Stereoselectivity in photocycloadditions

Buhr, S., Griesbeck, A. G., Lex, J., Mattay, J., Schroeer, J. Electronic control of stereoselectivity in photocycloaddition reactions. 7. Stereoselectivity in the Paterno-Biichi reaction of 2,2-diisopropyl-1,3-dioxole with methyl trimethylpyruvate. Tetrahedron Lett. 1996, 37, 1195-1196. 

Obata. T. Shimo, T. Yasutake. M. Shinmyozu, T. Kawaminami, M. Yoshida. R. Somekawa, K. Remarkable interaction effects of molecular packing on site- and stereoselectivity in photocycloaddition of 2-pyrones with maleimide in the solid state. Tetrahedron 2001, 57. 1531 -1541. 

Wiesner and coworkers have emphasized that while the equilibrium constants between the two diastereomeric photoexcited states and anionic intermediates respectively should be similar, there is no reason to expect that they must be numerically identical. Small differences in equilibrium constants could in some cases reverse the stereoselectivity of photocycloaddition with respect to metal reduction. The group of Cargill94 examined the validity of Wiesner s models by the photoaddition of tert-butylcyclohexenone 178 with ethylene. Irradiation at low temperature (—78 °C) afforded a mixture of three isomers 179-181, in which the photoproduct 179 is the major product while isomer 180, expected to be the major one based on the first model, was obtained as the minor isomer. This result seems to rule out the first model (it does not take into consideration the reversibility of the first bond formation in the intermolecular photoadditions), however, it is consistent with the second model (Scheme 39). 

The effect of substituents on the stereoselectivity of the intramolecular photocycloadditions of alkenes to cyclohexenones was systematically examined by Becker and coworkers84 who obtained high stereofacial selectivity in compounds 283a-c. However, small changes in the position, geometry or steric effect of the substituents have dramatically affected the selectivity, indicating the complexity in predicting the stereoselectivity in such system (Scheme 61). 

These results clearly show that the stereoselectivity in the photocycloaddition of 1-CN and 2-CN with 2- and/or 3-furylmethanol is reasonably explained by the hydrogen-bonding in the excited states. Probably, a partial electron transfer from furylmethanols (F-OH) to the excited singlet states of cyanonaph-thalenes (ArCN) contributes the formation of polar exciplexes [ArCN8 F-OH8+], where ArCN8 interacts with more acidic OH group of F-OH8 to produce hydrogen bonds in the excited states. 

A promising tool for controlling the regio- and stereoselectivity of photocycloaddition reaction (PCA) as well as the efficiency may be provided by assembling alkenes into a supramolecular structure with pre-organization of reactants such that the spatial arrangement of molecules would be favorable for the formation of only one cyclobutane isomer in a high yield. 

Termont, D., De Keukeleire, D., and Vandewalle, M. (1977) Regio- and stereoselectivity in [ 2 + 2] photocycloaddition reactions between cyclopent-2-enone and electron-rich alkenes. Journal of the Chemical Society, Perkin Transactions 1, 2349-2353. 

Bach, T., Bergmann, H., Grosch, B., and Harms, K. (2002) Highly enantioselective intra- and intermolecular [2 + 2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host host-guest interactions, product configuration, and the origin of the stereoselectivity in solution. Journal of the American Chemical Society, 124, 7982-7990. 

In contrast to photocycloaddition reactions of carbonyl compounds to electron-rich alkenes (which proceed with a low degree of stereoselectivity in the case of triplet excited carbonyls), reactions with electron-deficient alkenes, such as cyanoalkenes, are, although rather inefficient, but highly stereoselective [101]. Kinetic analysis showed that these reactions involve the... 

The stereoselectivity in the DCA-sensitized inter- and intra- molecular (2 r -I- 27t) photocycloaddition depends on the nature of solvents, the reaction temperature, and the concentration of the vinyl ether [76]. In benzene, formation of the cis isomer predominates and the reaction proceeds via a triplex. Involvement of triplexes in photoreactions will be discussed again in Sect. 3.6. In acetonitrile, the product ratio depends on the concentration of the vinyl ether and the reaction temperature. Formation of the tram isomer predominates at low concentration of the vinyl ether and at high reaction temperature ( > 100°C). At the high concentration of the vinyl ether, the cis and tram isomers are obtained in a 3 2 ratio (Scheme 20). 

Several additional examples of stereochemical control in [2 + 2] photocycloadditions which include systems containing preexisting stereogenic centers on the ground-state alkene are shown in equations (91)-<97)7 " The stereoselectivity in these systems is typically controlled by the nonbond inter-... 

Griesbeck, A. G., Mauder, H., Stadtmuller, S., Intersystem Crossing in Triplet 1,4 Biradicals Conformational Memory Effects on the Stereoselectivity of Photocycloaddition Reactions, Acc. Chem. Res. 1994, 27, 70 75. 

In photocycloadditions of 1,2-dioxine-4-ones A derived from menthone with unsymmetrically substituted alkenes moderate regioselectivity was observed. However, the cycloaddition reaction with cyclopentene proceeded highly stereoselective. " Chiral bicyclic lactams diastereoselectively react with ethylene furnishing optically active cyclobutanes. ... 

Griesbeck AG, MauderH, StadtmiillerS (1994) Intersystem crossing in triplet 1,4-biradicals conformational memmy effects on the stereoselectivity of photocycloaddition reactions. Acc Chem Res 27 70-75... 

It is known that the association of the molecules with double C=C bond in the solution can attribute to an increase in the quantum yield of photocycloaddition and influence its stereoselectivity. In our case it is reasonable to suggest that from (cis-2c)Mg dimers the cyclodimers are photosynthesized. In fact in the solution of 2cMg complex at concentration 1x10 M under the long-term influence of light at X>365 nm a new photoproduct is formed. Its absorption spectrum is represented in Figure 5. 

Ketones with homotopic faces have been widely used to determine the stereoselectivity in Paterno-Biichi reactions with E- and Z-isomers of electron-rich and electron-poor olefins. The photocycloaddition of... 

The overall results of substituent effects are observed in the products of a reaction, their rates of formation, and their stereochemistries. The purpose of this article is to apply very simple theoretical techniques to correlations and predictions of the rate and stereoselectivity effects of substituents in [2+2] photocycloadditions. The theoretical methods that will be used are perturbational molecular orbital (PMO) theory and its pictorial representation, the interaction diagram. Only an outline of the theory will be given below, since several more detailed descriptions are available. 4,18-34)... 

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