Solvent hydrogen-donating

In order to avoid competitive bimolecular photoreactions such as ketone reduction by hydrogen abstraction, poor hydrogen donating solvents are recommended (acetonitrile, acetic acid, tertiary alcohols). In those cases where ketene trapping is desired, solvents must also be miscible with water or other protic nucleophiles. 

In benzonitrile both types of processes occur in parallel with similar efficiencies. In hydrogen-donating solvents (toluene, ether, dioxane, ethanol), hydrogen abstraction processes prevail [toluene (114) -> (123)]. 

Phenolic compounds may enhance the rate of decomposition of aromatic ether, because the phenoxy radical may be stabilized by solvation (18) or hydrogen bonding (19) with phenolic compounds and may result in the subsequent hydrogen transfer reaction from hydrogen donating solvent or phenols (20). 

Once again, use of these donors as solvent may shift the reaction equilibrium towards the desired product. Since the reactivity of olefins is lower than that of carbonyl compounds, higher reaction temperatures are usually required to achieve acceptable TOFs, and then the relatively higher boiling hydrogen donating solvents mentioned above may be the best choice. 

For molecules in which intramolecular hydrogen-abstraction (Type II reaction) is structurally prevented, intermolecular abstraction from a hydrogen-donating solvent may take place (see Scheme 6). This type of reaction usually results in reduction of the carbonyl group (see Table VII). 

Because only a small number of stable selenoketones exist, studies of their decomposition are scarce. Irradiation of dwerf-alkylselones with UV light in hydrogen-donating solvents affords diselenides (80CJC6 87MI1), and the selone 37 behaves likewise. A second path of photolysis involves extrusion of molecular nitrogen from 37 to yield eventually tetramethyl-allene and elemental selenium, probably via the elusive... 

Note. Both the rearrangement In t-Butanol) and the double bond isomerization of (114) In Benzene) are quenched in a diffusion-controlled process by suitable triplet acceptors (e.g., naphthalene or 2,5-dimethylhexa-2,4-diene). The rearrangement (114) - (118) + (120) is also observed on irradiation in pyridine, the double bond isomerization (114) -> (122) in trifluorotoluene. In benzonitrile both types of processes occur in parallel with similar efficiencies. In hydrogen-donating solvents (toluene, ether, dioxane, ethanol), hydrogen abstraction processes prevail [toluene (114) - (123)]. 

When using photoinitiators such as phenyl acetophenone derivatives, addition of an hydrogen donating solvent (Table IV) to the system improved ultimately the percent grafting and, in addition, the concentrations of initiator required were lower. Moreover, the presence of THF is necessary with benzophenone derivatives. These results are in agreement with similar observations with respect to MMA polymerization in solution in fact, the rate of polymerization was enhanced in the presence of THF. 

Mechanisms of these reactions have not been established. It should be noted that all reported experiments were performed in the notoriously poor hydrogen-donating solvents benzene or acetone with excitation into S2 of the dione. Reactions at long wavelengths in good H-donating solvents or in the presence of olefins might exhibit normal a-diketone photochemistry as has been observed 73> with 68. 

In further experiments Bolsman, de Boer and Verhoeven [45] examined the photochemical decomposition of 2-iodo-2-nitroadamantane in several hydrogen donating solvents and found the formation of o-nitroalkyl radical. This was extended to simple a-iodonitroalkanes in solution (46). 

Ketenyl carbenes are proposed as the key intermediates in the photochemical decomposition of maleic and dichloromaleic anhydrides in cryogenic matrices. The former anhydride yields cyclopropenone and ethyne while the latter yields dichlorocyclopropenone and dichloroethyne. Dichloropropadienone is observed as a minor product. Maleimide (155) is excited into its triplet state on irradiation in solvents. In hydrogen-donating solvents the radical (156) is... 

The presence of a hydrogen-donating solvent is not essential for PCB photoreactivity Neat Aroclor 1254 thin films on glass (18 ug/100 cm ) were photolabile with surface deposits exhibiting half-lives of 50 min. 

A full report of the factors governing the stereoselectivity of photoreduction of some substituted cyclohexanones has now appeared.13 Irradiation of 3,3,5-trimethylcyclohexanone (10), for example, in hydrogen-donating solvents leads... 

Quinones. - Goerner has demonstrated that the yield of semiquinone radicals is low in the absence of hydrogen donating solvents. He notes that the quantum yield for decomposition is substantial in aqueous solution. A detailed account of the photohydroxylation reactions of 1,4-benzoquinone in aqueous solution has been published.The photochemical cyclization of some Diels-Alder adducts of benzo-l,4-quinones has been described. This has provided a path to complex molecules such as 3-bromotetracyclo[5.3.1.0 . 0 ]undec-10(12)-ene-9,l 1-dione. A biradical has been identified as the key intermediate in the photocyclization of phenylbenzoquinone. " A charge-transfer complex is formed initially on irradiation of 2-chloro-5-methoxybenzo-l,4-quinone in the presence of triethylamine and various solvents.2-Chloro-5-methoxyben-zo-l,4-quinone also undergoes addition to arylalkynes to afford oxetenes. The mechanism of this addition reaction was studied.A hydrogen abstraction is involved in the photochemical reactions of chloranil with 3-diketones. ... 

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