Robin B. Bedford

Palladacyclic catalysts in C–C and C–heteroatom bond-forming reactions

Palladacycles are amongst the most active catalysts in Heck-type carbon-carbon bond formation and related carbon-heteroatom bond forming reactions. For instance they have recently emerged as showing the highest activity in the Suzuki coupling of electronically challenging aryl chloride substrates. In addition to the high activity they display, their ease of synthesis, facile modification and comparative stability all act to enhance their appeal.

Iron(III) salen-type catalysts for the cross-coupling of aryl Grignards with alkyl halides bearing β-hydrogens

Iron(III) salen and related complexes are active catalysts for the coupling, under mild and simple reaction conditions, of aryl Grignard reagents with primary and secondary alkyl halide substrates bearing beta-hydrogens.

Silica-supported imine palladacycles : recyclable catalysts for the Suzuki reaction?

Silica-supported, imine-based palladacyclic catalysts have been synthesised and the crystal structure of complex 9, the triphenylphosphine adduct of the pre-supported precursor complex 8, has been determined. The solid-supported catalysts show considerably lower activity in the Suzuki reaction than their homogeneous counterparts. Poor recyclability of the silica-immobilised catalysts and the presence of active catalysts in solution indicate that imine-based palladacyclic catalysts are unstable with respect to liberation of zero-valent palladium species. Whilst the solid-supported complexes are not useful as catalysts, they do function as excellent mechanistic probes. Studies on model complexes give further information on the processes that cause the liberation of zero-valent species not only from the solid-supported catalysts, but also from homogeneous systems. In all cases it appears that a reductive-elimination event occurs to generate the active catalyst.

Orthopalladated phosphinite complexes as high-activity catalysts for the Suzuki reaction

The synthesis of a range of phosphinite ligands PR2(OAr) n(R = Ph, iPr), their simple complexes with palladium(II) and their palladacyclic complexes has been investigated. The crystal structure of one of the palladacyclic complexes, [{Pd(μ2-Cl){κ2-P,C-PiPr2(OC6H2-2,4-tBu2)}2], has been determined. The palladacyclic complexes show extremely high activity in the Suzuki coupling of aryl bromide substrates with phenylboronic acid and can also be used with alkylboronic acid substrates. A comparison of the phosphinite-based catalysts with equivalent phosphite- and phosphine-based systems highlights their superior activity. The orthometallation of the phosphinite ligand in the pre-catalyst appears to be crucial for optimal activity. While the phosphinite palladacycles are only moderately active in the coupling of activated and non-activated aryl chloride substrates, their tricyclohexylphosphine adducts prove to be highly active in the coupling of the deactivated substrate, 4-chloroanisole. This high activity compared with other palladacyclic systems is explained in terms of catalyst longevity. The orthometallated precatalysts appear to undergo a reductive activation process to generate zerovalent active catalysts via reductive elimination of the orthometallated ring with a phenyl introduced by the boronic acid. This implies that the true active catalysts contain 2-arylated ligands. Catalysts formed with such 2-arylated ligands tend to show markedly higher activity than their parent ligands.

Highly active catalysts for the Suzuki coupling of aryl chlorides.

Simple tricyclohexylphosphine adducts of palladium complexes with orthometallated N-donor ligands show by far the highest activity yet reported in the Suzuki coupling of aryl chlorides, even under aerobic conditions.

Alkyne insertion reactions of [RuH(κ2-S2CNEt2)(CO)(PPh3)2] : synthesis of alkenyl, alkynyl and enynyl complexes

Abstract The diethyldithiocarbamate hydride ruthenium complex [RuH(κ2-S2CNEt2)(CO)(PPh3)2] reacts with excess phenylacetylene to give the alkynyl complex [Ru(Cue606CPh)(κ2-S2CNEt2)(CO)(PPh3)2] via the intermediate alkenyl complex [Ru(CHue605CHPh)(κ2-S2CNEt2)(CO)(PPh3)2] and with 1,4-diphenylbutadiyne to give the enynyl complex [Ru{η1-C(Cue606CPh)ue605CHPh}(κ2-S2CNEt2)(CO)(PPh3)2]. The alkenyl complex [Ru(CHue605CHPh)(κ2-S2CNEt2)(CO)(PPh3)2] is a convenient precursor for alkynyl complexes of the type [Ru(Cue606CR)(κ2-S2CNEt2)(CO)(PPh3)2] (R=aryl, alkyl, C6H4-4-Cue606CH).

Phosphine and arsine adducts of N-donor palladacycles as catalysts in the Suzuki coupling of aryl bromides

Triarylphosphine and arsine adducts of imine- and amine-based palladacycles have been produced and the crystal structures of three examples have been determined, as has the structure of a parent imine-based palladacycle. The complexes were tested in the Suzuki coupling of an electronically deactivated aryl bromide and the phosphine adducts were found to show much greater activity than the parent palladacycles. Triarylphosphine adducts are preferable to trialkylphosphine adducts as they not only show higher activity but they are also more easily synthesised.

The Suzuki coupling of aryl chlorides in TBAB-water mixtures

Palladium acetate in a mixture of TBAB and water can be used as an effective catalyst for the Suzuki coupling of deactivated aryl chloride substrates.

Palladacyclic phosphinite complexes as extremely high activity catalysts in the Suzuki reaction

Phosphinite based palladacycles show extremely high activity nin the Suzuki coupling of both sterically hindered and electronically ndeactivated aryl bromides, especially in the presence of one equivalent of nfree ligand.

Di- and tri-alkylphosphine adducts of S-donor palladacycles as catalysts in the Suzuki coupling of aryl chlorides

The reaction of tricyclohexylphosphine with the S-based palladacycle [(Pd(mu-OAc)(kappa2-S,C-C(6)H(4)CH(2)SMe))(2)] gives several products, regardless of stoichiometry, one of which, [Pd(kappa1-OAc)(eta1-C(6)H(4)CH(2)SMe)(PCy3)2], has been characterised crystallographically. Despite this, catalysts formed in situ from di- and tri-alkylphosphines and [(Pd(mu-OAc)(kappa2-S,C-C(6)H(4)CH(2)SMe))2] show excellent activity in the Suzuki coupling of a range of deactivated, non-activated and activated aryl chloride substrates.