Cyclic ketones, « alkenals from states

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

When the alkyl chain is C3 or longer, cleavage of the C—C bond once removed from the C=0 group occurs with hydrogen migration. This is the same cleavage noted for aliphatic ketones that proceeds through a cyclic transition state and results in elimination of an alkene and formation of a stable ion. 

Trisubstituted cyclic alkenes have been kinetically resolved via a chiral dioxirane (4), generated in situ from the ketone and Oxone. A sequential desymmetrization and kinetic resolution of cyclohexa-1,4-dienes has also been achieved. The observed stereochemical results have been rationalized on the basis of a spiro-planar transition state model.93... 

K and in the solid state. The cyclic carbonyl ylide (77 R = Ph), obtained by irradiation of 2.3-diphenylnaphthoquinone 2,3-epoxide (78 R = Ph), has been trapped with suitable dipolarophiles. The carbonyl ylide (77 R = Me), derived from the 2,3-dimethyloxiran (78 R = Me) by irradiation in benzene, adds to allyl alcohol to give the hemiketal (79) by way of adduct 80. Other 2,3-dialkylnaphthoquinone 2,3-epoxides undergo similar photo-chemically induced carbon carbon bond cleavages to give dipolar or biradical species that have been trapped in turn with alkenes and ketones ... 

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