Current Organic Chemistry, 2012, 16, 73-88
73
Catalytic Applications of Polymer-Supported Molybdenum Complexes in Organic Transformations Mannar R. Maurya* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India Abstract: Procedures for covalent bonding of ligands to polymer support and their reactions with molybdenum precursors to obtain polymer-supported molybdenum complexes are outlined. These supported molybdenum complexes are used as catalysts for organic transformations such as oxidation of alkenes, phenols, allyl alcohol, ethylbenzene and benzoin, and for polymerization reaction. Higher activity, recyclability and high turn over numbers of most supported catalysts are confirmed in many cases which offer real prospect for technological developments.
Keywords: Polymer-supported complexes, catalysts, organic transformations, alkene epoxidation, catalytic applications of molybdenum complexes. 1. INTRODUCTION th
In 20 century, catalysts have played a vital role in chemical industries and emerged as a key factor for the economic development of countries. More than 99 % processes in petrochemicals, fine chemicals, pharmaceuticals, fertilizers and food industries involve the use of catalyst based technologies. Amongst the variety of catalytic active species known, the transition metal complexes catalyze the wide range of chemical reactions [1]. Most of the catalytic processes, e.g. oxidation, hydrogenation, polymerization etc., widely engaged in the manufacture of fine as well as bulk chemicals, are homogeneous in nature and produce a large amount of side waste materials that cause a serious problem to surrounding environment. There has been, therefore, a continuous necessity to control hazardous effect of such chemical processes or substances for the sound development of catalysts and catalytic processes [2]. The efficient use of solid supported catalysts can go a long way towards achieving these goals [3]. Various types of organic polymers or inorganic solids like zeolite/molecular sieve, silica, alumina, other metal oxides, and carbon have been used as solid support for the heterogenization of homogeneous catalysts. In the past few decades, the polymeric supports have revolutionized organic syntheses. The innovative discovery of Merrifield resin [4] has stimulated number of research groups [5,6] to investigate the prospects of polymer-supported catalysts. Since the catalytic action occurs at the specific site on solid surface, often called as ‘active site’, the uniform dispersion of active sites i.e. metal catalysts may be achieved on immobilizing them on polymersupport. This is desirable for the significant improvement of the catalytic action of a catalyst [7]. Further, polymer-bound heterogeneous catalysts offer many advantageous features such as environment friendly and easy recovery from the reaction mixtures, better thermal stability and reduced possibility of contamination of catalyst with products. Molybdenum can easily form intermediate complex with electron rich peroxo group, which in turn can transfer oxygen to the organic substrates. Therefore, molybdenum complexes, in the presence of oxidants like H2O2, tert-butylhydroperoxide (TBHP), may
*Address correspondence to this author at the Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India; Tel: +91 1332 285327; Fax: +91 1332 273560; E-mail:
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act as good oxidation catalyst. In fact, catalytic conversion of propylene to its epoxide by homogeneous molybdenum complex using TBHP, a key industrial process, is known for very long time [8]. Immobilization of catalytically active molybdenum complexes on suitable solid support may also maintain all these advances mentioned above and increase their turn over rates. In fact, Sherrington and collaborators have developed wide range of polymer-supported molybdenum complexes as catalysts that offer real prospect for technological applications [8]. Applications of polymer-supported molybdenum catalysts in the oxidation of various organic substrates and other reactions have grown rapidly in the past few years and this mini review highlights such studies. Organization of the review is such that details of supported catalysts developed are mentioned within the subsections of catalytic reactions studied. 2. CATALYTIC REACTIONS BY POLYMER-SUPPORTED COMPLEXES 2.1. Epoxidation of Aliphatic and Aromatic Alkenes Progress made in the epoxidation of alkenes using polymersupported metal (including molybdenum) complexes as catalysts has been reviewed [8,9]. Supported molybdenum catalysts for liquid phase alkene epoxidation also find space in the sub-section of a book chapter [10]. However, these reviews very briefly report on the polymer-supported molybdenum complexes. The epoxidation of alkenes is one of the most widely studied reactions in organic chemistry. Epoxides are very important precursors for the development of drugs, agrochemicals and additives. In the early nineties, Sherrington and collaborators developed polymer-supported molybdenum complexes for the oxidation of alkenes. Early studies mainly concentrate on the immobilization of molybdenum hexacarbonyl on polymer supports and study their catalytic activities because of their effective catalytic applications for the selective epoxidation of olefins using alkyl hydroperoxides as oxidant [11-15]. The catalysts anchored on polystyrene show highest activity for epoxidation [16] while those anchored on chelating resin exhibit very low activity. The reaction rates for polystyrene anchored molybdenum hexacarbonyl catalysts are also much better than that of the corresponding homogeneous analogues. Interaction of [MoVIO2(acac)2] with the chelating resins derived from cross-linked poly(chloromethylstyrene-divinylbenzene), cross-linked poly(glycidylmethylacrylate) and cross-linked poly(4© 2012 Bentham Science Publishers
74 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
O O CH2X
CH3
O
Y
O
N
N
O
O Mo O
O
O
tBu (8)
Y=
tBu N H
OH
O
O X=
O
Mo
O
Mo
O
(1)
N
N H
tBu
(4) O
N
(5) N
NH
N
N (2)
OH
(6)
NH
O NH
(7)
N N N
(3)
Scheme 1. Examples of polystyrene bound peroxido MoVI–complexes. Ball represents polymer back bone.
