Photochemical reactions extrusion

Azides undergo intramolecular cycloaddition to C—C double bonds present in rings to provide a variety of novel products. Schultz et al for example, reported thermolysis of azide (206) at 110 C to afford an isolable triazoline as a single diastereomer (Scheme 64).115 A related reaction, however, gave an isomer mixture. Photochemical nitrogen extrusion from the triazoline provided a dihydropyrrole in 97% yield. 

Cycloaddition of diazoalkanes to 6-6 bonds was among the first reactions tested on the carbon cages41 and affords isolable fullerene-fused pyrazolines as primary adducts. Photochemically induced extrusion of N2 from the latter yields 6-6-closed methanofullerenes having the functionalized bond embedded in a cyclopropane substructure, whereas the thermal process affords 6-5 open homofullerenes in which a methylene group is bridging the open junction between a six- and a five-membered ring.15-17... 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The double bond in thiirene, thiirene-1-oxide, and thiirene-1,1-dioxide is expected to increase ring strain and lead, in general, to enhanced reactivity in any reactions that would lead to strain relief. Such relief would result from ringopening reactions, extrusion of S, SO, or SO2, or additions to the double bond that convert sp - to sp -carbon. Moreover, conjugation between the 7t-bond and sulfur, with its substituents, might be expected to lead to absorption of light and photochemical reactions. 

Furans give 2,5-cycloadducts on irradiation with alkynes (equation 75) . Thiophens probably behave similarly , but the product isolated is a substituted benzene which arises by extrusion of sulphur from the adduct (equation 76). The photochemical reaction with thiophen involves a triplet excited state of the thiophen, but both furan and thiophen cycloadditions can also be brought about thermally , (compare the pyrrole reaction in equation 74). 

Thermal and photochemical reactions of thiiranes, thiirane 1-oxides, and thiirane 1,1-dioxides may result in a cleavage of a carbon-sulfur bond and further extrusion of sulfur, sulfur monoxide or sulfur dioxide, respectively. 

Irradiation of the dithiin (298) brings about rearrangement and forms the thiophene derivative (299). An extrusion of sulfur also takes place and yields the thiophene (300). The rearrangement is proposed to occur via the ring opened dithione (301) and the episulfide (302). Sulfur-sulfur interactions appear to be a vital component in the photochemical reactions of dithiin-1-oxide (303) to give benzaldehyde and the dithiin (304). Thio analogues of (6—4) pyrimidine-pyrimidinone photodimers have been synthesised. ... 

Beside carbon monoxide extrusion acyl radicals formed in a a-cleavage reaction can stabilize by subsequent hydrogen migration. Thus the a-trimethylsilylmethyl substituted cyclopentanone used by L. F. Tietze gives in a clean photochemical reaction the corresponding aldehyde with a vinylsilane moiety in its side chain. 

Photochemical nitrogen extrusion also is the key reaction in a cross coupling reaction between reactive sides of a polymer chains as described by H. Meier. In this case a thia-diazole linked to a polyethylene chain (prepared from 3-pentanone in five steps) serves as the chromophore and double denitrogenation leads to chain dimerization with formation of a 1,4-dithiine. 

Dihydrobiphenyls are formed on irradiation of bicyclo[2.2.2]octenediones. Extrusion of CO is the initial photochemical step. A CIDNP study has examined the photochemical reactions between l-acetylindole-2,3-dione and various hydrogen donors. ... 

The results of the photochemical reactions of dimethyl acetylene-dicarboxylate with furan,222 thiophene,18 and 1 //-pyrrole11 were found to differ widely. Furan yielded either a 1 1 or a 2 1 adduct, and thiophene yielded a sulfur-free reaction product, viz., dimethyl phthalate. These products were formed by (4 + 2)-cycloadditions, which—in the case of thiophene—was followed by extrusion of sulfur from the initial Diels-Alder adduct. 1//-pyrrole, however, reacted in a different way because a l//-azepine (195) was isolated. This product was most probably generated by (2 + 2)-cycloaddition of the acetylene to the... 

---