Reaction of sterically congested NHC–Zn(CH₂CH₃)₂ with substituted phenols leading to zincate complexes

Abstract

We report the reaction of a sterically congested NHC–Zn(CH₂CH₃)₂ Lewis adduct (1) prepared through reaction of an equimolar ratio of 1,3-di-tert-butylimidazol-2-ylidene and diethyl zinc, with various substituted phenols (4-tert-butyl-phenol, 2,6-di-tert-butyl-4-methyl phenol, and 1-bromo-4,6-di-tert-butyl phenol). The NHC–Zn dative bond was cleaved in each of the reactions with the substituted phenols to afford the corresponding ionic complexes of imidazolium cation and aryloxo-zincate, [{(4-CMe₃C₆H₄O)₂Zn(μ-OC₆H₄-4-CMe₃)}₂{(1,3-(CMe₃)_2-ImCH}₂] (2), [{(2,6-(CMe₃)₂-4-Me-C₆H₃O)₂}Zn{(1,3-(CMe₃)_2-ImCH}] (3), and [{(1-Br-3,5-(CMe₃)₂C₆H₂O)₂}_2-Zn{(1,3-(CMe₃)_2-ImCH}] (4), where 1,3-(CMe₃)_2-ImCH) is imidazolium carbocation. The molecular structures of 1–4 were established by X-ray diffraction analyses and from the solid-state structures of 2–4, it was confirmed that, in all the compounds, zinc ions are coordinated through substituted phenolate groups.

The treatment of a sterically congested NHC–Zn(CH₂CH₃)₂ Lewis adduct (1) obtained from 1,3-di-tert-butylimidazol-2-ylidene and diethyl zinc, with various substituted phenols (4-tert-butylphenol,2,6-di-tert-butyl-4-methyl phenol and 1-bromo-4,6-di-tert-butyl phenol) are described.

Keywords:

• N-heterocyclic carbene
• Imidazolium cation
• Aryloxo-zincate
• Metallacycle

Acknowledgments

This work was supported by the Department of Science and Technology, Ministry of Science and Technology, Government of India (DST) under the SERC Fast Track Scheme (SR/FT/CS-74/2010) and start-up grant from the Indian Institute of Technology Hyderabad. Kishor Naktode and Ravi K. Kottalanka thank the University Grants Commission, India for their PhD fellowships. We thank Prof. W. Massa, Germany for his help in solving the structure of 2. Generous support from K. Mashima, Osaka University, Japan is gratefully acknowledged.