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Issue 32, 2008
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Palladium catalysed alkynehydrogenation and oligomerisation: a parahydrogen based NMR investigation

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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–CPh[double bond, length as m-dash]CHPh)+ (4) also being shown to play a role in the alkyne dimerisation step.

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

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Article information


Submitted
12 Mar 2008
Accepted
09 May 2008
First published
25 Jun 2008

Dalton Trans., 2008, 4270-4281
Article type
Paper

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

J. López-Serrano, S. B. Duckett, J. P. Dunne, C. Godard and A. C. Whitwood, Dalton Trans., 2008, 4270
DOI: 10.1039/B804162H

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