Mechanochemical and slow-chemistry radical transformations: a case of diorganozinc compounds and TEMPO

From the green chemistry perspective, molecular solid-state transformations conducted under mild conditions are of great interest and desirability. However, research in this area lacked popularity in the previous century, and thus progressed slowly. In particular, the application of radical reactions in solid-state chemistry has been hampered by several long-standing challenges that are intrinsically associated with the apparent unpredictable nature of radical chemistry. We present a comparative study of model mechanochemical, slow-chemistry and solution radical reactions between TEMPO and homoleptic organozinc compounds (i.e., di-tert-butylzinc and diphenylzinc). In the case of the tBu(2)Zn/TEMPO reaction system only a dimeric diamagnetic complex [tBuZn(μ-TEMPO*)](2) is obtained in yields slightly varying with the method chosen. In contrast, when TEMPO is mixed with diphenylzinc in a 2 : 1 molar ratio a novel paramagnetic Lewis acid–base adduct [[Ph(2)Zn(η(1)-TEMPO)]·TEMPO] is isolated in high yields regardless of the applied methodology. This adduct is also formed in the slow-chemistry process when TEMPO is gently mixed with Ph(2)Zn in a 1 : 1 molar ratio and left for two weeks at ambient temperature. Within the next week the reaction mixture gives in high yield a diamagnetic dinuclear compound [PhZn(μ-TEMPO*)][PhZn(μ(2)-η(1):η(1)-TEMPO*)] and biphenyl. The analogous reaction conducted in toluene results in a much lower conversion rate. The reported results open up a new horizon in molecular solid-state radical transformations.