Cyclization photochemical mechanism

Benzothiepin (281), obtained from (282) by the action of a rhodium catalyst, has been cyclized photochemically to the cyclobutene (283), which can also be obtained directly from (282) by the action of heat, silver perchlorate, or radiation. For the thermal conversion of (282) into (283), deuterium-labelling studies ruled out an intermediate of type (284), in which rotatioh of the four-membered ring could occur, and the most likely mechanism appears to be a + 2j ring-opening to (281a) followed... 

Another approachl2a e makes use of bilenimincs which can be photochemically cyclized to isobacteriochlorins. A plausible mechanism for this ring-closure process involves the formation of an enamine tautomer from the imine at the 1-position, which is then cyclized in a photochemical 1 STc-n-isomcrization process followed by elimination of the leaving group at the 19-... 

Scheme 5.2. Proposed mechanism of the domino cyclization of photochemically induced reaction of 5-4.

Photochemical [2+2]cycloaddition between benzo[b]furan and 3-cyano-2-alkoxy-pyridines in benzene has been reported to follow a very interesting mechanism supported also by Frontier-MO calculations using the PM3 Hamiltonian. It is believed that the singlet excited state of the pyridine and the ground state benzofuran react to form a [2+2] adduct and is followed by ring opening to the cyclooctatriene, which cyclizes to the secondary endo- and exo-isomers shown below <00CC1201>. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Thermal or photochemical extrusion of nitrogen from 1-arylbenzotriazoles (114) leads to the formation of carbazoles (Scheme 14). The mechanism is believed to involve cyclization of a diradical (115) or an iminocarbene (116) to the 4a/f-carbazole (117) followed by an aromatizing hydrogen... 

A similar reaction has been observed experimentally (a) M. Chakraborty, C. A. Tessier, and W. J. Youngs, J. Org. Chem. 1999, 64, 2947. A photochemical C1C5 endiyne cyclization has recently been observed, but follows probably not a biradical mechanism via a fulvenediyl, but involves photoinduced electron transfer to the endiyne with subsequent cyclization of the radical anion (b) I. V. Alabugin and S. V. Kovalenko, J. Am. Chem. Soc. 2002, 124, 9052. 

The mechanism of this novel photochemical rearrangement involves a photochemical conrotatory cyclization, a novel 1,9-hydrogen shift, lateral ring opening, and finally photochemical conversion of the dienol ether to the conjugated ketone (Scheme 22). 

An interesting photochemical approach to 4,5-disubstituted N-alkylimidazoles consists of the photolysis of 2,3-dihydro-5,6-disubstituted-pyrazines that can be easily prepared from 1,2-diketones and 1,2-diamino-alkanes. For example, the preparative-scale photolysis, in absolute EtOH with high-pressure Hg lamp (Pyrex filter), of 5,6-dimethyl- or 5,6-diphenyl-2,3-dihydropyrazines 54, yields the corresponding N-methyl-imidazoles 57 in high yields (Scheme 12.16). The reaction mechanism involves the formation of an enediimine intermediate 55, followed by cyclization and re-aromatization [41]. 

A transient intermediate has been observed which decays with k = 12.6 s-1 in a first-order process. In view of its long lifetime the transient is supposed to be the cyclohexadienyl radical. A similar reaction mechanism had earlier been proposed for the photochemical cyclization of 5-(2-chlorophenyl)-1,3-diphenyl-1,2,4[1 //]-triazole561. Intramolecular aryl-aryl coupling has also been described for A2-l,2,4-triazol-5(l//)-one derivatives containing an 0-bromophenyl and a heteroaryl substituent in vicinal positions562. 

Photochemical and thermal isomerization and cyclization in bile pigments lead to 4-azaazulene derivatives. In Scheme 9, a proposed mechanism for the... 

There are a number of other mechanisms by which alkenes can undergo photochemical (2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cyclizes and accepts an electron to give the product cydobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycyclobutanes from alkoxy-ethylenes with dicyanonaphthalene as sensitizer (2.78). 

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

Palladium-catalyzed cyclization of the oxime derivative 47 provided a good yield of the pyrrole 48 (Equation 9) <1999CL45>. Similar reactions have been observed in connection with cyclization studies of related ketone trimethylhydrazonium salts <2005H(65)273>. Photochemical radical cyclization of 7,5-unsaturated ketone oximes has been reported to produce 1-pyrrolines <2005TL2373>. Similar 0-acetyloximes may also be annulated to 1-pyrrolines by treatment with acetic acid in the presence of 1,4-cyclohexadiene and naphthalene-l,5-diol, possibly proceeding via a radical mechanism <2002CL144>. 

---