Cyclohexene photoisomerization

Photoisomerizations of cycloheptenes and cyclohexenes are known as well, and photosensitized excitation and isomerization of these have been directly detected using nanosecond flash photolysis techniques19 and time-resolved photoacoustic calorimetry20. 

The smaller-sized cyclohexene and cycloheptene have also been subjected to enantiodifferentiating photoisomerization, although the corresponding ( )-isomers are short-lived transient species. Photosensitization of (Z)-cyclohexene 31Z with chiral benzene(poly)carboxylates affords trans-anti-trans-, cis-trans-, and cis-anti-cis-cyclodimers 34 (Sch. 32). Interestingly, of the former two chiral products, only the trans-anti-trans isomer is optically active and its ee reaches up to 68%, whilst the cis-trans isomer is totally racemic under a variety of irradiation conditions, for which two competing, concerted, and stepwise cyclodimerization mechanisms are responsible. Thus, the enantiodifferentiating photoisomerization of 31Z to the optically... 

Recent studies on the enantiodifferentiating photosensitization reveal that the singlet sensitization with appropriate enantiopure compounds leads to good-to-excellent enantiodifferentiating photoisomerization of cyclohexene, cycloheptene, and cyclo-octene through the corresponding... 

Both direct and sensitized irradiations of cyclohexene 78Z to give trans-anti-transcis-trans-, and cis-anti-cis- 2 + 2]-cyclodimers 80-82 in different ratios [59]. This photodimerization is believed to proceed through the initial formation of the highly strained ( )-isomer 78E, which is followed by the thermal concerted and/or stepwise cyclodimerization with 78Z, although no direct evidence for the intervention of 78E has been obtained and the cyclization mechanism(s) involved are not very clear. In this photocyclodimerization, the enantiodifferentiation occurs not in the cyclodimerization but in the initial photoisomerization step hence we classify this formally bimolecular reaction as a unimolecular enantiodifferenti-ating photosensitization. 

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. 

Further studies on the photoisomerization of ci5-cyclohexene and cycloocta-1,3-diene have been reported. Again the work has focused on enantiodiffer-entiation. In this case a series of optically active chiral sensitisers (3) have been used under conditions where solvent and temperature have been varied. Some of the o-disubstituted and tetra substituted amide sensitisers afford mixtures with enantioisomeric excesses of 14%. The influence of pressure and temperature on the asymmetric photochemistry of cyclooctene has been reported. A variety of chiral sensitisers were used. Some of these are shown in (4). Other work has shown that aromatic phosphates, phosphinates and phosphines (e.g. 5-8) can also sensitise the isomerism of cyclooctene. Moderate stationary-state ratios were obtained. 

Alk-l-enyl)cyclohexenes 9 photoisomerize regioselectively to either 6-vinylbicyclo[3.1.0]-hexanes 10 or bicyclo[5.1.0]oct-2-enes 11, depending on the position of a phenyl substituent (i.e. either on Cl of the ring or on the terminal vinyl carbon atom, respectively). " ... 

Upon sensitized irradiation, the triplet excited acyclic alkenes and large-ring cyc-loalkenes undergo E Z isomerization in both aprotic and protic media. Mediumring cycloalkanes cyclohexenes, cycloheptenes or cyclooctenes can, however, be protonated via the corresponding thermodynamically unstable (strained) /i-isomers formed initially by a Z —> E photoisomerization step (Section 6.1.1).662-664 For example, acid-catalysed irradiation of (Z)-l-methylcyclohexene (99) in the presence of /)-xylene as a triplet sensitizer in methanol affords the Markovnikov adduct 1-methoxy-l-methylcyclohexane (100) and the elimination product methylenecyclohexane (101), both in approximately 40% chemical yield (Scheme 6.41).671 The Zs-isomer intermediate exhibits extensive incorporation of deuterium in the presence of CH3OD. 

Singlet photosensitized polar addition of methanol to (A )-(>)-limonene (102) in nonpolar solvents afforded a mixture of the diastereomeric ethers 103 and 104 and the rearrangement product 105 (Scheme 6.42).677 The diastereomeric excess (de) of the photoadduct was optimized by varying the solvent polarity, reaction temperature and nature of the sensitizer. The first step of the reaction is the Z E photoisomerization (Section 6.1.1) of 102 to a highly strained /i-isomer, followed by protonation and methanol addition. The initial formation of a carbocation via the protonation step has been excluded under those reaction conditions. The Markovnikov-oriented methanol attack on the less-hindered (Rp)-(E)-102 compared with that of (Sp)-(E)-U)2 explains why 103 can be obtained in up to 96% de upon sensitization with methyl benzoate in a methanol solution. The hypothesis that Z E isomerization of the cyclohexene moiety affords a strained (reactive) alkene, whereas isomerization of the exocyclic double bond does not, was supported by the observation of an exclusive nucleophilic addition to the cyclohexene double bond. 

The copper(I) trifluorosulfonate catalyzed photoisomerization of cyclohexene (42a) leads to a mixture of three products, 91-93 (112,113). The... 

Especially interesting is the trans,anti,trans stereochemistry of the major cyclobutane product generated in the photodimerization of cyclohexene (eq 12). It was noted that the formation of this product may be the result of a preliminary CuOTf promoted cis-trans photoisomerization that generates a trans cyclohexene intermediate (eq 13). Since one face of the transC=C bond is shielded by a polymethylene chain, the trans-cyclohexene is restricted to suprafacial additions. Although a highly strained transcyclohexene intermediate could be stabilized by coordination with Cu, such a complex has not been isolated. 

Both aikenes and alkynes quench the excited state Pt2(pop)4. Aikenes such as trans-stilbcnc that have triplet energies of less than -57.7 kcal/mol quench the A2u State of Pt2(pop)4 by energy transfer. This quenching reaction is followed by emission from the excited state of stilbene, and also by photoisomerization of the stilbene. For cycloalkenes such as cyclohexene and cyclopentene that contain allylic C-H bonds, quenching reactions with Pt2(pop) proceed by hydrogen-atom abstraction ... 

Osaka University 16.5 Asymmetric Photoisomerization of Cycloalkenes Cyclooctene Cyclooctene Derivatives Cyclohexene and. . 16-12... 

A number of reviews have been devoted to the photoisomerism of alkenes, with the photochemistry of cycloalkenes treated as a subcategory.The geometrical photoisomerization of alkenes was reviewed also in the previous edition." However, the photobehavior of cycloalkenes is not a simple extension of that of acyclic alkenes, particularly in the cases of small- and medium-sized cycloalkenes. Medium-sized (E)-cycloalkenes produced photochemicaUy are highly constrained and rapidly react with alcohols in the presence, or even in the absence, of dilute acid. An account of the photoprotonation-driven cationic reactions, such as addition, isomerization, rearrangement, and fragmentation of cyclohexenes and cyclo-heptenes, in protic solvents has been published. The protonation of strained (E)-cycloalkenes was also reviewed in the previous edition. ... 

---