Syntheses, Structures, and Catalytic Reactions of Palladium Adducts of Chiral Diphosphines That Contain a Rhenium Stereocenter in the Backbone
摘要:
Reactions of the title diphosphines [(eta(5)-C(5)H(4)Phh(2))Re(NO)(PPh3)((CH2)(n)PPh2)] (n = 0, (R)-1; n = 1, recemic or (S)-2) with [PdCl2(PhCN)(2)] give palladium/rhenium chelate complexes [(eta(5)-C5H4PPh2)Re(NO)(PPh3)((mu-CH2)(n)PPh2)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)(2)AlH 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(diplienylphosphanyl)-1,1'-binaphthalene), the major product 2-phenyl-2,3-dihydrofuran is nearly racemic (less than or equal to12% ee).