Alcohols 1-6 photoaddition

Ultraviolet irradiation of CCXLV, which is obtained by the condensation of diphenylketene and diphenyldiazomethane, in hydroxylic solvents, effects its cleavage into the starting materials in the presence of carboxylic acids or alcohols, photoaddition takes place with the formation of benzilic acid derivatives.167 On the other hand, irradiation of CCXLV in benzene containing acetic acid and/or methanol produces CCXLVI and CCXLVII, respectively. Irradiation in the presence of diphenylmethane under nitrogen atmosphere yields phenylacetic acid. 

Flegel, M. Lukeman, M. Huck, L. Wan, P. Photoaddition of water and alcohols to the anthracene moiety of 9-(2 -hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer. J. Am. Chem. Soc. 2004, 126, 7890-7897. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

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

Numerous examples of the photoaddition of water, alcohols, and carboxylic acids to multiple carbon-carbon bonds have been reported. The photoaddition of water to pyrimidine derivatives is of probable biological significance (see Chapter 12) ... 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Many other miscellaneous additions of alcohols have been described. Polar addition of methanol to 2//-pyrroles264 and to azepines265 has been observed. Photoaddition of methanol to the 1,3,4-oxadiazole 320 is followed by cycloelimination of methyl benzoate (321) to give the ylid 322.266 An adduct (323) of the ylid and the 1,3,4-oxadiazole has been isolated. Photoaddition of methanol to the quaternary ammonium salt 324 results in ring expansion and the formation of the azonine 325.267... 

Describe the photoaddition of water and alcohols with alkenes. 

Nitramines are known to photodissociate from their jt,jt state to give aminyl and nitric oxide radicals in the presence of an acid the aminyl radicals are protonated to give aminium radicals, which can initiate addition to olefins. As a synthetic reaction, photolysis of nitramines in the presence of acids can be conveniently run under oxygen to give oxidative addition similar to those shown in equation 145 indeed TV-nitrodimethylamine is photolysed with triene 299 under such conditions to give a mixture of 301 and 302, similar to results observed in the oxidative nitrosamine photoaddition169. To simplify the isolation, the crude products are reduced with LAH to form the open-chain amino alcohol 303. Some other oxidative photoadditions of N-nitro dimethylamine to other olefins are reported. As the photoreaction has to use a Corex filter and product yields are no better than those shown by nitrosamines, further investigations were scarcely carried out. 

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

Although the excited states of alkynes are known to be attacked very rapidly by proton acids, the products obtained by photoaddition of alcohols (2.9B) often arise by a radical mechanism that results in hydrogen abstraction from the position adjacent to the alcohol hydroxyl group, rather than by an ionic mechanism, which would result in abstraction of the —OH proton. 

The overall course of reaction depends on the relative rate constants for the various secondary radical processes. Aliphatic ketones are often photoreduced to secondary alcohols (4.121, but although there are interesting features in the stereochemistry of the reduction, the method is not a worthwhile alternative to thermal reduction using hydride reagents, except in cases where the substrate is sensitive to basic conditions. Photoaddition of methanol is promoted in the presence of titaniurnfiv) chloride, both for acyclic and cyclic (4.33) ketones the titanium involvement probably starts in the early steps of the reaction, but the detailed mechanism is not known. Addition may also be a major pathway when cyclohexene is used as hydrogen source (4.341 unlike many other simple alkenes, cydohexene does not readily give oxetanes by photocycloaddition (see p. 126). 

Imines can be photoreduced by hydrogen donors such as propan-2-ol, but in some cases it is clear that a small amount of carbonyl compound (arising from hydrolysis of the imine) is necessary to initiate the process. For example, N-alkytirnines of benzaldehyde give elhane-1.2-diamine derivatives on irradiation in 95% ethanol (5.6). but they are inert when irradiated in a perfectly dry alcohol. The C=N chromophore is capable of abstracting a hydrogen atom, and both the photoreduclion of a tetrahydropyridine (5.7) and the photoaddition of p-xylene to an isoxazoline (5.6) occur by such a direct reaction. 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

Photochemical reactions of carbohydrates have been discussed.279 Therefore, only examples applicable to the synthesis of branched sugars are briefly described here. Although non-stereoselectivity and low yield are general defects of photoreactions, photoaddition of an alcohol to the enones 125 and 122a proceeded with the same stereo- and regio-selec-tivities as ionic addition ( see Section II,4,c), and in rather better yields,... 

Photoadditions of water and primary alcohols to unsaturated bonds have been reported, but insufficient compounds have been studied to allow any generalizations to be made. 

The first step in all these photoadditions must be hydrogen abstraction from the alcohol by the excited molecule this is then followed either by radical recombination to yield the substituted alcohol or further hydrogen abstraction to give the dihydro compound. 

The photoaddition of alcohols and phenols to alkenes has been observed.404 The equivalent intramolecular process results in cyclization and the formation of oxygen heterocycles. Irradiation of 2-allyl-4-(-butylphenol (388) affords 2,3-dihydro-2-methyl-5-<-butyl-benzofuran (389), whereas o-3-methylbut-2-enylphenol (390) gives 2,2-dimethylchroman (391). In both cases, therefore, the addition can be said to occur in a Markovnikov direction. 

In contrast to the ground-state hydration, the photoaddition of water and of several alcohols to the triplet excited states of m-nitrostyrenes affords the corresponding anti-Markovnikov products58. 

The photosensitized 1-4 addition of alcohols to hexenopyranosuloses first reported by B. Fraser Reid and coworkers [58 a] has been developed with other studies on photoadditions of oxycarbinyl species such as polyols, acetals, dioxolanes, aldehydes. A mechanistic study on this photoaddition has been recently detailed [58 b] showing that the important photochemical event is hydrogen abstraction from methanol, for example, to form the hydroxymethyl radical. 

Since alcohols are less effective as hydrogen donors than amines, a PET photoaddition can occur only when the oxidized component of the reaction is the alkene. Furthermore, if the photosensitizer is chiral, the polar addition would occur in an enantiodifferentiating manner to some degree. Thus, the photoaddition of 2-propanol to 1,1-diphenylpropene, when sensitized by chiral naphthalene(di)carbox-ylates, formed the anti-Markovnikov photoadduct with enantiomeric excesses of up to 58% [53]. Unfortunately, the reaction is far from attracting synthetic interest as the yields are still too low. 

Wan, P Davis, M.J., and Teo, M.A. (1989) Photoaddition of water and alcohols to 3-nitrostyrenes. Structure-reactivity and solvent effects. Journal of Organic Chemistry, 54, 1354-1359. 

This procedure provides a method for functionalizing the pyrrole ring 1n the 3-pos1t1on, normally a difficult synthetic step when conventional electrophilic substitution is used. The technique has been extended to addition of several aldehydes and acetone and to a number of pyrroles.4 The generality Includes photoaddition to imidazoles which are substituted in the 4-posltion. Pyrrole photoadduct alcohols are readily dehydrated to 3-alkenylpyrroles or oxidized to 3-acyl derivatives. 

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