Valence Bond Tautomerism

The phenomenon of tautomerism comprises many different types of which the prototropic tautomerism that we consider here is only one. Prototropic tautomerism exists when the two tautomers differ only in the position of a proton (this is, of course, an approximation there are other differences between two tautomers, for example, in precise bond lengths). Other important types of tautomerism include the following (1) anioniotropy, where the two tautomers differ only in the position of an anion, which moves from one place to another in the molecule (2) cationiotropy, where the two tautomers differ in the position of a cation (other than a proton), which moves from one place to another in the molecule (3) ring-chain tautomerism and (4) bond-valence tautomerism. 

Ratera et al. (2003) discovered valence tautomerism in the ferrocene connected through the ethylenic bond with perchlorotriphenylmethyl radical. As ascertained by Moessbauer spectroscopy, this species in the solid state exhibited a thermally induced intramolecular electron transfer resulting in the formation of ferrocenium and perchlorotriphenylmethyl anion moieties. The authors used the initial species in its trans form. If the cis form would be available, the possibility of rotation around the ethylenic bond would be interesting to disclose. According to the authors, the interconversion of the cation-radical and anion centers proceeds gradually. At ambient temperature, equilibrium composition of the tautomers is achieved. This peculiarity is important with respect to potential technical applications. 

The facility of 1H-azepines to form transition metal carbonyl complexes was realized soon after they were first synthesized. Variable temperature HNMR studies on the tricarbonyliron complex formed either by photolysis of 1-ethoxycarbonyl-l//-azepine with tricarbonyliron in THF, or by heating the azepine with nonacarbonyldiiron in hexane, demonstrated that it undergoes rapid reversible valence tautomerism and that there is considerable restricted rotation about the N—CO bond (B-69MI51600). The molecular geometry of the complex has been determined by X-ray analysis (see Section 5.16.2.2). 

Valence tautomerism is responsible for the photochromic transformation exhibited by the xanthenone (143). This phenomenon is a type of dynamic isomerism in which the primary change is a shift in the position of the valence bonds as illustrated in Scheme 16 (68JOC3469). 

FIGURE 6. One-dimensional representations of the potential energy surface (PES) of molecule 42 shown as a function of the interaction distance R. Situation 1 corresponds to the bicyclic molecule 42a, situation 2 to the open monocyclic molecule 42c, situation 3 to the no-bond homoaromatic molecule 42b with non-classical structure and situation 4 to a valence tautomeric equilibrium between 42a and 42c with the homoaromatic form 42b being the transition state. See text... 

It is interesting to consider further the relationship between situations 3 and 4. Situation 3 will be reached if the transition state in 4 is sufficiently stabilized so that its relative energy drops below those of the valence tautomeric forms 42a and 42c. In other words, situation 3 corresponds to a frozen transition state 7. A no-bond homoaromatic compound is simply the realization of a frozen TS. 

Figure 15 presents a schematic view of how the atomic subspaces Cl, C6 and Cl 1 of 1,6-methanojl Ojannulene (35) change upon an approach of Cl to C6. Bond paths (solid lines between atoms), bond critical points (dots) and the traces of the zero-flux surfaces S (A, B) (perpendicular to bond paths) that separate the atomic subspaces are shown in Figure 15a. Clearly, the subspace C11 extends less and less into the region between C1 and C6 until the surfaces of C1 and C6 coincide and a bond path between C1 and C6 is formed. At the same time, the Laplace concentration between Cl and C6 gradually increases and coverges to the one found for a three-membered ring. As shown in Figure 15b, this change corresponds to the valence tautomerism of the l,6-methano[10]annulene to bisnorcaradiene27,54.

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). 

Because the tropylium cation contains one uncoordinated olefinic bond, yet displays a single proton NMR signal in solution, rapid valence tautomerism or ring whizzing is proposed (18S). 

Much later, Ikeno and coworkers provided IR-spectroscopic evidence that Co(II) carbene complexes 325 exhibit considerable single bond character (Fig. 76). This complex can be depicted best as a valence tautomeric pair 325A and 325B, in which... 

Valence tautomerism - This refers to the interconversion of isomers simply by reorganization of bonding electrons and without any accompanying rearrangement including proton migration. For examples see Sections 2.2.5.3, 2.4.54, and... 

In addition to the above-mentioned ring contractions and trans-annular bonding of compounds of the type 10 in ionic reactions, photo-induced valence tautomerism has also been noted.38 Thus 10 or the various 2-substituted compounds can be converted to 44.35... 

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