Richard L. Wingad

Cyclopropenylidene carbene ligands in palladium C–C coupling catalysis

A palladium complex supported by a 2,3-diphenylcyclopropenylidene carbene ligand is a highly active and robust catalyst for Heck and Suzuki coupling reactions.

N,N‐Diphospholylamines—A New Family of Ligands for Highly Active, Chromium‐Based, Selective Ethene Oligomerisation Catalysts

A series of new diphosphazane (PNP) ligands that contain 2,3,4,5‐tetraethylphosphole or dibenzophosphole moieties has been synthesised. The new compounds have been screened for chromium‐catalysed, selective ethene oligomerisation by in situ combination with CrCl3(thf)3 and methylaluminoxane (MAO). The ligands derived from 2,3,4,5‐tetraethylphosphole produce highly active catalysts for ethene oligomerisation, which show excellent selectivity to C6 and C8 linear α‐olefins. Complexes of the form [Cr(CO)4L] were synthesised and studied by IR spectroscopy and single‐crystal XRD. Variable‐temperature NMR spectroscopy was used to investigate restricted PN rotation in compounds with bulky nitrogen substituents.

Chiral triaryl phosphite-based palladacycles and platinacycles: synthesis and application to asymmetric Lewis acid catalysis

The optically pure monophosphites P(OAr)(BINOLate) (7, where Ar = 2,4-di-tert-butylphenyl) have been prepared by treatment of PCl2(OAr) with R- or S-BINOL. Treatment of [PdCl2(NCMe)2] with 7 gave [PdCl2(7)2] (9) or the binuclear orthometallated complex [Pd2Cl2(7-H)2] (8) depending on the reaction conditions. Bridge cleavage reactions of 8 gave [PdCl(7-H)(L)] with L trans to carbon when L = PPh3 or 7 and cis to carbon when L = N-heterocyclic carbene. Treatment of [PtCl2(NCtBu)2] with 7 gave [PtCl2(7)2] (18) which upon further reaction with PtCl2 furnished a mixture of binuclear [Pt2Cl2(7-H)2] (17) and cis-[PtCl(7-H)(7)] (19). The palladium complexes containing cyclometallated 7 were screened for catalysis of 1,4-conjugate addition of phenylboronic acid to cyclohexen-2-one and the allylation of benzaldehyde with allyltributyltin. Conversions were generally high in each case but enantioselectivities were low (15% e.e. at best). The X-ray crystal structures of 8, 17 and [PdCl(7-H)(NHC)] (10a, where NHC = 1,3-(dimesityl)imidazolidin-2-ylidene) have been determined.

Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: carbene ligand rotation and application to the Stille reaction

Reaction of [Pd(PPh(3))(4)] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl(2)(PPh(3))(C(3)(Ar)(2))] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)(2)] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl(2)(C(3)(Ar)(2))](2) (Ar = Ph 7, p-(OMe)C(6)H(4)8, p-(F)C(6)H(4)9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl(2)(NCMe)(C(3)(Ar)(2))] (Ar = Ph 10, p-(OMe)C(6)H(4)11 and p-(F)C(6)H(4)) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl(2)(PR(3))(C(3)(Ar)(2))] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C(6)H(4), R = Ph 14, Ar = p-(F)C(6)H(4), R = Ph 15). Crystal structures of complexes 6·3.25CHCl(3), 10, 11·H(2)O and 12-15 are reported. DFT calculations of complexes 10-12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5-9 were tested for catalytic activity in C-C coupling (Mizoroki-Heck, Suzuki-Miyaura and, for the first time, Stille reactions) and C-N coupling (Buchwald-Hartwig amination) showing excellent conversion with moderate to high selectivity.

Towards the upgrading of fermentation broths to advanced biofuels: a water tolerant catalyst for the conversion of ethanol to isobutanol

Isobutanol is an ideal gasoline replacement due to its high energy density, suitable octane number and compatibility with current engine technology. It can be formed by the Guerbet reaction in which (bio)ethanol and methanol mixtures are converted to this higher alcohol in the presence of a suitable catalyst under basic conditions. A possible limitation of this process is the catalysts water tolerance; a twofold problem given that water is produced as a by-product of the Guerbet reaction but also due to the need to use anhydrous alcoholic feedstocks, which contributes significantly to the cost of advanced biofuel production. Isobutanol formation with pre-catalyst trans-[RuCl2(dppm)2] (1) has been shown to be tolerant to the addition of water to the system, achieving an isobutanol yield of 36% at 78% selectivity with water concentrations typical of that of a crude fermentation broth. Key to this success is both the catalysts tolerance to water itself and the use of a hydroxide rather than an alkoxide base; other catalysts explored are less effective with hydroxides. Alcoholic drinks have also been used as surrogates for the fermentation broth: the use of lager as the ethanol source yielded 29% isobutanol at 85% selectivity in the liquid phase.