H N
O
H N
CH2X O
N
Mo O
n
N
A
O
O
O
Mo
tBu O
O But
N
X=
N
9 N O
10
O
O
O N O
N
Mo O
OH O
C
H N
O
N tBu
N
11
12 Scheme 2. Proposed structures of molybdenum based catalysts [19,20].
vinylpyridine) give MoVI–complexes which on treatment with TBHP produce the corresponding hydroperoxy complexes. Scheme 1 presents polymer-supported complexes prepared [17,18]. These complexes are active catalyst for the oxidation of cyclohexene using TBHP as the oxidant. The stability of 2 (with ligand to metal ratio of < 2 : 1 ) allowed nine times recycling without any significant loss of molybdenum. However, some of them (with ligand to metal ratio of < 1 : 1) leach molybdenum and the leaching of molybdenum goes on increases with recycling. Polybenzimidazole also binds to molybdenum similarly to give supported catalyst, (PBI.Mo, 9; Scheme 2) which shows very good activity towards epoxidation of oct-1-ene and propene in the presence of TBHP [19]. Although the activity under the reaction conditions used is poor in the beginning, the progressive good selectivity and remarkable ageing behaviour over 10 recycles, makes its useful
for technology development. Other molybdenum based catalysts of functionalized polymers presented in Scheme 2 are also good for the epoxidation of various alkenes [20]. Polybenzimidazole has further been modified with 2aminomethylpyridine via epoxidized polybenzimidazole (Scheme 3) to support MoVI–complex for the epoxidation of cyclohexene in the presence of TBHP. The experimental activation energy of the catalyst EPBI-AMP.Mo (13) is higher than that of analogous [MoVIO2(acac)2] catalyzed reaction. Catalyst 13 is reusable up to nine cycles without loss of molybdenum but its activity slowly decreases after each cycle [21]. Ahn et al. have prepared polyimide-supported MoVI–catalyst by reacting polyimide bearing triazole residue (abbreviated as PIDAT) with [MoVIO2(acac)2] in ethanol [22, 23]. This shows high thermo-oxidative stability (up to ca. 400 oC) and is highly active
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 75
Cl H N
H N O H N
n
N N
H N
60 % NaOH
n
N EPBI
N
PBI
O H2N
N H N
H N
n
N N OH
N H
N
EPBI-AMP Scheme 3. 2-Aminomethylpyridine modified polybenzimidazole.
H N H N
N
N O O
O
O
Mo
N O
O
PS.AMP.Mo (14)
n
N
O
Mo
O
PBI.Mo (15)
Scheme 4. Proposed structures of polystyrene and polybenzimidazole bound MoVI–complexes. Only idealized structural units are shown. Original papers have shown trans-MoO2 structure which is not correct [25,26].
and selective for the epoxidation of olefins with TBHP. Further, catalyst can be recycled 10 times with no detectable loss of Mo from the support. Approximately similar activation energy for the epoxidation by immobilized as well as homogeneous Mo complex indicates that the active centre of the immobilized catalyst is easily accessible and the environment of the active centre in the resin is similar to that of the homogeneous catalyst [24]. Dioxomolybdenum(VI) complexes of 2-aminomethylpyridine functionalized poly(styrene-divinylbenzene) (14: PS.AMP.Mo1 with 53 % amination and PS.AMP.Mo2 with 90 % amination) resin and polybenzimidazole (PBI.Mo, 15) have also been isolated by reacting them with [MoVIO2(acac)2]; Scheme 4. The stabilities of all polymer catalysts have been assessed by using them in small scale reactions followed by their removal from reaction mixture and by using the residues as catalysts in epoxidation reactions. All three polymer catalysts are highly active and selective in 10 consecutive reactions. Supernatant solution after catalytic action always contains catalytically active Mo residue in small quantity in case of PS.AMP.Mo1 while other two catalysts are stable towards recycling [25,26]. Extensive reusability of these catalysts for the epoxidation of cyclohexene, 1-octene, limonene and -pinene using dry TBHP as oxidant has also been studied under different reaction conditions [27].
Other groups have also designed various benzimidazole containing polyfunctional ligands to support molybdenum complexes. Chavan et al. have designed benzimidazole-functionalized dendrons to immobilize molybdenum complexes [28]. These dendrons react with [Mo0(CO)6] and [MoVIO2(acac)2] to give corresponding molybdenum supported complexes (Scheme 5) which are oxidatively stable, highly efficient and selective catalysts for the epoxidation of cyclohexene using TBHP as oxidant. The conversion of cyclohexene is low (4 – 12 %) while the selectivity is as high as 99%. This activity is preserved or even increased with the number of recycles without leaching molybdenum. Polymer supported dioxomolybdenum(VI) catalyst PS-MT (19), prepared via PS-acac (18) as shown in Scheme 6 catalyzes, by TBHP, the oxidation of alkenes e.g. 1-octene, cyclooctene, (Z)cyclododecene, (E)-cyclododecene, (Z)-2-hexen-1-ol, (E)-2-hexen1-ol, (Z)-stilbene and (E)-stilbene selectively to the corresponding epoxide in toluene [29]. 1-Octene is least reactive substrate (only 10 % conversion) while epoxidation of internal and cyclic olefins proceeds much faster (20 – 78 % conversion). These results suggest that the oxidizing species are electrophilic, and hence the reaction rate increases with increasing electron density on the olefin. The E and Z olefins give the corresponding epoxides while retaining their configurations.
