Paterno-Biichi reaction diastereoselectivity

Hu, S. and Neckers, D.C. (1997) Rapid regio and diastereoselective Paterno-Biichi reaction of alkyl phenylglyoxylates. Journal of Organic Chemistry, 62, 564—567. 

The difference between the simple diastereoselectivities in photocycloaddition reactions following the singlet vs. the triplet route were studied by determination of the concentration dependence of the Paterno-Biichi reaction [155]. Carbonyl substrates which have both reactive singlet and triplet states, exhibit one characteristic substrate concentration where a 1 1 ratio of single and triplet reactivity, i.e., spinselectivity, could be detected. The shape of these concentration/diastereoselectivity corrrelations reflects the different kinetic contributions to this complex reaction scenario. 

The diastereoselective Paterno-Biichi reaction of benzoin 128 with benzo[ ]furan 82 was utilized to stereoselec-tively construct a [6-5-4] tricyclic heterocycle 129 (Equation 110). The observed diastereoselective excess was also explained <2004TL3877>. 

Solvent polarity can trigger a photoinduced electron-transfer (PET) (Section 5.2) step in the Paterno Biichi reaction of benzaldehyde to dihydrofuran, thus affecting the reaction regio- and diastereoselectivity (Scheme 6.106).899 Whereas the reaction in benzene proceeds via a triplet biradical intermediate, a radical ion pair is formed in acetonitrile. The electron transfer alters the charge distribution in the reactants, which promotes the formation of two different regioisomers with inverted diastereoselectivity. 

Hydrogen bonding between the ground-state reactants dihydropyridone (259) and an aldehyde 260 was found to be responsible for a high facial diastereoselectivity (>90% de) (see also Special Topic 6.3) in the Paterno Biichi reaction (Scheme 6.107).900 901 X-ray analysis, furthermore, revealed that the corresponding lactam moieties in the racemic product 261 are intramolecularly hydrogen-bonded (dashed). 

Recent reports emphasizing the role of 1,4-diradicals in triplet Paterno-Biichi reactions should also be mentioned. The first to be discussed concerns the chiral induction of photocycloadditions of various olefins to chiral phenylglyoxalates11 28,29, The high diastereoselectivity of these reactions often reaches de values of >96% and shows a characteristic temperature dependence with specific points of inversion. This behavior is a result of competition between enthalpy- and entropy-controlled partial selection steps. 

Irradiation of atropisomeric a-oxoamides (25) leads to intramolecular Paterno-Biichi reaction, affording bicyclic oxetanes with very high enantio- and diastereoselectivity. Remarkable differences in product selectivity are observed in solution and in the crystalline state. ... 

Effect of Hydrogen Bonding on Diastereoselectivity of Paterno-Biichi Reaction... 

The induced diastereoselectivity in a Paterno-Biichi reaction resulting from a stereogenic center in the alkene part was recently described by Bach and co-workers in the photocycloaddition of chiral silylenol ethers 67 with benzaldehyde 18. The substituents, R, at the stereogenic center were varied in order to evaluate the influence of steric bulk and possible electronic effects. In accord with the 1,3-aUyhc strain model, the facial diastereoselectivity was at a maximum with large (R = t-Bu, SiMejPh) and polar (R = OMe) substituents at the y-position of silyl enol ether (diastereomeric ratio of oxetanes 68 > 95 5). 

Buschmann, H., Scharf, H.-D., Hoffmann, N., Plath, M., and Runsink, J., Chiral induction in photochemical reactions. 10. The principle of isoinversion a model of stereoselection developed from the diastereoselectivity of the Paterno-Biichi reaction, /. Am. Chem. Soc., Ill, 5367, 1989. 

Adam, W., Stegmann, V. R., and Weinkoetz, S., Unusual temperature-dependent diastereoselectivity in the [2-1-2]-photocycloaddition (Paterno-Biichi reaction) of benzophenone to ds- and trans-cyclooctene through conformational control, /. Am. Chem. Soc., 123, 2452, 2001. 

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