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

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Clegg, W. and Conway, B. and Hevia, E. and McCall, M.D. and Russo, L. and Mulvey, R.E. (2009) 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. Journal of the American Chemical Society, 131 (6). pp. 2375-2384. ISSN 1520-5126 (https://doi.org/10.1021/ja8087168)

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Abstract

The new dialkyl(aryl) lithium zincates [(THF)2Li(C6H4−OMe)MeZnMe] (4), [(TMEDA)Li(C6H4−OMe)MeZnMe] (6), [(THF)3Li(C6H4−OMe)tBuZntBu] (7), and [(PMDETA)Li(C6H4−OMe)tBuZntBu] (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 1H, 13C{H}, and 7Li 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)2Li2Zn(C6H4−OMe)4] (5) is obtained as the result of a disproportionation process. This lithium-rich zincate has also been spectroscopically and crystallographically characterized.

ORCID iDs

Clegg, W., Conway, B., Hevia, E. ORCID logoORCID: https://orcid.org/0000-0002-3998-7506, McCall, M.D., Russo, L. and Mulvey, R.E. ORCID logoORCID: https://orcid.org/0000-0002-1015-2564;
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