Conjugated polyenes dimerization

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

The spirit is to show some of the results, but also to guide users of the approach by pointing to the problems and limitations of the method. The review covers some of the newer applications in the spectroscopy of organic molecules acetone, methylenecyclopropene, biphenyl, bithiophene, the protein chromophores indole and imidazole, and a series of radical cations of conjugated polyenes and polyaromatic hydrocarbons. The applications in transition metal chemistry include carbonyl, nitrosyl, and cyanide complexes, some dihalogens, and the chromium dimer. 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

While there is no doubt that as the degree of conjugation in the polymer increases, stable polyenic carbenium ions are formed in the acidic medium, the nature of the chain carriers involved in the formation of the dimeric ester and in the early sta s of its polymerisation is worth discussing. The authors invoked the formation of carbenium ions from the first step of the process but no proof of their presence, or of the presence of the dimer cation and dication was offered. An alternative mechanism based on activated ester molecules would be more plausible to us in view of the hi reactivity of the cyclopentadienyl cation and the low likelyhood that it would be formed in such mildly acidic conditions as those employed in these experiments. 

On the other hand, photochemical addition of alcohols to conjugated dienes, observed in the case of cholestadieiie-3,5, is probably related to the reactions observed when vitamin A acetate is irradiated in methyl alcohol. It is also possible that the photoisomerization of acyclic trienes to allenes is related to the as yet incompletely studied dimerization reactions of larger polyenes. 

Equations (15) and (16) define the so-called effective conjugation coordinate of ECC theory. The important conclusion that can be derived from Eqs. (11) and (17) is that only normal modes that contain an oscillation of the dimerization amplitude ( R vibration) can have relevant Raman cross sections. Then the two relevant lines of the Raman spectrum of polyenes are necessarily assigned to normal modes that involve a large contribution by the fl oscillation (FI modes in the ECC theory). More precisely, the treatment of the dynamical problem of polyenes in terms of ECC theory assigns the two strong Raman lines to two different combinations (in-phase and out-of-phase) of the R oscillation with C—H wagging vibration. 

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