Studies of the thermal and photochemical decomposition mechanisms of But2Se in the presence or absence of Me2Zn using deuterium labelled analogues

Abstract

Thermal decomposition of But2Se in helium produces 2-methylpropene and 2-methylpropane (8∶1) and no volatile selenium containing products, apart from the element. In hydrogen, the products are the same (8∶1 ratio of 2-methylpropene∶2-methylpropane). However, the 2-methylpropane produced by decomposing d18-But2Se in H2 contains significant amounts of hydrogen (d9∶d10 = 2.6∶1). In the codecomposition of d0- and d18-But2Se in helium, d0, d1, d9 and d10-2-methylpropane are all formed, but d9-But2Se is not a product. Photolysis of d0- and d18-But2Se in helium at room temperature produces 2-methylpropane and 2-methylpropene (1∶1) with the 2-methylpropane being d0, d1, d9 and d10. d9-But2Se is also formed along with small amounts of 2,2,3,3-tetramethylbutane and d0- and d18-But2Se2. Thermal codecomposition of Me2Zn with d18-But2Se in H2 produces d0 and d1-methane and d8-2-methylpropene together with traces of d8-2-methylpropane.These results are interpreted as indicating that thermally, But2Se decomposes via homolytic cleavage of the Se–C bonds and that the free But· so formed initiates a radical chain reaction involving H· as the chain carrier through its reaction with intact But2Se to give hydrogen, 2 × 2-methylpropene, Se and H·. The chain is initiated by But· abstracting H· from But2Se or from H2. Semi-empirical calculations carried out with a variety of levels of theory have confirmed that E–C bond cleavage is the first step in the decomposition of all the group 16 precursors studied and have been used to model the product distribution with time. They confirm the conclusions drawn on the basis of our experimantal observations, except that the H· abstraction reactions from intact But2Se probably do not occur by a concerted pathway, rather they are stepwise. The radical chain process also occurs in the presence of Me2Zn, but the radical chain initiator in this case is Me· from homolytic fission of the Zn–C bonds. Evidence is presented that these reactions occur on the ZnSe surface. The small amount of 2-methylpropane produced from reaction of Me2Zn with But2Se in H2 comes from hydrogenation of 2-methylpropene catalysed by ZnSe. Photochemically, homolytic cleavage of the Se–C bonds in But2Se again occurs, but ButSe· has sufficient lifetime to recombine with tBu· and the major hydrocarbon products are formed from radical–radical reactions.