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

The addition of o-carborane (1) to aldehydes 2 proceeded very smoothly in the presence of aqueous tetrabutylammonium fluoride (TBAF; 3 equiv) at room temperature, giving the corresponding carbinols 3 in high yields. The TBAF-mediated reaction was applied to the intramolecular cycloaddition of o-carboranyl aldehydes and ketones 4, and the corresponding five-, six-, and seven-membered carboracycles were obtained in good-to-high yields. Further, [3 + 2] annulation between o-carborane (dianionic C₂ synthons) and α,β-unsaturated aldehydes and ketones (dicationic C₃ 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.