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-XC(6)H(4)OTf, X = OMe, H, CO(2)Me, 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 PdCl(2)(1) + NaBH(4). The solid-state structures for two PdI-(aryl)(1a), 3, derivatives, aryl = p-MeOOC-C(6)H(4) (3a) and C(6)F(5) (3b) are reported.