Singlet oxygen reaction intermediates

Hasty, N.M. and Kearns, D.R., Mechanisms of singlet oxygen reactions. Intermediates in the reaction of singlet oxygen with dienes, /. Am. Chem. Soc., 95, 3380, 1973. 

This is the first example of the isolation of a compound of the type postulated as intermediate in the ketene-singlet oxygen reaction. Instead... 

Enamines which cannot undergo ene-like reactions add efficiently to singlet oxygen. The intermediate dioxetane can be cleaved under mild conditions to afford a ketone (and an amide). Such a sequence has been applied in a synthesis of testosterone (6.18) 624). 

Toutchkine, A., Aebisher, D. and Clennan, E.L. (2001). Substituent dictated partitioning of intermediates on the sulfide singlet oxygen reaction surface. A new mechanism for oxidative C-S bond cleavage in a-hydroperoxysulfides. J. Am. Chem. Soc. 123, 4966 4973... 

The identification of the major products of the photooxidation of phenothiazine upon its direct excitation does not support the idea of a N-hydroperoxy intermediate (Equation 5) (11). In fact, phenothia-zine-5-oxide (A) which has been first identified by comparing Rf values in thin layer chromatography of isolated and separately prepared samples (11) represents the best evidence of a singlet oxygen reaction (Equation 9) (12). 

Sulfone production itself is not a singlet oxygen reaction (26) the reaction requires most probably an electronically excited phenothia-zine intermediate since superoxide and MPT do not react in model reactions using K02 and crown ethers in pyridine. 

Since no dealkylation is observed in the reaction of MPT with singlet oxygen, an intermediate leading to a reaction sequence as postulated in Scheme 35 (30,33) does not seem realistic. 

The pyrazine I reacts with singlet oxygen to yield the endoperoxide 2, which is converted into the 1,3,6-oxadiazepine 4 on treatment with triphenylphosphanc. It is thought that the reaction involves deoxygenation to the bicyclic oxirane 3 as the key intermediate.320... 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

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]. 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

In addition, particularly micro-channel processing is demanded here as the reaction is dangerous owing to the explosive nature of the endoperoxide formed as intermediate. This handling of explosive or toxic compounds for Diels-Alder reactions is also described in [21]. Owing to the use of only small volumes, the hazardous potential can be substantially minimized. This was exemplarily shown for the addition of singlet oxygen. 

GL 24] [R 1] [P 26] The feasibility of safely carrying out the oxidation of cyclopentadiene by singlet oxygen to 2-cyclopentene-l, 4-diol was demonstrated [40]. The explosive intermediate endoperoxide was generated and without isolation used onsite for a subsequent hydration reaction. By reduction with thiourea the pharmaceutically important product 2-cyclopentene-l,4-diol was so obtained. 

Photochemical irradiation of dimethyl and diethyl sulphoxides yields the corresponding sulphone in the presence of air and a photosensitizer such as methylene blue in yields up to 99% . Sulphoxides are also oxidized when they act as traps for persulphoxides, the intermediate formed on reaction of a sulphide with photochemically generated singlet oxygen - , equation (9). Isotope studies have shown that such reactions proceed through a linear sulphurane intermediate . Persulphones also react with sulphoxides in a similar manner , equation (10). 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

The elementary reaction step, which involves the formation of singlet oxygen, is a reaction of superoxide anion radicals CO ), which are the reaction intermediates of the above oxidation when performed in an alkaline medium. In the presence of water they may be converted to hydrogen peroxyl radicals HOO as follows ... 

In contrast to some analogous structures, pyrido[2,l-f][l,2,4]triazine 108 is quite stable toward atmospheric oxygen. It reacts, however, with singlet oxygen in a cycloaddition reaction, and the intermediate 109 thus formed loses nitrogen resulting in the pyridone 110 (Scheme 8) <2001JHC205>. 

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