Intermediates photoaddition, radicals

Although less well studied, SET-promoted ground- and excited-state reactions driven by a-destanny-lation of intermediate cation radicals are also known. One example is found in the photoaddition of aUylstannane 26 to the pyrrolinium salt 28 (Scheme 17), which produces the pyrroUdine adduct 292 The quantum yield for the SET-promored addition of 26 to 28 is ca. tenfold greater than that for photoaddition of the corresponding allylsilane 27. The enhanced efficiency is a result of the larger rate of destannation vs. desilylation of the intermediate Jt-cation radical. 

Full details of the [3 + 2] photoaddition of nitrobenzene (296) to cyclohexene (297) to give the 1,3,2-dioxazole (298), stable at -70°C, have now been published.244 An analogous intramolecular cycloaddition has been proposed to account for the novel photocyclization of the nitrouracil 299 to the triazole 300 and the pathway is outlined in Scheme 9.245 A 2-azaallyt radical has been shown to be an intermediate in the photoaddition of benzo-... 

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... 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

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... 

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]. 

Photoaddition of neutral radicals R to aromatic rings occurs efficiently, especially for aromatic compounds having electron-withdrawing substituents. The addition of R to the radical anions of aromatic compounds affords the anion intermediates. The anion intermediates are converted to reductive addition products by protonation and to substitution products by elimination of an... 

In the photoaddition of acetone and other ketones to 1-, 2- and 1,2-di-methylimidazoles the products sire a-hydroxyalkylimidazoles (153) which are derived from the selective attack of excited carbonyl oxygen at C-5. In the case of 2-methylimidazole the products are the 4-mono- (8%) and 4,5-di- (14.5%) substituted compounds, but imidazole itself does not react. The suggestion that it is not a sufficiently electron-rich substrate is not particularly convincing. The reaction mechanism (Scheme 72) may reflect the greater radicd reactivity at C-5, and the comparative stabilities of the radical intermediates derived from carbonyl attack at this position. Hiickel calculations of radical reactivity indices show that, indeed, C-5 is more reactive, and the radical intermediate at C-5 is more stable than that at C-4, but a concerted cycloaddition could also give rise to the oxetane (152). Such an oxetane can be isolated in the photochemical addition of benzophenone to 1-acetylimidazole. 

Photoaddition of amines has also been reported and is often the result of initial electron-transfer from the amino-group. Photoadducts of triethylamine with styrene and a-methylstyrene have been obtained carbon dioxide reacts with the styrene radical anion intermediates to give carboxylic acids. In the photoreaction of arenecarbonitriles with aliphatic amines, both aminyl and... 

The mechanism of the photoaddition of CO to h -C5H5(CO)2Mo(jii-SMe)W(CO)j to break the metal-metal bond and form h -C5H5(CO)3Mo(/a-SMe)W(CO)j also may follow a radical mechanism because W(CO) and (h -C5H5)jM0j(CO)4 are also formed in the photoreaction, although an open dinuclear intermediate similar to that proposed in photosubstitution of (h -C5Hj)2Fe2(CO) (see 13.3.1.1.3) is also possible. 

The photochemical and photophysical behavior of the N,N-dimethylaminoalkyDstyrenes 37-42, in which the amino group is attached to the styrene ci- or p-carbon by an ethyl, propyl, or butyl polymethylene linker, has been investigated in the author s laboratory. The preparative results are summarized in Scheme 7 and display remarkable dependence upon the chain length and point of attachment to styrene. Whereas formation of the intramolecular adduct 43 upon irradiation of 37 requires a-C- H transfer from a Al-methyl to the styrene P-carbon, formation of adduct 45 upon irradiation of 40 requires a-C-H transfer from a A -methyl to the styrene a-carbon. The latter process results in formation of a secondary alkyl radical rather than the more stable benzyl radical intermediate. The (aminopropyl)sty-renes 38 and 41 both fail to undergo intramolecular photoaddition. The (aminobutyl)styrenes 39 and 42 form adducts which result from a-C—H transfer from a Al-methylene to the styrene P-carbon, the former reaction leading to a mixture of dias-tereomeric aminocyclopentanes 44 and the later reaction to ami-nocyclohexanes 46. 

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