Free radical reactions of allylic and propargylic derivatives
Addition of tert-butyl radical, generated from t-BuHgCl, to allyl or propargyl derivatives forms adduct radicals which can undergo [beta]-elimination with substituents such as halogen, PhS, PhSO[subscript]2 to form the alkyl-substituted propene or allene and an eliminated radical which regenerates the tert-butyl radical by displacement from t-BuHgCl. With [beta]-oxy substituents, such as O[subscript]2CR, OP(O)(OEt)[subscript]2, O[subscript]3SAr, the adduct radical can displace the alkyl radical from alkylmercurial to yield the [beta]-substituted alkylmercurials which spontaneously, or in the presence of nucleophiles, undergo an elimination reaction to yield the alkene or allene. Relative reactivities towards tert-butyl radical attack, such as k(allyl chloride) / k(propargyl chloride) ≅ 10, have been determined. A similar relative reactivity is observed in reaction with (t-Bu)[subscript]2CuLi, implicating attack by tert-butyl radical. With propargyl iodide, radical attack leads to iodine atom abstraction. Reaction of propargyl iodide with t-BuHgCl / h[upsilon], (t-Bu)[subscript]2CuLi, or (t-Bu)[subscript]3ZnLi leads to a mixture of hydrocarbons in which tert-butylallene is present in only trace amounts. Benzene is an important reaction product which seems to be formed via the cyclodimerization of two "propargyl" (C[subscript]3H[subscript]3•) radicals in the presence of metal ion. The pseudo-unimolecular rate constants, k[subscript] obs, of allyl derivatives with excess t-BuHgCl have been determined and the plot of log k[subscript] obs vs. [sigma][subscript] m gives a straight line with correlation coefficient of 0.994 and [rho] value of 3.24. Addition of t-Bu• to 1,3-butadiene epoxide in the presence of DABCO gives a 53% yield of t-BuCH[subscript]2CH=CHCHO with very high regio- and stereoselectivity. The reaction is thought to involve the loss of a proton from the intermediate t-BuCH[subscript]2CH=CHCH[subscript]2O•, followed by electron transfer from the ketyl radical anion to t-BuHgCl.
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