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 (CR)-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, PdBrCp-NCCeHUXphosphino-oxazoline, 2b), 8b, and PdCl(C 6H 4CH 2NMe 2)(MOP, 4b), 9b, have been determined.