A study of the acetolyses of 2-acetoxyalkyltellurium trihalides

Abstract

The oxidation of alkenes using tellurium tetrachloride is a two step process. The first step has been shown to be the anti addition of tellurium and oxygen across the double bond. This work examines the second step of the oxidation process, the breaking of the carbon-tellurium bond. Six 2-acetoxyalkyltellurium trihalides (X = Cl, Br, I) have been synthesized and their reactions in refluxing acetic acid have been studied. The reaction of the well defined 2-acetoxyalkyltellurium trihalides with carefully dried acetic acid yields haloalkyl acetates as major reaction products. The formation of the carbon-halogen bond occurs with retention of stereochemistry. Acetolyses of trans-2-acetoxycyclohexyltellurium trihalides yield the trans-2-halocyclo-hexyl acetates and cis- and trans-1,2-cyclohexyl diacetates. No cis-2-halocyclohexyl acetate is observed. The formation of these novel haloalkyl acetates is believed to involve intramolecular displacement of the tellurium by halide in a 1,2-shift reaction of the halide from tellurium to carbon. The product distribution between the haloalkyl acetates and the 1,2-alkyl diacetates shows a definite variation with increased reaction time. The relative amount of haloacetate in the product mixture decreases with increased reaction time. This observed variation in the product distribution suggests that the haloalkyl acetates undergo further reaction to yield the 1,2-alkyl diacetates. The formation of the diacetates is believed to occur through neighboring group participation. Neighboring group participation explains the loss of stereoselectivity in the formation of the diacetates. For example, cis-1,2-cyclohexyl diacetate and trans-1,2-cyclohexyl diacetate are observed in the product mixture from reaction of the trans-2-acetoxycyclohexyltellurium trihalides. The intervention of a dioxolenium intermediate also explains the observed differences in product composition with wet solvent which gives a product mixture consisting entirely of diacetates and dry solvent which provides the 2-haloalkyl acetate...