Valence tautomeric system

Ksenofontov V, Caspar AB, Giitlich P (2004) Pressure Effect Studies on Spin Crossover and Valence Tautomeric Systems. 235 23-64 Ksenofontov V, see Real JA (2004) 233 167-193... 

Pressure Effect Studies on Spin Crossover and Valence Tautomeric Systems... 

Dynamic Vibronic Problem for a Valence Tautomeric System... 

Small unsaturated rings are usually very reactive undergoing ring opening in a number of ways, and this characteristic has been utilized in heterocyclic synthesis. In their role as dienophiles or dipolarophiles, the initial cycloaddition is usually followed by a valence tautomerism resulting in a six-membered or larger ring system. Several examples exist, however, where this does not occur, and these are described below. 

Valence tautomerism of heterocyclic systems containing tetrazole rings has received much attention [76AHC(S1), p. 498 84CHEC-I(5)791 96CHEC-1I(4)621], The latest contributions to this area are considered in Section IV,C, which is concerned with the topic of azido-tetrazole tautomerism. 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and hve-membered heterocyclic azides. Tire tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

The knowledge of the valence tautomerization of benzene oxides to oxepins12 prompted several groups to synthesize oxepins by dehydrohalogenation of 7-oxabicyclo[4.1.0]heptane derivatives. Numerous examples have been described for the base-catalyzed elimination of hydrogen bromide from the 3,4-dibromo-7-oxabicyclo[4.1.0]heptane system. The reaction products are usually obtained as mixtures of oxepin 1 and benzene oxide 2. The 2,7-bis(hydroxy-methyl)oxepin 1 p obtained by this route can be converted to the 2,7-dicarbaldehyde with man-ganese(IV) oxide.23... 

When the valence tautomeric mixture of oxepin and benzene oxide is treated with singlet oxygen, the primary product is the 1,4-endoperoxide 3 which has proven to be too labile for isolation.219 Its formation can be rationalized by a 1,4-addition across the diene system of the benzene oxide structure 3 then rearranges to ba s-3,6,9-trioxatetracyclo[6.1.0.02 4.05 ]nonane (transbenzene trioxide, 4). 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

Irradiation of troponoid systems leads to a variety of photochemical reactions which include valence tautomerization, deep-seated rearrangement and dimerization. The photochemical transformations of troponoid systems as a general rule proceed readily in Pyrex vessels and thus must involve excitation of the intense, long wavelength absorption band (320-350 m/t) of the troponoid system. 

One of the important features of the structure and reactivity of arene oxides is the possibility of their involvement in valence tautomerism with the oxepin system. In benzene oxide (86) the two n bonds and the C—C a bond undergo facile disrotatory electrocyclic reaction to give a 6rc-oxepin system (96). 

Valence tautomers, benzene oxide 1 and oxepine 2 (Equation 1), as well as relative tautomeric systems, benzene sulfide-thiepine and o-xylene-2,7-dimethyloxepine, have been studied by a post-Hartree-Fock (HF) ab initio QCISD(r)/6-31G //MP2/6-31G method. In particular, the enthalpy calculated for a benzene oxide-oxepine system is 0.59 kj moF1 <1997PCA3371>. The calculated molecular orbital (MO) energies are in linear relationship to those from the photoelectron (PE) spectra <1996JCF1447>. Barrier to tautomerization for a benzene oxide-oxepine system is 29.4 kj mol-1. Protonation stabilizes the oxide form versus the oxepine <1997PCA3371>. 

A bimetallic iron tricarbonyl cation C8H9Fe2(CO)6 + (IV) is derived from cyclooctatriene (157). The Mossbauer resonance of spectrum of (IV) and the related cycloheptatrienyl-Fe2(CO)6 cation (V) (73) suggest that these cations may be considered as containing a bis(7r-allyl) and an allyl-diene system, respectively. In the second case (V), rapid valence tautomerism is invoked to account for the unique proton NMR signal in solution. 

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