Observation of Spontaneous C=C Bond Breaking in the Reaction between Atomic Boron and Ethylene in Solid Neon

A ground‐state boron atom inserts into the C=C bond of ethylene to spontaneously form the allene‐like compound H 2 CBCH 2 on annealing in solid neon. This compound can further isomerize to the propyne‐like HCBCH 3 isomer under UV light excitation. The observation of this unique spontaneous C=C bond insertion reaction is consistent with theoretical predictions that the reaction is thermodynamically exothermic and kinetically facile. This work demonstrates that the stronger C=C bond, rather than the less inert C−H bond, can be broken to form organoboron species from the reaction of a boron atom with ethylene even at cryogenic temperatures.