Contributions to the Enantioselective Heck Reaction Using MeO−Biphep Ligands. The Case Against Dibenzylidene Acetone

It is shown that the Pd-catalyzed enantioselective Heck reaction of p-XC6H4OTf, X = OMe, H, CO2Me, with dihydrofuran gives higher enantioselectivities when the chelating diphosphine MeO−Biphep, 1a, is replaced with its disubstituted analogue 3,5-di-tert-butyl MeO−Biphep, 1b. The phenylation of 5-methyl-2,3-dihydrofuran produces a new dihydrofuran containing a quaternary stereogenic center (ee, >98% with 1b, ca. 20% with 1a). Catalytic results for the reaction of phenyl triflate with dhf, together with stoichiometric oxidative addition reactions of aryl halides on Pd complexes of 1, show that the use of Pd(dba)(1), dba = dibenzylidene acetone, slows the oxidative addition relative to the reaction in which the Pd(0) precursor is generated from PdCl2(1) + NaBH4. The solid-state structures for two PdI(aryl)(1a), 3, derivatives, aryl = p-MeOOC−C6H4(3a) and C6F5 (3b) are reported

New Chiral Complexes of Palladium(0) Containing P,S- and P,P-Bidentate Ligands

New chiral complexes of Pd(0) containing either the bis(phosphine) (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(3,5-di-tert-butylphenylphosphine) (MeO-BIPHEP, 1) or the phosphine−sulfur chelate (2,3,4,6-tetra-O-acetyl-1-{(2-diphenylphosphino)benzyl)thio}-β-d-glucopyranose ((2-Ph2PC6H4CH2)−S−CHCH(OAc)CH(OAc)CH(OAc)CH(CH2OAc)O, 2) have been prepared, and the solid-state structure of one of these, Pd(benzoquinone)(2), has been determined. These Pd(0) complexes reveal interesting solution dynamics, as shown by 2-dimensional exchange spectroscopy. For the MeO-BIPHEP derivatives, one can obtain useful structural insights based on the observed restricted rotation around the aryl(3,5-di-tert-butylphenyl) P−C bonds

Enantioselective Homogeneous Catalysis and the “3,5-Dialkyl Meta-Effect”. MeO−BIPHEP Complexes Related to Heck, Allylic Alkylation, and Hydrogenation Chemistry

The enantioselectivities arising from a Pd-catalyzed Heck reaction (>98% ee) and an allylic alkylation (>90% ee) using a 3,5-di-tert-butyl-MeO−BIPHEP chiral auxiliary (1) are reported. Higher ee's are observed with the 3,5-dialkyl substituents than with the unsubstituted parent MeO−BIPHEP. It is proposed that the observed dialkyl “meta-effect”, on enantioselectivity, is the combined result of a more rigid and slightly larger chiral pocket and that this effect will have some generality in homogeneous catalysis. Detailed NMR studies on the allyl complex [Pd(PhCHCHCHPh)(1)]PF6 (5), and the model hydrogenation catalyst [RuH(cymene)(1)]BF4 (6), reveal restricted rotation about several of the P−C(ipso) bonds of the phosphorus substituents containing the 3,5-di-tert-butyl groups. The X-ray structure of 6 reveals that the cymene ligand is not symmetrically bound to the Ru atom. This observation is interpreted as an expression of the chiral pocket of 1. MM3* calculations on 6 support the NMR findings and reproduce the X-ray results