How substituents at boron atoms affect the CH-acidity and the electron-withdrawing effect of the ortho-carborane cage: A close look on the ¹H NMR spectra

Based on the data of 1H NMR spectra, the effect of various substituents at boron atoms in different positions of the ortho-carborane cage on its CH-acidity and the electron-withdrawing effect upon substitution at the carbon atom is considered. The introduction of chlorine atoms to the boron atoms opposite to the carbon atoms (positions 9 and 12) slightly reduces the CH-acidity of the carborane cage, whereas the introduction of bromine and, especially, iodine atoms lead to the significant increase in the CH-acidity in comparison with the parent orthocarborane. The introduction of halogen atoms to the boron atoms bound to both carbon atoms (positions 3 and 6) results in a significant increase in the CH-acidity and this increase is practically independent on the halogen electronegativity. Nevertheless, in the case of polychlorinated derivatives of ortho-carborane, the total effect of the substituent completely covers the decrease in CH-acidity when the first halogen atoms are introduced into positions 9 and 12. The introduction of alkyl substituents, regardless of the substitution position, leads to a decrease in the CH-acidity of the ortho-carborane cage, while the introduction of aryl substituents has the opposite effect.