Palladium-catalyzed asymmetric phosphination. Enantioselective synthesis of PAMP-BH₃, ligand effects on catalysis, and direct observation of the stereochemistry of transmetalation and reductive elimination

The complexes Pd(diphos)(o-An)(I) (o-An = o-MeOC₆H₄; diphos = dppe (3), (S,S)-Chiraphos (4), (R,R)-Me-Duphos (5), (R,S) -t-Bu-Josiphos (6), (R)-Tol-Binap (7)) were prepared. Complex 6 catalyzed the coupling of PH(Me)(Ph)(BH₃) (2) with o-AnI in the presence of base to yield PAMP-BH₃ (P(Me)(Ph)(o-An)(BH₃) (1)) in low enantiomeric excess. The course of stoichiometric reactions of 3-7 with 2 and NaOSiMe₃ depended on the diphosphine ligand. Complexes 6 and 7 gave PAMP-BH₃ (1) and Pd(0) species; no intermediates were observed. With 3, the intermediate Pd(dppe)(o-An)(P(Me)(Ph)(BH₃)) (10) was observed by ³¹P NMR, while 4 gave the isolable diastereomeric palladium complexes (Sp)-Pd((S,S)-Chiraphos)(o-An)(P(Me)(Ph)(BH₃)) (11a) and (R_P)-Pd((S,S)-Chiraphos)(o-An)(P(Me)(Ph)(BH₃)) (11b), whose absolute configurations were determined by X-ray crystallography after separation. The analogous Pd((R,R)-Me-Duphos)(o-An)(P(Me)(Ph)(BH₃)) diastereomers (12a,b) were also separated and isolated. Treatment of 4 with highly enantioenriched 2 (R or S) gave 11a or 11b in high diastereomeric excess with retention of configuration at phosphorus. P-C reductive elimination from either isomer of highly diastereoenriched 11 in the presence of excess diphenylacetylene yielded Pd((S,S)-Chiraphos)(PhC≡CPh) (14) and highly enantioenriched PAMP-BH₃ (1), with retention of configuration.