76 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
R1 Mo
R2 N NH
Mo
O N
N
R1
N H
R2
O
N O N
Mo
H3CO
R1 N
N
O
R2 N
O
N H
N O
Mo N HN
a: R1, R2 = 1,1,4,4-tetramethyltetramethylene (16) b: R1 = t-Bu, R2 = H (17)
R2 R2
Scheme 5. Molybdenum complexes supported on benzimidazole-functionalized dendrons [28].
Cl H2N
NH2
HN
DMF/ Air
NH2
NH
N MoO2(acac)2
O
Mo O
DMF/ Air
O O
PS-acac (18) OH
HS
DMF O
NH
N Mo S
O O
O PS-MT (19)
O
Scheme 6. Preparation of PS-MT (19) [29]. Note that MoO2 moiety in structures of both complexes is shown as trans in the original article [29] though it should have been drawn as cis.
Chloromethylated polystyrene cross-linked with divinylbenzene (Diaion IR-20), and functionalized with triethylenetetraamine followed by treating with alkali to convert remaining chloro group to
hydroxide binds with oxoperoxomolybdenum(VI) species to give peroxo complex, PS-Mo-y (20). This supported complex has been used several times as epoxidation catalyst in benzene using TBHP
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 77
O
O
O
O O
O
O
O
N
DGEBA (21) O
O
O
O
O
TGAP (22)
O
N
N
O
O O
O
n Novolac (Bakelite EPR 600, 23)
TGMDA (24)
Scheme 7. Various epoxy resins.
as oxidant. The activity of the catalyst gradually decreases on reuse but stabilizes after four cycles. The catalytic activity of 20 could be regenerated simply by treatment with 30 % aqueous H2O2 [30]. Molybdenum-doped polymeric epoxy resins have been prepared by polymerization of epoxide of 4,4’-methylene-bis-(N,Ndiglycidylaniline) (TGMDA) and N,N-diglycidyl-4-glycidyloxyaniline (TGAP), diglycidylether of bisphenol A (DGEBA, Bakelite EPR 164) and novolac (Bakelite EPR 600) (Scheme 7) using MoV(OEt)5 or molybdenum 2-ethylhexanoate (Mo(EH)n as polymerization initiator. The metal compounds release ligands on heating, thus initiating epoxy ring opening, followed by anionic polymerization and cross-linking of the thermosets [31]. These molybdenum doped (0.5 to 1.5 % Mo loading) catalysts are useful for the epoxidation of 1-octene, 2-octene, cyclohexene, styrene, (R)-(+)limonene and 1,2-dihydronapthalene, by TBHP in toluene at 90 oC. All these substrates give high conversion along with almost quantitative selectivity towards corresponding epoxide. Leaching of molybdenum content of catalysts is very low in these reactions. Epoxidation of propene catalyzed by TGMDA–MoV(OEt)5 (1.5 % Mo loading) gives 67–75% propene epoxide in five consecutive reactions. Long term performance of these catalysts is proved by using them continuously up to 120 times without any recondioning. Styrene oxide, an oxidation product of styrene, is manufactured industrially either by the reaction of benzene and ethylene oxide or by the epoxidation of styrene by NaOCl. Oxidation of styrene catalyzed by polymer-anchored complex PS-[MoVIO2(fsalohyba)·DMF] (25) gives five oxidation products (Scheme 8) with overall conversion of 17.5 % under optimized reaction conditions. The difficulty in transferring oxygen from the intermediate peroxo complex, PS-[MoVIO(O2)(sal-ohyba)], formed by the reaction between 25 and H2O2, may be the reason for poor oxidation ability of the catalyst. The selectivity of the reaction products are in the order: benzaldehyde (64.0 %) > 1-phenylethane-1,2-diol (16.4 %) > benzoic acid (8.7 %) > phenylacetaldehyde (4.8 %) > styrene oxide (4.2 %) [32]. Amongst all products, the formation of benzaldehyde in highest yield is possible because the styrene oxide formed may convert into benzaldehyde by nucleophilic attack of H2O2 to styrene oxide followed by a cleavage of the intermediate hydroperoxystyrene; Scheme 9. Benzaldehyde formation may also be facilitated by di-
O Phenylacetaldehyde (phaa) O
HO OH
Catalyst H2O2 1-phenylethane-1,2-diol (phed)
Styreneoxide (so)
Styrene
HO
O
Benzoic acid (bzac)
Benzaldehyde (bza)
O
O
O
O O
O Mo
Catalyst
N
O DMF
25 Scheme 8. Oxidation products of styrene.
rect oxidative cleavage of the styrene side chain double bond via radical mechanism [33]. Although benzoic acid may form through benzaldehyde, this is normally a slow process. The formation of phenylacetaldehyde, a product that may form by isomerisation of styrene oxide, is very low. Water present in H2O2 is probably responsible for the hydrolysis of styrene oxide to 1-phenylethane-1,2diol. The polymer-anchored ligand PS-Hhpbmz (Hhpbmz = 2-(2´hydroxyphenyl)benzimidazole readily reacts with [MoVIO2(acac)2 ] in DMF to give the complex [PS-[MoVIO2(hpbmz)2] (26, Scheme
78 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
H
O O
CH2
O O
OOH
+
OH
O
+
H2O
H Scheme 9. Possible reaction pathway for the oxidation of styrene to benzaldehyde.
O
O N
MoO2(acac)2
N
N
N
O
DMF
N
O Mo N
HO PS-[MoO2(hpbmz)2] (26)
PS-Hhpbmz Scheme 10. Preparation of 26 and its proposed structure [34].
