Formation of Gold(III) Alkyls from Gold Alkoxide Complexes.

The gold(III) methoxide complex (CNC)AuOMe () reacts with tris(-tolyl)phosphine in benzene at room temperature under O abstraction to give the methylgold product (CNC)AuMe () together with O=P(-tol) ((CNC) = [2,6-(CH Bu-4)pyridine]). Calculations show that this reaction is energetically favorable (Δ = -32.3 kcal mol). The side products in this reaction, the Au(II) complex [Au(CNC)] () and the phosphorane (-tol)P(OMe), suggest that at least two reaction pathways may operate, including one involving (CNC)Au radicals. Attempts to model the reaction by DFT methods showed that PPh can approach to give a near-linear Au-O-P arrangement, without phosphine coordination to gold. The analogous reaction of (CNC)AuOEt, on the other hand, gives exclusively a mixture of and (-tol)P(OEt). Whereas the reaction of (CNC)AuOR (R = Bu, -CHF) with P(-tol) proceeds over a period of hours, compounds with R = CHCF, CH(CF) react almost instantaneously, to give and O=P(-tol). In chlorinated solvents, treatment of the alkoxides (CNC)AuOR with phosphines generates [(CNC)Au(PR)]Cl, via Cl abstraction from the solvent. Attempts to extend the synthesis of gold(III) alkoxides to allyl alcohols were unsuccessful; the reaction of (CNC)AuOH with an excess of CH=CHCHOH in toluene led instead to allyl alcohol isomerization to give a mixture of gold alkyls, (CNC)AuR' (R' = -CHCHCHO (), -CHCH(CHOH)OCHCH=CH ()), while 2-methallyl alcohol affords R' = CHCH(Me)CHO (). The crystal structure of was determined. The formation of Au-C instead of the expected Au-O products is in line with the trend in metal-ligand bond dissociation energies for Au(III): M-H > M-C > M-O.