Diphosphine platinum(II) binaphtholates: Structures and application to molecular imprinting and Lewis acid catalysis

Interactions of chiral disphosphine and binaphtholate (BINOL) ligands were investigated in square-planar platinum(II) complexes containing matched and mismatched ligand pairs. X-ray crystallography revealed that both ligands adopt predicted, favorable conformations in the matched complex [(S,S)-chiraphos]Pt[(S)-BINOL]. However, in the mismatched case eight [(S,S)-chiraphos]Pt[( R)-BINOL] conformers were observed in the solid state; both ligands exhibited significant conformational flexibility. 3,3′-Methylation of BINOL did not affect the matched structure [(S,S)-chiraphos]Pt[( S)-Me₂BINOL], but NMR and X-ray data for the mismatched complex showed that Me₂BINOL adopts a novel C,O-binding mode in [(S,S)-chiraphos]Pt[(R)-Me₂BINOL] to avoid steric conflicts with the diphosphine's phenyl groups. The related complex (dppe)Pt[(R)-Me₂BINOL] exists as an equilibrium mixture of O,O′- and C,O-bound species in solution, which interconvert through O → C Pt migration accompanied by Me ₂BINOL enol-keto tautomerization.; Co-polymerization of metallomonomer (vinyl dppe)Pt[(R)-Bu ₂BINOL] with EGDMA produced a molecularly imprinted polymer (MIP). Removal of the imprinting ligand (R)-Bu ₂BINOL revealed chiral cavities in the MIP, associated with the immobilized Pt centers; subsequent exposure to rac-BINOL led to preferential rebinding of the imprinted enantiomer (69%ee) to Pt via ligand exchange. Both aggregate selectivity and extent of reaction increased with rebinding time and temperature, indicating that unreactive Pt sites are more selective than accessible sites. The kinetic selectivity of the least reactive sites in the MIP was determined to be 94%ee. Open, accessible Pt sites were proposed to be less selective than unreactive sites because their associated chiral cavities are less well-defined and therefore less capable of shape-based enantiodiscrimination.; Activation of P₂Pt[(S)-BINOL] [P₂ = dppe, (R)-BINAP] with two equivalents of triflic acid (HOTf) afforded Lewis acids P₂Pt(OTf)₂, which catalyze the Diels-Alder reaction of acryloyl-N-oxazolidinone with cyclopentadiene. Competition experiments monitored by 31P NMR probed the relative binding of counterions, water, dienophile, and cycloadduct to [P₂Pt] 2+. Observation of P₂Pt(OTf)₂-catalyzed Diels-Alder reactions by NMR at 195 K revealed that dppe and (R)-BINAP reactions proceed by different mechanisms: (dppe)Pt(OTf)₂ reacts rapidly with cyclopentadiene to give [P₂Pt(η5-Cp)] +[OTf−] and HOTf, which actually catalyzes the Diels-Alder reaction, but [(R)-BINAP]Pt(OTf)₂ does not decompose as quickly. Finally, the structure of a Pt₂(μ-BINOL) dimer observed whenever less than two equivalents of HOTf were added to (dppe)Pt[( S)-BINOL] was investigated by 2D NMR techniques.