Photocatalytic Isomerization Of N-sulfonyl Ynamides

Ketenimines,containing cumulated double bond sequence as C=C=N,can undergo a variety of transformations,such as the cycloaddition,electrocyclic ring-closure,sigmatropic rearrangements,nucleophilic,electrophilic,and radical addition.As such,they serve as versatile building blocks for the construction of various nitrogen-containing compounds which are difficult to be prepared via traditional methods.In this regard,many methods using ynamides as readily available starting materials have been developed for the synthesis of ketenimines,including the thermal rearrangement of ynamides,Pd-catalyzed aza-Claisen rearrangement,and base-promoted transformation of ynamides.However,the precedented methods require the use of forcing conditions,expensive catalyst,strong bases,or a combination thereof,hence leading to unsatisfactory functional group tolerance and poor accessibility.On the other hand,radical mediated processes have attracted more attentions in organic synthesis due to their mild reaction conditions and excellent functional group tolerance.Herein,this dissertation focuses on investigations of novel radical-mediated strategy for assembling ketenimines using readily available ynamides as the starting material.Specifically,with N-sulfonyl ynamides as substrates,fac-Ir（ppy）₃ as the photocatalyst,DCM as the solvent,various ketenimines can be obtained in almost quantitative yields under irradiation of 15 W of blue LEDs at room temperature.Compared with traditional methods,this method is featured by the production of isolable ketenimines without isolation.A wide range of functional groups,such as halide,ester,alkyne,amide,thioether,TMS and OTBS,are well tolerated under the conditions.Moreover,it is applicable to terminal,alkyl,and aryl ynamides.Synthetic utility of this method was also conducted,including the coupling with water,amines,and imines,which is valuable for the access of nitrogen-containing compounds.Mechanistic studies suggest a radical chain process,consisting ofβ-selective sulfonylation andβ-fragmentation.Of note,the method developed here is the first example of radical-mediated synthesis of keteneimines via an unprecedented isomerization of N-sulfonyl ynamides,which provides new opportunities for the keteneimine chemistry.