Alkene alcohol photoaddition

Describe the photoaddition of water and alcohols with alkenes. 

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. 

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). 

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. 

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. 

The intramolecular version of photoaddition of alcohols to alkenes was also reported. Mizuno and Otsuji reported the DCA-sensitized photocyclization of co-hydroxy-l,l-diphenylalkenes (n = 3-5) via exciplex in benzene [38]. The long chain cu-hydroxyalkenes (n = 6,10) do not give the corresponding cycHzed ethers. Gassman reported the photocyclization of unsaturated carboxylic acids by the use of a sensitizer system consisting of sterically hindered electron acceptors and biphenyl to give y-lactones (Scheme 10) [39]. 

Arnold reported that the photoaddition of methanol and trifluoroethanol to 1,1-diphenylethene occurs in the photoreaction using methoxynaphthalenes as an electron donating sensitizer [35, 113]. The proposed mechanism for this reaction is shown in Scheme 34. The radical anion of the alkene is first produced by photoinduced electron transfer from the electron donating sensitizer to the alkene and it is protonated in a Markownikoff fashion to form the 1,1-diphenylethyl radical. The resulting radical is then oxidized by the radical cation of the electron donating sensitizer to generate the cation of the alkene. Finally, a nucleophilic attack of alcohol on this cation affords the alkoxylated product. 

The photoaddition of phthalimides and related dicarbox-imides to alkenes has been the subject of detailed study over the years. In the presence of added methanol, the photoreactions of dicarboximides with alkenes also result in the formation of methanol-incorporated products. The efficiency of alcohol trapping by the N-methylphthalimide-alkene radical ion pair, which is involved in... 

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]. 

Some interesting photo-NOCAS-type reaction (photochemical nucleophile-olefin combination, aromatic substitution) have been reported by three groups. Arnold has developed the photo-NOCAS reaction as three components photoaddition.Xu et al., reported the intramolecular photocyclization of A-(co-hydroxyalkyl)-tetrachlorophthalimide (138, 141) with alkenes to give medium- and large-ring heterocycles (140,143). These photoreactions proceeded via 1, n-biradicals generated from the nucleophilic attack of alcohols to alkenes between the radical anions of phthalimides and the radical cations of alkenes. 

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