Cycloadditions forming 1,2-dioxetans

With electron rich olefins 1,2 cycloaddition forms relatively unstable dioxetanes which cleave to give carbonyl fragments ... 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

A further cycloaddition of interest is that of aldehydes and ketones to furans.289 The oxetane and dioxetanes formed from furan and benzophenone have been the subject of considerable study, and are now believed290 to have the detailed structure and stereochemistry... 

Singlet oxygen reactions involving rearrangements of initially formed endoper-oxides to dioxetanes have been reported [24, 25]. For example, 2-(2 -anthryl)-l,4-dioxene 9 initially formed an endoperoxide by a [2 + 4]-cycloaddition reaction, and later rearranged to a dioxetane when separated on silica gel with o-xylene [24],... 

In some instances the primary product of alkene photooxidatitxi is not the allylically rearranged hydroperoxide, but the dioxetane addition product, e.g. (56), which may or may not formed by concerted [2 + 2] cycloaddition. Some of these dioxetanes, e.g. (57), ate relatively stable, although most suffer cleavage to produce carbonyl compounds or other materials. For example, photooxidation of indene gives homophthaldehyde (58) which was not produced under identical reaction conditions from hydroperoxide (59). Isomeric hydroperoxides (60) and (61) were also isolated when the oxidation was carried out in methanolic solution (Scheme 14). 

Some substituted alkenes react with singlet oxygen to form a dioxetane in a -f- cycloaddition reaction. Most dioxetanes readily decompose to carbonyl compounds in an exothermic reaction that is accompanied by a bluish luminescence. The chemiluminescence will be dealt with in more detail in Section 7.6.4. 

The foregoing sequence is less effective with the -isomer (234), the total yield of product being 65%. The reason lies in the availability of axially disposed allylic hydrogen atoms on the side of the molecule that is sterically accessible to singlet oxygen. In other words, allylic hydroperoxidation pre-empts cycloaddition to the dioxetane. Consequently, although (15), (16) and (237) (17, 17 and 14% yield respectively) are formed as before, the cyclohexenecarboxaldehyde (238) is the major product (30% yield) (Equation (37)). 

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. 

For similar substrates, such as substituted styrenes, stilbenes, or 1-vinylthiophenes, in some cases bis-endoperoxides have been isolated similar to the indene and dihydronaphthalene cases vide infra). Also, terpene derivatives are a treasure house of polyfunctional substrates for which aU types of reaction modes have been observed. Substituted 3-vinylindoles 48 without aUyhc hydrogens give with Oj the endoperoxides 49 by [4 -I- 2]-cycloaddition unless the s-cis conformation is disfavored due to the substituent pattern. Product 50 is formed presumably by [2 -I- 2]-cycloaddition of singlet oxygen and subsequent 1,2-dioxetane cleavage. 

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