Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis

The decomposition of N-tosylhydrazones is a safe and convenient method for the generation of donor carbenes. However, alkynyl carbenes cannot be iso- lated by this route because they readily undergo intramo- lecular cyclization to pyrazoles as soon as formed from al- kynyl N-tosylhydrazones. Here, the use of alkynyl N-nosyl- hydrazones for the in situ generation of alkynyl carbenes and their coupling reaction with boronic acids under metal-free conditions is reported, giving rise to a wide array of di- and trisubstituted allenes. Preliminary mecha- nistic investigations demonstrated that g-protodeboration of propargyl boric acid was responsible for the initial allene formation. This methodology based on the nosyl group allows for novel transformations that involve an al- kynylcarbene transient species.