Palladium catalysed alkynehydrogenation and oligomerisation: a parahydrogen based NMR investigation

Abstract

The role phosphine ligands play in the palladium(II)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL′)(OTf)₂ (1a–e) [LL′ = dcpe (PCy₂CH₂CH₂PCy₂), dppe, dppm, dppp, cppe (PCy₂CH₂CH₂PPh₂)] are used. Alkyl palladium intermediates of the type [Pd(LL′)(CHPhCH₂Ph)](OTf) (2 and 3) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the α-H of the alkyl ligand of 2a (LL′ = dcpe) and linkage isomer 2e′ (LL′ = cppe) into trans-stilbene on the NMR timescale. Activation parameters (ΔH^≠ and ΔS^≠) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD₂Cl₂. All five of the phosphine systems described are less active than those based on bcope [where bcope is (C₈H₁₄)PCH₂–CH₂P(C₈H₁₄)] and ^tbucope [where ^tbucope is (C₈H₁₄)PC₆H₄CH₂P(^tBu)₂]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL′)(PhCH–CHPh–CPhCHPh)⁺ (4) also being shown to play a role in the alkyne dimerisation step.