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 c oupling 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 R3SnSn R3 compounds (R = Ph, p- and m-Tol) have been prepared via coupling of triaryltin chlorides. The stereochemical course of the reaction betwe en R3SnCl and (C4H7)Br (C4H7 = alpha-methylallyl, trans- and cis-croty l) has been extensively studied. Two distinct reactions are involved i n the overall process: (i) the coupling reaction, which gives rise ste reoselectively to the sole R3SnCH(CH3)CH=CH2 (alpha-isomer), and (ii) the subsequent isomerization of the alpha-isomer furnishing mixtures o f (alpha, trans, cis)-isomers. The occurrence of reaction ii depends u pon the nature either of the R group or the employed cosolvent. In cyc lohexane, 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 ternar y isomeric mixture. In tetrahydrofuran, 2-propanol, acetonitrile, and pyridine, the isomerization occurs to an extent which depends on the p olarity and the coordinating ability of the cosolvent itself. The obse rved stereoselection has been hypothesized to occur through one-electr on transfer from the zinc metal to the (C4H7)Br to form stereoselectiv ely an adsorbed CH2=CHCH(CH3)Br.-Zn.+ radical ion which is trapped by the R3SnCl reactant to form the alpha-isomer. Similarly, ditin compoun ds are thought to be formed by interaction of R3SnCl.-Zn.+ radical ion s with R3SnCl molecules.