The Mechanism of the Ru-Assisted C-C Bond Cleavage of Terminal Alkynes by Water Claudio Bianchini,* Juan A. Casares, Maurizio Peruzzini,* Antonio Romerosa, and Fabrizio Zanobini Contribution from the Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR,Via Jacopo Nardi 39 50132, Firenze, Italy ReceiVed January 16, 1996 X Abstract: The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl 2 (PPh 3 ) in THF at 60 °C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl 2 (CO) and toluene [PNP ) CH 3 CH 2 CH 2 N(CH 2 CH 2 PPh 2 ) 2 ]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl 2 {CdC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl- (CtCPh)(OH 2 ), and the (benzyl)carbonyl mer-(PNP)RuCl(η 1 -CH 2 Ph)(CO). Other intermediates such as the σ-acyl mer-(PNP)RuCl(η 1 -COCH 2 Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac,cis-(PNP)RuCl 2 {C(NC 5 H 10 )(CH 2 Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction. Introduction The hydration of alkynes to give carbonyl compounds is a well-known reaction 1 that can be promoted by either electro- philes (H + , Hg 2+ ) 2 or transition metal complexes. 3 In the peculiar case of terminal alkynes and iron-group metal promot- ers, the binary combination of the unsaturated hydrocarbon and water may lead to the cleavage of the C-C triple bond with formation of CO and the saturated hydrocarbon with one less carbon atom (generally in the form of carbonyl and alkyl ligands). 4 Although the first example of ruthenium-assisted C-C triple bond cleavage by water was reported more than ten years ago, 4a the overall mechanism of this important reaction, which involves two plentiful molecules of nature, is still rather obscure. From the mechanistic viewpoint, all of what is known may be summarized by reporting the following comment from the most recent edition of ComprehensiVe Organometallic Chemistry: “The hydrolysis of Vinylidene ligands typically leads to acyl or alkyl-carbonyl complexes and accordingly Vinylidene intermedi- ates, while not isolated, are almost certainly inVolVed in the synthesis of such complexes from terminal alkynes and halide complexes of ruthenium(II)”. 5 In this article we present a mechanistic study of the reaction between water and the model 1-alkyne, phenylacetylene, in the presence of a Ru(II) promoter. We are confident that the results obtained may constitute the final chapter for the mechanism of the metal-assisted C-C bond cleavage of 1-alkynes by water as well as provide new insight into many other reactions in which C-C bond cleavage is believed to occur Via metal- hydroxycarbene intermediates (i.e., Fischer-Tropsch chemistry). Experimental Section General Procedures. Tetrahydrofuran (THF), chloroform, and dichloromethane were purified by distillation under nitrogen over LiAlH 4 and P2O5, respectively. Piperidine and triethylamine were purchased from Carlo Erba, dried over KOH and distilled from BaO under an N 2 atmosphere prior to use. Oxygen-18 labeled water (95 atom % 18 O) was purchased from Aldrich. All the other reagents and chemicals were reagent grade and, unless otherwise stated, were used as received by commercial suppliers. All reactions and manipulations were routinely performed under a dry nitrogen atmosphere by using standard Schlenk-tube techniques. The solid complexes were collected on sintered glass-frits and washed with light petroleum ether (b.p. 40-60 °C) or n-pentane before being dried in a stream of nitrogen. The ligand CH 3CH2CH2N(CH2CH2PPh2)2 (PNP) 6 and the complexes X Abstract published in AdVance ACS Abstracts, April 1, 1996. (1) (a) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley, New York, 1992; pp. 762-763. 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An improved synthesis of the PNP ligands is reported: Bianchini, C.; Farnetti, E.; Glendenning, L.; Graziani, M.; Nardin, G.; Peruzzini, M.; Rocchini, E.; Zanobini, F. Organometallics 1995, 14, 1489. 4585 J. Am. Chem. Soc. 1996, 118, 4585-4594 S0002-7863(96)00139-4 CCC: $12.00 © 1996 American Chemical Society