Isolation and Reactivity of a Frustrated N‐Heterocyclic Carbene‐Borane

Abstract

The synthesis of the frustrated carbene‐borane 4‐(9‐borabicyclo[3.3.1]nona‐9‐yl)‐1,3‐di‐tert‐butylimidazolin‐2‐ylidene (6) was accomplished by deprotonation of the N‐heterocyclic carbene (NHC) 1,3‐di‐tert‐butylimidazolin‐2‐ylidene (1) at the 4‐position with n‐butyllithium, followed by addition of 9‐chloro‐9‐borabicyclo[3.3.1]nonane (BBN‐Cl). The abnormal carbene‐borane adduct 5, with the BBN‐Cl moiety at the 4‐position, was obtained from the reaction of 1 with BBN‐Cl; however, its conversion into 6 by treatment with K[N(SiMe3)2] (potassium hexamethyldisilazide, KHMDS) was not achieved. The reactions of 6 with the rhodium(I) complexes [Rh(COD)Cl]2 (COD = 1,4‐cyclooctadiene) and [Rh(CO)2Cl]2 afforded the complexes cis‐[(6)Rh(COD)Cl] (7) and cis‐[(6)Rh(CO)2Cl] (8), with the latter allowing assessment of the σ‐donor/π‐acceptor properties of 6 by IR spectroscopy. The reactivity of 6 towards carbon dioxide, phenylacetylene and acetonitrile was studied, affording the labile adduct 6·CO2 (9), whereas irreversible deprotonation and formation of the imidazolium phenylalkynylborate 10 and the imidazolium cyanomethylborate 11 was observed for PhCCH and CH3CN. The molecular structures of 5, 7, 8, 10, and 11 were established by X‐ray diffraction analyses.