) complex is observed (dotted lines). (b) Absorbance changes, over time, monitored at 400, 480 and 520 nm (addition of H2 O2 is indicated by arrows).

Scheme 3 Summary of species present during the oxidation of alkenes catalysed by 1/L-ascorbic acid.

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The EPR spectrum after addition of H2 O2 shows a 16-line spectrum also, but with an increased a-value of 76 G (Fig. 11a ii–iv). As for the absorption spectrum the intensity of this latter 16-line signal decreases with time. A subsequent addition of H2 O2 results in the recovery of the same intense featureless UV-Vis absorption spectrum (Fig. 10b) and in the intensity of the 16 line This journal is © The Royal Society of Chemistry 2008

EPR signal (Fig. 11b), both of which decrease again as the H2 O2 concentration decreases.

Fig. 11 EPR spectra of samples taken from the reaction mixture of the oxidation of cyclooctene (1 M) in CH3 CN catalysed by 1 (1 mM) and L-ascorbic acid (10 mM): (i) after stirring for 15 min prior to addition of H2 O2 (lower solid line, y-axis offset for clarity), (ii) 15 min after 1st addition of H2 O2 (solid line), (iii) 15 min after 2nd addition of H2 O2 (dashed line); (iv) 1 min after 3rd addition of H2 O2 (dotted) (9.47 G, attenuation/power 10 db/20.1 mW, conversion time 81.9 ms) (b) Signal intensity just before (solid squares) and immediately after (open circles) addition of H2 O2 (50 equiv. w.r.t. Mn2 ).

The 16-line EPR signal observed after addition of H2 O2 is assigned to a mixed valent MnIII,IV 2 species. However, the change in hyperfine coupling constant from 69 to 76 G indicates a change in the nature of the bridging ligand(s) as the a-value is very similar to that observed for complexes such as 2 (Fig. 1, at concentrations above 10 mM) in the presence of H2 O2 .22 Attempts to identify this MnIII,IV 2 species (and the MnIII 2 bis(mcarboxylato) species observed by UV-Vis spectroscopy) by ESIMS were unsuccessful: i.e. although 1 was converted completely within 30 min of the start of the reaction and a series of weak signals were observed, conclusive assignments could not be made. However, as suberic acid, which can form through further oxidation of the cis-diol product,11b hydrolysis of the g-lactone of L-ascorbic acid or dehydroascorbic acid or an oxidation product of either compound (e.g., C–C bond cleavage between the ketone This journal is © The Royal Society of Chemistry 2008

functional groups of dehydroascorbic acid or oxidation of the primary alcohol) are all potential sources of the carboxylato ligands, the formation of charge neutral species is possible and would preclude detection by ESI-MS. Oxalic acid. For the oxidation of cyclooctene catalysed by 1/oxalic acid changes in the UV-Vis absorption spectrum of the reaction mixture, and hence 1, are observed as the reaction proceeds, however as for salicylic acid, EPR signals are not observed at any stage. Surprisingly, when the reaction mixture is allowed to stand at r.t. overnight, the UV-Vis spectrum becomes that typical of a [MnIII 2 (m-O)(m-carboxylato)2 (tmtacn)2 ]n+ complex (Fig. 12b). The observation of a UV-Vis spectrum characteristic of [MnIII 2 (m-O)(mcarboxylato)2 ]2+ species demonstrates the presence of carboxylic acids capable of acting as bridging ligands. Attempts to identify this/these carboxylato ligand(s) (e.g., by ESI-MS, both positive and negative mode) where unsuccessful, however. As for the system 1/ascorbic acid, possible candidates for carboxylato ligands are oxalic acid or carboxylic acid(s) derived from overoxidation of the cis-diol product formed initially. Importantly, suberic acid can be formed during the catalytic oxidation of cyclooctene (see the Experimental section for details). Since both oxalic acid and suberic acid are diacids, a possible explanation for the absence of a clear signal due to a MnIII 2 bis(m-carboxylato) species in the ESIMS spectra is deprotonation of the non-coordinated carboxylic acid group, resulting in the formation of charge neutral species, which are not detectable by ESI-MS. Nevertheless, the evolution of the UV-Vis absorption spectrum with time shows that complex 1 undergoes conversion to a catalytically active species immediately upon addition of H2 O2 . The increase in molar absorptivity and the absence of any EPR signals suggests that the active catalyst is in the MnIII or MnIII 2 state rather than a MnII , MnII 2 or MnIII,IV 2 state. Remarkably, after ca. 3 h a sudden decrease in absorptivity is observed and the spectrum changes to that reminiscent of the [MnIII 2 (m-OH2 )2 (mRCO2 )2 (tmtacn)2 ]2+ ‘open’ complex (i.e. B, see Scheme 2). On standing overnight the spectrum of the reaction mixture changes further to that typical of a [MnIII 2 (m-O)(m-RCO2 )2 (tmtacn)2 ]2+ complex.

