Diynes electronic structure

The electronic structure of alkynes is related to that of alkenes, and the photochemistry of the two classes of compound reflects this similarity. Because the photochemistry of alkenes has received greater attention and has already been described in systematic form - it is not unexpected that the present account should point out the ways in which alkyne photochemistry parallels, or is markedly different from, that of alkenes. There is a considerable difference, however, in the range of compounds which has been studied in each class. Reports of photochemical reactions of alkynes very often refer to mono- or disubstituted acetylenes in which the substituents are alkyl, aryl or alkoxycarbonyl. There have been studies on diyne and enyne systems, but as yet there has emerged nothing in alkyne chemistry to match the wealth of photochemistry reported for dienes and polyenes. This reflects in part the greater tendency of the compounds containing the C=C bond to undergo photopolymerization rather than any other reaction on irradiation. Within this limitation there is a wide variety of reactions open to the excited states of alkynes, and quite a number of the processes have synthetic application or potential. 

Some of the colour changes are accompanied by isosbestic points in the optical spectra (Figs. 7a and c). Isosbestic points indicate a single-step transition process. In case of the diyne amphiphiles they imply a transition of the electronic structure of the backbone. Each polymer chain in its blue form is converted into another one in the red form without occurance of any intermediate state... 

In the case of the reaction between 2-diazopropane and diphenyldiacetylene, the reverse (as compared with other diynes) orientation of addition of the first molecule of the diazo compound with a predominant formation of 4-phenylethynylpyrazole is observed. Therefore, it is noteworthy that whereas the regioselectivity of the addition of diazoalkanes to alkenes is well studied audits products have, as a rule, the structure been predicted with respect to electron effects, the problem of orientation... 

The cyclic diyne (55) has C—C=C angles of 159.3° as determined by an X-ray analysis, which is close to the value found in cyclooctyne by electron diffraction 161 (see above). The carbon framework of the molecule is almost planar with the two CH2—CH2 units in an almost eclipsed arrangement. This and the transannular repulsion between the ii-moieties leads to the large value of 1.57 A for the C3—C4 single bond. All other structural parameters seem to be normal.164)... 

By reaction with buta-l,3-diyne, an unusual complex of a distorted cyclocumulene can be generated (equation 55). This structure is stabilized by coordination of the central double bond of the seven-membered ring to the electronically... 

The diyne ene (84) undergoes photochemical cyclization on irradiation at 350 nm. The product obtained was identified as the phenanthrene derivative (85). " A variant of the photo-Bergman cyclization has been reported following the irradiation of the enediynes (86). The reactions are brought about using electron transfer from cyclohexa-1,4-diene in acetonitrile. This affords the products (87) in the yields reported under the structure. The products are formed by cyclization via the fulvene radical anion intermediate (88). " ... 

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