Singlet state reaction and

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... 

A Z-substituted alkene has a lower-energy HOMO than C- and X-substituted alkenes, and it has a correspondingly lower-energy LUMO. Thus the interaction of the 7i orbital of the ketone 8.21 with the LUMO of the alkene 8.22 ought to be more important for a Z-substituted alkene than it was for C- or X-substituted alkene. In this interaction, the two larger lobes are on the carbonyl carbon 8.24 and on the (3 carbon 8.25, and it is these two which become bonded. Furthermore, this reaction is a singlet-state reaction, and, with /3-substituted acrylonitriles, it is stereospecific, with retention of configuration on the alkene component. 

In two cases, where the singlet- and triplet-state reactions have been carefully looked at and separated, this proves to be true. Thus coumarin 8.59, in the excited singlet state, dimerises to give only the HH dimer syn-8.60, but in the triplet state it gives both HH isomers syn-8.60 and anti-8.60, with only a trace of head-to-tail (HT) products.1119 Acenaphthylene also gives the syn dimer from the singlet-state reaction and a mixture of the syn and anti dimers from the triplet-state reaction.1120... 

Upon photolysis of CVCN in the presence of 150-fold excess of ethanethiol, MGCN is produced in 20% yield (only 1% of CVH was isolated). This may be the result of the radical substitution reaction, which implies that the C-CN bond homolysis takes place with (j) > 0.02. Because the intervention of the homolytic bond cleavage has not been seen elsewhere in our study, we assume that it is a singlet state reaction and that the recombination of the singlet radical pair within the solvent cage is rapid (Scheme 1.13) [66, 30]. 

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

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... 

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. 

When in addition the singlet spin state is not preserved even though the reduced dye-hole pair is formed from the two singlet states XD and xCo we have to consider similar free energy plots for the reverse electron transfer reactions lead-... 

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 irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

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. 

---