Diastereoisomer Interconversion in Chiral BiphepPtX₂ Complexes

Reaction of biphepPt(CO3) (biphep = 2,2‘-bis(diphenylphosphino)-1,1‘-biphenyl) with BINOL or HN(Tf)CHPhCHPhOH (TfNO) yielded square-planar biphepPtX2 (X2 = BINOL, N(Tf)CHPhCHPhO) complexes as a mixture of diastereomers (∼1:1). BiphepPtCl2 also reacted with Na2BINOL to generate biphepPt(BINOL) as a 1:1 mixture of diastereomers. With racemic BINOL or TfNO ligands, the mixtures were prone to isomerize to thermodynamic diastereomer mixtures (BINOL, 95:5; TfNO, >97:3) by an X2−X2 ligand−ligand exchange mechanism that was rapid at room temperature. With enantiopure ligands the X2−X2 ligand−ligand exchange process was degenerate and nonproductive. However, thermolysis of 1:1 mixtures of enantiopure biphepPt(BINOL) diastereomers (92−122 °C) cleanly established thermodynamic equilibrium by a process that involves biphenyl atropisomerism (ΔH⧧ = 27(2) kcal mol-1, ΔS⧧ = −5(5) eu). Two mechanisms for this process were considered, concerted stereoinversion via a planar seven-membered metallacycle, and one-arm-off prior to a biphenyl isomerization (anti disposed PPh2 units). In pyridine, a third mechanism for atropisomerism was identified and proposed to involve a five-coordinate pyridine intermediate (not observed) with an enhanced phosphine dissociation rate. Pyridine lowered the isomerization temperature of enantiopure complexes by ∼50 °C. X-ray structures of the thermodynamically favored biphepPt(TfNO) ((±)-4a) and the thermodynamically less favored biphepPt(BINOL) (λ(S)-5b) diastereomers were obtained, and a stereochemical model to explain the diastereoselectivity was formulated.