Trans double bonds, irradiated

A number of workers have reported a very high rate of disappearance of double bonds on irradiation of PBD. Using IR spectroscopy, Parkinson and Sears (346) determined G(cis) to be 15, G(-trans) 11-22, and G(cis-trans) 7. The last reaction is the isomerization of a cis double bond to a trans double bond via an allylic radical intermediate. The rate of consumption of 1,2-vinyl groups was found to depend on the initial vinyl content values from 0.2 to 40 were reported. Similarly, Golub (347,348) reported a G-value of 7.9 for consumption of double bonds, and... 

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

Methyl sorbate gives three dimeric products from sensitized irradiation, Eq. 52. The trans configuration of the reactant double bonds is retained in all of the products. 134> Over 80% of the product arises from... 

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

As an example of compounds containing —N=N— double bonds, photoisomerization of azobenzene has been extensively studied. Irradiation of azobenzene in neutral solutions is known to result in cis trans photoequilibration.102 Similar photochemical cis - trans isom-... 

The effects of irradiation damage on polymers has also been followed using FT-IR 219,235,236), by obtaining difference spectra after specific exposure durations in a variety of environments. For polyethylene 219), the difference spectra revealed an increase in the vinyl end groups and an increase of ketonic carbonyl and trans-vinylidene double bonds. It was found that for every free radical formed approximately 10 carbonyl groups and 2 carbon-carbon double bonds were formed 236 ... 

An important controlling factor is the conformation of the triene about the two single bonds that link the three double bonds (2.21) an s-c/s-s-cis conformation is required for 6-electron ring-closure, but 4-electron closure can occur also in an s-cis-s-trans conformation. The different conformations interconvert rapidly in acyclic trienes, but because each has different absorption characteristics, the wavelength of irradiation can influence the course of reaction dramatically. 2,5-Dimethylhexa-1,3,5-triene undergoes efficient cis trans isomerization at 254 nm (where the major absorbing species is the s-trans-s-frans conformation), but it cyclizes to a cyclohexadiene (2.22) using 313 nm radiation. 

Direct irradiation of tetramethylethylene 27 afforded octamethylcyclobutane 28 (Scheme 8) in good yield31. Stereo specific dimerization was obtained in the irradiation of concentrated solutions of Z- and E-2-butenes 29 and 33, respectively32. Irradiation of 29 afforded a mixture of the all-c product 30 and the cis-anti-cis 31 along with double bond migration (32) and Z E isomerization (33). Irradiation of E-2-butene 33 afforded 31 and trans-anti-trans 34 as the dimeric products. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

In 1957, Golub [47] first reported the cis-trans isomerisation of an unsaturated macromolecule. He showed the successful transformation of the cis 1,4 BR into corresponding trans units by means of ultraviolet irradiation in the presence of a suitable sensitiser, which may be any one of a wide variety of organic bromine or sulfur compounds. In the photoisomerisation of BR or other diene elastomers, there is direct excitation of 7t-electrons of the double bonds to an antibonding state in which free rotation and hence geometrical interconversion can occur. 

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