New Chiral Lewis Bases Derived from L‐Pipecolinic Acid Showing Stereocontrol Highly Dependent on the Catalyst Design in the Hydrosilylation of N‐Phenyl Ketimines

Abstract

AbstractChiral L‐pipecolinic acid based catalysts, the N‐functionalized pipecolinamides 4a–h and 5a–e, have been obtained in a straightforward two‐step synthesis starting from Boc‐L‐pipecolinic acid. The catalysts form a complex with trichlorosilane, which proved to be effective for the enantioselective addition to the standard N‐phenylimine substrate 6. We have tested the enantioselectivity of the ligands to establish the influence of N‐ and O‐functionalization on the chiral α‐amino acid. Whereas the N‐formylpipecolinamides give rise to R‐configured amines, the opposite result was obtained by using the urea‐pipecolinamide 5c, which proved to be an efficient activator of trichlorosilane. The observed reactivities and stereoselectivities have been rationalized not only on the basis of the conformational dependence of the ligands, but also by noncovalent interactions, which has allowed the structures of the transition states to be deduced. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)