Mechanistic Analysis of Protodeauration and Gold(I) and Palladium(II) Catalyzed Carbene Transformations

Abstract

In this work we demonstrated advancements in the understanding of gold(I) and palladium(II) catalysis. Initially, to better understand the elementary step of protodeauration present in most proposed mechanisms of hydrofunctionalization, a series of experiments analyzing the kinetics of protodeauration of gold β-styrenyl complexes when treated with excess acetic acid were performed. These experiments demonstrated the electronic dependence of protodeauration and led to a divergent model. Depending on the electron donating ability of the p-substitent on the gold β-styrenyl complex, protodeauration was shown to occur both via a σ-α and a π-α pathway, lending creedence to both proposed mechanisms. Subsequently, the kinetics of the gold(I) benzylidene transfer were studied as a function of the substituents on the alkene undergoing carbene transfer. Aliphatic and aromatic alkenes were found to rapidly and efficieintly partake in carbene transfer when treated with a gold sulfonium benzylide complex that generated the cationic gold9I) carbene in situ. This led to the proposal of a mechanism of a concerted, asynchronous, electrophilic benzyilidene transfer mechanism consistent with the experimental observations. The diastereoselectivity of the benzylidene transfer was interpreted in the transition state model involving a near anti-peripplanar arrangement of the carbene Au=C and alkene C=C bonds. Lastly, treatment of homobenzylic diazirines 3-(4-chlorobenzyly)-3H-diazirine and 3-benzyl-3H diazirine with a catalytic amount of PdCl2(PhCN)2 (15 mM Pd) at 25 °C led to consumption of the diazirine and yielding corresponding vinyl arenes as the exclusive organic product. Kinetic analysis of this palladium catalyzed diazirine activation demonstrated a first order dependence on both the diazirine and the PdCl2(PhCN)2. Upon further analysis the dimer formed in the presence of vinyl arenes, PdCl2[π(p-chlorostyrene)}2 as an example, also exhibited the catalytic ability shown by the PdCl2(PhCN)2. Upon addition of p-chlorostyrene and benzonitrile, it was deemed that the catalyst behaves independent of the additives and ligands have little to do with the catalytic activity, provided that they are weakly bound. Thus, this led to a proposed mechanism of an initial oxidative activation and addition of a diazirine by palladium to generate a diazometallacyclobutane followed by extrusion of nitrogen to form the palladium carbene, which then undergoes facile 1,2 elimination to liberate a vinyl arene as the exclusive organic product, providing the second reported example of catalytic activation of diazirines as carbene precursors.