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

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 (ZnR 2 ). To verify these postulates, we have prepared and studied the neutral organozinc radical complex MeO(CH 2 ) 3 Zn( t -BuNCHCHN- t -Bu) ( A ) and two diorganozinc radical-anionic complexes, K[R 2 Zn( 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 ZnMe 2 ( t -BuNCHCHN- t -Bu) ( 1a ) with potassium or by the nucleophilic addition of K( t -BuNCHCHN- t -Bu) to ZnR 2 (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 ( Et 2 O ) 1 / 2 } n, have been determined. Both complexes form linear coordination polymers containing alternating potassium cations and zinc−diazabutadiene anions.