Free-radical chain reactions of organic mercury and tin compounds

Tashtoush, Hasan
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Alkylmercury halides (RHgX, R = n-butyl, n-hexyl, (DELTA)('3)-butenyl, (DELTA)('5)-hexenyl, isopropyl, cyclohexyl, cyclopentylcarbinyl and 7-norbornyl) rapidly react with PhYYPh (Y = S, Se and Te), p-MeC(,6)H(,4)SO(,2)SePh, PhSO(,2)Cl, PhSH and BrCCl(,3) under sunlamp irradiation to give RYPh, RCl, RH and RBr, respectively. The reaction is completely inhibited by di-tert-butyl nitroxide, does not occur in the dark and yields a mixture of cyclopentylcarbinyl and (DELTA)('5)-hexenyl products from the (DELTA)('5)-hexenylmercurial. The reaction is believed to proceed via a radical chain mechanism involving R(.) and Hg(III) intermediates. The alkyl radical (R(.)) undergoes S(,H)('2) or atom transfer reactions with the substrate and the resulting radical (PhY(.), PhSO(,2)(.) or (.)CCl(,3)) reacts with RHgX to form the Hg(III) intermediate which then eliminates R(.). A similar mechanism is proposed for the reaction of dialkylmercurials (R(,2)Hg, R = n-butyl, isobutyl and (DELTA)('5)-hexenyl) with PhSSPh and PhSeSePh. However, benzylmercurials (PhCH(,2)HgQ, Q = Cl and PhCH(,2)) react with phenyl dichalcogenide differently from other alkylmercurials, in that bibenzyl is formed in a chain reaction. Bibenzyl is also the product of the photostimulated chain decomposition of PhCh(,2)HgQ (Q = PhCH(,2) and PhS). Evidence is presented that the reaction of benzylmercurials proceeds by a free radical chain mechanism involving S(,H)('2) attack of the benzyl radical at the benzyl mercury bond;Tri-n-butyl-1-alkenyltin compounds containing a (beta)-phenyl substituent, PhCH=CHSnBu(,3) and Ph(,2)C=CHSnBu(,3), undergo a photostimulated reaction with 2(DEGREES)- or 3(DEGREES)-alkylmercurials to give PhCH=CHR and Ph(,2)C=CHR, Bu(,3)SnCl and Hg(0). The reaction was established to proceed via a radical chain process involving (alpha)-addition of the alkyl radical followed by (beta)-elimination of Bu(,3)Sn(.) and electron transfer from the tin-centered radical to the alkylmercurials. The reaction has been successfully extended to other (beta)-substituted styrenes, PhCH=CHQ (Q = I, PhSO(,2), HgCl and PhS) where Q(.) can be reacted with RHgX to generate R(.). In addition, PhCH=CHSnBu(,3) reacts with CCl(,3)Z (Z = Cl, Br and SO(,2)Cl) under UV irradiation to give exclusively PhCH=CHCCl(,3). Similarly, 1-alkenyltin derivatives with or without a (beta)-phenyl substituent, react with sulfur-centered radicals (PhS(.), PhCH(,2)S(.), PhSO(,2)S(.) and p-MePhSO(,2)(.)) derived from the reaction of Bu(,3)Sn(.) with PhSSPh, PhCH(,2)SSCH(,2)Ph, PhSO(,2)Cl or p-MeC(,6)H(,4)SO(,2)SePh to give the corresponding sulfides and sulfones in excellent yields.