Olefinic photoisomerization

The sensitized photoisomerization of alkenes is complex, with a variety of sensitizer-olefin interactions operative.(78 83)... 

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

The close relationship between reactions (5) and (7) is easily illustrated by consideration of a case in which the two processes are virtually identical. Consider the photoisomerization of an olefinic compound. 

If a system reaches the point where further irradiation under constant conditions causes no further change in composition, it is said to have reached its photostationary state (pss). The (cis/trans)PSB ratio in olefin isomerization depends on the relative extinction coefficients of the cis and trans isomers (i.e., their relative opportunities to isomerize) and on the quantum yields for both directions of the photoisomerization, as shown in Equation 13.35. 

The behavior of stilbene radical cations in the semiconductor catalysis is in keeping with the result of photoisomerization of other olefins like 6-methylstyrene sensitized by electron acceptors like chloranil in polar solvents (48). The semiconductor photocata-lyzed isomerization of strained cyclobutanes to strained dienes (isomerization of quadricyclene to norbomadiene and similar reactions of complex cage compounds (49)) is related to the olefin isomerization discussed above. 

Carbon-carbon double bonds (olefins) present significantly higher rotational barriers—typically 25-65 kcalmol 1—than single bonds, providing kinetic stability of both cis and trans isomers. This stability, together with the possibility of their interconversion by photoisomerization, have been exploited in the construction of a wide variety of rotors—and even directional molecular rotary motors—in which the rotor and base are connected via an olefin. 

The reaction can be also applied to azoles (e.g., imidazoles, pyrazoles, triazoles, tetrazole) as the N -nucleophile for the hydroaminations of cyclic olefins. In this case, the presence of methyl benzoate sensitizes the photoisomerization of the double bond to form a highly strained ( )-cycloalkene. Protonation of this intermediate by triflic acid (TfOH, 20mol%) and addition of the azole nucleophile completes the reaction sequence [41]. As an example, the expeditious synthesis of l-(l-methylcy-clohexyl)- H-imidazole (27) in 72% yield is shown in Scheme 3.17 [41]. 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

Under this photolysis condition, the rates of photoisomerization of the initially formed silacyclopropanes to the silylalkenes are rather slow. Irradiation of a hexane solution of 10 in the presence of 1-butene followed by treatment of the photolysis mixture with dry methanol after irradiation was stopped, affords 2-(methylphenylmethoxysilyl)butane in 27% yield, along with a small amount of silylalkenes. Similar photolysis of 10 in the presence of internal olefins or cyclic olefins gives the respective silacyclopropanes as the main products, together with the photorearranged silylalkenes as minor ones. These silacyclopropanes cannot be isolated by distillation or by GLC because of their extreme kinetic instability, but the formation of the silacyclopropanes can be determined by proton NMR spectroscopy (52). 

In 1989, a different approach was published by Orito [60], in which elaidinization ((Z) —> ( ) double bond isomerization) is used to obtain ( )-MNA from a (Z, )-mixture ofdiastereomers (Scheme 4.6). Gannet had observed that the iodine-induced photoisomerization of the methyl ester of MNA (48) gave only a 7 3 ( /Z) mixture [59], but Orito obtained a better diastereomeric ratio (8 1) using nitrous acid. Remarkably, no double-bond migration to form the more stable trisubstituted olefin was observed. This discovery paved the way to a very simple and general synthesis of the acidic component of capsaicinoids. Thus, a Wittig reaction of the phosphonium salt of a 6-bromohexanoic acid (49) with isobutyraldehyde (SO) afforded a 1 11... 

The photolysis of /rani-dibenzoylstilbene episulfide affords three primary products, trans- and cw-dibenzoyl stilbene (73 % and 16 % respectively) and cis-dibenzoyl stilbene episulfide . The cis episulfide gave essentially pure trans olefin. From exposure time studies it was concluded that the desulfurization of the trans episulfide is 97 % stereospecific and that most of the cis olefin originates from photoisomerization of the trans olefin. It was concluded that the initial photochemical... 

Quina, F H., and Wliitten, D. G. Photochemical Reactions in Organized Monolayer Assemblies. 4. Photodimerization, Photoisomerization, and Excimer Formation with Surfactant Olefins and Dienes in Monolayer Assemblies, Crystals, and Micelles. J. Am. Chem. Soc. 99, 877 (1977). 

Cis-trans photoisomerizations have been studied in great detail and can serve as an instructive example for the use of state correlation diagrams in discussing photochemical reactions. They have been observed for olefins, azomethines, and azo compounds. 

Sensitized irradiation of cyclohexenes and cycloheptenes in protic media results in protonation. This phenomenon, which is not shared by other acyclic or cyclic olefins, has been attributed to ground-state protonation of a highly strained tran -cycloalkene intermediate. In aprotic media, either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of (2 -I- 2] dimers 49-51 as the primary products, with 50 predominating. The reaction apparently involves an initial cis-trans photoisomerization of cyclohexene followed by a nonstereospecific nonconcerted ground-state cycloaddition, promoted by the high degree of strain involved. In contrast, cycloheptene undergoes only a slow addition to the p-xylene used as sensitizer. 

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