ORGANOZINC ALPHA-DIIMINE RADICALS - SYNTHESIS AND REACTIVITY TOWARD ALKYL-HALIDES

Organozinc radicals, [RZn-t-BuDAB]., which are considered to be key intermediates in the regioselective alkylation reactions of R2Zn compounds with 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene (t-BuDAB), have been prepared via independent routes in good yield either from the reaction of K(t-BuDAB) with RZnCl or by the thermal conversion of stable R2Zn-t-BuDAB complexes (R = Me, CH2SiMe3). In solution the [Rzn-t-BuDAB]. radicals are in equilibrium with their C-C coupled dimers, which were isolated as solids. A preliminary X-ray diffraction study of one such dimer, 6a, shows the stereoselective coupling of two Me3SiCH2Zn-t-BuDAB moieties by a C-C bond. The [Rzn-t-BuDAB]. radicals are reactive toward alkyl halides, like allyl and benzyl bromide, as well as alpha- and beta-substituted halo esters, producing C-alkylated products in good yield. The reactions with the halo esters give, after a subsequent ring closure reaction, heterocyclic products, i.e. beta-lactams (with good to excellent diastereoselectivity, de 60-90%) and 2-pyrrolidinones.