Tetrabutylammonium Fluoride Promoted Novel Reactions of <i>o</i>-Carborane:  Inter- and Intramolecular Additions to Aldehydes and Ketones and Annulation via Enals and Enones

The addition of <i>o</i>-carborane (<b>1</b>) to aldehydes <b>2</b> proceeded very smoothly in the presence of aqueous tetrabutylammonium fluoride (TBAF; 3 equiv) at room temperature, giving the corresponding carbinols <b>3</b> in high yields. The TBAF-mediated reaction was applied to the intramolecular cycloaddition of <i>o</i>-carboranyl aldehydes and ketones <b>4</b>, and the corresponding five-, six-, and seven-membered carboracycles were obtained in good-to-high yields. Further, [3 + 2] annulation between<i> o</i>-carborane (dianionic C<sub>2</sub> synthons) and α,β-unsaturated aldehydes and ketones (dicationic C<sub>3</sub> synthons) proceeded very smoothly in the presence of TBAF to give the corresponding five-membered carbocycles in good-to-high yields. Detailed mechanistic studies revealed that the [3 + 2] annulation proceeded through kinetically controlled 1,2-addition and thermodynamically controlled 1,4-addition.