Cycloaddition reactions photochemical alkene dimerization

Next, we will treat cycloaddition reactions of alkenes with the bare diamond surface, after high temperature vacuum annealing, which results in the formation of carbon carbon bonds [39 4l]. For example, if the diamond (100) surface is heated in vacuum to 1000°C, hydrogen desorbs, leaving surface C-C dimers. These have appreciable double-bond character and can react with alkenes under conditions appropriate for the Diels Alder cycloaddition reaction. Either the [2-h2] or the [2-1-4] product can be formed, with the latter being the energetically favored pathway. This type of modification can also be carried out photochemically, and this approach is the one that is used more commonly, as discussed in the next section. 

As discussed in Section 22.5, the [2 + 2] cycloaddition is photochemically allowed. The yields are often only mediocre, but this reaction is still useful because there are few good methods to prepare four-membered rings. As illustrated in the following equations, the cycloaddition can be used to dimerize two identical alkenes or to cyclize different alkenes ... 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of alkenes to cyclobutanes (15-63) gives very poor results in most cases, except when photochemically induced. Woodward and Hoffmann, and Fukui have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, for example, mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three... 

Table 20.2 summarizes all An and 4m + 2 reactions. Other 4m processes will follow the rules for the 2 + 2 dimerization of a pair of alkenes, and 4m + 2 processes will resemble the 4 + 2 cycloaddition we know as the Diels-Alder reaction. Perhaps you can see the relationship to aromatidty (4m + 2) that plays a role in this analysis. The transition state for these cycloaddition reactions is cyclic and will be allowed only in the cases where the number of electrons makes the transition state aromatic, 4m + 2 electrons for thermal processes and 4m for photochemical reactions. 

Photochemical [2+2] cycloaddition of alkenes in the crystalline state is synthetically very useful because it usually produces only one stereoisomer predicted ftom the crystal structure. On the other hand, this stereospeciflcity of the reaction can be a disadvantage because of inaccessibility to other stereoisomers. In order to circumvent such a problem, we explored compelled orientational control of the photodimerization of particular compounds like ranj-cinnamic acids and anthracenecarboxylic acids [74-78]. During our study, photochemistry of fluoro- and chloro-substituted ranj-stilbene-4-carboxylic acids and their methyl esters and alkaline and alkaline earth salts in the crystalline phase was likewise studied in order to synthesize specific stereoisomers selectively (Scheme 41) [79]. Most of these stilbene compounds dimerized to give exclusively or mainly syn head-to-head cyclobutane dimers. Some were photochemicaUy inert. 

Further cycloadditions include the 1,3-dipolar cycloadditions as well as the respective [2-i-l]-reactions. The latter have already been mentioned in the reaction of carbenes photochemically generated from diazirines (Section 6.5.2.4). Experimental examples for [3-i-2]-cycloadditions have not yet been reported. Still theoretical considerations gave rise to the assumption that such reactions with, for example, azides, diazomethane or other classical 1,3-dipoles should readily take place on the surface dimers. The calculations further revealed that the cleavage of nitrogen under formation of the respective azacyclopropane as known for normal alkenes should also take place for the [3-i-2]-adducts bound to diamond. For the reaction with ozone (Section 6.5.2.3) it has not yet been clarified whether or not an ozonide is initially formed by [3-i-2]-cycloaddition, only then to be transformed into the carbonyl compound. 

Margaretha and co-workers have described the cycloaddition of some cyclohex-2-enones to acrylonitrile. The addition reactions show moderate regioselectivity and form mixtures of exo- and ewJ6)-5-oxobicyclo[4.2.0]-octane-7-carbonitriles. Madhavan and Pitchumani have reported the dimerization of 2-cyclohexenone confined in clay interlayers (cation-exchanged bentonite). The reaction is remarkably regioselective and affords the head-to-head dimer almost exclusively. The cyclic alkene (24) undergoes photochemical addition to enones to afford the adducts (25) and (26) in the yields shown. 

---