Olefins from ketone photolysis

Aryl pinacols are readily obtained from the photolysis of substituted benzophenones in isopropyl alcohol. Photochemical coupling of benzophenones in isopropyl alcohol has even been reported to occur in direct sunlight.7 It is known however, that some aryl ketones do not give the pinacol from photolysis in isopropyl alcohol.8 Nevertheless, aryl pinacols have also been prepared from electrochemical reduction,9 hydroxylation of olefins,10 photoreduction of ketones by amines,11 metal reduction,12 and by other routes.13 The phenanthrene ring system has often been... 

A mixture of the oxetan (26) and olefin dimer is obtained from the photolysis of ajS-unsaturated nitriles and aliphatic ketones. Oxetan formation... 

A lower molecular weight methyl ketone and an olefin are isolated as products of this reaction. That the enol is formed as a primary product which rearranges to the ketone follows from its detection in the IR spectrum of gaseous 2-pentanone upon photolysis. 3 In addition to the ketone and olefinic products, one usually obtains varying amounts of cyclobutanols. 

One novel and interesting method of generating a silacarbonyl ylide occurred through the addition of a carbonyl species with a silylene formed under photolytic conditions. Komatsu and co-workers (177) found that photolysis of trisilane (315) in solution with a bulky carbonyl species led initially to the formation of a silacarbonyl ylide followed by a dipolar cycloaddition of an olefinic or carbonyl substrate. Reaction of simple, nonbulky aldehydes led to only moderate yields of cycloadduct, the siladioxolane. One lone ketone example was given, but the cycloadduct from the reaction was prepared in very low yield (Scheme 4.89). 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

The photochemistry of these aliphatic ketones is distinguished from that of other aliphatic ketones by the occurrence of an intramolecular primary process which gives an olefin and a methyl ketone. This process is classified as a Norrish Type II process to distinguish it from the primary process which leads to free radicals. It occurs in the photolysis of many high aliphatic ketones and a similar process also occurs in the photolysis of many aldehydes 101 and esters.4 Primary processes which give rise to free radicals also occur. 

The silicon-carbon double-bonded intermediates generated photo-chemically from a-alkenyldisilane derivatives react with both enolizable and nonenolizable ketones to give olefins (98). For instance, the photolysis of a-styrylpentamethyldisilane (49) in the presence of one molar equivalent of acetone gives l-trimethylsilyl-2-phenyl-3-methyl-2-butene in 13% yield as a single product. No silyl enol ether to be expected from the reaction of the intermediate with the enol form of acetone is observed. Similar irradiation of 49 with acetophenone affords (E)- and (Z)-l-trimeth-... 

An interesting study has been made of the photolysis of a nitro-olefinic sugar.148b Irradiation, at 253.7 nm, of an acetone solution of cis-6,7,8-trideoxy-l,2 3,4-di-0-isopropylidene-7-nitro-o-D-gflIocfo-oct-6-enose (135) gave a complex mixture from which there were isolated the trans isomer 136 and the cis and trans isomers of 6,8-dideoxy-l,2 3,4-di-0-isopropylidene-a-D-gaZacto-oct-5-enos-7-ulose (137 and 138). Insofar as the formation of the two ketones involves the loss of... 

Scheme 3 Olefin (29) prepared from dienone (26) by standard organic reactions was converted to bromohydrin derivative (30), which on subjection to photolysis and dehydrohalogenation yielded ketal (32), whose conversion to a,P unsaturated ketone (34) was accomplished by hydrolysis, bromination and dehydrobromination respectively. The cyanation of (34) followed by hydrolysis and esterification produced ester (35), which on Clemmensen reduction and oxidation afforded lactone (37). Reduction of (37) followed by oxidation and esterification gave ketoester (38), which was converted to (39).

It is well known that olefins may form complexes with some metals [80, 81]. Mercuric acetate can form complexes with polyolefins selectively, being removed by acid treatment [82]. Such complexes have been used in the separation of unsaturated fatty acids from mixtures [83]. Subbarao [84] has protected olefins aginst reduction using silver complexes, and Cope used reversible platinum complex formation [85, 86] for the resolution of medium ring trans olefins. Landesburg [87] used the iron tricarbonyl complex of the alcohol (13) to protect it during oxidation to the ketone (14) which was regenerated from the complex by photolysis [88a]. 

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