Ruthenium Complexes of Six-Electron-Donor NUPHOS-Type Diphosphines: Highly Selective Catalysts for the Hydrocarboxylation of Terminal Alkynes

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,η2(C)-1,2,3,4-Me4-NUPHOS)][SbF6]2 (3a) and [(p-cymene)Ru(P,P,η2(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(μ-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.