Development of the First P-Stereogenic PCP Pincer Ligands, Their Metallation by Palladium and Platinum, and Preliminary Catalysis

The potentially tridentate P-stereogenic [P*CP*] ligands 1,3-{bis[(tert-butyl)(phenyl)phosphino]methyl}benzene and 1,3-{bis[(tert-butyl)(phenyl) phosphino]methyl}-2-bromobenzene have been synthesized as the protected phosphine-borane adducts. Deprotection with a secondary amine affords the free phosphine ligand which can be metallated by Pd and Pt with standard metal synthons. Two of the resultant [P*CP*] metal complexes have been characterized by X-ray crystallography. The complexes exhibit a C2 symmetric environment about the remaining binding site of the square-planar center, with t-Bu groups filling two quadrants of the open site. The Pd complexes can be converted by use of a Ag salt to the analogous aquo complex, which is catalytically active in the aldol condensation of methyl 2-isocyanoacetate and benzaldehyde. Preliminary results and comparisons with previously reported catalysts with more distal C-stereogenicity are presented.