Are Methyl Groups Electron-Donating or Electron-Withdrawing in Boron Clusters?Permethylation of o-Carborane

摘要

The concept that methyl groups attached to carbon atoms,heteroatoms,or pi-electron systems are electron donors (+I) is widely accepted in organic chemistry.This idea is also assumed in boron cluster chemistry,most probably as a consequence of the many common reactions in both areas.However,we show in this paper through partial methylation on o-carborane (1,see Figure 1),supported by theoretical calculations,that the methyl groups,contrary to the common belief,are electron-withdrawing (-I) in boron clusters.This result should not be strange,however,taking into account the difference in electronegativity between C (2.5) and B (2.0).In the past decade,much effort has been dedicated to the alkylation or peralkylation of closo-borates,the monocarborane anion,and the dicarbaboranes.Good success has been obtained with anionic clusters,but much less with the dicarbaboranes.