Cyclohexene photodimerization

The purpose of this section is to demonstrate the reaction rate constant and selectivity of a bimolecular reaction may be influenced by solute-solute clustering as the pressure is lowered towards the critical point. The solvent sensitive photodimerization of 2-cyclohexen-l-one is an ideal candidate as both the rate of dimerization and the product selectivity may be monitored. 

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. 

Either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of [2 + 2] dimers in nearly quantitative chemical yields (Scheme 6.57).557 Dimerization is interpreted in terms of initial photoinduced E Z isomerization of cyclohexene (Section 6.1.1), followed by a non-stereospecific ground-state addition of (/i)-cyclohexene to the Z-isomer. In contrast, copper(I)-catalysed photodimerization of cyclohexene produces only one major stereoisomer 129 (Scheme 6.57).706 According to... 

Stable cis- -phenyl-1 -cyclohexene 24 photodimerizes via Diels-Alder cycloaddition to trans adduct 25 (Equation 1.33) [66] and the photoexcitation of dihydrobenzofuran-fused cyclohexenone 26 in net furan gives the trans fused Diels-Alder adduct 27 (Equation 1.34) [67]. 

Photodimerization of cyclohexene sensitized by xylene gives a low yield of a mixture of stereoisomers. The reaction was interpreted as a Z—t E isomerization followed by a nonstereospecific ground state [2tt + 2tt] addition. ... 

Calculations by the semiempirical theoretical method ASED-MO of the photodimerization of cyclohexene and methane by decatungstate anions have been carried out [84], Calculated hydrogen abstraction activation energies for a C-H bond in methane are higher than for cyclohexene, but they are low enough that in the case of methane it can be suggested that dimerization could be looked for in future experiments. 

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. 

In contrast to acyclic olefins, sensitized photodimerization of cyclic olefins is often fairly efficient (Eq. 16.28). A major reason for this is that cis-trans isomerization is no longer an efficient side reaction. As shown with the example of cyclohexene, this is an excellent way to make cyclobutanes, but control of stereochemistry is problematical. 

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