Photoelectron transfer cyclization

Whereas the previous methods for the synthesis of pyrrolo-oxazines were based on traditional ionic cyclization, the preparation of 7-rncthyltctrahydro-l //-pyrrolo[2,1 -c][l,4]oxazin K3//)-one 176 was reported using a new radical cyclization induced by photoelectronic transfer catalysis. This reaction is proceeding under very mild conditions and in neutral medium (Scheme 26). 

Bauld and coworkers studied the [2+2] cycloaddition of A-vinyl carbazoles 86a and electron-rich styrenes 86b catalyzed by iron(III) catalysts A or B in the presence of 2,2 -bipyridine as a ligand, which was reported originally by Ledwith and coworkers (Fig. 21) [142, 143]. Deuterium-labeling studies provided support for the stepwise nature of the process, consisting of reversible SET oxidation of the electron-rich olefin to a radical cation 86 A. Nucleophilic addition of excess 86 leads to distonic radical cation 86B, which cyclizes to cyclobutane radical cation 86C. Back electron transfer affords cyclobutanes 87 and regenerates the catalyst. Photoelectron transfer catalysis gave essentially the same result, thus supporting the pathway. 

Benites PJ, Holmberg RC, Rawat DS, et al. Metal-ligand charge-transfer-promoted photoelectronic Bergman cyclization of copper metalloenediynes photochemical DNA cleavage via C-4 H-atom absorption. J Am Chem Soc 2003 125 6434-46. 

The results of these studies were compared with those obtained earlier for the stilbene amine photoaddition [7, 11] and amine-enone photoelectron-transfer-catalyzed cyclizations (Table 7). The effects of a-substituents on the a-CH deprotonation rates of aminium radicals measured in the stilbene-amine photoaddition... 

The addition of radicals, generated by photoelectron transfer, to an unsaturated functional group has received the most attention. The fate of the intermediate radical does not depend strongly on the method used to produce it. An interesting example of a photoreductive reaction of co-unsaturated ketones in the presence of hexamethylphosphoric triamide (HMPA, neat) or triethylamine (Et3N) in acetonitrile has been reported [33]. The thus formed <5, e-unsaturated ketyl radicals exhibit the same behavior as m-unsaturated radicals [34] and they cyclize to produce cy-clopentanols and cyclohexanols. A typical example is provided in Scheme 18. 

Interestingly, imides possess a photochemical behavior that is very similar to that of ketones. Especially, phthalimides behave like phenyl ketones with respect to some of their photophysical properties. Despite many similarities there are at least three important differences. First, phthalimides are more prone to photoelectron transfer (PET) processes than ketones. This property was very successfully applied in the synthesis of a variety of amino acid derivatives (see Section 6.2.3.2). Second, the cyclization of imides often affords 0,7V-acetals as primary products, and this obviously has some consequences for the stability and the follow-up reactions of these products. Third, in contrast to aryl ketones, phthalimides are not quantitatively converted into the triplet state, and thus they may react both from the singlet and the triplet excited state. 

SCHEME 30.13 Top photo-Bergman cyclization of copper metalloenediynes via metal-ligand charge transfer complex. Bottom relative energies of copper and ligand MOs proposed to be involved in the photoelectronic Bergman cyclization of Cu(I) and Cu(II)-metalloenediynes. Adapted with permission from Benites et al. [27b]. American Chemical Society. 

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