Wurtz-type reductive coupling reaction of allyl bromides and haloorganotins in cosolvent/H2O(NH4Cl)/Zn media as a route to allylstannanes and hexaaryldistannanes

Article abstract of DOI:10.1021/om00015a025

Twenty-one allylstannanes have been prepared via a simple Wurtz-type coupling reaction of allyl bromides and R₃SnX compounds (R = Me, Et, Pr, Bu, Ph; X = Cl, I, OH), Bu₂SnCl₂, and (Bu₂SnCl)₂O in cosolvent/H₂O (NH₄Cl saturated) media under the mediation of zinc powder. Also R₃SnSnR₃ compounds (R = Ph, p- and m-Tol) have been prepared via coupling of triaryltin chlorides. The stereochemical course of the reaction between R₃SnCl and (C₄H₇)Br (C₄H₇ = α-methylallyl, trans- and cis-crotyl) has been extensively studied. Two distinct reactions are involved in the overall process: (i) the coupling reaction, which gives rise stereoselectively to the sole R₃SnCH(CH₃)CH=CH₂ (α-isomer), and (ii) the subsequent isomerization of the α-isomer furnishing mixtures of (α, trans, cis)-isomers. The occurrence of reaction ii depends upon the nature either of the R group or the employed cosolvent. In cyclohexane, the α-isomer is exclusively obtained with R = Bu, while with R = Me, Et, and Pr it is found as a major component in the ternary isomeric mixture. In tetrahydrofuran, 2-propanol, acetonitrile, and pyridine, the isomerization occurs to an extent which depends on the polarity and the coordinating ability of the cosolvent itself. The observed stereoselection has been hypothesized to occur through one-electron transfer from the zinc metal to the (C₄H₇)Br to form stereoselectively an adsorbed CH₂=CHCH(CH₃)Br^?-Zn^?+ radical ion which is trapped by the R₃SnCl reactant to form the α-isomer. Similarly, ditin compounds are thought to be formed by interaction of R₃SnCl^?-Zn^?+ radical ions with R₃SnCl molecules.