3,5-dialkyl effect on enantioselectivity in Pd chemistry: Applications involving both bidentate and monodentate auxiliaries

The structural 3,5-dialkylphenyl effect on enantioselectivity is demonstrated for several Pd-catalyzed reactions including a ring-opening transmetalation, Heck arylation, and allylic alkylation. For these homogeneously catalyzed reactions the observed enantiomeric excesses (ee's) are found to improve by more than 15%. The ligands tested include MeO-Biphep and a P,N-phosphino-oxazoline-bidentate ligand containing 3,5-di-tert-butylphenyl substituents. Further, several derivatives of the monodentate auxiliary MOP ((R)-2-diarylphosphino-1,1'-binaphthyl) have been modified to include 3,5-dialkylphenyl substituents and these auxiliaries have been tested in Pd-catalyzed enantioselective hydrosilylation chemistry. For some, but not all of these MOP ligands, enhanced ee's of the order of 40-50% are found. Variable-temperature and 2-D NMR studies have been carried out on new model complexes and reveal selected restricted rotation around a number of the P-C(Ipso) aryl bonds. Solid-state structures for two of the new complexes, PdBr(p-NCC6H4)(phosphino-oxazoline, 2b), 8b, and PdCl(C6H4CH2NMe2)(MOP, 4b), 9b, have been determined.