Unsaturated carbonyl compounds radical attack

Concerning the mechanism of Cr(VI) oxidations, initial attack of Cr03 to form a symmetric intermediate was proposed.677 Hydrogen atom or hydride abstraction from the allylic position leads to resonance-stabilized allylic radical or carbocation, respectively, which is eventually converted to the unsaturated carbonyl compound. 

The key substrates for conjugate addition are the a, 3-unsaturated carbonyl compounds. When the double bond is inside a chain or ring these compounds are available via a wide variety of routes including the aldol reaction and are generally stable intermediates that can be stored for use at wiU. When the double bond is exo to the ring or chain (exo-methylene compounds), the unhindered nature of the double bond makes them especially susceptible to attack by nucleophiles (and radicals), This reactivity is needed for conjugate additions but the compounds are unstable and polymerize or decompose rather easily,... 

Radicals are soft most of them do not have a charge, and in most chemical reactions they react with uncharged molecules. Thus the Coulombic forces are usually small while the orbital interactions remain large. The clean polymerisation of methyl methacrylate demonstrates this typically soft pattern of behaviour. Radicals attack at the conjugate position of a,/3-unsaturated carbonyl compounds such as methyl methacrylate 7.1, rather than at the carbonyl group, and the attack... 

Attempts to attack the i4a-methyl group of lanosterol from a 9a-alkoxy-radical gave only a 9,io-seco-9-ketone [53], although the 7a-alkoxy radicai readily formed a cyclic ether with the I4a-methyl group [54]. Finally, a selection of secondary alkoxy-radicals derived from alcohols adjacent to quaternary centres e.g. la- or ij8-0H [55], 3j5-OH-4,4-dimethyl [56], 17/S-OH [36J, and 2i-OH-20-cyclic ketal [56]) gave fragmentation products which were either unsaturated carbonyl compounds, or acetoxy derivatives resulting from combination of alkyl and acetoxy radicals, e.g. ... 

Radical Anions. Radical anions are common intermediates in organic reactions they are easily prepared from compounds with low-enough LUMOs by the addition of an electron (from a dissolving metal or from a cathode, or the solvated electron itself). Those derived from carbonyl groups (421) dimerize at carbon 339 those derived from a,/ -unsaturated carbonyl compounds (422) dimerize at the /1-position,340 and pyridines dimerize predominantly at the 4-position.341 In each case, the odd electron has been fed into the orbital which was the LUMO of the starting material the site of coupling therefore should, and does, correlate with the site at which nucleophiles attack the neutral compounds. 

Conjugate addition of organometals and enol ethers to a,fS-unsaturated carbonyl compounds is one of the backbones in organic synthesis. A less common approach is the use of free radicals, which also add to unactivated double bonds. However, the regiocontrol of such additions is best achieved by means of electronically withdrawing groups (such as carbonyls, nitriles, sulfones, and nitro), which direct the radical attack to the fi-position. 

---