Valence ring closure

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

Ring contraction and intramolecular cyclization constitute a convenient route to ring-fused systems that would be difficult to synthesize in other ways. H- 1,2-Diazepines (538) undergo electrocyclic ring closure to the fused pyrazole system (539) (71CC1022). Azepines also undergo similar valence bond isomerizations. 

Intramolecular ring closure by valence isomerization to a carbon in the 3-position is not common, but known l,l-di-t-butyl-2-nitrosoethylene, stable at room temperature, cyclizes at 220 °C to the stable 4,4-di-f-butyl(4iT)oxazete (75AG(E)70). 

Trifluoromethyl-substitutedazimines are surprisingly stable compounds. They are accessible by 1,3-dipole metathesis from tnfluoromethyl-substituted azomethine imines and certain nitroso compounds [187, 188] On photolysis, an electrocyclic ring closure first gives the triaziridines, which are stable at room temperature. On heating above 80-100 C, a valence tautomenzation takes place and azimines are formed [189] (equation 43). 

Irradiation causes ring closure by valence isomerization of 1,3-diphosphacyclobutane-2,4-diyl 42 (R = 2,4,6-tri(tert-butyl)phenyl) to 2,4-diphosphabicyclo[1.1.0]butane 43 which on thermolysis yielded the gauche-1,4-diphosphabutadiene 44 <99AG(E)3028>. The same group of workers have isolated the carbene 45 (R as above) as the lithium salt of a trimethylalane complex 46 <99AG(E)3031>. 

Scheme 22 illustrates a special application of the azide-tetrazole ring closure described by Ponticelli et al. <2004JHC761>. The diazido compound 84 exists as an azide valence bond isomer. When this compound, however, is subjected to reduction by molybdenum hexacarbonyl, one azido group undergoes reduction selectively to an... 

Ring closure to the 5//-tetrazolo[5,l-t][l,3]oxazine skeleton has been reported by Hoornaert and co-workers <1996T8813>. These authors treated variously substituted 2,4-dichloro[l,4]oxazin-2-ones 133 with sodium azide. The fused tetrazoles 135 obtained were formed via the formation of their azide valence bond isomer intermediate 134. A similar approach proved to be suitable for the benzologues of these compounds. Thus, the benzoxazinone compounds 136 gave the tricyclic ring-closed tetrazoles 137. Both reactions yielding 135 and 137 proceeded in high yields (80-90%). 

The reaction sequence starts from tetrazolyldiazonium salt 42 prepared from aminotetrazole 41 by diazotation. This compound when reacted with arylformylacetonitrile 43 leads to the intermediate formation of the condensation product 44, which easily undergoes ring closure to 45. This tetrazolo[5,l-z][l,2,4]triazine compound, however, forms an equilibrium with the valence bond isomeric azide 46, which can participate in a different ring closure than the reverse route, and yields the tetrazolo[l,5-A][l,2,4]triazine product 47. The reaction was carried out with a series of various aryl derivatives and proceeded in good to excellent yields (68-87%). 

This reaction is directly analogous to that found with 1-benzoxepin and the valence tautomeric cyclobutene product obtained from (46) and was formed by a concerted disrota-tory ring closure mechanism. 

Ring closure to cyclobutanes via valence isomerization of cycloocta-l,3,5-trienes is also possible when carbonyl functions are present in the ethane fragment. Thus, reaction of cyclohepta-2,4,6-trienone (tropone) with diazopropane gives 8,8-dimethylcycloocta-2,4,6-trienone (16, R = H), which rearranges quantitatively to 8,8-dimethylbicyclo[4.2.0]octa-2,4-dien-7-one (17, R = H).67... 

Several other dienones were observed to be incapable of effective ring closure to 2//-pyran valence tautomers of 459.164,175,387 In other cases cyclic forms such as 460 and 463 were too unstable to be detected,30,164,388 but their existence, although in negligible concentrations, was proved on the basis of the structure of mutually isomerizing different open-chain forms28,99,388 391 or as adducts with tetracyanoethene30 and maleic anhydride.237,392 The com-... 

If a 2f/-pyran or 2//-thiopyran derivative 459 possesses in substituents A or B an active methylene fragment, for example PhCH2, RCOCH2 or N02CH2, the corresponding valence-bond tautomers 460 or 462 may undergo intramolecular ring closure to substituted benzenes. The equilibria shown in Scheme 22 are shifted toward such products. 

Like its hydrocarbon analogue, it exists in thermal equilibrium with its octa-fluorobicyclo[4.2.0]octatriene valence isomer 52, as shown in Eq. (3) Keq = 0.003 at 20°C in acetone-d6) (123). Valence isomer 52 can be prepared independently, but has a half-life of only 14 min at 0°C (124). OFCOT undergoes a photochemical ring closure to give a 20 1 mixture of the anti- and syn-tricyclic valence isomers 53 and 54, which in turn can be converted thermally back to OFCOT at 150°C (125). 

The bicyclic isomers 109 are clearly formed by electrocyclic ring closure of their initially formed monocyclic valence isomers and not from the nucleophilic attack of the transition metal anion on traces of 52 in equilibrium with OFCOT, as shown by the low-temperature reaction of [Fe(f/5-C5H5)-(CO)2] . At -78°C, the l9F NMR spectrum of the crude reaction mixture indicates exclusive formation of the monocyclic complex 108b, but on warming the solution to room temperature the resonances of the bicyclic valence isomer 109b grow in (184). 

Photocycloaddition of l,2-dichloro-l,2-difluoroethene to hexafluoroben-zene produces an unstable ortho adduct that during the irradiation undergoes further ring closure, yielding 3,4-dichloro-l,2,3,4,5,6,7,8-octafluorotricy-clo[4.2.0.02,5]oct-7-ene [156,157], This was further elaborated into perfluorocy-clooctatetraene [156] and into its valence isomer, perfluorobicyclo[4.2.0] octa-2,4,7-triene [157],... 

Marvell, E. N., G. Caple, and B. Schatz Thermal valence isomeriza-tions Stereochemistry of the 2.4.6-octatriene to 5.6-dimethyl-1.3-cyclohexadiene ring closure. Tetrahedron Letters 1965, 385. 

Comparison of the NMR spectra of the isomeric systems (48) and (49) with that of (47) indicates that (47) exists in an equilibrium with its open azido form whereas (48) and (49) are almost exclusively present as the tetrazolo valence tautomer (76JHC881). This equilibrium for (47) is solvent dependent (76JHC881), it is influenced by the substituents present on the molecule (81JOC843), and it may be an attempt by the molecule to counteract the instability introduced by ring closure to the o-quinoid structure (47). 

It seems necessary to accept an intermediate species with either zwitterionic or closely related character. The most recent proposal [2gg] invokes the known conformational features of the pregnan-20 0ne side-chain (p. 12). The individual steps are considered to be (i) formation of the i7a-bromoenolate anion (21) with the side-chain in its most stable conformation (ii) rotation about the C i7) C(20) bond to minimise dipole interactions between the i7a-Br and 20-oxygen substituents 22 (iii) expulsion of Br , perhaps assisted by the anti-perlplanar -0 entity to give the unsaturated epoxide (23) or its valence tautomer, the zwitterion 24) (iv) cyclopropanone ring closure and (v) base-promoted rupture to give the Favorskii product. 

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