Valence isomer

FIGURE 11.8 EPR spectrum of [4Fe-4S]3+ valence isomers. The experimental spectrum (solid trace) of Allochromatium vinosum HiPIP is simulated as a sum of four slightly different spectra presumably presenting four of the possible six valence isomers. Arrows indicate the g s of the four components with relative concentration (from left to right) 8%, 68%, 12%, and 12%. 

Photochemistry of fluorinated aromatic systems has made an important contribution to the study of valence isomers because it has been possible to isolate and characterise some species on which there had previously only been speculation. Some of these, as might be expected, are very unstable (sometimes, treacherously so) towards reverting to 

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Dewar benzene is a valence isomer of benzene, to which it reverts on heating. 

Tetrazolo[l,5-(2]pyrimidine (/) exists in equilibrium with its valence isomer 2-azidopyrimidine (2). 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

Finally, there is a series of publications by Prinzbach and co-workers on 4,7-dihydro-l//-l,4,7-triazonine 42b (80CB3127, 80CB3161, 88CB757, 89AG1386). Tliis compound, obtained by a 3electron system, better represented as 42c. 

Consequently, structures 85b and 85c must be considered resonance structures rather than valence isomers. Hyperfine coupling constants were computed for a series of dithiazolyl radicals and related compounds [96MRC913]. An absolute mean deviation of 0.12 mT with respect to experimental data is reported for 10 sulfur hyperfine coupling constants obtained from UB3-LYP/TZVP calculations. 

Since both oxepin and its valence isomer benzene oxide contain a x-tb-diene structure they are prone to Diels-Alder addition reactions. The dienophiles 4-phenyl- and 4-methyl-4//-l,2,4-triazole-3,5-dione react with substituted oxepins at room temperature to give the 1 1 adducts 7 formed by addition to the diene structure of the respective benzene oxide.149 190,222... 

Benzo-3-thiatricyclo[4.1.0.02-7]heptene (I, X = H) is a valene -type valence isomer of 2 and 3 however, 1 isomerizes to 3 in photochemical and silver(I)-promoted reactions.75 However. when 1 is heated in carbon tetrachloride at 150°C 3 is not produced but instead the isomeric 1,2-disubstituted 2a,7b-dihydrobenzo[f>]cyclobuta[r/]thiophene. When a rhodium(I)... 

Azabicyclo[3.2.0]hepta-3,6-dienes are the 4 5 bicyclic valence isomers of 1//-azepines. In some cases there is an equilibrium between the bicycle and the azepine, whereas with other 1//-azepines photolysis yields an azabicycloheptadiene that can be isolated and characterized. For example, ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylate (1), the photoinduced valence isomer of ethyl l//-azepine-l-carboxylate (2), undergoes a clean, exothermic, first order, electrocyclic ring opening (AG = 20 kJ mol-1) to the parent 1//-azepine 2 on heating at 113-143C in an inert solvent (e.g., hexadecane).101... 

Attempts to prepare 2-aminoazepinium salts by the action of trityl tetrafluoroborate on azepin-2-amines, e.g. 18, resulted in ring contraction to stable 2-azabicyclo[4.1.0]hepta-2.4-dienium salts, e.g. 3-(pyrrolidin-l-yl)-7-trityl-2-azabicyclo[4.1,0]hcpta-2,4-dicnium tetrafluoroborate (19), the valence isomers of 7-(dialkylamino)-3-trityl-3//-azepinium tetrafluoroborates 64 with Meerwein s reagent the 2-ethyl derivative (77%) is formed. 

The thermally induced electrocyclic ring opening of 2-alkyl- and 2-acyl-3.4-benzo-2-azabicyclo-[3.2.0]hepta-3,6-dienes 2 to l//-l-benzazepines 1 (see Section 3.2.1.4.1.1.) are photorever-sible.23,37 38 Also, l-acyl-1//-benzazepines 1 (R = acyl), in refluxing xylene in the presence of silver(I) tetrafluoroborate, are in thermal equilibrium with their valence isomers the 2-acyl-3,4-benzo-2-azabicyclo[3.2.Oj nepia-3,6-uienes 2 (R = acyl).23-38... 

Irradiation of 3,5,7-triphenyl-4/7-l,2-diazepine gives the valence isomer 2.111,112... 

Irradiation of the same benzodiazepine 3 affords ethyl sumably by way of the tricyclic valence isomer 7. 64... 

The parent thionine system 1 up to now has not been prepared probably because the C-S bond in valence isomeric forms is too weak giving rise to facile rearrangement or decomposition. The obvious synthetic route, photochemical transformation of cyclooctatetraenccpisulfide 2 (9-thiabicyclo[6.1.0]nona-2,4,6-triene), does not lead to 1, but intriguingly to another valence isomer, the sulfur-bridged homotropylidene system 3.20... 

