Mechanism of the gold(III)-catalyzed isomerization of substituted allenes to conjugated dienes: a DFT study.

The mechanism of gold(III) [Au(III)]-catalyzed isomerization of alkyl-substituted allenes to conjugated dienes in the presence of a nitroso compound (additive) was studied quantum mechanically using hybrid density functional PBE0 with 6-31G** basis set for lighter atoms and (aug)-ccpVDZ basis set and LANL2 electron core potential for Au atom. Several pathways, involving the nitroso compound in a free or bound state to the gold-allene (GA) complex, were investigated. Calculated results reveal that the unbound nitroso compound acts as a better proton transferring agent in the isomerization process and utilizes its own nitrogen atom to carry the proton. While comparing the efficiency of other basic reagents to carry out the process, it appeared that the moderate basicity of the nitroso compound plays a crucial role to reduce the activation barrier of the reaction pathway. A similar pathway was also investigated using a gold(I) [Au(I)] catalyst and found to be less favorable than the process catalyzed by a Au(III) catalyst. All these facts agree well with the experimental reports for the reaction.