Photochemical ring opening

The reverse reaction, the photochemical ring opening of sphopyranes (22b), takes place by absorption ia the short-wave uv region of the spectmm and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is ia the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colorless to a colored solution (87,88). These photochromic reactions can be used for technical appHcations (89). 

Vitamins are classified by their solubiUty characteristics iato fat-soluble and water-soluble groups. The fat-soluble vitamins A, E, and K result from the isoprenoid biosynthetic pathway. Vitamin A is derived by enzymic cleavage of the symmetrical C q beta-carotene, also known as pro-vitamin A. Vitamins E and K result from condensations of phytyldiphosphate (C2q) with aromatic components derived from shikimic acid. Vitamin D results from photochemical ring opening of 7-dehydrocholesterol, itself derived from squalene (C q). 

Figure 3.24 Resonance Raman Stokes and anti-Stokes difference spectra of the photochemical ring opening of 1,3-cyclohexadiene. Anti-Stokes spectra were obtained with 284-nm pump and probe wavelengths, while the two-color Stokes spectra were generated with a 284-nm probe and a 275-nm pump. The line at 801 cm is due to the cyclohexane solvent. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Figure 3.25 Schematic reaction coordinates for the photochemical ring opening reactions of 1,3-cyclohexadiene. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Conlin studied the photochemical ring opening of 1, l-dimethyl-2-phenyl-cyclobut-3-ene to yield a siladiene and its recyclization (Eq. 61). The act for cyclization was 9.4 kcal mol-1.113... 

Schultz and Kulkarni have explored the possibilities of using this process in asymmetric synthesis by trapping the ketene generated from these photochemical ring-opening... 

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. 

B. Photochemical Ring Opening 1. Spiropyran Ring Opening... 

To summarize the data obtained for BIPS and NIPS in polar solvents. Scheme 5 describes the photochemical ring-opening reaction, where the reaction forms the merocyanine excited singlet state in around 11 psec or less and the stable ground-state merocyanine distribution also forms on the picosecond time scale. The reaction is similarly rapid in nonpolar solvents except that the assigned merocyanine 5i state is not observed as a transient form. The state assigned to the 5i of the merocyanine, which is observed in polar solvents, may equally be assignable to another transient. The exact identity is not proven beyond doubt. 

Only much later could Parker and Spoerri relate the 247 mju absorption maximum to the Bb band of phenanthrene The observations of Lewis, Magel and Lip-kin of Smakula and the finding of Parker and Spoerri were finally correlated in 1963 by Moore, Morgan and Stermitz , who suggested the 4a,4b-dihydrophanthrene structure (i) for the yellow intermediate of Lewis et al. and reported on the thermal and photochemical ring opening processes,... 

The potential usefulness of 3//-l,2,4-diazaphospholes for the synthesis of other phosphorus heterocycles was demonstrated using 234 (Scheme 8.55) which, after UV irradiation at —40°C, gave a 5 1 mixture of 4,5-dihydro-3//-phospholes 235 and 236. The latter compound is the hrst example of the previously unknown 2H-phosphirene system (288). This reaction probably proceeds via a photochemical ring opening of 234 and formation of a phosphavinyl carbene. 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

Photochemical ring opening of 4,5-epoxy-2,3,4,5-tetraphenyl-2-cyclo-pentenone (66 R = = Ph) (prepared by oxidation of... 

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