Carbonyl groups photolysis

Studies have also been conducted on compounds containing a thio-carbonyl group. Photolysis of dimethylthiocarbamates produces128,129 mixtures of deoxy sugars and alcohols (see Table XVI). As the alcohols can be reconverted into dimethylthiocarbamates, and these re-irradiated, the synthesis and irradiation of these compounds offers a useful pathway to deoxy sugars. A simple mechanism for this reaction, based on the two possible a-cleavages resulting from thiocarbonyl excitation, is shown in Scheme 23. In addition, the possibility has been... 

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... 

Figure C2.3.9. Product distribution of dissymmetrical ketone photolysis as influenced by cefyltrimethylammonium chloride (CTAC) micelles. The initial ketone, A(CO)B is photolysed to lose the carbonyl group and to produce tliree products, AA, AB and BB. These data are for benzyl (A) 4-methylbenzyl (B) ketone. Product AA is 1,2-diphenylethane, product BB is 1,2-ditolylethane and product AB is l-phenyl-2-tolyl-ethane. At low CTAC concentration, in the absence of micelles, a random distribution of products is obtained. In the presence of micelles, however, the AB product is heavily favoured. Adapted with pennission from 1571.

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. 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

In this case, the grafting reactions start with the photolysis of carbonyl groups. 

In the case of compound 4 photolysis with a medium pressure mercury lamp results in exclusive cleavage of the C—Sn bond fi to the carbonyl group, while the terminal C—Sn bond remains intact. The cyclic ketone 5 and the a, /Lunsaturated ketone 6 are produced in equal if low yields (18%)19. The reduced reactivity of remote C—Sn bonds in such compounds appears to be a general feature of their chemistry, and is also observed in acyclic systems. Thus the photochemistry of organotin compounds with carbonyl groups in the /3-position can be classified as follows ... 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Protection of the carbonyl group of an orf/zo-acyolxyacetophenone can also be achieved by synthesizing its enol acetate again, the PFR is more efficient than in the unprotected molecule. After photolysis of the phenyl ester, intramolecular transacetylation takes place, leading to the corresponding acetoxyphenones (Scheme 73) [206]. 

Kirmse and Horner160 have shown that the quantum yield, in photolysis with monochromatic light, of C6H5COCHN2 is lowered considerably by the presence of additional double bonds in conjugation with the carbonyl groups and by para- and mearoyl groups attached to the carbon atom of the diazo group. 

The photolysis of aldehydes and ketones has been the subject of many investigations, and is adequately dealt with in standard texts on photochemistry. In a few instances, excitation of the carbonyl function can result in cyclization and the formation of heterocyclic systems, although this is usually accompanied by other processes more characteristic of the carbonyl group. 

A final example of an n-n reaction will be cited to illustrate still another type of reactivity of the n—r excited state. This is the photolysis of frans-dypnorie oxide (XXXII) to afford as the major product the unsaturated alcohol XXXIV (22). In this case the mechanism, as outlined in Chart VI, involves hydrogen abstraction from the methyl group by the m-situated excited carbonyl group the odd electron containing... 

Since type II reactions require the presence of hydrogens y to the carbonyl group, it follows that alternating QH4—CO copolymers which lack these hydrogens will be substantially more stable towards photolysis at room temperature. 

We have also confirmed our previous results on the effect of methoxylation of phenolic hydroxyl groups within lignin. On the basis of our model studies, we suggest that the most likely explanation for this is that triplet carbonyl groups are quenched statically by hydroxyl groups within the lignin structure on timescales less than 20 ns thus reducing the amount of transient detected in our laser photolysis experiments. 

As already mentioned, the element-centered radicals formed through the photolysis of organoelement a-ketones are capable of attacking the most electronegative carbonyl group... 

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