Unique stereocontrol in carborane chemistry: skeletal alkylcarbonation (SAC) versus exoskeletal alkylmethylation (EAM) reactions.

ABSTRACT

Reactions between the arachno-6,9-C2B8H14 (1) dicarbaborane and acyl chlorides, RCOCl (2), are subject to stereocontrol that completely changes the nature of the reaction products. While most chlorides produce the 8-R-nido-7,8,9-C3B8H11 (3) tricarbollides (by skeletal alkylcarbonation=SAC), bulky RCOCls (2; where R=1-adamantyl, 2 a; 1-mesityl, 2 b; 9-anthranyl, 2 c; 1-naphthyl, 2 d) in 1,2-dichloroethane (DCE) in the presence of triethylamine at 40-60 °C gave a series of entirely different 1-R-2-CH3-closo-1,6-C2B8H8 (4) dicarbaboranes upon acidification with conc. H2SO4 (by exosleletal alkylmehylation=EAM). Both types of reactions seem to proceed via a common [8-R-nido-7,8,9-C3B8H10](-) (3(-)) anion which in the EAM case is unstable because of steric crowd and undergoes rearrangement via the isomeric [R-nido-7,8,10-C3B8H10](-) tricarbollide structures which, on protonation, undergo reductive extraction of one CH vertex to generate the 2-CH3 substituent in structure 4.