Discussion Hydroxybenzoic acid promoted oxidation of alkenes by 1 Although it is tempting to consider the involvement of the proximal –OH group of salicylic acid in ligation to the manganese ions, or as proton donor/acceptor group in rationalising the selectivity towards epoxidation over cis-dihydroxylation for the system 1/salicylic acid, it should be noted that the selectivity achieved with 3-hydroxybenzoic acid is essentially the same as with salicylic acid (Table 1, entry I and 6).11 UV-Vis spectroscopy shows that the mononuclear complex 8 is not present during the catalytic oxidation of cyclooctene by 1/salicylic acid, in contrast to ESI-MS studies. However, it should be noted that for the MnIII 2 bis(m-carboxylato) complex 4 generated in CH3 CN solution from 1/salicylic acid by reduction with H2 NNH2 , the mononuclear complex 8 was present as a major signal in its mass spectrum. Furthermore, 18 O-labeling results show similar incorporation of the oxygens in the cis-diol and epoxide products for the reaction catalysed by 1/salicylic acid as was found for Dalton Trans., 2008, 6283–6295 | 6291

Fig. 12 Catalysed oxidation of cyclooctene (1 M) in CH3 CN at 298 K with 1 (1 mM) and oxalic acid (10 mM). (a) UV-Vis spectra between 0–2 h. (b) UV-Vis absorption at 2.0, 3.3, 6.9 h and after 20 h. (c) Changes in absorbance at 360, 396, 494 and 960 nm with time. (d) Time profile showing concentration of cyclooctene (squares, solid line), epoxide (circles, dotted line) and cis-diol (triangles, dashed line).

other carboxylic acids. Thus, for the catalytic system 1/salicylic acid the results suggest that the mechanism is essentially the same as that described for 1/CCl3 CO2 H,11b and that the differences in spectroscopic features of the reaction mixture observed are due to the involvement of the hydroxyl group in the electronic structure of the complex. Hence, although the mononuclear complex 8 was isolated, UVVis spectroscopy demonstrates that this mononuclear species is not present during the catalytic oxidation reaction by the system 1/salicylic acid. Instead, just like for all other (substituted) benzoic acids, tentatively, a ‘open’ dinuclear MnIII 2 bis(carboxylato) complex (i.e. B, Scheme 2), which does not bear a m-oxido bridge is present (cf. A, Scheme 2). L-Ascorbic

acid promoted oxidation of alkenes by 1

Berkessel and coworkers reported the epoxidation of terminal alkenes by the combination of Mn-tmtacn and L-ascorbic acid.20 In the present study these results were confirmed, however, especially when cyclooctene is used as substrate with L-ascorbic acid as additive, substantial cis-dihydroxylation is observed in addition to 6292 | Dalton Trans., 2008, 6283–6295

epoxidation. That the system 1/L-ascorbic acid shows higher levels of cis-dihydroxylation for cyclooctene than for terminal alkenes, is in agreement with the trends observed for the substrate scope of the 1/carboxylic acid promoted catalytic oxidation of alkenes,11a where the highest selectivities towards cis-dihydroxylation were observed with electron rich cis-alkenes. The absence of a lag period for the system 1/L-ascorbic acid can be attributed to the reducing power of L-ascorbic acid. Whereas for other carboxylic acid promoted systems the reduction of 1 by H2 O2 is facilitated by protonation of 1 by the carboxylic acid, from both EPR and UV-Vis spectroscopy it is clear that L-ascorbic acid is a sufficiently powerful reductant to reduce 1 directly to lower oxidation states which can engage in ligand exchange to form active species. From UV-Vis spectroscopic data it is clear that a MnIII 2 bis(m-carboxylato) complex is present when the concentration of H2 O2 is low. However, the observation of a MnIII,IV 2 bis(carboxylato) species by EPR and UV-Vis spectroscopy may be misleading in terms of the direct involvement of such a species in the catalysis. The propensity of manganese carboxylate complexes to engage in disproportionation and self-exchange reactions should not be overlooked.1 Indeed, the one-electron This journal is © The Royal Society of Chemistry 2008

