Dioxetanes with singlet oxygen

Exceptions to this rule are compounds of the diamantylidene type (59) they form 1,2-dioxetans with singlet oxygen in spite of the presence of allyUc hydrogen atoms because an ene reaction would produce a hydroperoxide with a very strained double bond (forbidden by Bredt s rule). 

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]. 

Certain compounds react with singlet oxygen in a different manner, giving dioxetanes as products.129... 

The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen. This effect was demonstrated by the photooxygenation of / ,/ -dimethyl styrene (20). This substrate, apart from the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides as shown in Scheme 9. 

The cyclobutadiene structure (36), like cycloalkyne (37), also formed an adduct with singlet oxygen (dioxetane 143), which rearranged to a dione product (144 equation 31) (79LA473). 

In fact, the oxidation with singlet oxygen of alkenes not possessing allylic hydrogens is known to involve a 1,2-dioxetane intermediate15 16 (see Section 9.2.2). 

Kinetically stabilized azetes also show a high tendency for cycloaddition with a variety of other reagents. Cycloaddition of 96 with singlet oxygen produced an isolable dioxetan adduct 97, which decomposes at 25"C into tert-butyl cyanide and a 1,2-dione. 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

In the photo-oxygenation of enol ethers, where the ene reaction and [2 + 2]-cycloaddition compete, polar solvents favour cycloaddition whereas nonpolar solvents favour ene product formation [681, 683-685]. For example, 2,3-dihydro-4-methyl-4/f-pyran reacts with singlet oxygen to yield both a 1,2-dioxetane and an allylic hydro-... 

When phenylimidazoles react with singlet oxygen, although there is some evidence that 2,5-endoperoxides or zwitterionic intermediates may be formed initially, the products which are isolated (Scheme 45) appear to arise from the decomposition of hydroperoxides or dioxetanes (8lT(S9)19l). 

Electron-rich olefins, especially vinyl ethers, " ketene acetals, thioalkyl-substituted olefins,and enamines, " react readily with singlet oxygen to give dioxetane cleavage products. Under carefully controlled temperature conditions, the intermediary 1,2-dioxetanes can be isolated. The first 1,2-dioxetanes prepared in this manner were the cis and trans isomers (8a) and (8b), respectively (Eq. 14). 

In the oxidation of indene [39] and of 1,2-diphenylcyclobutene [25] with singlet oxygen generated by irradiation of the solutions of the compounds in the presence of sensitizers, dioxetanes are the probable intermediates in the conversion of the unsaturated hydrocarbons into dialdehydes and diketones, respectively. Different products may be formed depending on the solvents used (equation 65) [25]. 

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. 

---