Valence-bond isomerization

On the other hand, 2-arylthiazoles are easily isomerized to 3-aryliso-thiazoles in 407o yield upon irradiation with a high-pressure mercury lamp, in benzene solution in the presence of iodine (738). A valence bond isomerization was proposed among several alternatives to account for these results. 

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. 

In contrast to pyrazolenines, there are only a few publications on the photochemistry of isopyrazoles and they concern exclusively their iV-oxides (390). Irradiation of (390) affords the iV-oxides of pyrazolenine (391) (70CC289). Bicyclic intermediates (392) and (393 Scheme 36) are believed to be implicated in this reaction (75MI40400). The final step is similar to that reported from studies of the valence bond isomerization of pyrazolenines (68JA173). 

The reaction between zwitterion 74 and aryldiazonium salt at room temperature provides the fluoroborate salt 75. Upon treatment with a base the salt 75 undergoes a valence bond isomerization to yield triazolo-pyridazine 76 (Scheme 2) <2000CC1785>. 

Upon treatment with a base the zwitterionic triazine derivatives 112 undergo a valence bond isomerization yielding l,2,3-triazolo[4,5-d]pyridazine derivatives 113 <00CC1785>. 

The ring transformation of the fused triazolium salt 133 to the pyrazolopyridine derivative 134 proceeds via ring opening according to a valence-bond isomerization <1999JOC5499> and was already discussed in Section 11.16.5.1. 

Because of the presence of two azide groups in positions adjacent to the ring nitrogen atoms in compound 13a, valence bond isomerization can result in formation of 6-azido-7-methyltetrazolo[l,5-A pyridazine 14a, 6-azido-8-methyltetrazolo[l,5-A pyridazine 15a, and the bis-tetrazole compound 16a. Calculations have been carried out by using hybrid density functional theory (B3LYP/6-311+G(d,p)) and complete basis set treatments (CBS-4M). All calculations revealed that the 8-methyl derivative 15a is the most stable isomer. Similar studies on the triazide derivative 13b, however, indicated that in this case the equilibrium is shifted to the 7-methyl form 14b. All these conclusions proved to be in entire agreement with the experimental findings (see Section 11.18.3.2.). 

The fused tetrazoles 10 containing various aryl groups in position 6 were boiled in acetic acid in the presence of either triphenylphosphine or copper and gave rise to the amino-substituted triazines 12. The reaction proceeds obviously via valence bond isomerization of 10 to the azide 11 and is regarded as a useful synthetic route to the diamino compounds 12. 

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%). 

The tetramethylammonium triazine-olate 55 was treated with sodium azide, whereupon nucleophilic exchange of the trinitromethyl substituent took place to give the azide intermediate 56, which spontaneously underwent valence bond isomerization to the fused tetrazole 57. 

Irradiation of the six-membered heterocycles can, as mentioned above, lead to valence bond isomerization. Pyridine gives only one of the two possible Dewar pyridines, namely (19 equation 8). Derivatives of the alternative Dewar pyridine have been prepared as outlined in equation (9) and compound (20) in turn is converted on further irradiation into the azaprismane derivative (21). 

Several bicyclic azetidine systems have been synthesized via photochemically induced valence bond isomerization of a heterocycle. Photolysis of 2-amino-5-chloropyridine in... 

The reaction of 2-pyrones with Grignard reagents is complex305"310 the mechanism may be rationalized as in Scheme 14 and includes a knowledge of valence-bond isomerism of 2//-pyrans (see Section V,E). 

Azepines (228) result from spontaneous valence-bond isomerization of azanorcaradienes (227) which are themselves made by reaction of arenes with nitrenes (see CHEC 5.16.4.1.3). 

Photolytically induced valence bond isomerism of heterocycles is a useful method 2-amino-5-chloropyridine gives (1) (61JA2967), and (2) — (3) illustrates the conversion of a seven-membered ring into a 4,5-fused derivative (71JOC1934). Benzazetidines are available by ring contraction (4 — 5) (80CC471). 

Progress in the areas of photoscrambling and valence bond isomerization reactions in aromatic156 and heteroaromatic compounds157 has led to the study of new cyclic systems with strained olefinic double bonds.158 Dewar benzene, Dewar thiophene, benzvalene, and their heteroaromatic counterparts substituted with fluoro and/or perfluoroalkyl substituents have been synthesized and their reactions with phenyl azide have been investigated. 

The synthesis of medium-ring nitrogen heterocycles, such as azepines, azocines, and azonines, has been reviewed <1991T9131>. 1/7-Azepines 321 result from spontaneous valence-bond isomerization of azanorcaradienes 320 (Scheme 163), which are themselves made by reaction of arenes with nitrenes. Oxepins are prepared by an analogous method (323 324) the starting material is made from the dibromide 322 (Scheme 164) <1964AGE510>. 

---