reduction of the MnIII 2 complex to a MnII,III 2 state followed by twoelectron oxidation by H2 O2 or disproportionation reactions of the MnIII 2 complexes would lead to appearance of a MnIII,IV 2 state at high H2 O2 concentrations. Furthermore, at high concentrations of 1 (> 5 mM) in the presence of excess carboxylic acid additives the appearance of a similar 16-line EPR signal was observed upon addition of H2 O2 to 1/RCO2 H, suggesting that the MnIII,IV signal observed in the presence of L-ascorbic acid may not hold direct relevance to the catalysed oxidation of the alkene substrate itself. Oxalic acid promoted oxidation of alkenes by 1 The catalyst system 1/oxalic acid was found to provide the selective epoxidation of a terminal alkene (i.e. 1-octene), in agreement with the report of De Vos and coworkers.21 From the time course of the reaction it is clear that a lag period, which is normally observed for the combination 1/carboxylic acid, is not present. As for L-ascorbic acid, it is possible that oxalic acid facilitates the reduction35 of 1 and thereby activates the catalyst towards ligand exchange to form the active state. With cyclooctene as substrate, cis-dihydroxylation was observed in addition to epoxidation. However, substantial amounts of the cis-diol are formed only after 3 h at room temperature (and after ca. 4–5 h at 0 ◦ C), indicating that two distinct catalytic systems are operating. Furthermore, an abrupt change in the UV-Vis spectrum of the reaction mixture coincides with the onset of cisdihydroxylation activity. Importantly, if a second batch of oxalic acid (1 mol%) is added after 2.5 h, the occurrence of this second phase is suppressed, i.e. cis-dihydroxylation is not observed at any stage and only epoxidation takes place. While the nature of the catalytically active species during the first 3 h of the reaction remains elusive, during the latter period of the reaction, the oxalic acid system bears close resemblance to that of other alkanoic and benzoic carboxylic acids. UVVis spectroscopy indicates the presence of a dinuclear MnIII 2 bis(m-carboxylato) complex. However, the identity of the bridging carboxylato ligand(s) is as yet unclear. A possible candidate is suberic acid (formed from cyclooctene by C=C cleavage).

Summary For salicylic acid it is apparent that although the proximal hydroxy group has a profound influence on the coordination chemistry and spectroscopy of the MnIII 2 bis-(m-carboxylato) complex it forms upon reduction of 1, the mechanism by which salicylic acid enhances the activity of 1 in the oxidation of alkenes is essentially the same as that of other carboxylic acid co-catalysts.11b The effectiveness of L-ascorbic acid and oxalic acid in suppressing the catalase activity of Mn-tmtacn in order to attain effective epoxidation of alkenes was reported by the groups of Berkessel20 and De Vos,21 respectively. In our hands, and under somewhat modified conditions these additives gave effective epoxidation of (terminal) alkenes also. However, when cyclooctene was used as substrate, the combination of 1 and L-ascorbic acid or oxalic acid resulted in a catalytic system capable of cis-dihydroxylation in addition to epoxidation, as for the other carboxylic acid additives we examined earlier.11 For the substrate 1-octene the system 1/salicylic acid yields primarily the epoxide product (Table 1, entry 4). Both oxalic acid This journal is © The Royal Society of Chemistry 2008

