Ketones cyclic, photochemical reactions

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

The great similarity in the photochemical reactions in the cyclic ketones with five, six, four and seven carbon atoms has been noted by several workers (3,5,27). The only reaction that is common to all four is... 

The data that are currently available on the photochemistry of the substituted cyclic ketones pertain only to alkyl substituted five- and six-membered ring ketones. These indicate that the presence of one or more alkyl substituents does not lead to any new photochemical reactions. The general reactions 37-39 may be written as follows ... 

The photochemical reactions of three a/3-unsaturated cyclic ketones have been reported. In these cases, it appears that the reaction is that of the olefinic group although the wavelength of the incident radiation is at the ultraviolet absorption of the carbonyl group. Photolysis of 2-cyclo-pentenone (16), carvone (XXXV) (7,12) and y-tropolone methyl ether... 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

It is interesting to note that the development of photochemical reactions for the undergraduate laboratory has continued, and Haas 4 has reported one such experiment dealing with the photochemistry of cyclic ketones. 

Bamford CH, Norrish RGW (1935) Primary photochemical reactions. Part VI. The photochemical decomposition of certain cyclic ketones. J Chem Soc 455-459... 

The primary and fastest photochemical reaction of acyclic and some cyclic a, P-unsaturated ketones is cis-trans isomerisation. This isomerisation is not expected for cyclopent-2-enone or cyclohept-2-enone because of high strain energy, but it has been reported for cis-cyclooct-2-enone and cis-cyclohept-2-enone. It is thought to occur from triplet excited state. 

Mella, M., Freccero, M., and Albini, A., The photochemical reaction of benzene-1,2,4,5-tetracar-bonitrile with the acetals of cyclic and bicyclic ketones, /. Org. Chem., 59,1048, 1994. 

Photocycloaddition Reactions ofEnones. Cyclic a,(3-unsaturated ketones are another class of molecules that undergo photochemical cycloadditions.188 The reactive... 

Photochemical excitation of cyclic a-amino ketones (41 and 43) leads to the formation of bicyclic azetidines and azetidinols by abstraction of a hydrogen y to the ketone followed by cyclization. Production of (42) (72CC1108) and (44) (75TL2463) is believed to occur as a singlet state reaction. 

D. S. Weiss, in A. Padwa (ed.), Organic Photochemistry, vol. 5, Dekker (1981). This detailed review covers the photochemical alpha-cleavage reactions of cyclic ketones. 

Acyclic a,jS-unsaturated ketones do not provide much photochemistry, except for facile geometric isomerization about the double bond. Although some also isomerize to /3,y-unsaturated ketones, others, such as mesityl oxide, apparently undergo no net reaction upon absorption of irradiation. Their low reactivity might result from a rapid relaxation to an appreciably more twisted geometry in their triplet states than can be obtained in the cyclic enones. Below are some examples of typical photochemical behavior.426,427... 

Substitution of the acetate group at the C-3 position of the /3-sultam 105 can occur by reaction with silyl enol ethers in the presence of zinc iodide or zinc chloride. When the diazo compound is used, after desilylation with tetrabutyl-ammonium fluoride (TBAF), photochemical cyclization gives the bicyclic /3-sultam 106 as a mixture of two cis/ fra -diastereoisomers. When silyl enol ethers derived from cyclic ketones are used, the substitution product is stabilized by a retro-Michael-type reaction leading to open-chained sulfonamides 107 (Scheme 31) <1997LA1261>. 

Benzophenone (Amax = 340 nm, log e = 2.5, n-ir electronic transition) can be used as a photochemical reagent and eq. 4.25 shows a radical Michael-addition reaction with benzophenone. The formed benzophenone biradical (triplet state, Tx) abstracts an electron-rich a-hydrogen atom from methyl 3-hydroxypropanoate (62) to generate an electron-rich a-hydroxy carbon-centered radical [III], then its radical adds to the electron-deficient (3-carbon of a, (3-unsaturated cyclic ketone (63) through the radical Michael addition. The electrophilic oxygen-centered radical in the benzophenone biradical abstracts an electron-rich hydrogen atom from methyl 3-hydroxypropanoate (62) [70]. So, an a-hydroxy carbon-centered radical [III] is formed, and an electron-deficient a-methoxycarbonyl carbon-centered radical [III7] is not formed. 

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