A Study of Fundamental Reaction Pathways for Transition Metal Alkyl Complexes. I. The Reaction of a Nickel Methyl Complex with Alkynes. II. The Mechanism of Aldehyde Formation in the Reaction of a Molybdenum Hydride with Molybdenum Alkyls

I. This study reports the rapid reaction under mild conditions of internal or terminal alkynes with methyl (acetyl- acetonato) (triphenylphosphine) nickel (1) in either aromatic or ether solvents. In all cases vinylnickel products 2 are formed by insertion of the alkyne into the nickel-methyl bond. These complexes may be converted into a variety of organic products (e.g. alkenes, esters, vinyl halides) by treatment with appropriate reagents. Unsymmetrical alkynes give selectively the one regioisomer with the sterically largest substituent next to the nickel atom. In order to investigate the stereochemistry of the initial insertion, ax-ray diffraction study of the reaction of 1 with diphenylacetylene was carried out. This showed that the vinylnickel complex formed by overall trans insertion was the product of the reaction. Furthermore, subsequent slow isomerization of this complex, to a mixture of it and the corresponding cis isomer, demonstrated that this trans addition product is the kinetic product of the reaction. In studies with other alkynes, the product of trans addition was not always exclusively (or even predominantly) formed, but the ratio of the stereoisomers formed kinetically was substantially different from the thermodynamic ratio. Isotope labeling, added phosphine, and other experiments have allowed us to conclude that the mechanism of this reaction does involve initial cis addition. However, a coordinatively unsaturated vinylnickel complex is initially formed, which can undergo rapid, phosphine-catalyzed cis-trans isomerization in competition with its conversion to the isolable phosphine-substituted kinetic reaction products. II. The reaction of CpMo(CO)3H (la) with CpMo(CO)3R (2, R = CH3, C2H5) at 5O°C in THF gives the aldehyde RCHO and the dimers [CpMo(CO)3]2 (3a) and [CpMo(CO)2]2 (4a). Labeling one of the reactants with a methylcyclopentadienyl ligand it was possible to show that the mixed dimers MeCpMo (CO)3-CO)3MoCp (3b) and MeCpMo (CO)2≡(CO)2MoCp (4b) are the predominant kinetic products of the reaction. Additionally labeling the carbonyl ligands of la with 13CO led to the conclusion that all three of the carbonyl ligands in la end up in the tetracarbonyl dimers 4a if the reaction is carried out under a continuous purge of argon. Trapping studies failed to find any evidence for the intermediacy of either [CpMo(CO)3]- or [CpMo(CO)3]+ in this reaction. A mechanism is proposed that involves the initial migration of the alkyl ligand in 2 to CO forming an unsaturated acyl complex which reacts with la to give a binuclear complex containing a three center-two electron Mo-H-Mo bond. This complex then selectively looses a carbonyl from the acyl molybdenum, migrates the hydride to that same metal, and forms a metal-metal bond. This binuclear complex with the hydride and acyl ligands on one metal reductively eliminates aldehyde, and migrates a carbonyl ligand, to give 4a directly. The other product 3a is formed by addition of two molecules of free CO to 4a.