Fast halogen abstractions from alkyl halides by alkyl radicals. Quantitation of the processes occurring in and a caveat for studies employing alkyl halide mechanistic probes

Abstract

Abstract : Reactions of nucleophilic tin anionoids with alkyl halides are broadly applied for formation of tin-carbon bonds. The mechanisms of these reactions have been studied by the use of alkyl halide mechanistic probes. This report demonstrates how qualitative mechanistic probes can give misleading information about the extent of electron transfer processes in reactions of nucleophiles with alkyl halides. Second order rate constants for halogen atom transfer (k sub RX) in benzene at 50 C were determined for reactions of octyl radical with tert-butyl, iopropyl and cyclohexyl iodides and bromides and with ethyl iodide, n-butyl bromide, tert-butyl chloride, and carbon tetrachloride using two methods. In Method A, an alkyl iodide and tributylstannane were allowed to compete for octyl radical in radical chain reactions; in Method B, an alkyl halide competed with 1-(oxononoxy)-2(1H)-pyridine (1) for octyl radical. The values of k sub RX were calculated from the product distributions, the reactant ratios and the known rate constants for reaction of tributylstannane or 1 with octyl radical. The possibility that rearranged products can be formed in reactions of alkyl halide mechanistic probes with nucleophiles via a sequence involving radical chain isomerization that converts the probe halide into a rearranged halide followed by nucleophilic attack on the isomerized halide is discussed as are possible chain terminating reactions. The conclusion is reached that the percentage of rearranged substitution products formed in reactions of alkyl halide mechanistic probes with nucleophiles can give misleading information about the number of radical initiating events.