and L-ascorbic acid additives give slightly higher conversion then salicylic acid (Table 2 and 3, respectively), however, for all three additives the epoxide is the main product in agreement with earlier reports.11,20,21 Several key differences between the systems 1/L-ascorbic acid, 1/oxalic acid and 1/RCO2 H, are notable and demonstrate clearly that more than one possible mechanism for enhancing the reactivity of the Mn-tmtacn system may be available. For the systems 1/L-ascorbic acid and 1/oxalic acid, the conversion of cyclooctene was seen to proceed immediately upon addition of H2 O2 . This is in stark contrast to the 0.5–3 h lag-period observed for the system 1/RCO2 H,11 which we demonstrated was due to the delayed reduction of 1 and subsequent formation of catalytically active MnIII 2 -bis(m-carboxylate) complexes. This difference in behaviour can, however, be rationalised on the basis of the inherent ability of both L-ascorbic and oxalic acid to act as reductants. For L-ascorbic acid this ability is manifested in the reduction of 1 observed by UV-Vis and EPR spectroscopy prior to addition of H2 O2 . For oxalic acid, although reduction is not observed prior to addition of H2 O2 , 1 is reduced quickly upon addition of H2 O2 to the reaction mixture, in contrast to the system 1/RCO2 H. Indeed, for dehydroascorbic acid very low activity was observed over the entire course reaction, primarily due to its inability to reduce 1 and the absence of a proton source to enable H2 O2 to engage in reduction of H1+ either. Oxalic acid behaves somewhat differently compared to all other carboxylic acids examined. Although 1 undergoes reduction and presumably ligand exchange reactions in the very early stages of the catalysis (observed as a rapid change in the UV-Vis spectrum), only epoxidation is observed during the first 3 h of the reaction and not until a sudden change in the UV-Vis spectrum of the reaction mixture (during the second phase of the 1/oxalic acid promoted reaction) are substantial amounts of cis-diol formed in addition to the epoxide product. In contrast, for all other carboxylic acids examined, cis-dihydroxylation and epoxidation commenced and proceeded simultaneously. The sudden change in selectivity accompanied with a sudden change in the UV-Vis spectrum of the reaction mixture of the catalytic system indicates the sudden formation of a different active species. The change in reactivity is very much likely to be due to the decomposition of oxalic acid as addition of a second equivalent of oxalic acid after 2 12 h results in a near complete suppression of this change in selectivity (i.e. cis-dihydroxylation is supressed). It is clear that at least the possibility of formation of dinuclear MnIII 2 -bis(m-carboxylato) species either early (L-ascorbic acid) or later in the reaction (oxalic acid) is indicated in the present study. However, when L-ascorbic acid is used as additive, a mixed-valent MnIII,IV 2 species is observed at high H2 O2 concentration also. The hyperfine splitting constant indicates that this mixed-valent species contains different bridging ligands than present in 1 (which contains three m-oxo bridges), possibly two carboxylato bridges.

Conclusions Additives such as oxalic acid have been proposed to induce the formation of mononuclear species36 which can engage in the catalysed oxidation of alkenes. However, from the data presented here, it is clear that definitely for salicylic acid and very likely for L-ascorbic acid and oxalic acid (that is, during the second phase Dalton Trans., 2008, 6283–6295 | 6293

of the reaction) dinuclear MnIII 2 bis(m-carboxylato) complexes are present and are likely to be involved in the catalysed cisdihydroxylation and epoxidation of cyclooctene. Unfortunately, the exact nature of the bridging carboxylato ligands in the case of the L-ascorbic and oxalic acid promoted reactions remains elusive. Nevertheless, it is clear that in confronting the challenge of determining the mode of action of additives in altering the reactivity of a catalyst, it is not only necessary to consider a single possible role but rather the possibility of multiple roles. In the case of ascorbic and oxalic acid, the primary role in enhancing reactivity is the facilitation of the initial reduction of 1, thereby allowing for ligand exchange to form the active complex. Secondly, the two additives provide carboxylato ligands either directly, from decomposition products of the additives themselves or via oxidation of the substrate. Finally, it is clear that in the case of these additives more than one catalytic species may be in operation over the course of the reaction complicating mechanistic studies considerably This finding holds considerable implications as to the approaches that can be taken in unravelling mechanistic details in transition metal catalysed oxidation chemistry; not least that it is insufficient to focus on only one aspect or technique in mechanistic studies.

Acknowledgements We thank Ms T. D. Tiemersma-Wegman, Mr C. Smit, Ms A. van Dam, Dr A. P. Bruins and Mr A. Kiewiet for assistance with GC and ESI-MS analysis and the Dutch Economy, Ecology, Technology (Grant EETK01106) program for financial support. We thank Prof. J. Reedijk and Dr J. G. Roelfes for discussions and valuable suggestions.

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