Triplet oxygen, with alkenes

Scheme 3.22. Reaction of triplet oxygen with reactive alkenes and cyclopropanes [87-89].

However, it has been recently suggested that oxirane intermediates also play a part in this reaction, producing some of the minor products (Scheme 59)152. Dienes do not appear to be good substrates for this reaction, at least not with triplet oxygen, as the cation-radical Diels-Alder dimerization is much faster unless the alkene is sterically hindered153. 

Highly strained alkenes often behave chemically as 1,2-diradicals, and can therefore readily react with triplet oxygen (Scheme 3.6). 1,2-Quinodimethanes can also react as diradicals, and can undergo a formal [2 + 4] cycloaddition with triplet oxygen to yield cyclic peroxides (first example, Scheme 3.22). That this reaction also proceeds in the dark strongly indicates that no singlet oxygen is involved [87]. 

The triplet sensitizer may react with the 02 triplet or an oxidizable reactant (Figure 6.10), eg with alkenes, dienes, and aromatic and other unsaturated organic compounds. For many sensitizers the reaction with oxygen is so efficient that the former process is favoured over the latter [125]. 

Fig- 4. Summary of probable mechanisms for the reaction of atomic oxygen with saturated and unsaturated hydrocarbons in the gas phase. The reaction of 0( D) with an alkene is believed to form the expoxide directly, thus skipping the formation of a triplet biradical. 

The Z-alkene in the other half 257 was made from a furan 262. Cycloaddition with singlet oxygen (rose Bengal is a dye that sensitises the conversion of normal triplet oxygen to the singlet) leaves one necessarily Z-alkene in the five-membered ring 264. One possible mechanism for the decarboxylation of the cycloadduct 263 is given 265. 

This system is not a very efficient one and is replaced in artificial luminescent devices by various activated carboxylic acids with an a-methine proton or a double bond. The reactive chromophores react with normal triplet oxygen in the ground state the corresponding reaction of alkenes depends on light-activated... 

Alkenes and 1,3-dienes also react with triplet oxygen in the presence of ArjN, Lewis Acids or PhjC with ultraviolet irradiation (Barton el al., 1972, 1974, 1975 Hayes et al., 1978) to form cyclic peroxides such as [19]... 

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