A general entry into the azonine system, c.g. 4, is by photoinduced electrocyclic ring opening of the bicyclic valence isomer 3,9 11-13 which is readily prepared by earboethoxynitrene addition to cyclooctatetraene. 

The free radical catalyzed generation of valence isomers has been described to account for the products observed during the thermal re-... 

The steric and dipole-dipole effects of the CF3 group on valence isomerization in the Dewar pyridine-azaprismane-pyridine system have been studied. These reveal themselves in the high stability, compared to the pyridine, of the valence isomer arising out of the large activation energy for rearomatization. The transformation of a 1-Dewar to 2-Dewar pyridine was observed (89T3115). 

Before discussing Beckwith s data on that basis, a brief mention must be made of investigations by Lown s group (Naghipur et al., 1989, 1990, and other papers cited there) who claim to have observed the formation of benzoxathiete (10.63) and its valence isomer monothio-l,2-benzoquinone (10.64) in the aprotic diazotization of 2-[(2,-acetoxyethyl)sulfinyl]aniline (10.62). Scheme 10-82 is an abbreviated form of the mechanism proposed by the authors. A more detailed experimental study is clearly required. 

Kobayashi and Mutai75 have recently reported an interesting rearrangement of the 1,4-dithiin sulfone (53) to the thiophenes (54) and (55) (equation 12). While 54 presumably arises as a result of simple photoextrusion, the rearranged thiophene (55) is postulated to arise via the valence isomer (56), followed by cyclization to the thiophene, concomitant with, or preceded by, loss of S02. Some support for the intermediacy of the thioketone (56) was revealed by the isolation of the pyrrole (57), when the photolysis was conducted in n-butylamine. Compound 57 presumably arises by cyclization of the iV-butylimine analog of 56 initially formed. 

Photolysis in acetonitrile gives ortho- (10) andpara-phenylbenzaldehyde (11), as well as cis- (12) and fmws-diphenylstilbene (13). The mechanism for product formation is not clear, but the norcaradiene valence isomer is thought to be the most likely precursor, arising either via a secondary... 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Heydt H (2003) The Fascinating Chemistry of Triphosphabenzenes and Valence Isomers. 225 215-249... 

The planar hexagons of [114] and As [115] have the highest energies of the five valence isomers. 

The 1,4,2-diselenazine (56), formed when a 1,3-diselenolium salt reacts with ammonia and iodine, readily fragments to a 1,2-diselenete or its valence isomer, a 1,2-diselenone. This species affords a 1,4-diselenin either by dimerisation or on trapping with DMAD <96CC2375>. 

For related reviews see a) Balaban AT, Banciu M, Ciorba V (1987) Annulenes, benzo-, hetero-, homo- derivatives and their valence isomers, CRC, Boca Raton, FL, Vol 2, p 146 b) Moore JS (1997) Acc Chem Res 30 402 c) Diederich F (1994) Natoe 369 199... 

HEAVY ALKYNE ANION RADICALS The Only example of Compounds of this type, the anion radical of the valence isomer of distannyne 51-K, was recently synthesized by Power by the reduction of chlorostannylene 52 with potassium graphite in THF (Scheme 2.39). ... 

Real-time photoelectron-spectroscopic analysis of the band intensities revealed that the optimum reaction conditions required a temperature of 1100 K for the complete elimination of four molecules of N2. The formation of the valence isomer Ph—Si=N, which is predicted to be 400 kJ mol-1 less stable, can be excluded.53... 

The HPLC-MS/MS assay was also successfully applied to the measurement of UV-induced dimeric pyrimidine photoproducts [123, 124]. The latter lesions were released from DNA as modified dinucleoside monophosphates due to resistance of the intra-dimer phosphodiester group to the exonuclease activity during the hydrolysis step [125, 126]. The hydrolyzed photoproducts exhibit mass spectrometry and chromatographic features that allow simultaneous quantification of the three main classes of photolesions, namely cyclobutane dimers, (6-4) photoproducts, and Dewar valence isomers, for each of the four possible bipyrimidine sequences. It may be added that these analyses are coupled to UV detection of normal nucleosides in order to correct for the amount of DNA in the sample and obtain a precise ratio of oxidized bases or dimeric photoproducts to normal nucleosides. 

Thus, 4,5-benzo-3-thiatricyclo[4.1.0.02 7]heptene (46), which is regarded as a valence isomer of benzo[8]thiepin (4), appears to be an attractive precursor to 4. The most obvious method for the synthesis of 46 would be the application of the elegant Katz method 37> to the benzvalene synthesis. The well established success of the preparation of 43 36) by the Katz method makes this a likely route to 46. 

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