Study On The Reaction Of Alkynyl Diazomethanes In The Synthesis Of Allenes

Diazo compounds are simple nitrogen compounds,has been widely used in organic synthesis.Diazo compounds can undergo insertion,coupling,cyclization,rearrangement,and other reactions.Due to its unstable and explosive characteristics,diazo compounds bring people a lot of difficulties in preparation.N-tosylhydrazones is one of the research hotspots of diazo chemistry in recent years.It can produce Bamford-Stevens reaction under base conditions and in situ generation of diazo compounds to participate in subsequent reactions.However,the decomposition temperature of N-tosylhydrazones is usually higher than 70 ?,and its participation in the reaction is generally around 100 ?,which greatly limits its development.Despite the N-tosylhydrazones has obvious advantages as diazo surrogates of alkyl-,alkenyl-,or aryldiazomethanes,N-tosylhydrazones have been scantily used as precursors of alkynyldiazomethanes,because of the preferred intramolecular cyclization to pyrazoles,thus the study of alkynyl diazomethane lags far behind that of diazo compounds.Compared with N-tosylhydrazones,N-nosylhydrazones can produce diazo compounds at lower temperature due to its steric and electronic effects.From the reaction of N-nosylhydrazones,it can be seen that o-nitrophenylsulfonyl group has more advantages as a leaving group than methylbenzenesulfonyl group.This leads us to see the potential of alkynyl N-nosylhydrazones as alkynyl diazo precursors to participate in the reaction.Allenes are crucial structural motifs in a variety of bioactivemolecules,drug molecules and natural products,and are important members of the organic synthesis family.It has special physical and chemical properties and can participate in various organic reactions such as cycloaddition,migration,oxidation,and electrophilic addition,because of the conjugated double bond of the allene.There are also many methods for the synthesis of allenes,despite great achievements in the allene synthesis by metal-mediated direct coupling of two fragments,analogous metal-free reactionsremain rare.In this paper,we use the N-nosylhydrazones in situ generation of alkynyl diazomethane under base conditions,coupling with boric acid under metal-free conditions to form allenes.This reaction overcomes the synthesis of pyrazoles by N-tosylhydrazones self-rings,and synthesizes di-and tri-substituted allenes with two readily available starting materials,with high tolerance of reactive functional groups and a wide range of substrates.