Cluster-mediated conversion of diphenylacetylene into alpha-phenylcinnamaldehyde. Construction of a catalytic hydroformylation cycle based on isolated intermediates

The present paper deals with a rational attempt to achieve the hydroformyla tion of diphenylacetylene onto a hydride triruthenium cluster complex incor porating the 2-(methylamino)pyridyl group (abbreviation: MeNpy) as a hemila bile ancillary ligand [note: in all species discussed below, the bridgehead mu(2)-N atom is linked to the centers labeled as Ru(1) and Ru(2), whereas the pyridyl nitrogen is bound to Ru(3)]. The complex Ru-3(mu-H)(mu-MeNpy)(C O)(9) (1) is shown to react cleanly with diphenylacetylene to give the alke nyl complex Ru-3(mu-MeNpy)(mu-PhC=CHPh)(CO)(8) (2), the structure of which is reported. The reaction of 2 with 1 equiv of PPh3 proceeds to completion within less than 3 min at 25 degrees C, giving two propenoyl complexes, nam ely, Ru-3(mu-MeNpy)(mu-O=C-PhC=CHPh)(PPh3)(CO)(7) (3) (48% yield) and Ru-3( mu-MeNpy)(mu-O=C-PhC=CHPh)(PPh3)(2)(CO)(6) (4) (19% yield), both fully char acterized by spectroscopic methods and X-ray analysis. Complex 3 is an addu ct of 2 with PPh3. The incorporation of the phosphine has caused a migrator y CO insertion of the alkenyl group. The phosphine occupies an equatorial c oordination site on Ru(1), in cis position relative to the nitrogen atom of the amido bridge. The newly formed propenoyl group occupies an equatorial bridging position across the Ru(1)-Ru(3) edge, with the acyl oxygen bound t o Ru(1), in cis position relative to both the bridgehead nitrogen atom and the phosphine. The molecular structure of the second propenoyl compound, Ru -3(mu-MeNpy)(mu-O=C-PhC=CHPh)(PPh3)(2)(CO)(6) (4), is formally derived from the previous one, 3, by a simple substitution of an equatorial CO of Ru(2) by PPh3. The use of a 2-fold amount of phosphine for the above reaction mo difies only slightly the relative abundance of 3 (30%) and 4 (44%). This in dicates that 3 is not the kinetic product of the reaction between 2 and a p hosphine. Further reaction of 4b with CO induces loss of one PPh3 and incor poration of two CO ligands. This produces the open 50e cluster Ru-3(mu-MeNp y)(mu-O=C-PhC=CHPh)(PPh3)(CO)(8) (5), in which the bridging propenoyl group now spans the open edge Ru(1)-Ru(2) (the remaining phosphine occupies an e quatorial site cis to the acyl oxygen). Treatment of 2b with CO (1 atm, 25 degrees C, 20 min) also promotes migratory CO insertion, giving the 50e pro penoyl complex Ru-3(mu-MeNpy)(mu-O=C-PhC=CHPh)(CO)(9) (6b), whose structure has been determined. The propenoyl group spans the open edge Ru(1)-Ru(2). Although stable in CO-saturated solutions under CO atmosphere, the complex reverts rapidly to 2 within 30 s cinder inert atmosphere. Treatment of 6 wi th CO/H-2 gas mixtures under ambient conditions produces alpha-phenylcinnam aldehyde with concomitant recovery of 1, showing that the hydroformylation of diphenylacetylene can be achieved in a stepwise manner through the cycli c reaction sequence 1 --> 2 --> 6 --> 1. Under nonoptimized catalytic condi tions, the amount of alpha-phenylcinnamaldehyde obtained corresponds to abo ut eight cycles. The metal-containing species recovered in the reactor thro ugh the catalytic runs is isolated and formulated as the bimetallic carboxa mido complex [Ru{-C(O)-MeNpy}(CO)(3)](2) (7). Thus, it appears that deactiv ation of the system has taken place via CO insertion into the metal-amide b ond.