Umkehrung des Selektivitätsprinzips bei radikalischen Additionen an Alkene

Recent Advances in Radical Addition to Alkenylboron Compounds

The addition of carbon-centered radicals to alkenylboron compounds provides a useful method for the construction of organoboron reagents which are versatile reagents in chemical synthesis. While the first examples of this type or process appeared 70 years ago, until recently, attention to this type of reaction has been limited. A number of examples of this reactivity have been demonstrated recently, and strategies are emerging that allow for tuning the reagent structure in order to modulate reactivity. This review surveys recent advances in this class of reaction.

Photocycloaddition of cyclohex-2-enones to acrylonitrile

The photocycloaddition of cyclohex-2-enones 1a, 1b, and 5a–5c to acrylonitrile (2) in benzene has been studied. The reactions proceed with moderate regioselectivity and with preferential formation of diastereomeric mixtures of endo- and exo-5-oxobicyclo[4.2.0]octane-7-carbonitriles 3, 4, and 6–8. Very similar relative rates of conversion to products are observed in the photocycloadditions of 1b, 5a, and 5b to 2 and methacrylonitrile (9), respectively. In contrast, 5,5-dimethylcyclohex-2-enone (1b) forms cycloadducts with 2,3-dimethylbut-2-ene (12) as efficiently as with the unsaturated nitriles, but the 2,5,5-trialkylcyclohex-2-enones 5a and 5b do not react with this alkene at measurable rates of conversion. A dual path mechanism is presented that involves either exciplex formation between triplet-excited cyclohexenones and electron-rich alkenes or radical addition of the former to electron-deficient alkenes bearing a substituent, exercising a resonance effect.Key words: [2+2]-photocycloaddition, cyclobutane, cyclobutanecarbonitrile, radical addition to unsaturated nitriles.