Reductive elimination and isomerization of organogold complexes. Theoretical studies of trialkylgold species as reactive intermediates

Abstract

Trialkyl(phosphine)gold complexes R3AuL are involved in facile reductive elimination as well as cis-trans isomerization. Deuterium labeling studies show that elimination, but not isomerization, proceeds via two competing pathways. The intermolecular route predominates in nonpolar solvents such as decalin and benzene, whereas dimethyl sulfoxide (Me2SO) and dimethylformamide promote the intramolecular reactions. Kinetic studies support trialkylgold species, R3Au, formed by the rate-limiting dissociation of phosphine, as the common intermediate in both cis-trans isomerization and reductive elimination. Capture of R3Au by Me2SO prevents its association with other alkylgold species to promote further intermolecular reactions, and only intramolecular processes leading to isomerization and reductive elimination are observed in this solvent. We calculate from the kinetic results that the coordinatively unsaturated intermediate Et(CH3)2Au undergoes isomerization between Tshaped configuration 100 times faster than reductive elimination. Molecular orbital calculations indicate that the potential energy surface for (CH3)3Au is determined by the orbital degeneracy of the symmetrical C3* geometry, and favor distortion to Tand Y-shaped configurations of lower energies. The former represent minima, and the Y-shaped configurations are saddle points for the cis-trans isomerization of the T's and serve as exit channels through which reductive elimination proceeds. Deuterium labeling studies show that methyl-methyl coupling between dimethylcuprate(I) and methyl iodide or trifluoromethanesulfonate occurs without scrambling. If trimethylcopper species are formed as intermediates similar to the gold analogue, then the reductive elimination of methyl groups must also proceed via T-shaped configurations.

5,5′-Diuracilyl Species from Uracil and [AuCl4]−: Nucleobase Dimerization Brought about by a Metal

“…We assume that formation of the diuracilyl products is the consequence of a reductive elimination process of two cisoriented uracil entities, each bonded to the Au III center through C5. Similar reactions are documented for Au III alkyl compounds [12] as well as a compound of 2-(2'-thienyl)pyridine. [13] Formation of the 5-chlorouracil species could occur in an analogous manner.…”

Gold-Catalyzed Cycloisomerization Reactions Through Activation of Alkynes

“…This is well illustrated by the difference between organocopper(I) and organogold(I) complexes, the former being readily oxidized to a copper(III) species while the latter is relatively inert [6]. Likewise, reductive elimination from gold(III) is quite disfavored [7,8]. These phenomena are consistent with the rare observation of red-ox cycles with these metals, and their relative tolerance to oxygen, particularly with gold(I).…”

Organogold chemistry: II reactions

“…The mixed alkyl complexes Me zRAu(PPh3 ) and [MezAuR z]-undergo rapid isomerization in solution (37,40). It follows the pathway discussed above and shown in Scheme 2.…”

“…Detailed studies have established that cis elimination occurs here too, but it is preceded for R 3AuL by dissociation (36,37). The greater stability of complexes in which L is an ylide is attributed to lessened tendency for dissociation (38).…”