Investigation of the Heck reaction and the associated catalytic systems using XAFS and Phosphorus NMR techniques

Rayner, Graham (2002) Investigation of the Heck reaction and the associated catalytic systems using XAFS and Phosphorus NMR techniques. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The main body of work was conducted on the Heck reaction between 2-methylprop-22-en-1-01 and PhX (X = C1, Br and I) to form 1-phenyl-2-methyl-propanal catalysed by the systems already examined in this thesis (the systems were also extended to include other phosphine ligands (tri(1-napthyl)phosphine and tris(o-methoxyphenyl)phosphine) and 3b as catalyst). The results showed a variety of catalytic properties, from tricyclohexylphosphine which prevents any catalysis, to tri(t-butyl)phosphine which promotes catalysis in all reactions. Reactions catalysed by palladium acetate and tri-o-tolylphosphine, showed that the main phosphorus species present in solution were different, compared with using 1 as catalyst. As the reactions which were catalysed proceeded, the extent of halogenation to the palladium increased. Halogenated species present for reactions catalysed by palladacycles were of the form: (P-C)PdX(solv), ([P-C)PdX₂^-] and [(P-C)P(m-X)] (where (P-C) is the palladated ring) and Pd(PR₃)₂(Ph)X, [Pd(Ph)(PR₃)(m-X)]₂ and [PdX(PR₃)(m-X)]₂ for reactions catalysed by palladium acetate and phosphine ligand. It was noted for uncatalysed reactions that there was negligible or no coordination of halogen to the metal, which suggests that this is an important early step in the catalytic cycle. Enforced, since heating the catalytic species (1) with PhI led to a stable mixture of species, whilst heating with alcohol led to decomposition of the metal precatalytic complex and liberation of free phosphine. Also, Pd(P^tBu₃)₂ was isolated from the PhBr reaction catalysed by Pd(OAc)₂ / P^tBu₃ and characterised by X-ray crystallography.