WURTZ-TYPE REDUCTIVE COUPLING REACTION OF ALLYL BROMIDES AND HALOORGANOTINS IN COSOLVENT H2O(NH4CL)/ZN MEDIA AS A ROUTE TO ALLYLSTANNANES AND HEXAARYLDISTANNANES

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)20 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 = alpha-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 (alpha-isomer), and (ii) the subsequent isomerization of the alpha-isomer furnishing mixtures of (alpha, trans, cis)-isomers. The occurrence of reaction ii depends upon the nature either of the R group or the employed cosolvent. In cyclohexane, the alpha-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 alpha-isomer. Similarly, ditin compounds are thought to be formed by interaction of R3SnCl.-Zn.+ radical ions with R3SnCl molecules