N
N
O
N
Mo
N O
O
O N
Mo
N
O
O O
O
O
O
PS-[MoVIO(O2)2(3-pybmz)] (28)
PS-[MoVIO(O2)2(2-pybmz)] (27)
Scheme 11. Schematic representation of the proposed molecular formula of PS-[Mo VIO(O2) 2(L)] )] (L = 2-pybmz, 27; L = 3-pybmz, 28) [35].
H N
DMF, (Et)3N
+ Cl
N
SH
CH3COOC2H5
H N + N
HCl
S
Htmbmz PS-S-tmbmz (29) Scheme 12. Reaction of Htmbmz with chloromethylated polystyrene [36].
10). Oxidation of styrene, catalyzed by 26 using H2O2 as oxidant also gives all five oxidation products (Scheme 8) but their selectivity follows the order: benzaldehyde (81.6 %) > phenylacetaldehyde (10.3 %) > benzoic acid (3.8 %) > styrene oxide (3.1 %) > 1phenylethane-1,2-diol (1.3 %) with 23.4 % conversion of styrene [34]. Peroxomolybdenum species, prepared in situ by stirring MoVIO3 with an excess of 30% H2O2, reacts with polymer-anchored ligands, PS-2-pybmz (2-pybmz = 2-(2-pyridyl)benzimidazole) and PS-3-pybmz (3-pybmz = 2-(3-pyridyl)benzimidazole) to give complexes of formula PS-[MoVIO(O2)2(2-pybmz)] (27) and PS[MoVIO(O2)2(3-pybmz)](28), respectively; Scheme 11. These diperoxo complexes lose one of the peroxo groups easily but it does not preclude their use as catalysts. However, only 3.8 – 6.6 % conversion of styrene has been achieved with these catalysts in the presence of TBHP. This conversion slightly improved (13.5 to 16.5 %) with aqueous 30 % H2O2 but the obtained high selectivity of styrene oxide with TBHP (42.7 – 51.2 %) reduced considerably (7.5 – 13.2 %). This trend is similar to that obtained by other catalysts mentioned above. Selectivity of other four products is also similar. The (unstable) diperoxo structure reformed in the reaction mixture
in the presence of H2O2, and the catalytic cycle turns between the mono- (less) and diperoxo (more active) species. The reusability after regeneration has an added advantage of using them as catalyst [35]. The reaction of 2-thiomethylbenzimidazole (Htmbmz) with the chloromethylated polystyrene yields a polymer-bound ligand designated as PS-ligand. The analytical results suggest that the S/N ratio does not fully agree with the formation of PS-S-tmbmz (29, Scheme 12) and product(s) corresponding to a higher relative % of sulfur also form. The product formed in the reaction of Htmbmz with benzyl chloride, where the sulphur atom acts as the nucleophile in the reaction, confirms the condensation of Htmbmz with benzyl chloride. However, it is possible that, before and after the Htmbmz binds to the solid, the thiol group oxidizes to disulphide and other oxidation products possibly also being formed; details of these are discussed in the original publication [36]. As the exact nature of the molybdenum complex obtained from 29 is not known, this is designated as PS-[MoVIO2(ligand)n] (30). PS-[MoVIO2(ligand)n] (30) has been used for the oxidation of styrene and cyclohexene by H2O2. A maximum of 68 % conversion
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 79
O
OH
OH
H2O2
O
+
+
Catalysts
+ OH
2-Cyclohexene-1-ol
Cyclohexeneoxide
2-Cyclohexene-1-one
Cyclohexane-1,2-diol
Scheme 13. Oxidation products of cyclohexene.
H N Cl
CH3CN
N
+ N hu
Mo(CO)6 + THF 30 min
N
reflux, 48 h * Mo(CO)6 / THF
reflux (16 h)
N N CO
OC Mo
PS-[(im)Mo0(CO)5] (31)
CO
OC CO Scheme 14. Outline of the formation of 31 [37].
Cl
(CH2)n
DMF, 100 oC
(CH2)n
+ Ph2P
PPh2
P Ph2 Cl
12 h
PPh2
n = 1, 2 hu Mo(CO)6 + THF 30 min
* Mo(CO)6 / THF
reflux (3 h)
(CH2)n P Cl Ph2
PPh2 CO
OC
n = 1 : PS-[(dpm)Mo0(CO)5] (32)
Mo
n = 2 : PS-[(dpe)Mo0(CO)5] (33)
CO
OC CO
Scheme 15. Preparation of phosphine based polymer-bound molybdenum carbonyls [38].
of styrene has been obtained under optimized conditions where the selectivity of three major products obtained varied in the order: benzaldehyde (66.0 %) > 1-phenylethane-1,2-diol (29.5 % > styrene oxide (3.1 %). With 66.0 % conversion of cycohexene, the selectivity of three main products (Scheme 13) follows the order: cyclohexane-1,2-diol (83.4 %) > 2-cyclohexene-1-one (13.2 %)> cyclohexeneoxide (2.3 %). Catalyst does not leach during the catalytic reactions and is recyclable at least three times without much loss in their activities. Molybdenum hexacarbonyl directly reacts with imidazole and phosphine bound polystyrene (2 % cross-linked with divinylbenzene) to give polymer supported molybdenum carbonyl catalysts (PS-[(im)Mo0(CO)5] (31; Scheme 14), PS-[(dpm)Mo0(CO)5] (32;
Scheme 15) and PS-[(dpe)Mo0(CO)5] (33; Scheme 15) [37,38]. Other polymer supported molybdenum carbonyl catalysts, PS[(en)Mo0(CO)4] (34, en = ethane-1,2-diamine), PS[(detn)Mo0(CO)3] (35, detn = N1-(2-aminoethyl)ethane-1,2diamine) and PS-[(pipz)Mo0(CO)5] (36, pipz = piperazine) covalently bonded to polystyrene have also been prepared similarly and characterized. [39,40]. The chloromethylated polystyrene has also been modified to aldehydic polystyrene which on reaction with ethane-1,2-diamine provides modified polymer-supported ligand (PS-ien). The functionalized ligand gives PS-[(ien)Mo0(CO)4] (37) according to Scheme 16 [41].
