Are Methyl Groups Electron-Donating or Electron-Withdrawing in Boron Clusters? Permethylation of o-Carborane
Journal of the American Chemical Society2005Vol. 127(29), pp. 10158–10159
Citations Over TimeTop 10% of 2005 papers
Francesç Teixidor, Gemma Barberà, A. Vaca, Raikko Kivekäs, Reijo Sillanpää, Josep M. Oliva, Clara Viñas
Abstract
That methyl groups attached to carbon atoms are electron donors must not be generally assumed. In boron clusters, Me groups on boron are electron withdrawing. At the B3LYP/6-31G* level of theory, it has been proven that the Mulliken charge on each boron after substitution of -H by -Me increases by +0.18 unit. This leads to a high build-up of positive charge upon permethylation, then hampering it. Experimentally, this is proven by the synthesis of 9-I0.707H0.293-12-Cl0.566H0.434-3,4,5,6,7,8,10,11-Me8-1,2-C2B10H2, in which positions 9 and 12 were first methylated and then attacked by nucleophiles. This is substantiated by the synthesis of 3,6,8,9,10,12-Me6-1,2-C2B10H6 under the same experimental conditions but with time control.
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