Synthesis of B-alkyl derivatives of o-carborane by alkylation under electrophilic conditions; Scope and limitation

The reaction of o -carborane with 13 alkylation agents in CS 2 over AlCl 3 using different ratios of reactants was investigated. Alkylation succeeded with CH 3 X, C 2 H 5 X and i-C 3 H 7 X whereas other used agents failed. Only i-C 3 H 7 Br alkylated in nitromethane. A rich mixture resulted even with o -carborane: RX 1 : 1 ratio. In the presence of H 2 SO 4 o -carborane was isopropylated with i-C 3 H 7 OH, no other alcohols or olefins reacted. Alkyls enter stepwise B-9(12) > B-8(10) > B-4(5,7,11) positions, facilitating always the introduction of further alkyl into the skeleton. On ethylation, the most ethylated product was 4,5,7,8,9,10,12-(C 2 H 5 ) 7 -1,2-C 2 B 10 H 5 . Seven B-alkyl and polyalkyl derivatives were isolated as individua. With C 6 H 5 Br-AlCl 3 no reaction occurred at 27 °C, while only a transfer of Br into 9-position was observed at 127 °C. Possible mechanisms of B-substitution under electrophilic conditions are discussed.