Photoaddition reaction

In this section the photoaddition of water, alcohols, carboxylic acids, and miscellaneous other small molecules to various substrates will be considered. 

Comparatively little was previously known about photoaddition reactions to unsaturated substrates of different structure. Elements of water add to the A5 6 in the pyrimidine ring of 1,3-dimethyluracil (CXXXYII)301 and probably of nucleosides and nucleotides of cytosine261,307 upon irradiation with ultraviolet light to give products such as CXXXVIII. CXXXVIII reverts to CXXXVII under the influence of acid, alkali, or heat. 

Alkyl amines add photochemically to olefins a condensation occurs between the a-carbon atom of the amine and the terminal carbon atom of 1-olefins by what is probably a free-radical, chain reaction.291 Internally illumination of solutions, containing piperidine and octene-1, with a mercury discharge tube led to the formation of 2-n-oetylpiperidine (CXLV). 

The photochemical action of nitrobenzene on unsaturated compounds in sunlight is a field already investigated by Vechiotti and Zanetti.295 Recently, Buchi and Ayer32 have evaluated the reaction of nitrobenzene with olefins as a possible synthetic route to the heterocyclic class of compounds, 1,3,2-dioxazolidines (CXLVI). Photoexcited 

The photoaddition of aldehydes to o-quinones is a general reaction leading to the formation of products which are to be regarded as derivatives of methylene ethers of o-dihydroxy compounds (A).198 194 247 Such compounds exhibit ring-chain tautomerism and are generally assigned open-chain structures (B).188 242 

Although the mechanism of the reaction is unknown, Moore and Waters188 have shown that addition of a carbonyl radical to a carbonyl double bond occurs when benzaldehyde is irradiated in the presence of phenanthraquinone, yielding, ultimately, the hydroquinone monobenzoate. 

Ground-state alkenes generally undergo electrophilic addition with alcohols in the presence of a Bronsted acid catalyst, yielding the Markovnikov product  

The excited states of alkenes can undergo sensitised addition reactions  

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Azepine, 3,5-bis(t-butyl)-l-ethoxycarbonyl-photoaddition reactions, with oxygen, 7, 523... 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

The photoaddition reaction of 2-phenylbenzimidazole with Michael acceptors was investigated <96JHC1031> as was the preparation and cycloaddition-reactivity of benzimidazole-2-carbonitrile oxide <96AJC199>. The nitration of 1-methylbenzimidazole was found to give only the 5- and 6-nitrated products as a mixture of isomers in 87% yield... 

Thus it can be seen that the heavy-atom effect, so useful in spectroscopy, can also be an important tool in photochemistry, not only to facilitate the study of a reaction mechanism, but also to control the major reaction product. This general system and other related 2w + lit photoaddition reactions will be considered in more detail in Chapter 10. 

An elegant example of the use of photochemistry in complex organic synthesis is the preparation the bollweevil phenomone (sex attractant). The key step in both the Zoecon Corporation synthesis and the USDA synthesis involves the formation of a cyclobutane ring by a photoaddition reaction ... 

Thymine also undergoes a photoaddition reaction with water to form the anti-Markovnikov product ... 

The efficient photoaddition reactions between pyrrolinium perchlorate (133) and ben-zyltrimethylsilane (equation 67) or the allylsilane (equation 68) are examples of the initial electron transfer induced desilylation processes. 

The iminium salt photoaddition reaction has been applied to the synthesis of N-heterocyclic systems (equation 70)126,127. 

Dimethyl-5-methoxyangelicin 11 and 4,4 -dimethyl-5-methoxypsoralen 12 were added separately to 12 synthetic oligonucleotides, and their interaction and subsequent photoaddition reaction on UVA irradiation was studied <2000PCB4992>. The kinetics constant of the overall photoaddition process was determined accurately, the different intercalation sites were identified, and the value of the kinetics constant relative to each type of site was determined. 

An example of the equivalent (photoaddition) reaction following hetero-molecular photoassociation is provided by the photochemical addition of maleic anhydride to anthracene." Livingston and coworkers100 have shown that the anthracene triplet state is not involved in this reaction and that, in terms of Eq. (47) in the appropriate form, q%. = 0.03. However, if the excited complex XMQ formed directly by light absorption in the charge-transfer band is the reactive intermediate, this produces the adduct with a computed efficiency of 347 . 

Photoaddition reactions can destroy the original sensitizer, form a new sensitizer, or form a substance that quenches the desired reaction completely. Since substrate would also be consumed, its measured rate of disappearance will be anomalously high and that of product appearance too low. The importance of ensuring that the sensitizer acts as a true catalyst in a photo-reaction is demonstrated by a study88 in which 1,2-benzanthracene (13) was used to sensitize the dimerization of cyclohexadiene. The quantum yield of dimer formation was found to decrease at higher sensitizer concentration ... 

The most widely used method for synthesizing bicyclic azetidines is by photoaddition reactions, primarily of the [2 + 2] type. Scheme 3 lists several examples of this method (37) (81JA3136), (38) (75JA7288), (39) (77TL431), (40) (74JHC453). Cycloadditions to carbon-nitrogen double bonds have been reviewed (78MI51200). 

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

The reactive state of anthracene involved in the photoaddition reaction between anthracene and CC14 has been a point of controversy. An attempt has been made to establish the reactive state by populating anthracene triplet state by triplet-triplet energy transfer process ... 

Perhaps the best-known photoaddition reaction of benzene is that with chlorine to produce hexachlorocyclohexane (3.37). of which one steroisomer is widely used as an active component in insecticides. However, this reaction does not involve the excited state of benzene chlorine absorbs light and cleaves homolytically to give chlorine atoms, which then attack the ground state of benzene, leading to overall addition. 

Photoreduction and Photoaddition Reactions of Heterocyclic Compounds , D. G. Whitten, in Photochemistry of Heterocyclic Compounds , ed. O. Buchardt, vol. 4 of General Heterocyclic Chemistry Series , Wiley, New York, 1976, pp. 524-573. 

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