Synthetic and structural insights into the zincation of toluene: direct synergic ring metallation versus indirect nonsynergic lateral metallation

摘要

Toluene has been directly zincated by reaction with the heteroleptic sodium zincate [(tmeda)NaACHTUNGTRENUNG(tBu)- ACHTUNGTRENUNG(tmp)ZnACHTUNGTRENUNG(tBu)] (1) to afford a statistical mixture of the meta and para regioisomers of [(tmeda)NaACHTUNGTRENUNG(C6H4Me)- ACHTUNGTRENUNG(tmp)ZnACHTUNGTRENUNG(tBu)] (2) (TMEDA= N,N,N',N'-tetramethylethylenediamine, TMP=2,2,6,6-tetramethylpiperidide). Interestingly, the methyl group of toluene has been left untouched by the mixed-metal base. The crystal structures of both regioisomers have been elucidated by using X-ray crystallography, which revealed a common motif, in which zinc lies in the same plane as the tolyl ring and binds strongly to the carbon that has experienced the deprotonation, leading to a covalent s bond; whereas sodium adopts a perpendicular disposition, p-engaging with the aromatic ring. DFT studies of the reaction of 1 with toluene showed that of the four possible regioisomers, the experimentally observed meta and para ones are the most thermodynamically preferred products due to a large extent to the combined (synergic) effect of the zinc and sodium and the different way in which each metal binds to the tolyl anion. In contrast, the more straightforward benzyl-based zincate [(tmeda)2NaACHTUNGTRENUNG(CH2Ph)ZnACHTUNGTRENUNG(tBu)2] (3) is obtained through an indirect route, in which toluene is first laterally metallated by monometallic butylsodium to afford benzylsodium, which then forms a mixed-metal compound by co-complexation with tBu2Zn. Zincate 3 has been characterised by X-ray crystallography in the solid state and by NMR spectroscopy in deuterated THF solution.