| 18682866 |
Lopez-Serrano J, Duckett SB, Dunne JP, Godard C, Whitwood AC: Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation. Dalton Trans. 2008 Aug 28;(32):4270-81. Epub 2008 Jun 25.
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 (PCy (2) CH (2) CH (2) PCy (2)), dppe, dppm, dppp, cppe (PCy (2) CH (2) CH (2) PPh (2))] are used. Alkyl palladium intermediates of the type [Pd (LL')(CHPhCH (2) Ph)](OTf) ( and ) 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 (LL' = dcpe) and linkage isomer ' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (DeltaH ( not equal) and DeltaS ( 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 CD (2) Cl (2). All five of the phosphine systems described are less active than those based on bcope [where bcope is (C (8) H (14)) PCH (2)-CH (2) P (C (8) H (14))] and (t) bucope [where (t) bucope is (C (8) H (14)) PC (6) H (4) CH (2) P ((t) Bu)(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. |
1(0,0,0,1) |