Alkynyl N‐Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis. Issue 38 (27th June 2017) - Yang, Yang
Abstract: The decomposition of N ‐tosylhydrazones is a safe and convenient method for the generation of donor carbenes. However, alkynyl carbenes cannot be isolated by this route because they readily undergo intramolecular cyclization to pyrazoles as soon as formed from alkynyl N ‐tosylhydrazones. Here, the use of alkynyl N ‐nosylhydrazones 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 mechanistic investigations demonstrated that γ‐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 alkynylcarbene transient species. Abstract : N ‐Nosylhydrazones : Formation of a pyrazole byproduct is the frequently encountered side reaction in the decomposition of alkynyl‐ N ‐sulfonylhydrazones, which represents a long‐standing challenge for chemists to generate acetylenic carbenes for subsequent applications. This was resolved through introduction of a nosyl group in place of the tosyl moiety of alkynyl‐ N ‐tosylhydrazones, thus generating the required acetylenic carbenes for the coupling/isomerization reactions with aryl/alkyl boronic acids.