Syntheses, Structures, and Catalytic Reactions of Palladium Adducts of Chiral Diphosphines that Contain a Rhenium Stereocenter in the Backbone

Reactions of the title diphosphines [(η5-C5H4PPh2)Re(NO)(PPh3)((CH2)nPPh2)] (n=0, (R)-1; n=1, racemic or (S)-2) with [PdCl2(PhCN)2] give the palladium/rhenium chelate complexes [(η5-C5H4PPh2)Re(NO)(PPh3)((μ-CH2)nPPh2)PdCl2] (n=0, (S)-5; n=1, racemic or (S)-6) in 75–92% yield. The crystal structure of racemic 6 shows a twisted-boat conformation of the chelate ring, giving a chiral pocket very different from that in a related rhodium chelate. However, NOE experiments suggest a similar ensemble of conformations in solution. Catalysts are generated from various combinations of a) Pd(OAc)2 and (R)-1 or (S)-2 (1 : 2), b) (S)-5 or (S)-6 and (R)-1 or (S)-2 (1 : 2), or c) (i-Bu)2AlH with the preceding recipes. These factors effect the Heck arylation of 2,3-dihydrofuran with phenyl trifluoromethylsulfonate. In contrast to analogous reactions with (R)-binap (=(R)-2,2′-bis(diphenylphosphanyl)-1,1′-binaphthalene), the major product 2-phenyl-2,3-dihydrofuran is nearly racemic (≤12% ee).