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

Abstract

The subtle nature of reactivity of the free stable nitroxyl radical TEMPO towards tBu2Zn and Ph2Zn was explored in an uncommon comparison of wet, mechanochemical and slow-chemistry reactions.

A Case Study on the Desired Selectivity in Solid‐State Mechano‐ and Slow‐Chemistry, Melt, and Solution Methodologies

“… Speciation of the reaction products involving the Ar 2 Zn/TEMPO system for different Ar substituents and synthetic procedures. Previous studies on the reactivity of TEMPO towards (a,b) Ph 2 Zn [30] confronted with current results.…”

“…As mentioned previously, initially our investigations on the (C 5 F 5 ) 2 Zn/TEMPO reaction system were encouraged by our recent advances in understanding the reactions between Ph 2 Zn and TEMPO [30] . In the light of the reported results, a rough difference is the lack of the SET reaction in the (C 5 F 5 ) 2 Zn/TEMPO system.…”

“…As a part of our ongoing interest in the radical chemistry of the non‐redox active metal‐based organometallic complexes [38–44] only very recently we demonstrated that TEMPO reacts smoothly with Et 2 Zn [45] (in solution) or t Bu 2 Zn [46] (in solution or solid state) compounds in solution by the single electron transfer (SET) step affording diamagnetic zinc complexes incorporating TEMPO anion (TEMPO*). Moreover, in the case of the solid‐state reactions involving the Ph 2 Zn/TEMPO system, mechano‐ and slow chemistry appeared as mutually complementary methods for the synthesis of the paramagnetic [Ph 2 Zn( η 1 ‐TEMPO)⋅TEMPO] and diamagnetic [PhZn( μ 2 ‐ η 1 : η 1 ‐TEMPO*)][PhZn( μ 2 ‐TEMPO*)] products (Figure 1a,b, respectively) [30] . The aforementioned mechano‐ and slow‐chemistry reactions for the Ph 2 Zn/TEMPO system indicated that the secondary coordination sphere N−O⋅⋅⋅H−C π (Figure 1a) interactions may have profound effect on the reaction outcomes [30] .…”

“…Moreover, in the case of the solid‐state reactions involving the Ph 2 Zn/TEMPO system, mechano‐ and slow chemistry appeared as mutually complementary methods for the synthesis of the paramagnetic [Ph 2 Zn( η 1 ‐TEMPO)⋅TEMPO] and diamagnetic [PhZn( μ 2 ‐ η 1 : η 1 ‐TEMPO*)][PhZn( μ 2 ‐TEMPO*)] products (Figure 1a,b, respectively) [30] . The aforementioned mechano‐ and slow‐chemistry reactions for the Ph 2 Zn/TEMPO system indicated that the secondary coordination sphere N−O⋅⋅⋅H−C π (Figure 1a) interactions may have profound effect on the reaction outcomes [30] . Now, we turned our attention to (C 6 F 5 ) 2 Zn as an arylzinc derivative exhibiting a monomeric structure [32] in the solid state (in contrast to the dimeric form of [Ph 2 Zn] 2 ), [47] resistant to SET reactions and not prone to the N−O⋅⋅⋅H−C π interactions.…”

“…Remarkably, there are a number of reports comparing a solution versus mechanochemical synthesis [11,28–30] and one work on a mechanochemical versus melt protocol [31] for the same reaction, and to the best of our knowledge no comparative studies in four different reaction media (i. e., solid‐state mechano‐ and slow chemistry, melted phase, and solution) were reported to date. Herein, we report comprehensive studies in four different reaction media involving a moderately exothermic model reaction system based on a non‐redox active metal compound with moderate Lewis acidity, that is, a monomeric bis(pentafluorophenyl)zinc, [32] [(C 6 F 5 ) 2 Zn], and 2,2,6,6‐tetramethylpiperidinyl oxide (TEMPO) as a stable nitroxyl radical, which is widely used in various chemical transformations [33,34] .…”