Palladium-Catalyzed Asymmetric Phosphination. Enantioselective Synthesis of PAMP−BH₃, Ligand Effects on Catalysis, and Direct Observation of the Stereochemistry of Transmetalation and Reductive Elimination

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Abstract

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 (RP)-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.

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