Evidence for reductive elimination of H₂ in the decomposition of primary arsines

Abstract

The decomposition of o-CH₃C₆H₄AsD₂ (o-tolyl AsD₂) in the gas phase at 900K gives toluene with 0-3 D atoms in the methyl group and/or D on theortho carbon. These experimental data, together with calculations carried out in the PM3 system show that the only low energy pathway for decomposition ofo-tolylAsD₂ involves loss of D₂ followed by reaction ofo-tolylAs with intacto-tolylAsD₂ to giveo-tolylAsD•.o-tolylAsD• can reductively eliminate toluene or can undergo a rearrangement too-HDAsC₆H₄CH₂• for which the calculated free energy of activation at 900K is very similar to that for reductive elimination, hence explaining the multiple deuteriation of the methyl group of toluene. Calculations on the decomposition of^tBuAsH₂ show that this too decomposes by loss of H₂ to give^tBuAs with a very low free energy of activation.^tBuAs decomposes via β-H abstraction to 2-methylpropene and AsH. There is no unimolecular process with a low free energy of activation that leads to 2-methylpropane, so it is proposed that this product arises mainly from bimolecular H transfer from^tBuAsH₂ to^tBuAs to give^tBuAsH• which can lose 2-methylpropene or^tBu•.^tBu• abstracts H from an AsH species to give 2-methylpropane. A number of experimental results on the decomposition of^tBuAsH₂ are rationalized in terms of these mechanistic pathways.