Cluster-Mediated Conversion of Diphenylacetylene into α-Phenylcinnamaldehyde. Construction of a Catalytic Hydroformylation Cycle Based on Isolated Intermediates

Abstract

The present paper deals with a rational attempt to achieve the hydroformylation of diphenylacetylene onto a hydrido triruthenium cluster complex incorporating the 2-(methylamino)pyridyl group (abbreviation: MeNpy) as a hemilabile ancillary ligand [note: in all species discussed below, the bridgehead μ 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 (μ-H)(μ-MeNpy)(CO) 9 ( 1 ) is shown to react cleanly with diphenylacetylene to give the alkenyl complex Ru 3 (μ-MeNpy)(μ-PhC CHPh)(CO) 8 ( 2 ), the structure of which is reported. The reaction of 2 with 1 equiv of PPh 3 proceeds to completion within less than 3 min at 25 °C, giving two propenoyl complexes, namely, Ru 3 (μ-MeNpy)(μ-O C−PhC CHPh)(PPh 3 )(CO) 7 ( 3 ) (48% yield) and Ru 3 (μ-MeNpy)(μ-O C−PhC CHPh)(PPh 3 ) 2 (CO) 6 ( 4 ) (19% yield), both fully characterized by spectroscopic methods and X-ray analysis. Complex 3 is an adduct of 2 with PPh 3 . The incorporation of the phosphine has caused a migratory CO insertion of the alkenyl group. The phosphine occupies an equatorial coordination 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 to 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 (μ-MeNpy)(μ-O C−PhC CHPh)(PPh 3 ) 2 (CO) 6 ( 4 ), is formally derived from the previous one, 3, by a simple substitution of an equatorial CO of Ru(2) by PPh 3 . The use of a 2-fold amount of phosphine for the above reaction modifies only slightly the relative abundance of 3 (30%) and 4 (44%). This indicates that 3 is not the kinetic product of the reaction between 2 and a phosphine. Further reaction of 4b with CO induces loss of one PPh 3 and incorporation of two CO ligands. This produces the open 50e cluster Ru 3 (μ-MeNpy)(μ-O C−PhC CHPh)(PPh 3 )(CO) 8 ( 5 ), in which the bridging propenoyl group now spans the open edge Ru(1)−Ru(2) (the remaining phosphine occupies an equatorial site cis to the acyl oxygen). Treatment of 2b with CO (1 atm, 25 °C, 20 min) also promotes migratory CO insertion, giving the 50e propenoyl complex Ru 3 (μ-MeNpy)(μ-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 under inert atmosphere. Treatment of 6 with CO/H 2 gas mixtures under ambient conditions produces α-phenylcinnamaldehyde with concomitant recovery of 1, showing that the hydroformylation of diphenylacetylene can be achieved in a stepwise manner through the cyclic reaction sequence 1 → 2 → 6 → 1 . Under nonoptimized catalytic conditions, the amount of α-phenylcinnamaldehyde obtained corresponds to about eight cycles. The metal-containing species recovered in the reactor through the catalytic runs is isolated and formulated as the bimetallic carboxamido complex [Ru{−C(O)−MeNpy}(CO) 3 ] 2 ( 7 ). Thus, it appears that deactivation of the system has taken place via CO insertion into the metal−amide bond.