Closer Insight into the Reactivity of TMP-Dialkyl Zincates in Directed ortho-Zincation of Anisole: Experimental Evidence of Amido Basicity and Structural Elucidation of Key Reaction Intermediates

The new dialkyl(aryl) lithium zincates [(THF)(2)Li(C6H4-OMe)MeZnMe] (4), [(TMEDA)Li(C6H4- OMe)MeZnMe] (6), [(THF)(3)Li(C6H4-OMe)(BuZnBu)-Bu-t-Bu-t] (7), and [(PMDETA)Li(C6H4-OMe)(BuZnBu)-Bu-t-Bu-t] (8) have been prepared by co-complexation reactions of lithiated anisole with the relevant dialkylzinc compound and the relevant Lewis base. These new heterobimetallic compounds have been characterized in solution using H-1, C-13{H}, and Li-7 NMR spectroscopy, and the molecular structures of 6 and 8 have been elucidated by X-ray crystallography. In 6 the distinct metals are connected through the anisole ligand which binds in an ambidentate fashion (through carbon-zinc and oxygen-lithium contacts) and also through, one of the methyl groups, to close a [LiOCCZnC] six-membered ring; whereas 8 displays an open structure where anisole connects the two metals (in the same mode as in 6) but with the tert-butyl groups exclusively bonded terminally to zinc.. Reactivity studies of zincates 4 and 7 with the amine TMP(H) supply experimental evidence that these heterobimetallic compounds are intermediates in the two-step deprotonation reaction of anisole by TMP-dialkyl zincates and show the relevance of the alkyl groups in the efficiency of TMP-dialkyl zincate bases. In addition, important solvent effects have also been evaluated. When hexane is added to THF solutions of compounds 4 or 7, the homoleptic tetraorganozincate [(THF)(2)Li2Zn-(C6H4-OMe)(4)] (5) is obtained as the result of a disproportionation process. This lithium-rich zincate has also been spectroscopically and crystallographically characterized.