Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation

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 alpha-H of the alkyl ligand of 2a (LL' = dcpe) and linkage isomer 2e' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (Delta H-not equal and Delta S-not equal) 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 (t)bucope [where (t)bucope is (C8H14) PC6H4CH2P(Bu-t)(2)]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL')(PhCH-CHPh-CPh= CHPh)(+) (4) also being shown to play a role in the alkyne dimerisation step.