Mechanism, regiochemistry, and stereochemistry of the insertion reaction of alkynes with methyl(2,4-pentanedionato)(triphenylphosphine)nickel. A cis insertion that leads to trans kinetic products

Article abstract of DOI:10.1021/ja00401a016

This study reports the rapid reaction under mild conditions of internal and terminal alkynes with methyl(2,4-pentanedionato)(triphenylphosphine)nickel (1) in aromatic and ethereal solvents. In all cases vinylnickel products (2) are formed by insertion of the alkyne into the nickel-methyl bond. The regiochemistry is unusual; unsymmetrical alkynes give selectively the one regioisomer with the sterically largest substituent next to the nickel atom. So that the stereochemistry of the initial insertion could be investigated, an X-ray diffraction study of the reaction of 1 and 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 cis addition. However, a coordinatively unsaturated vinylnickel intermediate is initially formed, which can undergo rapid, phosphine-catalyzed cis-trans isomerization in competition with its conversion to the isolable phosphine-substituted products.