Photodimerization of Olefins

Small-ring cyclic alkenes cannot deactivate by cis-trans isomerization. For instance, in contrast to acyclic alkenes, cyclopentene (48) therefore undergoes photosensitized [2 + 2] cycloaddition. For cycloalkenes with a six-membered or larger ring, a trans form becomes possible for molecules such as cyclooctene (2), photosensitized cis-trans isomerization is the more efTicient reaction path. (Cf. Section 7.1.2.) 

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. 

Irradiation of butadiene in isooctane yields the isomeric 1,2-divinylcyclo-butanes 53a and 53b and various other products, dependent on reaction conditions. The triplet-sensitized photoreaction yields 4-vinylcyclohexene (54) in addition to the divinylcyclobutanes tcf. Example 6.11), and the product ratio depends on the triplet energy of the sensitizer  

When is in excess of 60 kcal/mol,. s-rm/i.r-butadiene ( V = 60 kcal/ mol), which strongly predominates in the thermal equilibrium, is excited and produces mainly divinylcyclobutanes. When the V of the sensitizer is not high enough to excite. r-rran.s-butadiene, energy transfer to j-c -butadiene ( V = 54 kcal/mol) occurs instead, yielding vinylcyclohexene. If V 50 kcal/mol, non vertical energy transfer to a twisted diene triplet is believed to occur (Liu et al., 1%5). 

Intramolecular photoadditions can also occur as so-called x[2 -P 2] cycloadditions, as demonstrated for 1,5-hexadiene (55) the terminal carbon atom of each double bond adds to the internal carbon atom of the other double bond in such a way the resulting a bonds cross each other. 

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Similar relationships have been established with regard to the 1,3-dipolar addition and the photodimerization of olefins 62>. The HO—LU... 

Fig, 7.27 a and b. Orbital relationships in 1,3-dipolar additions and photodimerization of olefins, a) 1,3-Dipolar addition, b) Photodimerization of olefins... 

Photodimerization of olefins, 19 132 Photodissociation of metal-metal bonds, 19 106, 107, 124, 125 Photoelectrons... 

The examples of ex situ steady-state X-ray photodiffraction utihzed to follow the photodimerizations of olefin bonds in a single-crystal-to-single-crystal (or nearly so) manner are ubiquitous in the chemical literature. The interest of sohd-state chemists in this reaction dates back to the work of Cohen and Schmidt [30, 31], and it has become much of a guinea pig in organic solid-state photochemistry. In 1993, Enkelmann and collaborators published two seminal papers in the Journal of the American Chemical Society [32] and in Angewandte Chemie [33], where they presented a series of structures of a-tra s-cinnamic acid crystals reacted to various extents. These reports laid the way for a plethora of later studies on the olefin photodimerization reaction. The convenience of the high conversion and the simple mechanism, combined with the relatively small structural perturbation that it requires, has turned this reaction into a very useful tool to probe intermolecular... 

Recent studies on the photodimerization of olefinic crystals several examples, which deviate from accepted topochemical principles, have been... 

Photodimerization involves 1 1 adduct formation between an excited nd a ground state molecule. Olefinic compounds, aromatic hydrocarbons, conjugated dienes, oc-unsaturated compounds are known to dimerize when Cxpdsed to suitable radiation. Photodimerization of olefinic compounds Can occur by either (a) 1,2-1,2 addition, (b) 1,2-1,4 addition or... 

Fujita, M., Yoshizawa, M., Takeyama, Y., Kusukawa, T., Cavity-directed, highly stereoselective [2+2] photodimerization of olefins within self-assembled coordination cages. Angew. Chem. Int. Ed. 2002, 41, 1347-1349. 

As observed for the photoreduction of methyl orange [218] and the oxidative photodimerization of olefins [80] catalyzed by CdS powder, the dependence of rate on CdS concentration increases approximately linearly and reaches a plateau at about 3 g L. Surprisingly, in the same concentration range the ratio of addition to hydrodimer product (20a 21a) decreases from 2 to about 1. This can be rationalized by considering that the specific rate which is the measured rate divided by the amount of catalyst employed, decreases with increasing catalyst concentration (Figure 32). 

A preparative study of four-center type photopolymerization has established the empirical rule that the crystalline state [2- 2] photodimerization of olefins can be widely extended to the crystalline state photopolymerization of diolefins with a rigid linear structure containing conjugated double bonds separated by the 1,4-position of an aromatic ring, such as stilbazol to DSP, or cinnamic derivatives to I,4-phenylenediacrylic (p-PDA) derivatives (O. During the past eighteen years, a great number of photopolymeric diolefin crystals have been found by the use of this empirical rule (see Table I). 

The [2+2] photodimerization of olefins is the best studied reaction in the context of SCSC reactions in both co-crystals and single-component solids. Co-crystals that exhibit SCSC [2+2] photoreactivity have been achieved using both the host-guest and linear template approach. The reactive solids have involved neutral and ionic [49] species, and have been constructed primarily using hydrogen bonds. 

The photodimerization of olefinic systems such as cinnamic acid and stilbenes has been studied and reported for many years a2>. 

M. Yoshizawa, Y. Takeyama, T. Okano, M. Fujita, Cavity-directed synthesis within a self-assembled coordination cage highly selective [2-1-2] cross-photodimerization of olefins, J. Am. 

Coordination assemblies also show unique properties and functions through their architectures. Coordination cage 5 shows the stereo- and regiocontrol of intermolecular [2- -2] photodimerizations of olefins as well as the acceleration of the reactions (Figure 18a and This result means... 

Table 2 Yields of products obtained in the photodimerization of olefins Olefin Product Yield %...

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