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

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 .