Photochemical cycloreversion

A few studies have been carried out on the parent four- and five-membered cyclic sulfones—for thietane 1,1-dioxide (30) by Scala and Colon65 and for thiolane 1,1-dioxide (sulfolane) (31) by Honda and coworkers66 and, later, by Schuchmann and von Sonntag67. In the former compound, the major photochemical process, in the vacuum UV range, is the initial production of a trimethylene (C3H6) biradical and S02 (equation 9). In both the solid- (77 K) and gas-phase photolyses, formation of a triplet biradical appears to be favored. As well as the expected cyclopropane and propylene, ethylene is also obtained during these photolyses, presumably by a cycloreversion process (equation 10). 

Scheme 6. Photochemical [2 + 2] cycloreversion of the dodecadehydro[18]annulene in furan as the solvent, followed by [4 + 2] Diels-Alder cycloaddition, gives the oxanorbornadiene adducts 38-40 [66]...

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

The adducts 41 from 1 and ketones or thiobenzophenone undergo interesting photochemical cycloreversion to afford a silanone or silanethione intermediate 42 in addition to silene 43 both of these intermediates are trapped by ethanol, as shown in Eq. (14).68 71 In the reaction with the thiobenzophenone adduct 41 (R = Ph, X = S), the intermediate silene 43 (R = Ph) was detected by Si NMR.71... 

Saltiel and Nghim have provided a nice example of the photochemical 2, + 2, cycloreversion(130) ... 

Photochemical fragmentation 165 166+167 is also feasible. Isopropyl phosphate and isopropyl methyl phosphate are expectedly found after reaction of the photolysis solution with water or methanol. It would appear that 166 (and possibly also 167) can arise directly from 164 and not only by photochemical cycloreversion of 165 since 166 is formed together with 165 even at wavelengths in the range where 165 is known to be stable. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Cycloreversions of a 1,2-disilacyclobutane (42) and a 1,2-digermacyclobutane (43) have been induced in solution both thermally and photochemically fragmentation of sterically congested (42) follows Scheme 5 paths a and b, respectively fragmentation of (43) yields (46) (which photodissociates to 48) in each case. " ... 

Scheme 11.4 Photochemical reaction of 14 with singlet oxygen, leading to the cluster opened fullerene 19. (i) O2 hv, benzene (II) [2+2] cycloreversion, keto-enol tautomerization.

A new approach to the cyclodeca-1,5-diene systems [cf. (281) and (282)] characteristic of germacranolides involves a photochemical-cycloaddition and thermal-cycloreversion sequence shown in Scheme 27. ... 

It should be noted that t,c,t-TPCB" decomposes into o-St2, but t,t,t-TPCB" does not upon warming up to 90 K. On the other hand, photochemical cycloreversion of both TPCB occurs through cleavage of two C—C bonds to give Tr-(t-St)2 or t-Sf /t-St pair. This may account for the different interaction between two phenyl (Ph) groups through the... 

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

The cyclization and cycloreversion of l,2-bis(heteroaryl)ethene systems under photochemical conditions has received considerable attention due to the ability of these compounds to function as molecular switches, and the... 

Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122], The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,10-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [123]. Photodissociation of the 1,4-endoperoxide from l,4-dimethyl-9,10-diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and l,4-dimethyl-9,10-diphenylanthracene, in their electronically excited state [124]. 

The molecular geometry of the lepidopterene skeleton remains a feature of unique importance for the observed photochemical reaction. The photolytic An + 27i cycloreversion generates an electronically excited product in which the diene and dienophile moieties are bound to face each other in an arrangement which, subsequent to deactivation to the Franck-Condon ground state, is an ideal one for bond formation. 

Crystal reaction study mechanistic tools, 296 computer simulation, 297 electronic spectroscopy, 298 electron microscopy, 298 electron paramagnetic resonance (EPR), 299 nuclear magnetic resonance (NMR), 298 Raman spectroscopy, 299 Crystal reaction study techniques crystal mounting, 308 decomposition limiting, 309 polarized IR spectroscopy, 309 temperature control, 308 Cycloreversions, adiabatic photochemical involving anthracenes, 203 excited state properties of lepidopterenes, 206... 

These results are most likely interpreted in terms of the photochemical cycloreversion of the thiadisiletane 106 leading to the formation of the silene 109 and silanethione 110, though both of them were not isolated. 

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