N-Heterocyclic Carbene-Borane Adducts with Chiral (R)-Chloroethyl and Vinyl Substituents.

This research provides how chiral imidazolium salts {1,3-bis[1'-chloro-1'-phenylpropan-2'-yl]-imidazolium} (1a) and enantiopure {1,3-bis[(R)-1-chlorobutan-2-yl]-imidazolium} [1b] can be utilized in diverse synthetic pathways to obtain new carbene-borane adducts (2a-2d): {1,3-bis[1'-chloro-1'-phenylpropan-2'-yl]-imidazolyl-2-ylidene-borane} (2a), {1,3-bis[(Z)-1'-phenylpropen-2'-yl]-imidazolyl-2-ylidene-borane} (2b), {1,3-bis[(R)-1-chlorobutan-2-yl]-imidazolyl-2-ylidene-borane} (2c), and {1,3-bis[but-1-en-2-yl]-imidazolyl-2-ylidene-borane} (2d). The carbene-borane adducts were synthesized and characterized by ¹³C, ¹H, and ¹¹B nuclear magnetic resonance spectroscopy and time-of-flight mass spectrometry. The X-ray crystal analyses of compounds 2a and 2b were performed, and to understand the structure and interactions of 2a, a computational study was carried out. The effect of N-substituents in the NHC-borane adducts was clearly observed in the C-B bond lengths obtained by single-crystal X-ray diffraction, where the C-B bond is longer for adducts with N-(R)-chloroethyl substituents than for vinyl substituents. The analysis of the reduced density gradient and the bond critical point calculations of 2a showed intramolecular proton-hydride and Cl···N interactions. These chiral imidazolium salts could have applicability in the development of new materials and possibly in pharmaceutical research.