Carbon from ketone photolysis

As might be expected, a complex mixture of lactones corresponding to abstraction of a hydrogen atom from various sites along the methylene chain was obtained from the photolysis. The mixture of lactones was converted by dehydration, ozonolysis, and hydrolysis to a mixture of ketones. It was found that no functionalization occurs with ester side chains of less than nine carbon atoms. This is probably due to the inability of the carbonyl to approach any methylene closely enough to abstract a hydrogen. The data for side chains of nine carbons or greater is presented in Table 3.11. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

Early work on the gas-phase interaction of methylene with carbon-hydrogen bonds showed selective reactivity in the order, tertiary C-H > secondary C-H > primary C-H. Methylene from diazomethane was found to be less selective than methylene from ketone . The differences in the relative reactivity of methylene from the two sources have been attributed to excess translational energy of methylene from CH2N2 . Recently, however, it has been shown that the photolysis of ketene produces more triplet CH2 than the photolysis of diazomethane cf. preceeding section). Part of the reported selectivity must be attributed to triplet-methylene effects. 

The photolysis of ketones leads to reactions that we will call Norrish type 11 in Chapter 16. The reaction involves carbonyl excitation followed by hydrogen atom abstraction from a y-carbon. Imbedded carbon monoxide, in the form of an ester, placed between the ketone and the y-carbon is known to accelerate the decay of the resulting 1,4-biradical by allowing a fragmentation pathway. To ascertain the lifetime of the radical intermediates formed from the photolysis of the following a-keto ester, the incorporation of a radical clock was performed. Upon photoly-... 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

The nitrites aie most conveniently prepared from the corresponding alcohols by treatment with nitrosyl chloride in pyridine. The crude nitrites can be precipitated by addition of water and recrystallized from appropriate solvents. However nitrites prepared from carbinols in which the adjacent carbon is substituted by halogen, free or esterified hydroxyl or a carbonyl function are very readily hydrolyzed and must be recrystallized with great care. In general the photolysis gives higher yields if purified and dried nitrites are used which do not contain acids or pyridine, although occasionally the addition of small amounts of pyridine is recommended in order to prevent hydrolysis of the nitrite. Traces of acids do in fact catalyze the thermal decomposition of secondary nitrites to equimolar amounts of alcohol and ketone. ... 

Intramolecular hydrogen abstraction from a g carbon occurs when certain ketones are irradiated. For e.g., vapours of Hexan-2-one on photolysis gives an alkene and a ketone (via the enol form). It is referred to as Norrish type II cleavage. 

Metal acetylacetonates quench triplet species generated by flash photolysis of aromatic ketones and hydrocarbons.330-333 More recently, these reactions have been studied from a synthetic standpoint. Triplet state benzophenone sensitizes photoreduction of Cu(MeCOCHCOMe)2 by alcohols to give black, presumably polymeric, [Cu(MeCOCHCOMe)] . This reacts with Lewis bases to provide complexes of the type CuL2(MeCOCHCOMe) (L = bipyridyl/2, ethylenediamine/2, carbon monoxide, Ph3P). Disubstituted alkynes yield Cu(C2 R2 XMeCOCHCOMe) but terminal alkynes form CuQR acetylides.334 The bipyridyl complex of copper(I) acetylacetonate catalyzes the reduction of oxygen to water and the oxidation of primary and secondary alcohols to aldehydes and ketones.335... 

Photolysis of cyclohexanone gives rise to carbon monoxide, ethylene, propylene (1), cyclopentane, 1-pentene (3), and 5-hexenal (29). Cyclo-hexenyl cyclohexanone, water, and a polymer have also been reported as products, especially when the photolysis is conducted in the temperature range from 100°-300° in the presence of short wavelength radiation (3). At 3130 A. and over the temperature range of 100°-300° the ketone that is decomposed is almost fully accounted for in the products (5) and the stoichiometry of the products fits the eq. 15-18 (3,27) ... 

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. 

Recent studies of the photolysis of bicyclic ketones have led to the preparation of unusually strained systems by the eUmination of carbon monoxide from excited ketone molecules. Irradiation of Formula 286 gives bicydo [2.2.0[hexane (Formula 287) in low yield (118). Mercury-photosensitized decomposition of bicyclo [2.2.1 ]heptan-2-one (Formula 258) gives bicyclo [2.2.1 [hexane (Formula 289) in 20% yield (119). Camphor (Formula 290) undergoes a similar mercury photosensitized decomposition to 1,5,5-trimethylbicyclo [2.2.1 [hexane (Formula 291) (10%) (119). 

Carbon-bromine cleavage from upper triplet states has also been observed by two-color photolysis of 55 and for the ketones 83-85 [69]. In the latter cases cleavage is followed by a neophyl-type rearrangement and decarboxylation yielding the arylmethyl radical (Scheme 4). 

Oxadiazoline (59), on irradiation in benzene, forms an equilibrium mixture with its synthetic precursors diphenylketene and diphenyldiazomethane. Ring opening occurs on irradiation in the presence of alcohols (66AHC(7)183). Photolysis of oxadiazolinones (60a) proceeds similarly to thermolysis and a ketone R COR2 is produced concurrently with loss of nitrogen and carbon dioxide. In the presence of cyclohexene the major product is the hydrazone (63) which is believed to arise from a bimolecular photoreaction of the alkene with the oxadiazolinone. Similar photoreactions occur with 2-phenylpropene and diethyl ether (78S535). 

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-... 

Carbon-oxygen bond heterolysis is responsible for the observed photolyses of 9-aryl-9-xanthenols ° and llH-benzo[b]fluoren-11-ol. - - Evidence for the formation of the 9-fluorenol radical cation as well as the 9-fluorenyl cation has been obtained from a laser flash photolysis study of 9-fluorenol. 1, l-Di-2-thienylethanol undergoes light-induced dehydration to give i,i-di-2-thienylethylene.Single electron transfer pathways, however, are implicated in the ring cleavage reactions of a,j8-epoxy ketones in the presence of allyltributyltin - or alkylamines. 

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