Syntheses, structures, and catalytic reactions of palladium adducts of chiral diphosphines that contain a rhenium stereocenter in the backbone
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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)(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)2AIH 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).
author list (cited authors)
Kromm, K., Hampel, F., & Gladysz, J. A.