Quadricyclene valence isomerization

Cassar and Halpern (85) provided evidence that the Rh -catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism ... 

QEUC, see Quasi-molecular enlarged unit cell Quadricyclene, 20 323 valence isomerization of, 20 304 Quadricyclene, isomerization of, 24 146, 148 Quadrupolar interactions, 33 205-209 Quadrupole splitting, 26 126, 134, 140-142 Qualitative studies of simple reactions, 32 116 Quantitative treatment, structure effects, 29 155-162... 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbomadiene to quadricyclene conversion.

Both the bidentate and monodentate processes probably play a role in the catalysis of symmetry-restricted reactions. For some reactions, hke the cyclobutanation of olefin Hgands suggested in olefin metathesis bidentate coordination is essential to the critical transformation. In other processes, such as the valence isomerization of quadricyclene 9) to norbornadiene (10) 10), both paths are available. 

We have thus far discussed rather generally the nature of restrictive ligand fields. We noted that these restrictions, even when small, could be sufficient to block molecular transformations which are, by themselves, thermodynamically only marginally favorable. Thus, simple olefins coordinated to metals that prefer square planar (with and lower metal complexes) and octahedral coordination should not be expected to undergo [2i-j-2s] cycloaddition. For molecular transformations which are energetically more favorable, the situation is different. Here, the energy provided by the exothermic [2 - -2s] process can reasonably be expected to compensate for attending restrictive field effects. We shall now consider the valence isomerization of quadricyclene (9) to norbomadiene (10). 

Quadricyclene undergoes facile valence isomerization to norbomadiene in the presence of a variety of metal complexes, including rhodium(I)... 

The silver ion, then, does not exhibit the same degree of back-bonding that the more familiar transition elements do. Since back-bonding is an essential factor in the forbidden-to-aUowed process and, in particidar, in direct oxidative addition, silver s function in this chemistry could differ. It may be that the silver ion (and other similar metallic species) stands apart from the other transition elements (W, Mo, Cr, Fe, Co, Ni, Rh, etc.) in its mode of catalysis. In the valence isomerization of quadricyclene, some oxidation occurs as evidenced by the deposition of metallic silver 45). Certainly, irreversible redox cannot be a feature of the actual catalytic path, since silver s role is definitely catalytic and the isomerization itself precludes it i.e., the oxidation state of the system remains fixed). Some electron transfer, however, clearly proceeds and may be a critical feature of the catalysis. One could speculate on the possibility of intermediate ion radicals generated through electron transfer from a reactant to Ag(I) followed by electron recapture by the rearranged species in the catal5dic system. 

Valence Isomerism.—Photochemical energy storage via the photosensitized valence isomerization of norbornadiene to quadricyclene has continued to attract attention. The effects of excitation wavelength and suppression of side reactions have been studied and the efficiency of various photosensitizers has been compared. It has been concluded that such systems are technically feasible but economically non-competitive at present. Similar valence... 

A somewhat more convincing case for forbidden-to-allowed catalysis is found in the impressive work of Hogeveen and Volger in a study of metal-catalyzed valence isomerization of quadricyclene (XI) (22). 

However, unlike simple alkenes, this reaction produces predominately trans-isomers since the trans-isomer is much more stable that its cis-isomer. A third example is the valence isomerization of quadricyclene through a similar mechanism, which yields norbomadiene on irradiated SC particles, including CdS, TiOj, and ZnO [Eq. (4)] [105] ... 

Recent work (20, 21) in our laboratory has focused upon the use of transition metal compounds to sensitize the energy-storing valence isomerization of norbornadiene, NBD, to quadricyclene, Q (Reaction 3). In particular we have found that a catalytic amount of CuCl functions as an effective and quite specific sensitizer for this transformation. Conversions of greater than 90% have been achieved since Cu(I) is ineffective as a catalyst for the energy-releasing reverse reaction. Spectral and photochemical evidence support a mechanism which features a 1 1 ClCu—NBD complex as the photoactive species. As illustrated in Figure 3, an obvious consequence of complexation is a shift of the absorption spectrum of the system into a region accessible to the 313-nm irradiation used. Possible pathways by which the photo-excited complex relaxes to Q have been discussed (12). 

Sensitized isomerization of norbornadiene Valence isomerization of norbomadiene to quadricyclene has been suggested as a possible means of photochemical conversion and storage of solar energy. The isomerization of... 

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