Support effect on stereoselectivities of vinylogous mukaiyama-michael reactions catalyzed by immobilized chiral copper complexes

Abstract

Chiral bis(oxazoline)- and azabis(oxazoline)-copper complexes, used as homogeneous catalysts or immobilized onto laponite, are able to catalyze vinylogous Mukaiyama-Michael reactions between 2-(trimethylsilyloxy)furan and several electron-deficient alkenes. A study of the support effect has been conducted and different changes on the diastereoselectivities and enantioselectivities has been observed. The behavior of the catalyst is different, depending on the structure of the substrate (Michael acceptor). When diethyl benzylidenemalonate was used, the major diastereomer was that with syn configuration, but the homogeneous and the heterogeneous catalysts lead to opposite enantiomers (-80% ee in solution and 38% ee in the heterogeneous phase). This change represents a support effect of ∼1.8 kcal/mol. With N-(E)-but-2-enoyloxazolidinone, the most relevant change is in the diastereomer preference. In solution, the anti isomer is the major one (anti/syn = 98/2); however, in contrast, syn isomer is preferred with the immobilized catalyst (anti/syn = 19/81). This syn preference has not been previously reported in the literature. Finally, in the case of α,β-unsaturated ketones, the homogeneous catalysts are not able to induce enantioselectivity, whereas the immobilized ones lead to moderate values (up to 70%), similar to those values described in the literature with organocatalysts. © 2013 American Chemical Society.