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 Bu^t₂Se 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-Bu^t₂Se in H₂ contains significant amounts of hydrogen (d⁹∶d¹⁰ = 2.6∶1). In the codecomposition of d⁰- and d¹⁸-Bu^t₂Se in helium, d⁰, d¹, d⁹ and d¹⁰-2-methylpropane are all formed, but d⁹-Bu^t₂Se is not a product. Photolysis of d⁰- and d¹⁸-Bu^t₂Se in helium at room temperature produces 2-methylpropane and 2-methylpropene (1∶1) with the 2-methylpropane being d⁰, d¹, d⁹ and d¹⁰. d⁹-Bu^t₂Se is also formed along with small amounts of 2,2,3,3-tetramethylbutane and d⁰- and d¹⁸-Bu^t₂Se₂. Thermal codecomposition of Me₂Zn with d¹⁸-Bu^t₂Se 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, Bu^t₂Se decomposes via homolytic cleavage of the Se–C bonds and that the free Bu^t· so formed initiates a radical chain reaction involving H^· as the chain carrier through its reaction with intact Bu^t₂Se to give hydrogen, 2 × 2-methylpropene, Se and H^·. The chain is initiated by Bu^t· abstracting H^· from Bu^t₂Se 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 Bu^t₂Se 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 Bu^t₂Se in H₂ comes from hydrogenation of 2-methylpropene catalysed by ZnSe. Photochemically, homolytic cleavage of the Se–C bonds in Bu^t₂Se again occurs, but Bu^tSe^· has sufficient lifetime to recombine with ^tBu^· and the major hydrocarbon products are formed from radical–radical reactions.