80 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
DMSO, NaHCO3 O
155 oC, 6h
Cl
MeOH, reflux 6h N
NH2
H2N
CO NH2 i) Dioxane, reflux 1h Mo
N
ii) Mo(CO)6, reflux (45 min)
CO
OC
NH2
CO PS-[(ien)Mo0(CO)4] (37) Scheme 16. Preparation of PS-[(ien)Mo0 (CO)4 ] (37) [41].
All these complexes catalyze the oxidation of a range of alkenes to respective epoxides in the presence of TBHP in CCl4. Table 1 provides detail of alkenes conversion and yield of epoxides. In
some cases, PS-[(ien)Mo0(CO)4] provides other products in addition to epoxides. All these catalysts are recoverable and reusable several times.
Table 1. Epoxidation of Alkenes with tert-BuOOH Catalyzed Polymer-Supported Molybdenum Carbonyl Catalysts in CCl4 Under Refluxed Condition
% Conversion
Reaction
(% Epoxide)
Time (h)
[PS-[(im) Mo0(CO)5]
89 (98)
2.5
[37]
[PS-[(im) Mo0(CO)5]
100 (100)
4
[37]
[PS-[(im) Mo0(CO)5]
89 (89)
20
[37]
[PS-[(im) Mo0(CO)5]
94 (94)
8
[37]
[PS-[(im) Mo0(CO)5]
98 (98)
9
[37]
[PS-[(im) Mo0(CO)5]
96 (96)
10
[37]
[PS-[(im) Mo0(CO)5]
98 (98)
16
[37]
[PS-[(im) Mo0(CO)5]
100 (100)
15
[37]
[PS-[(im) Mo0(CO)5]
97 (93% cis, 4% trans)
15
[37]
[PS-[(im) Mo0(CO)5]
100 (100)
9
[37]
PS-[(dpm)Mo0(CO)5]
93 (93)
4
[38]
PS-[(dpm)Mo0(CO)5]
100 (100)
3
[38]
PS-[(dpm)Mo0(CO)5]
85 (85)
21
[38]
PS-[(dpm)Mo0(CO)5]
89 (89)
7
[38]
Catalyst
Alkene
Ref.
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 81 Table 1. contd…
% Conversion
Reaction
(% Epoxide)
Time (h)
PS-[(dpm)Mo0(CO)5]
93 (93)
8
[38]
PS-[(dpm)Mo0(CO)5]
18 (18)
17
[38]
PS-[(dpm)Mo0(CO)5]
100 (100 % trans)
22
[38]
PS-[(dpm)Mo0(CO)5]
77 (61% cis, 16% trans)
22
[38]
PS-[(dpm)Mo0(CO)5]
98 (98)
8
[38]
PS-[(dpe)Mo0(CO)5]
92 (92)
3
[38]
PS-[(dpe)Mo0(CO)5]
100 (100)
2.5
[38]
PS-[(dpe)Mo0(CO)5]
62 (62)
24
[38]
PS-[(dpe)Mo0(CO)5]
100 (100)
6
[38]
PS-[(dpe)Mo0(CO)5]
97 (97)
4.5
[38]
PS-[(dpe)Mo0(CO)5]
64 (64)
20
[38]
PS-[(dpe)Mo0(CO)5]
100 (100 % trans)
22
[38]
PS-[(dpe)Mo0(CO)5]
72 (57% cis, 15% trans)
22
[38]
PS-[(dpe)Mo0(CO)5]
88 (88)
3
[38]
PS-[(en)Mo0(CO)4]
92(92)
1.25
[39]
PS-[(en)Mo0(CO)4]
100(100)
2.5
[39]
PS-[(en)Mo0(CO)4]
96(96)
2.5
[39]
PS-[(en)Mo0(CO)4]
80(80)
1.5
[39]
PS-[(en)Mo0(CO)4]
56(56)
5
[39]
PS-[(en)Mo0(CO)4]
98(98)
3.5
[39]
PS-[(en)Mo0(CO)4]
99(99)
4.5
[39]
Catalyst
Alkene
Ref.
