Enantioselective synthesis and application to the allylic imidate rearrangement of amine-coordinated palladacycle catalysts of cobalt sandwich complexes

Abstract

The reaction of (η5-(N,N-dimethylaminomethyl)cyclopentadien- yl)(η4-tetraphenylcyclobutadiene)cobalt with sodium tetrachloropalladate and (R)-N-acetylphenylalanine gave planar chiral palladacycle di-μ-chloridebis[(η5-(Sp)-2-(N,N- dimethylaminomethyl)cyclopentadienyl,1-C,3′-N)(η4- tetraphenylcyclobutadiene)cobalt]dipalladium [(Sp)-Me 2-CAP-Cl] in 92 % ee and 64 % yield. Enantiopurity (>98 % ee) was achieved by purification of the monomeric (R)-proline adducts and conversion back to the chloride dimer. Treatment with AgOAc gave (Sp)-Me 2-CAP-OAc which was applied to asymmetric transcyclopalladation (up to 78 % ee). The (R)-N-acetylphenylalanine mediated palladation methodology was applicable also to the corresponding N,N-diethyl (82 % ee, 39 % yield) and pyrrolidinyl (>98 % ee, 43 % yield) cobalt sandwich complexes. A combination of 5 mol % of the latter [(Sp)-Pyrr-CAP-Cl] and AgNO3 (3.8 equiv) is a catalyst for the allylic imidate rearrangement of an (E)-N-aryltrifluoroacetimidate (up to 83 % ee), and this catalyst system is also applicable to the rearrangement of a range of (E)-trichloroacetimidates (up to 99 % ee). This asymmetric efficiency combined with the simplicity of catalyst synthesis provides accessible solutions to the generation of non-racemic allylic amine derivatives. Enantioselective C-H activation: (R)-N-Acetylphenylalanine mediated enantioselective palladation gave a bulky cobalt sandwich complex based amino-palladacycle (Sp)-Pyrr-CAP-Cl in more than 98 % ee (see scheme). Following activation with silver nitrate, the use of 5 mol % of the product catalyst resulted in the highly enantioselective rearrangement of (E)-trichloroacetimidates. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.