Electron transfer Paterno-Buchi reaction

Bach and coworkers investigated the photocycloaddition of 7V-acyl, 7V-alkyl enamines 125 with benzaldehyde [125]. The 3-amido oxetanes 126 were formed with excellent regioselectivity (analogous to reactions with enolethers—vide supra) and good diastereoselectivity (Sch. 41). Enamines, not deactivated by acylation at the nitrogen atom are poor substrates for Paterno-Buchi reactions due to preferred electron transfer reactivity (formation of the corresponding enamine radical cation and subsequent reactions). 

Photoinduced [2+2] cycloaddition (the Paterno-Buchi reaction) of 1-acetylisatin with acyclic enol ethers afford the spiro(3f/-indole-3,2 -oxetane)s 43 with moderate regio- and diastereoselectivity via the mi triplet state of the isatin derivative without involvement of single electron transfer <02JCS(P1)345>. 

Transformations of arene substituents can also be achieved by transfer of an electron from a ground state donor to the excited state of the arene ring, or from an excited state donor to the ground state arene. Thus the retro Paterno-BUchi reaction of the benzo-phenone-tetramethylethylene oxetane (373) can be induced by irradiation in the presence of triethylamine as the electron donor. This yields 1,l-diphenyl-2-methylpropene the regiochemistry of this fragmentation is the opposite to that obtained by irradiation of electron acceptor arenes in the presence of (373), in which case benzophenone is the product. 

Eckert, G. and Goez, M., Photoinduced electron-transfer reactions of aryl olefins. 1. Investigation of the Paterno-Buchi reaction between quinones and anetholes in polar-solvents, /. Am. Chem. Soc., 116, 11999, 1994. 

Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Paterno-Buchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies involving quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate which appear on the ns/ps time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse ( c=108/s) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

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