Synthesis, Structure, Dynamics, and Selective Methylation of Platinum and Palladium Diphosphametallacyclobutane Complexes

Treatment of M(dppe)Cl2 (M = Pd, Pt) or Pt((R,R)-Me-DuPhos)Cl2 with IsHPCH2PHIs (1; Is = isityl = 2,4,6-(i-Pr)3C6H2) and 2 equiv of NaOSiMe3 gave the mononuclear diphosphametallacyclobutane complexes M(dppe)(IsPCH2PIs) (M = Pd (2), Pt (3)), or Pt((R,R)-Me-DuPhos)(IsPCH2PIs) (4). Dynamic processes involving phosphorus inversion and rotation about the P–C(Is) bonds in 2–4 were characterized by variable-temperature NMR spectroscopy, which suggested that each existed as a single C 2-symmetric diastereomer in solution, consistent with their solid-state structures determined by X-ray crystallography. The MiniPhos derivative IsMePCH2PMeIs (5) was prepared as a 5.5/1 rac/meso mixture by sequential arylation and methylation of Cl2PCH2PCl2. Alternatively, the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl) mediated alkylation of secondary phosphines in the presence of NaOSiMe3 to yield selectively meso-5 either from PHMe(Is) and CH2I2 or from 1 and MeI. Recrystallization and chromatography yielded diastereomerically enriched rac-5 and meso-5. Treatment of M(dppe)(OTf)2 (M = Pd, Pt) or Pt((R,R)-Me-DuPhos)(OTf)2 with meso-5 gave the dications meso-[M(diphos)(IsMePCH2PMeIs)][OTf]2 (M(diphos) = Pd(dppe) (6), Pt(dppe) (8), Pt((R,R)-Me-DuPhos) (10)). Similar reactions of rac-5 yielded the dications rac-[M(diphos)(IsMePCH2PMeIs)][OTf]2 (M(diphos) = Pd(dppe) (7), Pt(dppe) (9)) and a 1/1 mixture of the C 2-symmetric diastereomers [Pt((R,R)-Me-DuPhos)(IsMePCH2PMeIs)][OTf]2 (11a,b). Treatment of 2 and 3 with 2 equiv of methyl triflate gave the dications 6–9 as ca. 1/1 meso/rac mixtures, and methylation of 4 selectively gave one of the C 2-symmetric diastereomers, 11a. These alkylations proceeded via the observable monomethylated intermediates [M(diphos)(IsMePCH2PIs)][OTf] (12–14).