Cumulated dienes, cycloaddition

Why do some [2 + 2] cycloadditions occur thermally and others photochemically What is special about fluoroalkenes and cumulated dienes The answers are complex, but it appears that most thermal [2 + 2] cycloadditions,... 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

In the context of the ethynyl sulfones 6.30 Padwa s cycloadditions of phenylsulfonyl-substituted propa-1,2-dienes like 6.31 with diazomethane should be mentioned briefly (Padwa et al., 1993a, with many references on allenes used as dienophiles). The example in (6-22) demonstrates the regiospecificity of the cycloaddition to the more reactive electron-deficient 71 bond. The ready reactivity of cumulated double bonds is based on the relief of strain when the 1,2-propadiene... 

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