1.3- Cyclohexadienes, irradiation

Cyclopentadiene behaves differently than the cyclohexadienes in that its radiolysis leads to high molecular weight polymer via a cationic mechanism89, whereas such compounds are not formed in high yield from cyclohexadienes irradiated in the liquid phase. 

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

Intramolecular dipolar azide-olefin cycloaddition of 723 took place upon heating in benzene to afford 724 (83JA3273). An alternative rearrangement process can take place upon photolysis of 724 to give 725. Mesylation of 4-(3-hydroxypropyl)-2,4,6-trimethyl-2,5-cyclohexadiene-l-one (78JA4618) and subsequent treatment with sodium azide in DMF afforded the respective azide 726 which underwent intramolecular cycloaddition to afford the triazoline 727 (83JOC2432). Irradiation of 727 gave the triazole derivative 728 (Scheme 126). 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

As described ( 6.5.3.1) l-phenylbora-2,5-cyclohexadiene (cyclic divinylborane) reacts with FeCCOj under irradiation to the Fe(CO)3 complex. The dinuclear borabenzene complex [(C5H5BPh)Fe(CO>2]2 is found as a by-product. It is to be compared with the well-known [Tj -CpFe(CO)2]2. CjHjBPh reacts spontaneously with Co2(CO)u to form three products. Above 60°C (C5H5BPh)Co(CO)2 is the only product . At 30°C yellow isomers III and IV having partially hydrogenated borabenzene ligands form above 40°C III isomerizes to IV. 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Irradiation of 1,3-cyclohexadiene in the presence of a sensitizer affords dimers (19) and (20) as well as two other dimers 24-26 ... 

The photoequilibrium between 1,3-cyclohexadienes and 1,3,5-hexatrienes 3t5,3i6) s key step jn synthesis of vitamin D, as shown in the formation of vitamin D3 (R = C8H17) via a [l,7]sigmatropic H-shift from previtamin D which is obtained by irradiating provitamin D (3.9) 317). 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Irradiation of mixtures of cyclohexene with 1,3-cyclohexadiene leads to high yield of dimers (G = 6.3)86. Schutte and Freeman87 found that radiolysis of 1,3-cyclohexadiene dissolved in various solvents gives dimers mainly via cationic Diels-Alder addition,... 

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

Photolytic. PCB-1260 in a 90% acetonitrile/water solution containing 0.2 to 0.3 M sodium borohydride and irradiated with UV light (X = 254 nm) reacted to yield dechlorinated biphenyls. After 2 h, about 75% of the congeners were destroyed. Without sodium borohydride, only 10% of the congeners had reacted. Products identified by GC include biphenyl, 2-, 3- and 4-chlorobiphenyl, six dichlorobiphenyls, three trichlorobiphenyls, l-phenyl-l,4-cyclohexadiene, and l-phenyl-3-cyclohexene (Epling et ah, 1988). 

Saturation of double bonds can also be achieved by catalytic transfer of hydrogen [74, 76], Styrene was quantitatively reduced to ethylbenzene by 1,4-cyclohexadiene in the presence of iodine under ultraviolet irradiation [76], and /ru j-stilbene to 1,2-diphenylethane in 90% yield by refluxing for 46 hours with cyclohexene in the presence of palladium on charcoal [74],... 

Irradiation of 138 (Z > 370 nm) together with 1,3-cyclohexadiene, 1,4-diphenylbutadiene, l-carbomethoxy-4-phenylbutadiene and co-nitro-... 

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. 

On the positive side, there are photochemical reactions that are essential for human health. One of these is the formation of vitamin D (the antirachitic vitamin) by irradiation of ergosterol. This photochemical reaction is an electro-cyclic ring opening of the cyclohexadiene ring of ergosterol of the type described in Section 28-2D. The product, previtamin D2, subsequently rearranges thermally to vitamin D2 ... 

The ring fission of 1,3-cyclohexadienes observed in the vitamin D series (Sec. IVA) is a general reaction for 1,3-cyclohexadienes and 2,4-cyclohexadienones (Sec. IIB). Irradiation of 1,3-cyclohexadiene (Formula 330) gives 1,3,5-hexatriene (Formula 331) (142,143), and a-phellendrene (Formula 332) gives a triene which is probably Formula 333 (143). The evidence for Structure 333 is not compelling. Methyl dehydro-... 

The ring fission of 1,3-cyclohexadienes to trienes is reversible. The photocyclization, however, is not always simple. Even in the absence of oxygen side products are formed. Irradiation of 1,3,5-hexatriene (Formula 345) gives benzene, hydrogen, and 1.2,4-bexatriene (Formula 346) in addition to 1,3-cyclohexadiene (149). 

Irradiation of 2,4,6-octatriene (Formula 352) in ether gives irans-5,6-dimethyl-l,3-cyclohexadiene (Formula 353) in low yield (155). 

Cyclic 1,3-Dienes. The first examples of photocyclization of 1,3-dienes to cyclobutenes were provided by the irradiation of pyrocalciferol and isopyrocalciferol (139) (see Sec. IVA). The reaction, however, is not general for 1,3-cyclohexadienes which usually undergo ring fission on irradiation. Recently, a method for circumventing this difficulty in at least one instance has been reported (156). Irradiation of 1,4-di-carbomethoxy-l,3-cyclohexadiene (Formula 356) in ether using diazo-... 

The most striking photocyclization of a 1,3-cyclohexadiene is one which occurs with concomitant shift of a hydrogen atom. Irradiation of 1,2,3,4,-5-pentaphenyl-l,3-cyclohexadiene (Formula 359) in the absence of oxygen in benzene solution gives l,2,3,5,6-pentaphenylbicyclo[3.1.0]-hex-2-ene (Formula 360) (158). 

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