Relative Reactivities of the Small Closo Carboranes 1,6-C₂B₄H₆ and 2,4-C₂B₅H₇ and of closo-1,10-C₂B₈H₁₀ toward “Electrophilic” Reagents

The relative reactivities of the closo carboranes C₂B_nH_n+2 (n = 4, 5, 8), and some of their derivatives, toward electrophilic reagents of the type RX/AlCl₃ (RX = CH₃Cl, C₂H₅Cl, Cl₂, Br₂) are reported from competition studies. Among the three parent carborane compounds, closo-2,4-C₂B₅H₇ is the most reactive toward an electrophilic type of substitution. Alkyl substituents on closo-2,4-C₂B₅H₇ enhance the reactivity of the compound toward an electrophilic substitution, whereas halogen substituents decrease the reactivity. However, in the closo-1,6-C₂B₄H₆ system, the reactivity of the chloro-substituted compound, 2-Cl-1,6-C₂B₄H₅, toward an electrophilic substitution is greater, at the 4-position, than that of the parent carborane. The nature (halogen or alkyl) and cage position of a substituent on closo-2,4-C₂B₅H₇ appear to have little or no influence on the site of “electrophilic” substitution.