82 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
Table 1. contd…
% Conversion
Reaction
(% Epoxide)
Time (h)
PS-[(en)Mo0(CO)4]
100(100)
5
[39]
PS-[(en)Mo0(CO)4]
100 (97.5% cis, 2.5% trans)
5
[39]
PS-[(en)Mo0(CO)4]
97(97)
3
[39]
PS-[(detn) Mo0(CO)3]
95(95)
1.5
[39]
PS-[(detn)Mo0(CO)3]
100(100)
2
[39]
PS-[(detn)Mo0(CO)3]
95(95)
7
[39]
PS-[(detn)Mo0(CO)3]
74(74)
2.5
[39]
PS-[(detn)Mo0(CO)3]
49(49)
4
[39]
PS-[(detn)Mo0(CO)3]
100(100)
4
[39]
PS-[(detn)Mo (CO)3]
100(100)
5
[39]
PS-[(detn)Mo0(CO)3]
100(100)
5
[39]
PS-[(detn)Mo0(CO)3]
100 (97% cis, 3% trans)
5
[39]
PS-[(detn)Mo0(CO)3]
100(100)
2
[39]
PS-[(pipz)Mo0(CO)5]
94(94)
1.5
[40]
PS-[(pipz)Mo0(CO)5]
100(100)
4.5
[40]
PS-[(pipz)Mo0(CO)5]
92(29)
10
[40]
PS-[(pipz)Mo0(CO)5]
75(75)
5
[40]
PS-[(pipz)Mo0(CO)5]
89(89)
4.5
[40]
PS-[(pipz)Mo0(CO)5]
92(92)
7
[40]
PS-[(pipz)Mo0(CO)5]
85(85)
7
[40]
PS-[(pipz)Mo0(CO)5]
100(100 % trans)
10
[40]
Catalyst
0
Alkene
Ref.
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 83 Table 1. contd…
% Conversion
Reaction
(% Epoxide)
Time (h)
PS-[(pipz)Mo0(CO)5]
97 (90 % cis, 7 % trans)
10
[40]
PS-[(pipz)Mo0(CO)5]
97(97)
3
[40]
PS-[(ien)Mo0(CO)4]
94(94)
1.5
[41]
Catalyst
Alkene
PS-[(ien)Mo0(CO)4]
Ref.
[41]
PS-[(ien)Mo0(CO)4]
95(73)
2
[41]
PS-[(ien)Mo0(CO)4]
75(52)
2
[41]
PS-[(ien)Mo0(CO)4]
35(35)
2
[41]
PS-[(ien)Mo0(CO)4]
94(94)
4
[41]
PS-[(ien)Mo0(CO)4]
9(9)
4
[41]
PS-[(ien)Mo0(CO)4]
100(100 % trans)
5
[41]
PS-[(ien)Mo0(CO)4]
100 (77% cis, 23% trans)
5
[41]
PS-[(ien)Mo0(CO)4]
100(91)
2
[41]
O OH
OH
P
OH N
N
OH P
OH OH
O N
N
O
OH
OH
P
OH N
N
OH P
OH OH
O Scheme 17. Phosphonomethyl and hydroxyethyl containing chelating polymer resins [42].
2.2. Oxidation of Allylic Alcohols Microreticular (gel type) polystyrene with 2% divinylbenzene cross-linking and microreticular (MR-type) polystyrene with 10% divinylbenzene cross-linking have been functionalized with
bis(phosphonomethyl)-amino (PMA) and bis(hydroxyethyl)amino (HEA) moiety to prepare chelating polymer resins; Scheme 17. Molybdenum(VI) complexes of these resins, prepared by the reaction of respective resin with aqueous solution of K2MoO4, catalyze the epoxidation of (E)-geraniol in the presence of TBHP in benzene
84 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
Polymer Mo(VI) catalyst OH
O OH
TBHP, C6H6
2,3-Epoxygeraniol
(E)-Geraniol Scheme 18. Oxidation of (E)-geraniol.
Me O
O O
Me Me
N
Mo
Me
H
N
Me
O
O
Mo
O
H
O O O
O 38 39
Me Me
Me
H N Mo
O
H
O
O
N
O
O Mo
O
O
O
O O 41
40 Scheme 19. Chiral dioxomolybdenum(VI) complexes used for the oxidation of geraniol [43].
to give 2,3-epoxygeraniol; Scheme 18. Conversion of the substrate is 89 – 98 % with 71 – 81 % selectivity of the product. The olefinic site close to the hydroxyl group of (E)-Geraniol has the preferential epoxidation regardless of the functional group and the type of polymer matrix [42]. Catalytic epoxidation of geraniol has also been reported by Cazaux and Cae by chiral MoVI –complexes (Scheme 19) immobilized on chloromethylated polystyrene [43]. The yields as well as ee are poor and the polymer catalysts are unstable. 2.3. Oxidation of Phenol and Related Derivatives Liquid phase oxidation of phenol using simple oxidants such as O2 and H2O2 has been reported using different types of heterogeneous catalyst. For example, the oxidation of phenol catalyzed by polymer-anchored complex 25 using H2O2 as an oxidant in CH3CN results in two major products, catechol and p-hydroquinone; Scheme 20. These are the expected products according to the directing effect of the phenolic OH group. However, under optimized conditions, conversion is only ca. 10 % at 80 oC with 59 % selectivity towards catechol and 39 % towards p-hydroquinone. Remaining phenol slowly polymerizes during oxidation [32]. OH
OH
OH OH +
OH Scheme 20. Oxidation products of phenol.
