A New Synthesis of N-tert-Butylvinylketenimines and Their Cycloaddition Reactions

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Abstract
The [4+2] cycloadditions of vinylheterocumulenes such as vinylketenes and N-substituted vinylketenimines with olefins have presented a preparative method for the direct synthesis of cyclohexenone derivatives. Vinylketenes can readily react with electron-rich olefins to afford cyclohexenone derivatives. On the other hand, vinylketenimines can react with electronpoor olefins to afford N-substituted cyclohexenonimines, which are hydrolyzed to the corresponding cyclohexenone derivatives. Consequently, vinylketenes and N-substituted vinylketenimines complement each other as the cycloaddition dienic partner in the [4+2] cycloadditions. Cycloaddition with vinylketenes was published.1) However, the cycloaddition with vinylketenimines has scarcely been investigated because of the lack of a general synthetic method of N-substituted vinylketenimines. To our best knowledge, N-p-tolylvinylmethylketenimine is only a vinylketenimine which has hitherto been prepared.2) Herein, we wish to report a new synthetic method of N-tert-butylvinylketenimines (1), in which allylic chloride is reacted with tert-butyl isocyanide in the presence of Pd(II)(OAc)2 together with 1,5-diazabicyclo[5.4.0]undec-5-ene (DBU), and their cycloadditions with electronpoor olefins producing N-tert-butylcyclohexenonimine derivatives (2).