And valence tautomerism

Stetter and ReischP described the preparation of 2,6-diphonyl-P5Tylium perchlorate (57) from l,5-diphenyl-2-penten-4-yn-l-one (56a) and concentrated sulfuric acid in the cold, followed hy perchloric acid, in 60% 5rield. The process may involve hydration to a 1,5-enedione, or an ion 56b related to 52, and valence tautomerism. 

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... 

The last major section deals with tautomerism, including prototropic tautomerism, ring-chain tautomerism, and valence tautomerism. 

MNDO, AMI, and PM3 study (with full geometry optimization, in the gas and aqueous phases) of the protonation tautomerization and valence tautomerization equilibria of some pyrrole macromolecules is described in the work <2002JMT(589)43>. 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

The first step is the photolysis of the azide giving a nitrene which forms a dihydrobenzazirine 13 by a [1+2] cycloaddition with the arene the latter isomerizes yielding 1//-azepine. An alternative synthesis of l//-azepines 14 is based on bicyclic aziridines 15, which are transformed to 14 by addition of bromine to the double bond, two-fold dehydrobromination and valence tautomerization of the resulting 13. The bicyclic aziridines 15 are available from cyclohexa-1,4-dienes via l-alkoxycarbonylamino-2-iodocyclohexenes 16 and their SNi-cyclization [15]. 

The UV-vis-near-IR absorption spectrum of the solution of the protonated polymer exhibits MLCT bands and valence tautomerization similar to those of 1,8-Fc2Aq as shown in Fig. 3.32, indicating the formation of fulvene complexes in the main chain in solution. Infrared spectrum of the protonated polymer also shows the similarity with that of l,8-Fc2Aq, namely, half the CC triple bond remains. 

The rate-determining step is thus the dimerization of a coordinatively unsaturated 002(00)3 fragment. An alternate preparation of 00 (00)12 utilizes the reaction of a mixture of 002(00)3 and a soluble cobalt(II) salt of a weak acid (e.g., cobalt 2-ethylhexanoate) with hydrogen under pressure (145). Attempts to elucidate the structure of 001 (00)12 from its v(C0) frequencies (94) have not been entirely conclusive, and valence tautomerism has even (93) been postulated to... 

Structural tautomerism breaks down into several categories, which include the most common prototropic tautomerism, ring-chain and valence tautomerism, and charge mesomerism. Only the first and the last have been successfully treated in software - everyone is familiar with the CAS Registry III tautomer normalisation conventions. It quickly became apparent, if not obvious, that in the case of a data handbook such as Beilstein, it was imperative that different tautomeric forms of a compound for which different data have been published must be registered differently the situation is analogous to that of registering stereoisomers as separate compounds. ... 

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