Catalytic oxidation of 2,6-di-tert-butylphenol by polystyrenedivinylbenzene (XAD-4) resin supported complexes XAD[MoVIO2(saloxim)2], (42) XAD-[MoVIO2(salpn)] (43) and XAD[MoVIO2(salphen)] (44) [Hsaloxim = salicylaldoxime, H2salpn = 1,3-propylenebis(salicylaldimine) and H2salphen = ophenylenebis(salicylaldimine)] in the presence of TBHP as an oxidant gives two major products, 2,6-di-tert-butylbenzoquinone (BQ) and 3,3’,5,5’-tetra-tert-butyldiphenoquinone (dPQ); Scheme 21. Controlling the reaction conditions such as solvent, temperature and catalyst, controls the selectivity of the oxidized products of 2,6-ditertbutylphenol. Amongst these catalysts, 43 gives BQ exclusively in 80 % yield in acetonitrile. Oxidation of 2,6-di-tert-butylphenol decreases only slightly in three subsequent runs [44]. 2.4. Oxidation of Ethylbenzene The development of clean and economically viable processes for the selective oxidation of ethylbenzene to value added product acetophenone [45,46] has attracted research interest. Oxidation of ethylbenzene has mostly been carried out using molecular oxygen as oxidant. With mild oxidant usually acetophenone is obtained as major product while strong oxidants such as TBHP and H2O2 give several oxidized products. Polymer-anchored complex 25 has been used to catalyze the oxidation of ethylbenzene, by H2O2, to give benzaldehyde, phenyl acetic acid, styrene and 1-phenylethane-1,2-diol as shown in Scheme 22 [32]. Reaction conditions have also been optimized for the maximum oxidation of ethylbenzene by varying the amount of oxidant (moles of H2O2 per mole of ethylbenzene), catalyst and temperature of the reaction. With 12.8 % conversion, the selectivity
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 85
O
OH But
But
But
But
But But O
+
O But
O
But dPQ
BQ Scheme 21. Oxidation products of 2,6-di-tert-butylphenol.
HO
O
O
HO OH
Catalyst +
H2O2 Benzaldehyde (bza)
Ethylbanzene
+
+ Phenylacetic acid (phac)
Styrene
1-Phenylethane-1,2-diol (phed)
Scheme 22. Oxidation products of ethylbenzene.
Br OH O
OH
Catalyst
OH +
H2O2 / KBr / H2SO4
Br
O
O
Br
Scheme 23. Oxidative bromination of salicyalldehyde.
of reaction products follows the order: benzaldehyde > phenyl acetic acid > styrene > 1-phenylethane-1,2-diol. Although same reaction products with 12.5 % conversion of ethylbenzene has also been obtained using catalyst 26, reaction products follow different selectivity order: benzaldehyde (74.3 %) > 1-phenylethane-1,2-diol (12.8 %) > phenyl acetic acid (8.5 %) > styrene (3.4 %) [34]. Catalyst 30 gives better conversion (ca. 17 %) of ethylebenzene along with the formation of acetophenone in small amount and no formation of styrene. Here, the selectivity of different products follows the order: benzaldehyde (69.7 %) > phenylacetic acid (22.2 %) > acetophenone (4.6 %) > 1-phenylethane-1,2diol (3.5 %) [36]. 2.5. Oxidative Bromination of Salicylaldehyde
O
O O Mo
N
N
N O
N
PS-[MoVIO2(hmbmz)2] (45) Scheme 24. Proposed structure of PS-[Mo VIO2(hmbmz)2] (45) [50].
2.6. Oxidation of Methyl Phenyl Sulfide
Enzymes vanadate-dependent haloperoxidases catalyze the oxidative halogenation of hydrocarbon, eq. 1 [47,48]. Though oxidative bromination catalyzed by model vanadium complexes is very common [47,48], molybdenum analogues have also been reported for such catalytic reaction [32,49,50]. hal- + H2O2 + RH + H + Rhal + 2H2O
ertness of polymer support towards HClO4 provides extra stability to the complex during catalytic reaction [50].
(1)
Dioxomolybdenum(VI) complex, 25 mimics haloperoxidases activity in that 92 % conversion of salicylaldehyde into 5bromosalicylaldehyde and 3,5-dibromosalicylaldehyde (Scheme 23) using H2O2 / KBr in the presence of H2SO4 in aqueous solution with the turn over rate per hour of 213 has been achieved. Here, selectivity of 5-bromosalicylaldehyde is 83 % [32]. PS-[MoVIO2(hmbmz)2] (45) also catalyzes the oxidative bromination of salicylaldehyde. A quantitative conversion of salicylaldehyde to 5-bromosalicylaldehyde selectively has been obtained under the best suited conditions of catalyst (0.050 g), 70 % aqueous HClO4 (15 mmol), H2O2, (15 mmol), KBr (10 mmol) and water (10 ml) for 5 mmol of substrate. Addition of HClO4 at a time causes slow decomposition of catalyst. However, this decomposition could be stopped by adding HClO4 successively in portions. Further, in-
Oxidation of sulfides (thioethers) to sulfoxides has been considered an important catalytic reaction [51]. Vanadium complexes are known to catalyze the oxidation of organic sulfides (thioethers) by H2O2 to sulfoxides and molybdenum complexes behave similarly to some extent. The oxidation of methyl phenyl sulfide catalyzed by polymer-anchored catalyst 26 gave a mixture of methyl phenyl sulfoxide and methyl phenyl sulfone in acetonitrile at ambient temperature; Scheme 25. The sulfur atom in methyl phenyl sulfide is electron rich species and facilitates the electrophilic oxidation to give sulfoxide and further to sulfone. Under the best suited conditions, a maximum of 75.1 % conversion with 75.9 % selectivity towards sulfoxide and 24.1 % towards sulfone has been achieved [34]. 2.7. Oxidation of Benzoin Polymeric resin (XAD-4) and alumina (A) supported molybdenum complexes are also effective catalysts for the oxidation of benzoin to benzil in the presence of TBHP [52]. Catalyst 42 gives benzil (42.8 %), methylbenzoate (9.7 %) and benzoic acid (25.7 %) with 78.2 % conversion of benzoin while other catalysts, XAD-
86 Current Organic Chemistry, 2012, Vol. 16, No. 1
Mannar R. Maurya
O S
Catalyst
CH3
O
O S
S CH3
H2O2
CH3
+
Scheme 25. Oxidation products of methyl phenyl sulfide. OCH3
O
O
OH
H TBHP C O
C
C Catalyst
OH
C
+
+
O
O Benzil
Benzoin
Methylbenzoate
Benzoic acid
Scheme 26. Oxidation products of benzoin. DME CF2COOC3H5
+
Mo(CO)6
F2 C
Reflux, 18 h
O C O O
OC Mo OC
O 47
Scheme 27. Example of 3-Allylíc molybdenum polymer-anchored complex [54] used for the polymerization of 1,3-butadiene. Ball represents styrenedifluorobutenoate copolymer or styrene-divinylbenzene-difluorobutenoate terpolymer.
