Radicophilic alkenes

These reactions of 1-Me resemble that of (dichloromethylene)cyclopropane [31] and radicophilic alkenes with a capto-dative substitution pattern [32]. Thus, it is not surprising that 1-Me reacts with a-ferf-butylthioacrylonitrile (18), yielding the two isomeric cyclobutane derivatives 19a, b (ratio 2.2 1) as a mixture of two diastereomers each [29] (Scheme 5), and this reaction occurs under milder conditions than the [2-1-2] cycloaddition of 18 onto methylenecyclopropane. 

Nucleophilic radical 79, derived from bromide 78, is expected to add intra-molecularly to the eleetrophilic double bond to yield a-acyl radical 80, but it can also add to the radicophilic alkene group to yield captodative species 81 (Scheme... 

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ab initio calculations indicated that the radical cation lies 84 kJ mol-1 above the propene radical cation and is separated from it by a barrier of 35 k.l mol Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophilic properties. 

Photoelectron, UV and NMR spectroscopy, as well as MNDO calculations all predict a reduced HOMO-LUMO gap for alkenes with captodative substitution, and an enhanced reactivity of the p carbon [43]. This explains their radicophilic behavior and their high reactivity in cycloaddition processes, the diradicaloid transition states of which are stabilized when the mechanism is asynchronous cf. Sec. 3.3.7). 

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