Photoaddition polar

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... 

Acid-catalyzed photohydration of styrenes19 and additions to cyclohexenes20 leading exclusively to the Markovnikov products are also possible. Sensitized photoaddition, in contrast, results in products from anti-Markovnikov addition. The process is a photoinduced electron transfer21 taking place usually in polar solvents.22,23 Enantiodifferentiating addition in nonpolar solvents has been reported.24 The addition of MeOH could be carried out in a stereoselective manner to achieve solvent-dependent product distribution 25... 

Micellar Catalysis. Non-polar molecules such as aromatic hydrocarbons are practically insoluble in water. In a micellar suspension they concentrate in the non-polar interior of the micelles where they can reach relatively high concentrations, even though their overall concentration may be very low. The quantum yields of bimolecular reactions like photoadditions are therefore greatly increased in micellar suspensions. 

High regioselectivity was found as well in the intermolecular photoaddition of cyclo-hexenone 78 to alkenes 79. The regioselectivity was examined by several research groups63 and found to be affected by the solvent polarity, the temperature and steric effects55 (Scheme 18). 

Houck and coworkers postulate that the origin of the regioselectivity is at the biradicalforming step and directly affected by the polarity of the alkene. The /J-carbon, considered as nucleophilic, adds rapidly to the less substituted side of the electron-deficient alkene, whereas a position considered as an a-acyl radical (more electrophilic than an alkyl radical) adds rapidly to the less substituted side of electron-rich alkenes. The calculated relative energies for the addition of jtjt triplet acrolein to different substituted alkenes at the first bond-forming step (Table 3) are found to be in good agreement with experimental values determined in the photoaddition of cyclohexenone to the related alkene. 

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. 

As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (Sj -> Tj) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. 

The observed regioselectivity can be perturbed to varying degrees by choice of reaction parameters. Solvent polarity can play a role in the control of regioselectivity, as would be predicted by the polar exci-plex model. The regioselectivity of the dimerization of cyclopentenone (equation 11) produces a larger proportion of the head-to-head adduct in more polar solvents. The photoaddition of enone (11) to al-kene (12) also displays a pronounced solvent dependence (equation 12). A consequence of the solvent effect is that nonpolar solvents tend to produce products which would be predicted from the polar exci-plex model, while more polar solvents result in somewhat more of the minor product but do not cause complete reversal of the regioselectivity. 

Organized media have also been used to influence the regiochemical outcome in the reactions. Photoaddition of 3-n-butylcyclopentenone with various terminal alkenes has shown a pronounced preference for the alignment of the enone and the alkenes with their polar groups toward the surface of the micelle. This effect is most pronounced in the case of 1-acetoxy-I-heptene, which gives exclusively the head-to-tail adduct in cyclohexane solvent but a 2.3 1 mixture in favor of the head-to-head isomer in the presence of potassium dodecyl sulfate (equation 13). 

A wide variety of photoadditions to unsaturated oxygen and sulfur heterocycles has been reported. It has, however, proved difficult to classify these processes, especially as the reaction mechanisms are not fully understood in all cases. Most additions of solvent to oxygen heterocycles arise via hydrogen abstraction pathwyas, often initiated by added ketone. Polar addition is relatively rare in these compounds the addition of methanol to... 

Enantiodifferentiating anti-Markovnikov polar photoadditions of alcohols to 1,1-diphenyl-l-alkenes 107 and 108 sensitized by optically active naphthalene(di)car-boxylates 41-43, 71, 72, and 91 were investigated in detail (Scheme 19) [70], Since this photoaddition involves the attack of alcohol to a radical cationic species of the substrate alkene [71], the use of polar solvents is desirable for obtaining the adduct in a high yield. However, in polar solvents, the radical ionic sensitizer-substrate pair produced upon photoexcitation is immediately dissociated by solvation, and no chirality transfer is expected to occur. Thus the optical and chemical yields are often conflicting issues, and therefore the critical control of solvent polarity is essential for obtaining the optically active product with an appreciable ee in reasonable chemical yield. In fact, the initial attempts on 107, employing naphthalenecarboxylate sensitizers with chiral terpenoid auxiliaries (a-c and f) and a pentane solvent afforded a best ee of 27% for adduct 110 (R = Me), but in < 2% yield [70a]. 

Figure 8 Microenvironmental polarity control upon enantiodifferentiating polar photoaddition of alcohol (ROH) to aromatic olefin (D) sensitized by naphthalenedicarboxylate with saccharide auxiliaries (A ) the local polarity is enhanced around the saccharide moieties, facilitating electron transfer from exited sensitizer (A ) to substrate olefin (D) to produce a radical cation (D -h). The radical cation produced cannot escape from the high polarity region around the saccharide to the low-polarity bulk solution and is accordingly attacked by ROH in the chiral environment of saccharide to produce the adduct in high ee.

The quantum yield for photodimerization of thianaphthene 1,1-dioxide is enhanced in the presence of bromoethane.188 The increase is not due to solvent polarity it does, in fact, appear to be the result of enhanced intersystem crossing to the reactive triplet. Cycloaddition reactions of the thione group have also attracted attention, particularly those occurring in pyrimidine thiones. The isolation of thietans, previously proposed as intermediates in the photoaddition of electron-deficient alkenes to thiouracil derivatives, has now been realized 187 in this way, for example, the dihydrouracil (259) is converted into the adduct... 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

Photoinduced reactions of cyclic a-diketones with different alkenes takes place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. Photoaddition of electron deficient silyl ketene acetals to 2-, 3- and 4-acetylpyridine generates oxetanes as major products. The reaction is favoured in non polar solvents. The photoreaction between silyl enol ethers and henzil affords [2 + 2] cycloaddition products, while in the case of 9,10-phenanthrenequinone [4 + 2] cycloacidition predominates. Photocycloaddition of p-henzoquinones to hicyclopropylidene affords spirooxetanes (21) as the primacy products further irradiation leads to rearranged spiro[4.5]deca-6,9-diene-2,8-diones. With 9,10-anthraqui-none, in addition to the spirooxetane, a spiro[indan-l,l -phthalan]-3 -one is also obtained. ... 

Two separate studies on photocyclization of allenes include examples of how Lewis acids and the polarity of the solvent affect the regioselectivity of the addition of activated olefins and 1,1-dicyclopropylallene, and provide details of the intramolecular photoaddition of allenes to a,p-cyclohexenones. Copper(i) trifluoromethanesulphonate has been found to catalyse the intramolecular photocyclization of several monocyclic (3- and Y"(pent-4-enyl)allyl alcohols for instance, the allyl alcohol (19) forms the tricyclic hydrocarbon (20), which is an... 

Arnold, D. R., Wong, A. J., and Cameron, T. S., Radical ions in photochemistry. 12. The photoaddition of olefins to cyano aromatic compounds in polar solvents, PureAppl. Chem., 52,2609,1980. 

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