Chemical-promoted oxidative polymerization of modified-cobalt salen complexes, efficient catalysts for the dynamic HKR

Abstract

FeCl 3 and NOBF 4 have been successfully tested as chemical oxidants to promote the polymerization of thiophene-or pyrrole-modified chiral cobalt salen complexes. The iron oxidation procedure particularly delivered an insoluble polymeric species in nearly quantitative yield that was tested for its propensity to promote the dynamic hydrolytic kinetic resolution of epibromohydrin. This catalyst allowed the synthesis of the expected diol in high yield and up to 85% ee. As an insoluble powder, it was either efficiently recycled in a batch reactor via simple filtration or dispersed in a silica support and reused in a flow procedure for up to five cycles. A copolymerization strategy delivered an even more reliable polymerized complex that was repetitively used without showing any loss in activity or in enantioselectivity for five reuses in this transformation.

Chiral Salen Complexes for Asymmetric Heterogeneous Catalysis: Recent Examples for Recycling and Cooperativity

“…It is worth mentioning the implementation of flow procedures, for which the stability of these catalysts is particularly adapted. This has already been demonstrated in the past for more conventional asymmetric catalytic reactions, [39,122] and these methods would be particularly suitable for the development of multicatalysis . 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57…”

“…It is worth mentioning the implementation of flow procedures, for which the stability of these catalysts is particularly adapted. This has already been demonstrated in the past for more conventional asymmetric catalytic reactions, and these methods would be particularly suitable for the development of multicatalysis.…”

“…Moreover, this catalyst was engaged in a multisubstrate recovery procedure with various terminal epoxides, thereby demonstrating its usefulness for the transformation of diverse substrates. Chemical oxidants such as FeCl 3 or NOBF 4 achieved also the polymerization of such complexes . Copolymerization of a 50/50 mixture of salen monomers possessing Co−OAc or Co−BF 4 active sites led again to a very stable catalyst which was dispersed in silica and reused in a flow procedure for up to five cycles.…”

“…Chemical oxidants such as FeCl 3 or NOBF 4 achieved also the polymerization of such complexes. [39] Copolymerization of a 50/50 mixture of salen monomers possessing CoÀ OAc or CoÀ BF 4 active sites led again to a very stable catalyst which was dispersed in silica and reused in a flow procedure for up to five cycles.…”

Chiral salen complexes in polymeric main‐chains for heterogeneous asymmetric catalysis – a brief account

“…For instance, FeCl3 or NOBF4 delivered insoluble polymeric species in nearly quantitative yield, from Co II -containing monomers, under an air atmosphere, or directly from Co III -precursors. [23] Accordingly, the polymers proved efficient catalysts for the dynamic HKR of epibromohydrin, leading to the expected scalemic diol with high yield and enantioselectivity. The polymerized catalyst could be recovered and reused leading to stable values of ee but large variations in the conversion went along with the recycling, as a proof for its chemical alteration during the HKR.…”