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

Abstract

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.