Photoaddition nucleophilic

The photoadditions proceed through 1,4-diradical intermediates. Trapping experiments with hydrogen atom donors indicate that the initial bond formation can take place at either the a- or (3-carbon of the enone. The excited enone has its highest nucleophilic character at the (3-carbon. The initial bond formation occurs at the (3-carbon for electron-poor alkenes but at the a-carbon for electron-rich alkenes.191 Selectivity is low for alkenes without strong donor or acceptor substituents.192 The final product ratio also reflects the rate and efficiency of ring closure relative to fragmentation of the biradical.193... 

Primary and secondary amines can add to Cjq as nucleophiles (Section 3.3). Tertiary amines can not form similar addition products, rather an electron transfer imder formation of zwitterions is often observed (Section 3.3). However, a photochemical reaction of tertiary amines with Cjq is possible and leads to complex mixtures of addition products [52-62]. The product distribution strongly depends on conditions such as temperature and the presence of either light or oxygen. If oxygen is thoroughly excluded, 9 is the major product (Figrue 6.8) in the photoaddition of triethylamine [56, 59]. It can be isolated in low yields. 

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. 

The photoaddition to aromatic ring is simply classified in Table 13. Direct attack of nucleophiles such as cyanide anion and carbanion to aromatic rings in the excited species has been recognized by SRN2 reactions [Eq. (4)], which were discussed in the past three decades [27,28], We will discuss briefly some examples in this section, including intramolecular photocyclization such as photo-Smiles rearrangement. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

It is widely believed that enone-alkene photoadditions proceed through an exciplex (excited complex). For cyclopentenone and 7, the exciplex forms from the photoexcited enone in its triplet state and the glycal in its ground-state. The regiochemistry of addition probably reflects a preferred alignment of the addends in the exciplex. Because a photoexcited enone probably has considerable diradical character, it is reasonable to assume that the more electrophilic a-keto radical would prefer to bind to the more nucleophilic portion of the alkene, since this would maximise attractive interactions within the complex. The reaction of 7 with cyclopentenone would thus favour the formation of diradical 22, which would then ring-close to cyclobutane 14 (Scheme 6.7). 

One of the most useful and widely used applications in the synthesis of natural product derivatives relies on the efficient photoaddition of RS-H onto a double bond (a reaction known as thiol-ene coupling) [55], The reaction exploits the weakness of the S—H bond that can be cleaved homolytically under irradiation (atca. 254nm). The electrophilic sulfur-centered radical attacks a nucleophilic double bond, thus starting a radical chain reaction. 

Photoaddition of nucleophiles to heterocycles is often observed.1 Irradiation of acridine and its quaternary salt in ethanol produced 9,9 -bisacridan (151) as the major product besides a little acridan and 9a-hydroxyethylacridan.207 The reaction mechanism is most likely a hydrogen abstraction from the alcohol by the excited molecule, followed by competitive radical combination to yield 151. Irradiation of an aqueous solution of alloxan monohydrate and its derivatives produces an alloxantin-type dimer (152 R = H, Me, Et) by combination (at the 5-position) of a radical intermediate.208... 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

A variety of photoaddition reactions of nucleophiles to electron-rich substrates in the presence of electron acceptors were developed in the early 1970s to 1980s. Arnold and his coworkers reported that the photoaddition of methanol to 1,1-diphenylethene occurs in the presence of methyl p-cyano-benzoate in an anti-Markownikoff manner [33]. Photoaddition of nucleophiles such as water, alcohols, acetic acid, and cyanide anion to electron-rich arylalkenes were also reported [34-37]. The photoaddition reaction proceeds via the mechanism as shown in Scheme 6. 

Gassman reported the photoaddition of nucleophiles to radical cations of highly strained aliphatic polycyclic molecules such as tricyclo[2.2.1.0 ]hexane and related compounds. The radical cation of bicyclo [1.1.0] butane is postulated as a key intermediate (Scheme 12) [45-46]. 

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. 

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... 

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. 

Similar effects, although with rather subtle differences, are also observed in the furazan and thiadiazole pyrimidine derivatives (91) and (92). These two molecules have quite different chemical properties, as evidenced both by the extremely facile nucleophilic substitution of the amino-group in (91), which is less easy in (92), and also by the ready photoaddition to the... 

The photoaddition of alcohols to epoxides which is catalyzed by Fe203 (Fe3+ and other metal ions) [39], Scheme 14(c) functions similarly. The metal catalysis of this reaction was detected when the solvent and reagent methanol had been distilled from EDTA to remove metal ion traces and the reaction found to proceed orders of magnitude slower in the purified solvent. Photo-electron transfer from the epoxide to Fe(III) as acceptor generates an oxonium cation that is highly susceptible to nucleophilic attack by the alcohol. Since the catalysis by metal ions in this case is quite general, their functioning simply as Lewis acids cannot be excluded. 

When an electron acceptorlhat can absorb incident Ughtwas used, i.e., 1-naphthonitrile (CNN), 1,4-dicyanonaphlhalene (DCN), 9-cyanophenanthrene (CNP), 9,10-dicyanoanthracene (DCA), and A -methylphthaUmide (MPI), an electron transfer from D to the excited state of A occurs to give the cation radical of D (Scheme 6.9A). This is a case where the electron acceptor plays a photosensitizer, which is absorbing an incident light to activate D. Electron-transfer photosensitization is well known to be effective for the photoaddition of weak nucleophiles such as alcohols and water. 

Structure, stability and essentiality of the intermediates R-FlredH, depend very much on the nature of the residue R (see below). Since H2Fired undergoes fast autoxidation (722), photodehydrogenations are in general reversible with respect to the flavin chromophore, while photoaddition and photodealkylation are not. Furthermore, photoadditions are necessarily nucleophilic, whereas the character of photodehydro-... 

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