[MoVIO2(salphen)] (44), A-[MoVIO2(saloxim)2] (45) and A[MoVIO2(salphen)] (46) are selective for benzil formation; Scheme 26. The catalytic activity of alumina supported complexes follows the order: 46 > 45; this order is followed by polymer supported complexes as well. This has been correlated with the increasing order of the Lewis acidity of the coordinated ligand. Molybdenum complexes catalyze the oxidation reactions with TBHP by polarizing the O – O bond in the hydpoperoxide. Lewis acid action of the molybdenum complex due to Lewis acidity of the coordinated ligand influences the withdrawal of the electrons from the peroxidic oxygen of the hydroperoxide and thus facilitates the heterolysis of the O – O bond in the hydroperoxide oxidation reaction. Polymer-supported molybdenyl thioglycolate (PS-MT, 19) has been used as oxygen atom transfer reagent to oxidize benzoin [53]. Thus, the stoichiometric reaction of one equivalent PS-MT and two equivalent of benzoin afforded one equivalent of benzil after 10 h. Here, supported dioxomolybdenum(VI) complex converts to oxomolybdenum(IV). PS-MT (19) also catalyzes the oxidation of benzoin using air or pyridine N-oxide as oxygen sources. Thus, using PS-MT to benzoin molar ratio of 1:10 and 15 equivalent of air or pyridine N-oxide in DMF or deoxygenated methanol, reaction completes in 48 h (for air) or 24 h (for pyridine N-oxide) with the complete conversion of benzoin to benzil. Leaching of molybdenum during reaction is less than 0.01 % based on Mo originally present in the resin. 2.8. Polymerization Reaction 3-Allylíc molybdenum polymer-anchored complexes (Scheme 27) polymerize 1,3-butadiene and give polymers having ca. 75% of 1, 2 units and 25% of cis 1, 4 units. The overall rates of the polymerization processes induced by polymer-supported catalysts are lower than that obtained with similar unsupported trifluoroacetate catalyst. Moreover, the polymerization rate decreases much by anchoring the catalyst on a soluble polymer than on a cross-linked resin [54].
3. CONCLUSIONS AND FUTURE PERSPECTIVES Synthesis of polymer-supported molybdenum complexes and their use as catalysts for various reactions have been presented. The main advantages of the use of these supported catalysts are their high activity and recyclability, confirmed in most cases for several cycles. Amongst various substrates studied, polymer-supported molybdenum catalysts have shown excellent catalytic activities for the oxidation of alkenes. Extensive recycle ability studies on the epoxidation of various alkenes with tert-butyl hydroperoxide (TBHP) as oxidant using some of the polymer-supported molybdenum catalysts under different reaction conditions qualify them for future technological developments. Oxidative bromination of organic substrates using supported molybdenum complexes has also shown very encouraging results. In general, synthesis, characterization and application of polymer-supported molybdenum complexes find only limited mention in the literature. One of the main reasons is possibly the difficulty in full characterization of the catalysts supported on solid, though recently some detail studies on the characterization of catalysts have been taken up. However, no studies have been taken up in the line of characterization of intermediate species formed during catalytic reactions in order to provide complete catalytic reaction mechanism. Since molybdenum in most supported complexes exists in the oxidation state of +VI, the 96Mo NMR could be applied with some success for this purpose. In conclusion, chemistry of polymersupported molybdenum complexes is very interesting and has future for technological developments. ACKNOWLEDGEMENTS I am thankful to Council of Scientific and Industrial Research and Department of Science and Technology, Government of India, New Delhi for financial support. I also acknowledge the contributions of my collaborators, Dr. Sweta Sikarwar, Dr. Umesh Kumar,
Catalytic Applications of Polymer-Supported Molybdenum Complexes
Current Organic Chemistry, 2012, Vol. 16, No. 1 87 [27]
Dr. Manish Kumar, Dr. Aarti Arya (I.I.T. Roorkee) and Prof. J. Costa Pessoa (I.S.T., Lisbon, Portugal) for developing polymersupported catalysts.
[28]
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88 Current Organic Chemistry, 2012, Vol. 16, No. 1 [53]
Mannar R. Maurya
Arroyo, P.; Gil, S.; Muñoz, A.; Palanca, P.; Sanchis, J.; Sanz, V. Polymersupported molybdenyl thioglycolate as oxygen atom transfer reagent. J. Mol. Catal. A: Chem., 2000, 160, 403-408.
Received: December 29, 2010
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Revised: February 11, 2011
Accepted: February 16, 2011
