View Online / Journal Homepage
Dalton Transactions
Dynamic Article Links
Cite this: DOI: 10.1039/c2dt11995a
COMMUNICATION
www.rsc.org/dalton
Iron(III) tetrakis(pentafluorophenyl)porpholactone catalyzes nitrogen atom transfer to C C and C–H bonds with organic azides† Downloaded by Beijing University on 09 January 2012 Published on 21 December 2011 on http://pubs.rsc.org | doi:10.1039/C2DT11995A
Lei Liang, Hongbin Lv, Yi Yu, Peng Wang and Jun-Long Zhang* Received 20th October 2011, Accepted 21st November 2011 DOI: 10.1039/c2dt11995a We have demonstrated that iron porpholactones could be effective catalysts for nitrogen atom transfer reactions such as aziridination of alkenes and amidation of alkanes using organic azides. Porpholactones, as analogues of porphyrins, in which one pyrrole is replaced by an oxazolone ring, are regarded as “intermediates” to porphyrins and chlorins.1 With lactone moiety replacement, partial saturation on porphyrin periphery results in restricted p-conjugation and lowered molecular symmetry on energies of the frontier molecular orbitals (MOs) predicted by an iterative ¨ extended Huckel (IEH) calculation.1b Compared with porphyrins, the energy of a porpholactone HOMO (the highest occupied molecular orbital) is slightly lower, supported by the anodic shift of the first oxidation potential of porpholactones relative to that of the corresponding porphyrins, indicating that porpholactones are better at stabilizing the high valent metal complexes than the corresponding porphyrins.2 However, the effects of particular structures on catalysis remains unexplored, although “porphyrin-like” properties had been recognized for 20 years.1b In contrast to various kinds of metalloporphyrin catalyzed reactions, only two examples of oxidation of alkenes and sulfides by metalloporpholactones have been reported.3 Thus, to expand the application of metalloporpholactones in catalysis is of importance. Herein, we report the first example of using iron porpholactone [Fe(F20 -TPPL)Cl] (F20 -TPPL = mesotetrakis(pentafluorophenyl)porpholactonato dianion) (Fig. 1) as a catalyst for nitrogen atom transfer to C C and C–H bonds with organic azides.
Fig. 1 Iron porphorinoid catalysts used in this work. Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, P.R. China. E-mail:
[email protected]; Fax: +86-10-62767034 † Electronic supplementary information (ESI) available: Experimental procedures and characterization and spectral reproduction for new compounds. See DOI: 10.1039/c2dt11995a
This journal is © The Royal Society of Chemistry 2012
Nitrogen atom transfer reactions catalyzed by metalloporphyrins through metal nitrene intermediate(s) to C–H and C C bonds attract much attention in organic synthesis.4 For the choice of nitrogen source, there has been a growing interest in the use of organic azides, because the by-product is nitrogen gas whereas the other nitrogen sources such as PhINTs, bromamineT or chloramine-T would lose PhI, NaCl and NaBr moieties. Cenini,5 Che,6 and Zhang7 groups pioneered the development of nitrene transfer reactions such as inter- and intramolecular aziridination of alkenes, amidation of sulfides, and amidation of alkanes with organic azides catalyzed by ruthenium, cobalt and iron porphyrins. In this work, at the outset of the project, we aimed to identify a suitable azide as the “nitrogen source” using styrene and ethylbenzene as model substrates. Through optimization of reaction conditions (Table S1, ESI†), typical reactions were carried ˚ molecule out in 1,2-dichloroethane at 80 ◦ C in the presence of 4 A sieves for 12 h. We found that only p-toluenesulfonyl azide (TsN3 ) and methylsulfonyl azide (MsN3 ) allowed for aziridination of styrene and gave the corresponding aziridines in 87 and 61% yield (Table 1, entries 1 and 2, respectively). For ethylbenzene, only p-nitrophenyl azide is effective for amidation of a saturated C–H bond (Table 1, entry 10). Using PhINTs as the nitrogen source, [Fe(F20 -TPPL)Cl] shows no reactivity towards styrene and ethylbenzene (Table 1, entries 7 and 14). Control experiments in the absence of [Fe(F20 -TPPL)Cl] or only using FeCl3 as catalysts were carried out.8 As shown in Table S2,† for the aziridination of styrene, without metal salts or only using FeCl3 , we isolated styrene aziridine in yields of 30 and 52%, respectively, which are lower than that obtained by [Fe(F20 -TPPL)Cl]. For amidation of ethylbenzene, control experiments could not afford the amide product (Table S3, ESI†). In addition, we found sunlight to have a negligible effect on these reactions. To investigate the ligand effect on the reactivity of N atom transfer to alkenes and alkanes, we chose [Fe(F20 -TPP)Cl] and [Fe(F15 TPC)] (F20 -TPP = meso-tetrakis(pentafluorophenyl)porphyrino dianion and F15 -TPC = meso-tris(pentafluoro-phenyl)corrolato trianion) as the catalysts and styrene, p-chlorostyrene and ethylbenzene as the substrates. As shown in Table S4,† [Fe(F20 TPPL)Cl] exhibited higher reactivity (88, 80 and 88%, entries 1, 5 and 8, respectively) than [Fe(F20 -TPP)Cl] and [Fe(F15 -TPC)] (entries 2, 3, 6, 7, 9 and 10). Replacing iron with manganese resulted in a decrease of the reactivity of aziridination of styrene to 53% yield (entry 4, Table S4, ESI†). A time course plot (Fig. S1, ESI†) for the aziridination of styrene by [Fe(F20 -TPPL)Cl] and Dalton Trans.
View Online Table 1 Organic azides for catalytic aziridination of styrene and amidation of ethylbenzene by [Fe(F20 -TPPL)Cl]a
Table 2 Aziridination of alkenes with TsN3 by [Fe(F20 -TPPL)Cl]a
Downloaded by Beijing University on 09 January 2012 Published on 21 December 2011 on http://pubs.rsc.org | doi:10.1039/C2DT11995A
Entry Yield (%)b Entry Sub.
Sub.
1
styrene 87
8
ethylbenzene 0
2
styrene 61
9
ethylbenzene 0
3
styrene n.dc
10
ethylbenzene 88
4
styrene 0
11
ethylbenzene 0
5
styrene 0
12
ethylbenzene 0
6
styrene 0
13
styrene 0
14
7
PhINTs
Alkene
Product
Yield (%)b
Yield (%)b
Entry Azide
1
87
2
81
3
87
4
88
ethylbenzene 0
5
80
ethylbenzene 0
6
89
7
