Journal Publications

Simon Doherty, Julian G. Knight, Rakesh K. Rath, William Clegg, Ross W. Harrington, Colin R. Newman, Robert Campbell and Hiren Amin, Organometallics 2005, 24, 2633.

Abstract: The ruthenium-p-cymene complexes [(p-cymene)Ru(1,2,3,4-Me₄-NUPHOS)Cl][SbF₆] (2a) and [(p-cymene)Ru(1,4-Et₂-2,3-cyclo-C₆H₈-NUPHOS)Cl][SbF₆] (2b) have been prepared by reaction of [(p-cymene)RuCl₂]₂ with the corresponding NUPHOS diphosphine in the presence of NaSbF₆. The chloro ligand can be abstracted from these monocations to afford [(p-cymene)Ru(P,P,^h2(C)-1,2,3,4-Me₄-NUPHOS)][SbF₆]₂ (3a) and [(p-cymene)Ru(P,P,^h2(C)-1,4-Et₂-2,3-cyclo-C₆H₈-NUPHOS)][SbF₆]₂ (3b), respectively, in which the diphosphine coordinates as a six-electron donor, bonded through both diphenylphosphino groups and one of the double bonds of the butadiene tether. In stark contrast, it proved markedly more difficult to abstract the chloro ligand from either the BIPHEP or the MeO-BIPHEP monocations [(p-cymene)Ru(BIPHEP)Cl][SbF₆] (4a) and [(p-cymene)Ru(MeO-BIPHEP)Cl][SbF₆] (4b), and even after prolonged reaction times at elevated temperature [(p-cymene)Ru(BIPHEP)][SbF₆]₂ (5a) and [(p-cymene)Ru(MeO-BIPHEP)][SbF₆]₂ (5b) formed as a 30% mixture with unreacted 4a and 4b, respectively. The structures of 2a, as its perchlorate salt, and 2b have been determined by single-crystal X-ray crystallography and are compared with that of their BIPHEP counterpart 4a. Unfortunately, it has not been possible to prepare the corresponding dppb complex [(p-cymene)Ru(dppb)Cl][SbF₆] to undertake a comparative study, since [(p-cymene)RuCl₂]₂ reacts with dppb under the same conditions as those used to prepare 2a,b to afford the bridged dimer [{(p-cymene)RuCl₂}₂(m-dppb)] (6), the identity of which has been confirmed by a single-crystal X-ray study. Interestingly, 3a undergoes rapid hydrolysis in the presence of pyridine to give [(p-cymene)Ru{Ph₂(O)PC(H)MeCMeCMeCMePPh₂}][SbF₆] (7), which contains an unusual unsymmetrical bisphosphine monoxide pincer ligand formed by oxidation of one of the diphenylphosphino groups of 1,2,3,4-Me₄-NUPHOS and a highly regioselective syn addition of Ru and H across the butadiene double bond proximate to the phosphine oxide. Dications 3a,b catalyze the regioselective anti-Markovnikov addition of benzoic acid to 1-pentyne and 1-octyne to give the corresponding alk-1-en-1-yl esters with cis-to-trans ratios as high as 95:5, while the corresponding BIPHEP and MeO-BIPHEP complexes were significantly less selective, catalyst mixtures formed from 4b giving a 70:30 mixture of cis and trans alk-1-en-1-yl ester. In contrast, selectivity was reversed with catalyst mixtures generated from 6, which were 90% selective for Markovnikov addition to 1-octyne, as might have been predicted for a ruthenium catalyst coordinated by a single phosphine, albeit one-half of a bidentate diphosphine. Catalysts based on NUPHOS diphosphines are also highly active and selective for anti-Markovnikov addition of benzoic acid to phenylacetylene and give (Z)-styryl benzoate in yields of up to 85% and selectivities as high as 99:1 with no evidence for the formation of terminal olefin. In our hands, solutions formed by activation of 4a,b with AgSbF₆ catalyze the regio- and stereoselective anti-Markovnikov hydrocarboxylation of phenylacetylene, which was somewhat surprising considering that an earlier report has claimed that 5b reacts with phenylacetylene to form a stable catalytically inactive cyclometalation/insertion product.