Photochemical transpositions

In the reactions described in Section III,C, a three-atom fragment in the heterocycle is replaced by the same atoms of the reagent and with the same sequence of atoms. This type of reaction has been referred to as an inter-molecular transfragment reaction (67JA6911) and therefore can be clearly distinguished from molecular rearrangements, such as the Dimroth reactions (68MI1 see also Chapter IV) and photochemical transposition reactions. 

Singh, B. and Ullman, E.F. (1967) Photochemical transposition of ring atoms in 3,5-diarylisoxazoles. An unusual example of wavelength control in a photochemical reaction of azirines. Journal of the American Chemical Society, 89 (26), 6911-6916. 

Photochemical transpositions of steroids into the naphtho[2,1-f]quinoline system include a low-yielding photo-Beckmann (H.Suginome and T. Uchida, Bull, chem Soc. Japan, 1974, f , 687) and the conversion of 17-nitrite esters to the 17-a hydroxamic acids, reduction of which affords the lactam (S.H. Imam and B.A. Marples, Tetrahedron Letters, 1977, 2613). 

Photochemical transposition (ring isomerization) of the carbon atoms in benzene is known to involve a benzvalene intermediate.784,785 Gas-phase photolysis of o-xylene (174) at 254 nm, for example, leads to the benzvalene 175 and subsequently to m-xylene (176) at low conversion (<10% Scheme 6.75).797 A para-isomer is formed upon prolonged irradiation. 

Photochemical transposition reactions of some heteroaromatic compounds have already been discussed in Section 6.2.1. Scheme 6.183 shows the photoisomerization of 1-methylpyrazole (391), which may involve competition between electrocyclic ring closure and cleavage of the N1 N2 bond to give the same product (1-methylimidazole,... 

Nishimoto and co-workers reported a theoretical smdy on the photochemical rearrangement of isoxazoles to oxazoles. In a later smdy of the thermal and photochemical transposition of oxazoles, the authors concluded that these rearrangements involve the formation of azirine intermediates. ... 

The photochemical transposition reactions of substituted benzenes were first reported in 1964 by Wilzbach and Kaplan " for the xylenes (Eq. (46.1)) and Burgstahler etal." for the di-t-butyl benzenes (Eq. (46.2)). 

A similar observation has been made for the photochemical transposition reactions in the gas phase of the six isomers of dimethylpyridine. For these compounds, the mechanistic proposal is that the nitrogen atom is the unique one that forms the one-atom bridge in the 6-aza-bicylo[2.1.0]hexenyl biradical and that it migrates around the periphery of the five carbon atoms. 

Photochemical cycloadditions of furans have also been reported occasionally. The photochemical reaction of furan and 3-cyano-2-methoxypyridines (in benzene solution) results in the formation of 1 1 adducts 16 and 17 (accompanied by a transpositional pyridine and a pyridine dimer) <99JCS(P1)171>. 

The 15N NMR spectra of fully and selectively labelled 5-phenyltetrazoles (Table XVIII, note 0 demonstrate that photochemical ring transposition does not occur in the system [30]. 

Various thermal and photochemical rearrangements of the benzene ring have been reviewed including valence isomerization and ring transpositions, and, fairly brieffy, a range of other reactions that can be classed formally as rearrangements. 

According to the observed photochemical products and the results of deuterium labeling studies, the six isomeric phenylisothiazoles and phenyl-thiazoles can be organized into a tetrad of four isomers that interconvert mainly via P5, Pg, and P, transposition pathways and a dyad in which 5-phenylthiazole 52 transposes via P5 and P7 pathways to 4-phenylisothiazole 48 (Scheme 8), the only isomer that did not yield a transposition product upon irradiation in benzene solution. With one minor exception, no interconversions between the tetrad and dyad were observed. In that case, in addition to transposing to members of the tetrad, 5-phenylisothiazole 49 also transposed to 5-phenylthiazole 52, the first member of the dyad, in less than 1 % yield. This conversion was assumed to occur via a P4 permutation process. 

Several new syntheses of vinylsilanes have been described. Tris(trimethyl-silyl)aluminium undergoes 5yn-addition to alkynes alternatively the same -isomers can be obtained by photochemical isomerisation of Z-1-alkenyl-silanes. Other methods described involve treatment of the lithium salts of hydrazones with trimethylsilyl chloride, Wurtz-type coupling with vinyl bromides, and reaction of acetylenes with a silyl-copper reagent followed by an electrophile. Using the hydrazone method, a route has been devised for 1,2-carbonyl transposition within ketones (Scheme 17). ... 

Primary photochemical reactions produced under direct absorbed light action, having as result free radicals or transposition products formation ... 

McGowan, C.A., Schmieder, A.-K., Roberts, L., and Greaney, M.F. (2007) Synthesis of the gunacastepene A-B hydroazulene ring system through photochemical ring transposition. Org. Biomd. Chem., 5,1522-1528. 

Substituted benzvalenes provide a reasonable intermediate for these transposition reactions, as their photochemical formation (make bonds AC and BD) followed by thermal or secondary photochemical reversal (break bonds AB and CD) leads to the transposition of atoms B and C (Eq. (46.4)). 

First, the isomers react with very different efficiencies with an order of reactivity of 6 (para) > 7 (meta) > 8 (ortho). Estimates of the relative reactivity of the isomers from low conversion data are 6 7 8 = 32 4 1 moreover, the reactions do not occur with high efficiency. Quantum yields of disappearance for 6 and 7 were only 0.025 and 0.003, respectively. Second, a photostationary state can be reached starting with either 6 (para) or 7 (meta) which is the same for both 7% para, 26% meta, and 67% ortho. The ortho isomer reacts too slowly to reach this photostationary state, and the mass balance slowly drops. Third, low conversion data indicate that both 1,2- and 1,3-photo transpositions are occurring in the primary photochemical event. Thus, from Figure 46.1, in the photolysis of 6 (para) both the 7 (meta) and the 8 (ortho) isomers are being formed simultaneously in a ratio of 4.2 1. No delay in the formation of the... 

Photochemical reaction of benzo-analogs, quinohne-2-thiones (47), in the presence of a variety of alkenes gives 1 1 photoadducts via a thietane intermediate (Scheme 7). Irradiation of (47) in the presence of electron-deficient alkenes gives thiols (48) and affords 1 2 adducts (50) with alkyl vinyl ether. On the other hand, irradiation with other electron-rich alkenes, such as isobutene or tetramethylethylene, gives various products (48-51). The transpositional isomer, isoquinoHne-2-thiones (52), also give many types of adducts (53-56) upon irradiation in the presence of electron-rich alkenes. 

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