Electrophilic Deauration of Gold(I) a-Styrenyl Compounds with Carbon and Silicon Electrophiles

Abstract

Electrophilic deauration of gold(I) alkenyl complexes involving electrophilic attack of the C=C-bond with delivery of the electrophile to C is not well understood and direct experimental evidence is lacking. For this reason, we sought out bulky silylium and triarylcarbenium ions that would bias toward electrophilic -deauration in gold(I) alkenyl complexes and probe said mechanism.We found reaction of the styrenyl complex (P)AuC(Ph)=CH2 [P = P(t-Bu)2o-biphenyl; 1a] in CD2Cl2 at -80 °C with [Ph3C][B(C6F5)4] forms a 3.4:1:3.9 mixture of the-electrophilic deauration product, (E)-1,3,3,3-tetraphenylpropene, styrene, and the cationic bis(gold) complex (3a). Reaction of 1a with [(4-C6H4OMe)3C][B(C6F5)4] (5), in an equimolar mixture in CD2Cl2 at 25 °C, forms a 3.3:1:4.9 mixture of (E)-1-phenyl-3,3,3-trisanisylpropene (6a), styrene, and the cationic bis(gold) complex (3a), which followed first-order kinetics to ~3 half-lives. A plot of log(k’X/k’H), where k’ is the rate of -alkylation, plotted versus the Hammett  constant displayed a great fit with R2 > 0.99 anda large negative reaction constant  = -3.44). This large negative reaction constant indicates the rate of the electrophilic deauration of para-substituted -styrenyl gold compounds is strongly dependent and highly sensitive to the electron-donating ability of the aryl groups and is consistent with a significant loss of electron density at C in the transition state of -electrophilic deauration. Though this is the case, the superior fit to the Hammett  constant and apparent zero-order rate constant of 5, make the intimate details of the mechanism somewhat unclear.