Decomposition and reactivity of tellurium alkyls in the liquid and gas phases; dihex-5-enyltellurium and dipent-4-enyltellurium as mechanistic probes

Abstract

The new tellurium alkyls, dihex-5-enyltellurium and dipent-4-enyltellurium have been prepared and pyrolysed in the gas and liquid phases. in the liquid phase, at 200°C some decomposition occurs but the main isolated products are the tellurium alkyls, bis(cyclopentylmethyl)tellurium and 2-methyltelluracyclopentane respectively. A further product, cyclopentylmethylhex-5-enyltellurium is observed as an intermediate in the rearrangement of dihex-5-enyltellurium, whilst (2-telluracyclopentylmethyl)pent-4-enyltellurium together with pent-1-ene and 1,4-pentadiene is obtained from dipent-4-enyltellurium. These products are interpreted as providing direct evidence for initial homolytic cleavage of the Te-C bonds followed by cyclisation of some of the formed radicals and radical chain reactions. In the gas phase, at 500°C similar products to those obtained in the liquid phase are formed from dipent-4-enyltellurium, although not compounds containing more than one Te atom. Dihex-5-enyltellurium, however, decomposes completely in the gas phase at 700°C to give a mixture of hydrocarbons. Substantial quantities of methylcyclopentane and methylenecyclopentane again confirm that a free radical pathway makes a major contribution to the mechanism. The origin of the other products, especially cyclohexene (the major C 6 product) and cyclohexane is also interpreted in terms of a free radical mechanism leading to the 6-tellurahex-1-enyl radical which cyclises to give the 3-telluracycloheptylradical. This radical rearranges to cyclohexyl Te· which in turn acts as the source of cyclohexene and cyclohexane by H· abstraction or addition. There is little evidence that mechanisms other than free radical operate for decomposition of these metal alkyls.