Microwave-promoted Tandem Cyclization Reactions Using 1,5-enynes As Substrates

In recent years,radical-initiated C-H functionalization has attracted much attention in organic synthesis,which enables the construction of carbon-carbon and carbon-heteroatom bonds efficiently without the use of organometallic reagents.Free radical tandem reactions are efficient and environment-friendly,which high atomic economy and significant advantages in the synthesis of complex natural products.Herein,radical tandem cyclization reactions play an important role in organic synthesis and are widely used in selective C-H functionalization of unactivated alkanes.The use of inexpensive and readily available non-polluting initiators for C-H bond activation reactions in metal-free catalytic systems has good prospects for application.This paper focuses on the study of C-H bond activation reaction of1,5-enynes by free radical process under the promotion of microwave as follows:(1)The reaction of 1,5-enynes with cyano groups and cyclohexane as starting materials and benzoyl peroxide(BPO)as initiator(15 mol%)was carried out under microwave irradiation at 100 ? for 20 min.The reaction was initiated by BPO to generate cyclohexyl radicals,which attacked the double bonds of olefins and then completed the cyclization reaction with the carbon-carbon triple bonds of alkynes.The reaction system has some advantages such as simplicity,good tolerance of functional groups,high product yields and so on.(2)The 1,5-enyne with cyano groups was reacted with benzaldehyde as starting materials,benzoyl peroxide(BPO)as the initiator and dichloromethane as the reaction solvent at 120 ? for 20 min under microwave radiation.The benzoyl radical attacked the carbon-carbon double bond of the alkyne and then underwent a tandem cyclization reaction with the alkyne to obtain 1H-indene-dicarbonitrile derivatives in high yield.