Photoaddition anti-Markovnikov

In the photoaddition of 2-pyrrolidone the 5-alkyl isomer (69) always predominates, usually in a ratio of 2 1. The formation of anti-Markovnikov 1 1 adducts, telomers, and dehydrodimers of structure (71) supports a free radical mechanism. Similarly, formamide undergoes olefin addition under... 

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

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... 

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 anti-Markovnikov photochemical addition of malononitrile onto styrenes in the presence of lithium carbonate and a cyanoarene has been reported. In this case, excited 9-cyanophenanthrene (9-CP) oxidizes an olefin by PET, and the resultant radical cation adds to the malononitrile anion. The resulting radical is then reduced by 9-CP - and protonated to afford the end products [29]. This represents one of the rare examples involving the photoaddition of a carbanion to an olefin, and is explained by the PET-induced Umpolung of the latter, which thus becomes electrophilic. 

The photoaddition of secondary phosphines onto alkenes is one of the straightforward approaches for the synthesis of organophosphorus compounds. An anti-Markovnikov phosphorus-centered free radical addition onto the olefin has been... 

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. 

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]. 

Another approach is to develop synthetic strategies for attaching carbohydrate residues to different parts of cyclodextrin molecules. Photoaddition of thiols to allyl ethers in an anti-Markovnikov fashion is used as a key step in the multiple attachment of carbohydrates to cyclodextrin cores (164). 

For a review, see Wan, P. and Yates, K., Photogenerated carbonium ions and vinyl cations in aqueous solution. Rev. Chem. Intermed., 5, 157, 1984 See also McEwen, J. and Yates, K., Photohydration of styrenes and phenylacetylenes. General acid catalysis and Bronsted relationships, /. Am. Chem. Soc., 109, 5800, 1987 3-Nitrostyrenes undergo anti-Markovnikov addition Wan, R, Davis, M.J. and Teo, M.-A., Photoaddition of water and alcohols to 3-nitrostyrenes. Structure-reactivity and solvent effects, /. Org. Chem., 54,1354,1989. 

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