EVIDENCE FOR REDUCTIVE ELIMINATION OF H-2 IN THE DECOMPOSITION OF PRIMARY ARSINES

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 th e ortho carbon. These experimental data, together with calculations ca rried 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 reacti on of o-tolylAs with intact o-tolylAsD(2) to give o-tolylAsD ..o-tolyl AsD . can reductively eliminate toluene or can undergo a rearrangement to o-HDAsC6H4CH2 . for which the calculated free energy of activation at 900K is very similar to that for reductive elimination, hence expl aining the multiple deuteriation of the methyl group of toluene. Calcu lations on the decomposition of (t)BuAsH(2) show that this too decompo ses by loss of H-2 to give (t)BuAs with a very low free energy of acti vation. (t)BuAs decomposes via beta-H abstraction to 2-methylpropene a nd AsH. There is no unimolecular process with a low free energy of act ivation that leads to 2-methylpropane, so it is proposed that this pro duct arises mainly from bimolecular H transfer from (t)BuAsH(2) to (t) BuAs to give (t)BuAsH . which can lose 2-methylpropene or (t)Bu ..(t)B u . abstracts H from an AsH species to give 2-methylpropane. A number of experimental results on the decomposition of (t)BuAsH(2) are ration alized in terms of these mechanistic pathways.