Equilibria valence isomerization

With respect to the calculated kinetic and thermodynamic parameters one is able to make the following statement about the intercombination of the different equilibria Valence isomerization and tetraphospha Cope rearrangement are independent equilibria exhibited by different energies of activation (Fig. 4). 

The formation of cyclopropane derivatives has also been invoked to explain the mass spectrometric behaviour of 4-phenyl-substituted 2,3-dihydrofurans (114). It was found that the main fragmentation peaks in the mass spectra of 114 and 115 (R R, R" = H,Me, R = Ph) can be interpreted in terms of an equilibrium (valence isomerization) between the parent ions (Scheme 16). 

Cycloheptatrienes are in many cases in rapid equilibrium with an isomeric bicy-clo[4.1.0]heptadiene. The thermodynamics of the valence isomerism has been studied in a number of instances, and some of the data are given below. Calculate the equilibrium constant for each case at 25°C. Calculate the temperature at which K= for each system. Are the signs of the enthalpy and entropy as you would expect them to be Can you discern any pattern of substituent effects from the data ... 

Prinzbach and Limbach have studied the valence isomerism between N-substituted azepines 14b and benzeneimines 14c (76CB3505) although 14b is much more stable (actually it is the only form detected by NMR), the compound could react, depending on R, as 14c with diazomethane. Later, Prinzbach et al. reported the study of the equilibrium 14b (90% )/14c (10%) in the case of R = p-tosyl [the compound has the following C-substituents 3,6-dichloro-4,5-di(methoxycarbonyl)] in the solid state (X-ray) only 14b is present [86CB616],... 

The tendency to undergo valence isomerization is generally of fundamental importance regarding the stability of the compounds. In the case where an equilibrium exists between the eight-membered ring and the bicyclo[4.2.0]octatriene, decomposition may readily occur by a [2 + 2] cycloreversion process, particularly if molecular nitrogen or a cyano compound can be eliminated. 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

As in the case of the 1,2-dioxins, the 1,2-dithiins exist in various states of saturation, oxidation, and benzoannelation (cf. Scheme 1, 17-27) and they have been studied in detail both theoretically and experimentally. Not only were the conformations of the ring and attached substituents investigated, but the valence isomerism of 1,2-dithiin by both NMR and high-level ab initio molecular orbital (MO) calculations and the dithiol/disulfide equilibrium by MP2 calculations were also examined. The latter equilibrium has been applied successfully as a luminescent molecular switch (cf. Section 8.10.2.1). Finally, as a very interesting 1,2-dithiin derivative, the synthesis, structure, and reactivity of the (-l-)-camphor-derived analog 25 and its sulfoxide 26 and sulfone 27 have been reported. Both the synthesis and the antimalarial activity of the 2,3-dioxabicyclo[3.3.1]nonane pharmacophore 28, which contains the 1,2-dioxane moiety, have been reviewed recently <2006BML2991>. 

An area of particular interest is the valence isomerization of c/s-dienones to 2H-pyrans. The H spectrum of the 2H-pyran (30), prepared by irradiation of trans-j8-ionone, showed a series of weak signals in addition to the expected pyran spectrum (66JA619). These were assigned to cis-jS-ionone (31) which exists in equilibrium with the pyran (Figure 2). Integration of the AB patterns of H-3 and H-4 for the two compounds allowed the equilibrium concentrations and hence the equilibrium constant for the isomerization to be determined. 

The next homolog in the cyclic series, 1,3,5-cyclooctatriene, also closes in a readily reversible transformation to bicyclo[4.2.0]octadiene (Equation 12.63). Cope and his collaborators reported this valence isomerization in 1952. They were able to separate the isomers, which revert to the equilibrium mixture of 85 percent 51 and 15 percent 52 on heating at 100°C for 1 hr.113 Huisgen has reported activation parameters of A Hi = 26.6 kcal mole-1, AS = — 1 cal... 

Paracyclophane, which exists as the minor component of an equilibrium with its valence-isomeric Dewar benzene, is the smallest member of this series to exhibit sufficient stability to be studied directly [3]. In order to further stabilize the [5]paracyclophane unit, Bickelhaupt [4] recently synthesized a benzannulated version, namely [5](l,4)naphthalenophane (2, Scheme 1). Even though this compound also exists in equilibrium with the Dewar naphthalene 1 from which it was produced, the proportion of the cy-clophane (35 % of the mixture) is higher than that in any previously reported case. A particularly interesting feature of this system is that there are two observable bridge conformers of 2 in solution in a ratio of 95 5. Unfortunately, despite considerable effort, assignment of the two conformers was not possible. 

A further example is the reversible valence isomerization equilibrium between the dipolar 8,8-diformylheptafulvene (41a) and the less dipolar 8a//-cyclohepta[Z)]furan-3-carbaldehyde (41b) [212]. 

A N2 elimination from the arylazide leads to an arylnitrene 18 which, following attack on the ortho-C-atom, rearranges into 2//-benzazirine 19. The azepine system is formed from the benzazirine by two routes. In the first, valence isomerization of 19 leads to the azacycloheptatetraene 20 which is converted into 17 by amine addition to the heterocumulene system. In the second, the amine adds directly to 19, producing the dihydrobenzazirines 21 which yields the l//-azepine 22 by electrocyclic ringopening. 17/-/3i/-Azepine rearrangement affords the product 17. It has been demonstrated that the highly strained 1-azatetraene 20 is an intermediate in the photolysis of phenylazide at 8 K and that it is in equilibrium with 1//-benzazirine in the case of naphthylazide [17]. 

The thermolysis of 8, 10, and 12 at 140°-175°C in inert solvents (e.g., benzene) does indeed afford the rearrangement products 9, 11, and 13, respectively, the reactions leading to equilibria that are easily detected by H— NMR spectroscopy 10). Equilibrium is reached rapidly, e.g., within 2 hours at 175°C. Heating an independently synthesized sample of 11 leads to the same equilibrium as 10. All reactions are clean, no traces of side products being observed. The fluxional behavior of 8 and 12 is strictly thermoneutral, i.e., the equilibrium constants have the value K = 1.0 within experimental error. The valence isomerization 10 11 is... 

A direct spectroscopic proof of the valence isomeric rectangular structure of cyclobutadiene [48] was obtained from the investigation of 1,2,3-tri-t-butyl-cyclobutadiene [49] in which one outer t-butyl group was perdeuteriated (Maier et al., 1982). The peak for the doubly populated olefinic site (C-l, C-3) in [49] shows a temperature dependent splitting of 0.453-0.297 ppm between -96°C and — 62°C, indicating isotopic perturbation of a fast equilibrium (46) of valence isomers. Carbon C-l substituted with the deuteriated t-butyl group is shifted upheld. 

Typical for disubstituted 1,2-dithietes is their valence isomerization, which results in the formation of 1,2-dithiones. The equilibrium favors the 1,2-dithiete with electron-withdrawing substituents such as CF3. The reaction with 2,3-dimethylbut-2-ene to give a hexasubstituted 2,3-dihydro-l,4-dithiine proceeds, however, as a (4 + 2) cycloaddition via the 1,2-dithione. 

Valence Isomerism ° Norcaradienes. The dicyano-substituted compound (U5) (see p. 38) exists only as the norcaradiene, and at 110°C it undergoes thermolysis to a benzylfulvene rather than valence isomerization. Protonation of the cycloheptatriene (330) at — 60°C gives an equilibrium in which only the norcaradiene (331) is detectable by n.m.r. spectroscopy. The n.m.r. 

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