Singlet state reactions

Turro and McDaniel(as) have shown that this reaction probably occurs from the excited singlet state (reaction could not be quenched by 1,3-pentadiene) and is stereospecific,... 

That oxetane formation results from a singlet state reaction follows from the following evidence (a) Acetone fluorescence is quenched by addition of the olefin, (b) oxetane formation is relatively insensitive to piperylene, and (c) cis-trans isomerization of the olefin is quenched at high olefin concentrations but oxetane formation is not affected. Since oxetane formation was... 

A remaining class of dimerizations is represented by the reactions shown in Eqs. 36—39. These are singlet state reactions, and they are all characterized by stereospecificity and regiospecificity. Production of the trans isomer from the steroidal dienone in Eq. 39 is anomalous and poses a... 

Singlet-State Reactions with Hydride Donors. 628... 

Photochemical excitation of cyclic a-amino ketones (41 and 43) leads to the formation of bicyclic azetidines and azetidinols by abstraction of a hydrogen y to the ketone followed by cyclization. Production of (42) (72CC1108) and (44) (75TL2463) is believed to occur as a singlet state reaction. 

For singlet state reaction, the quantum yield r is given by... 

For a reaction in solution occurring at a diffusion-controlled rate, Arc 109 I mol-1 s-1, and if Air is negligible and AtSc 9x 108s then for [Bl = 0.1 M, the efficiency of singlet state reaction is (equation 7.14)... 

The reaction can occur from the excited singlet and the triplet state as evidenced from the quenching of reaction in 2-pentanone and 2-hexa-none with piperylene. Piperylene can quench the reaction only partially, suggesting singlet state reaction mechanism for the unquenched fraction. Photoenolization resembles Type II process in that a y-hydrogen migrates to the carbonyl oxygen. 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

The direct photoisomerizations are singlet-state reactions, as shown by their independence of oxygen and the addition of triplet quenchers. Ilge et a/.31 103 demonstrated that no potential barriers in the photoisomerizations, EE - EZ, and EZ — ZZ, were found. In contrast, there was a small barrier in the photoisomerization of ZZ - EZ or ZE a veiy weak fluorescence was detected at 77 K, and the fluorescence of the ZZ isomer disappeared at temperatures above 130 K. 

The results from quenching by fumaronitrile clearly suggest the presence of a reactive singlet state. Reaction from the singlet state could occur through direct fission of the C-Cl bond which is feasible in terms of the energy of the singlet state... 

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