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The Role of Neutral and Radical Anionic Organozinc Complexes in the Alkylation Reactions of 1,4-Diaza-1,3-butadienes with Diorganozinc Compounds

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journal contribution
posted on 08.07.1997, 00:00 by Evelien Rijnberg, Jaap Boersma, Johann T. B. H. Jastrzebski, Miles T. Lakin, Anthony L. Spek, Gerard van Koten
We have earlier postulated the intermediacy of organozinc radical species in the regioselective alkylation reactions of 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene (t-BuNCHCHN-t-Bu) with diorganozinc compounds (ZnR2). To verify these postulates, we have prepared and studied the neutral organozinc radical complex MeO(CH2)3Zn(t-BuNCHCHN-t-Bu) (A) and two diorganozinc radical-anionic complexes, K[R2Zn(t-BuNCHCHN-t-Bu)] (R = Me (8a), Et (8b)). A was prepared in situ by the reaction of t-BuNCHCHN-t-Bu with bis(methoxypropyl)zinc. Complexes 8 were prepared by reducing the 1:1 coordination complex ZnMe2(t-BuNCHCHN-t-Bu) (1a) with potassium or by the nucleophilic addition of K(t-BuNCHCHN-t-Bu) to ZnR2 (R = Me, Et). The resulting radical-anionic complexes are thermally unstable and readily undergo an intermolecular single-electron transfer, giving a mixture of the metallacyclic heteroleptic zincate complexes K[ZnR(t-BuNCHCHN-t-Bu)] (9) and K[ZnR(t-BuNC(R)CHN-t-Bu)] (10). The ratio in which the latter complexes are formed depends on the R group and on the concentration and the temperature. Alternatively, 9 has been prepared separately from the reduction of [ZnR(t-BuNCHCHN-t-Bu)]2 (4) with 2 equiv of potassium. The crystal structures of two complexes of 9, i.e. {9a(THF)}n and {9c(Et2O)1/2}n, have been determined. Both complexes form linear coordination polymers containing alternating potassium cations and zinc−diazabutadiene anions.