Aldehydes Paterno-Buchi photocycloaddition

The Paterno-Buchi photocycloaddition of silyl 0,X-ketene acetals (with X—O, S, Se) and aromatic aldehydes was intensively investigated by Abe and coworkers in the last decade [62]. The regioselectivity of the reaction (71 vs. 72) is highly affected by the heteroatom (Sch. 19) [63,64]. The regioselectivity is rationalized by (a) the relative stability of the 1,4-biradicals and (b) the relative nucleophilicity of sp2-carbons in the respective 0,X-ketene acetal. 

TABLE 1.79. ERYTHRO a-AMINO-P-HYDROXY METHYL KETONES FROM PATERNO-BUCHI PHOTOCYCLOADDITION OF 2,4,5-TRIMETHYLOXAZOLE WITH ALDEHYDES OR a-KETOESTERS ... 

Abe, M., Fujimoto, K., and Nojima, M., Notable sulfur atom effects on the regio- and stereoselective formation of oxetanes in Paterno-Buchi photocycloaddition of aromatic aldehydes with silyl 0,S-ketene acetals, /. Am. Chem. Soc., 122, 4005, 2000. 

Abe, M., Torii, E., and Nojima, M., Paterno-Buchi photocycloaddition of 2-siloxyfurans and carbonyl compounds notable substituent and carbonyl (aldehyde vs. ketone and singlet- vs. triplet excited state) effects on the regioselectivity (double-bond selection) in the formation of bicyclic exo-oxetanes, /. Org. Chem., 65, 3426, 2000. 

Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

The Paterno-BUchi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes. The excited state of the ketone is 11-71, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. The orientation usually observed for C- and X-substituted alkenes is shown for benzophenone 8.15 and 2-methylpropene 8.16. 

Another synthetically important photochemical reaction is the Paterno-Buchi reaction, i.e., the photocycloaddition of ketones and aldehydes to olefins. This is a milestone in organic photochemistry and involves attack of the n,ji triplet of the carbonyl compounds to an alkene in the ground state, mostly in the triplet multiplicity, although reactions via the singlet are well known. With nucleophilic olefins, the reaction occurs through the initial formation of a CO bond, in the opposite case, formation of a C-C bond occurs first. The use... 

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