Unique Stereocontrol in Carborane Chemistry: Skeletal Alkylcarbonation (SAC) versus Exoskeletal Alkylmethylation (EAM) Reactions

Reactions between the arachno ‐6,9‐C 2 B 8 H 14 ( 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‐C 3 B 8 H 11 ( 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‐CH 3 ‐ closo ‐1,6‐C 2 B 8 H 8 ( 4 ) dicarbaboranes upon acidification with conc. H 2 SO 4 (by exosleletal alkylmehylation=EAM). Both types of reactions seem to proceed via a common [8‐R‐ nido ‐7,8,9‐C 3 B 8 H 10 ] − ( 3 − ) anion which in the EAM case is unstable because of steric crowd and undergoes rearrangement via the isomeric [R‐ nido ‐7,8,10‐C 3 B 8 H 10 ] − tricarbollide structures which, on protonation, undergo reductive extraction of one CH vertex to generate the 2‐CH 3 substituent in structure 4 .