Photooxygenation, of alkenes

Fig. 32 Examples of Wagnerova Type II photooxygenations of alkene, arene, and alkane substrates.

Stratakis, M. (2003). Selective photooxygenation of alkenes by zeolite confinement. In 13th European Symposium on Organic Chemistry, Medimond, V. (ed.), pp. 15-20. Medimond Publishing Co., Bologna... 

SCHEME 10. Intermolecular isotope effects in the photooxygenation of alkenes 20 vs 22... 

TABLE 7. Geminal selectivity in the photooxygenation of alkenes 45-48 bearing an aUyUc functionality ... 

SCHEME 16. Mechanism of the geminal selectivity in the photooxygenation of alkenes bearing a large alkyl substituent in allylic position... 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

The regioselectivity in the ene reaction has been reviewed.1295 The intrazeolitic photooxygenation of alkenes to produce hydroperoxides has attracted considerable... 

Oxygen may also be used as the oxidant for epoxidation. For example, a selective epoxidation, in the absence of metal ions, has been performed in excellent yield by treatment of alkenes with oxygen in the presence of a large excess of benzaldehyde36. Photooxygenation of alkenes, in the presence of transition metal catalysts, is a very general and synthetically useful reaction which leads to hydroxy-epoxidation (equation 4)37-39. 

The asymmetric photooxygenation of alkenes in chiral-supported zeolites was also reported [144,145] ... 

Epoxy alcohols. A few years ago Mihelich1 was granted a patent for preparation of epoxy alcohols by photooxygenation of alkenes in the presence of titanium or vanadium catalysts. Adam et al.2 have investigated this reaction in detail and find that Ti(IV) isopropoxide is the catalyst of choice for epoxidation of di-, tri-, and tetrasubstituted alkenes, acyclic and cyclic, to provide epoxy alcohols. When applied to allylic alcohols, the reaction can be diastereo- and enantioselective. The reaction actually proceeds in two steps an ene reaction to provide an allylic hydroperoxide followed by intramolecular transfer of oxygen catalyzed by Ti(0-i-Pr)4. The latter step is a form of Sharpless epoxidation and can be highly stereoselective. 

Epoxy alcohols have been synthesized by carrying out the photooxygenation of alkenes in the presence of the transition metal complexes derived frm Ti, V and Mo (for an example see equation 64b)." The hydroperoxides formed tom alkenes during the photooxygenation function as oxygen transfer reagents and precursors for the allylic alccAol intermediates. 

A much more efficient method is the one-pot synthesis of epoxy alcohols by photooxygenation of alkenes in the presence of transition metal complexes of titanium, vanadium or molybdenum. This method has been described for a variety of alkenes84 91. 

The vibrational deactivation of 102 has been elegantly exploited to achieve highly stereoselective photooxygenation of alkenes in which one site is protected by a nearby C H bond acting as a quencher.146 Replacement of the protective C H group by C D significantly reduces the selectivity of attack. The authors anticipate that this new concept of stereocontrol may prove to be general and may facilitate chiral control in a variety of phototransformations. 

Oxidation of Aliphatic Compounds. - A general review of the use of supra-molecular systems as microreactors for photochemical reactions contains a section dealing with the photosensitized oxidation of alkenes included in zeolites, nation membranes and vesicles. Particular consideration is given to the possibility of controlling the form and environment of the sensitizer and substrate so that the reaction selectively follows an energy-transfer or an ET pathway. The same authors have also provided a more substantial review on the same theme. Recent developments in relation to the stereochemistry and mechanism of the ene photooxygenation of alkenes by O2 have also been reviewed. ... 

A new electron-transfer mediator for photooxygenation of alkenes (to give allylic hydroperoxides) and methylarenes (to give aromatic aldehydes) is 9-mesityl-10-methylacridinium perchlorate (1). ... 

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