Mechanistic Study of β-Hydrogen Elimination from Organoplatinum(II) Enolate Complexes

Abstract

A detailed mechanistic investigation of the thermal reactions of a series of bisphosphine alkylplatinum(II) enolate complexes is reported. The reactions of methylplatinum enolate complexes in the presence of added phosphine form methane and either free or coordinated enone, depending on the steric properties of the enone. Kinetic studies were conducted to determine the relationship between the rates and mechanism of β-hydrogen elimination from enolate complexes and the rates and mechanism of β-hydrogen elimination from alkyl complexes. The rates of reactions of the enolates were inversely dependent on the concentration of added phosphine, indicating that β-hydrogen elimination from the enolate complexes occurs after reversible dissociation of a phosphine. A normal, primary kinetic isotope effect was measured, and this effect was consistent with ratelimiting β-hydrogen elimination or C-H bond-forming reductive elimination to form methane. Reactions of substituted enolate complexes were also studied to determine the effect of the steric and electronic properties of the enolate complexes on the rates of β-hydrogen elimination. These studies showed that reactions of the alkylplatinum enolate complexes were retarded by electron-withdrawing substituents on the enolate and that reactions of enolate complexes possessing alkyl substituents at the β-position occurred at rates that were similar to those of complexes lacking alkyl substituents at this position. Despite the trend in electronic effects on the rates of reactions of enolate complexes and the substantial electronic differences between an enolate and an alkyl ligand, the rates of decomposition of the enolate complexes were similar to those of the analogous alkyl complexes. To the extent that the rates of reaction of the two types of complex are different, those involving β-hydrogen elimination from the enolate ligand were faster. A difference between the identity of the rate-determining step for decomposition of the two classes of complexes and an effect of stereochemistry on the selectivity for β-hydrogen elimination are possible origins of the observed phenomena.

Reactivity and kinetic–mechanistic studies of regioselective reactions of rhodium porphyrins with unactivated olefins in water that form β-hydroxyalkyl complexes and conversion to ketones and epoxides

“…C-O reductive elimination from the d 6 metal center usually occurs through an S N 2 route and a direct C-O elimination pathway. 31 Groves reported a facile C-O reductive elimination from b-hydroxyalkyl complexes of rhodium tetraphenyl porphyrin in KOtBu/C 6 D 6 by a proposed S N 2 pathway. 19 The origin of the selective formation of ketone in water and epoxide in KOH/DMSO may be that the nucleophilicity of the alkoxy group is dramatically reduced in water due to the solvation.…”

Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

“…β-H elimination could occur via a five-coordinate intermediate, 32–34 although, in coordinatively saturated systems it has been proposed to occur via alkoxide dissociation 35–37 or in bimolecular fashion. 38,39 Given the strain of the chelates in complex 4 , phosphine dissociation may also be possible, 32,33,40,41 opening up a cis coordination site for β-H elimination. 42–44 Further studies are necessary to determine the mechanism in the present system.…”

On the Mechanism of the Palladium‐Catalyzed β‐Arylation of Ester Enolates

“…After ligand substitution (Scheme 3), palladium enolate III may follow two pathways: 1) reductive elimination to give a-arylated product 3 a, or 2) bÀH elimination to give olefin complex IVa. [24] Bond rotation provides isomeric complex IVb, which can undergo olefin insertion either into the Pd À H or the Pd À Ar bonds. Previous calculations with PCy 3 as the ligand indicated that the former insertion is much-more-kinetically favored than the latter.…”