Formation and Cycloaddition Reactions of a Reactive Boraalkene Stabilized Internally by N ‐Heterocyclic Carbene

The synthesis of element−carbon double bonds is of great importance for the development and understanding of reactive π‐bonded systems in chemistry. The seven‐membered heterocyclic system 4 b is readily made by internal C−H activation at a pendent isopropyl methyl group of the respective [(IPr)C 6 F 5 BH] + borenium ion. Subsequent deprotonation with the IMes carbene gives the neutral cyclic boraalkene system 5 b . The B=C double bond in compound 5 b adds carbon dioxide, CS 2 , sulfur dioxide, phenyl isocyanate, an acetylenic ester or two NO molecules to give the corresponding four‐membered ring annulated heterocycles. With sulfur or selenium 5 b gives the respective three‐membered ring systems. N 2 O reacts with 5 b to give a mixture of the related oxaborirane 18 and a unique [B]OH containing diazoalkane product 19 .