Evidence for Reductive Elimination of H_2 in the Decomposition of Primary Arsines

摘要

The decomposition of o-CH_3C_6H_4AsD_2 (o-tolyl AsD_2) in the gas phase at 900K gives toluene with 0-3 D atoms in the methyl group and/or D on the ortho carbon. These experimental data, together with calculations carried out in the PM3 system show that the only low energy pathway for decomposition of o-tolylAsD_2 involves loss of D_2 followed by reaction of o-tolylAs with intact o-tolylAsD_2 to give o-tolylAsD·. o-tolylAsD· can reductively eliminate toluene or can undergo a rearrangement to o-HDAsC_6H_4CH_2· 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 ~tBuAsH_2 show that this too decomposes by loss of H_2 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 ~tBuAsH_2 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 ~tBuAsH_2 are rationalized in terms of these mechanistic pathways.