Cyclohexenone, photochemistry

The effect of an a-silyl group attached to tertiary amine on 2-cyclohexenone photochemistry has been clarified by Mariano s group42,43, who illustrates the importance of... 

Substituent effects on the -(aminoethyl)cyclohexenone photochemistry were carried out to study the relative kinetic acidities of the tertiary aminium radical47. The ease of the methylene hydrogen to be removed as H+ increased in the order of X = alkyl < Si(CH3)3 < C=CH (equation 13). 

Acetophenone sensitization and cyclohexadiene quenching were held to demonstrate (not very conclusively) that the unsensitized process involves triplet intermediates, as in Scheme 7. A similarity to cyclohexenone photochemistry... 

The conversion of the enone (83) into the products shown in Scheme 8 upon irradiation in t-butyl alcohol is quite inefficient ( = 0.017).46 This study was aimed to solve one of the problems which has bedevilled the study of cyclohexenone photochemistry, namely that of the concertedness or non-concertedness of the rearrangement to the lumiketone products. When optically active starting material (83) was used all the products were optically active. The formation of... 

SCHEME 6 Differences in cyclohexenone photochemistry in solution and the solid state. 

Photochemical transformations of conjugated cyclohexenones, 317 Photochemical transformations of non-conjugatea ketones, 292 Photochemistry of cyclobutanones, 293 Photolysis of nitrites, 253... 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

The photo-[4+2] cycloaddition of furan with Pummerer s ketone 102 [70,71] gives evidence for the intermediacy of the highly twisted enone intermediate 103, and a biradical cycloaddition pathway (Sch. 23). The structures of the endo and exo products 104 were confirmed by X-ray crystallography [72,73]. In a related comparison of cyclohexenone and cyclopentenone photochemistry, conditions that gave [4+2] adducts for the cyclohexenone produced only [2+2] adducts from cyclopentenone [74]. 

Studies of the low-temperature photochemistry of umbellone (288) established a dual pathway for its conversion to thymol (291). Besides the opening of (288) to ketene (289), its direct conversion to the cyclohexadienone (290) was also postulated. However, this reaction depends on the substitution pattern, and in the case of lumisantonin (292) only traces of cyclohexenone (294) were detected in the photolysis... 

Zimmerman, H. E., Kutateladze, A. G., Novel Dissection analysis of Spin Orbit Coupling in the Type B Cyclohexenone Photorearrangement What Controls Photoreactivity Mechanistic and Exploratory Organic Photochemistry, J. Org. Chem. 1995, 60, 6008 6009. 

Enones also undergo cycloadditions to allene under conditions of surface photochemistry using silica gel as a medium. Several examples have been reported, among them is the photocycloaddition of cyclohexenone to allene63. 

In the dimorph photochemistry of 4-p-cyanophenyl-4-phenyl-cyclohexenone, there was the observation of a discontinuous change in stereoselectivity at 16% conversion. This led us to consider whether the phenomenon of reactions occurring in stages might be general. Before this, it had been common to assume that at some point, in a solid-state reaction, the crystal structure would be destroyed, and the reaction would be that of a heterogeneous solid or a liquid and thus be of no use. 

Yoon, U.C., Kim, J.U., Hasegawa, E., and Mariano, P.S., Electron-transfer photochemistry of a-silylamine-cyclohexenone systems. Medium effects on reaction pathways followed, J. Am. Chem. Soc., 109,4421, 1987. 

Su, Z., Mariano, P.S., Falvey, D.E., Yoon, U.C,. and Oh, S.W, Dynamics of anihnium radical a-heterolytic fragmentation processes. Electrofugal group, substituent and medium effects on desi-lylation, decarboxylation and retro-Aldol cleavage pathways, /. Am. Chem. Soc., 120, 10676,1998. Hasegawa, E., Xu, W, Mariano, P.S., Yoon, U.C., and Kim, J.U., Electron transfer induced photoadditions of the silylamine EtjNCHjTMS to a,P-unsaturated cyclohexenones. Dual reaction pathways based on ion pair selective cation radical chemistry, /. Am. Chem. Soc., 110, 8099, 1988. Yoon, U.C., Kim, J.U., Hasegawa, E., and Mariano, P.S., Electron transfer photochemistry of a-silylamine cyclohexenone systems. Medium effects on reaction pathways followed, /. Am. Chem. Soc., 109, 4421,1987. 

---