Photochemical Reactions in the Interior of a Zeolite. Part 5: The Origin of the Zeolite Induced Regioselectivity in the Singlet Oxygen Ene Reaction

doi:10.1016/S0040-4020(00)00514-7

Tetrahedron

Volume 56, Issue 36, 1 September 2000, Pages 6945-6950

https://doi.org/10.1016/S0040-4020(00)00514-7 Get rights and content

Abstract

Photooxidations of several new alkenes in the interior of methylene blue doped NaY are reported. The results suggest that the novel regiochemistry of these reactions can be rationalized by invoking both cation complexation with the alkenes and electrostatic interaction between the cation and the pendant oxygen on the developing perepoxide.

Introduction

Supramolecular control of photochemical reactions provides a powerful tool to control and alter photochemical behavior.2., 3. Zeolites, in particular are attractive hosts because of their well-characterized structures and properties.4., 5. Faujasites NaY and NaX (Scheme 1) have attracted considerable attention because of their availability and pore structure which can easily accommodate organic molecules of interest to the organic chemist. NaY and NaX have the same topographical structure and only differ in their Si/Al ratio. These crystalline porous aluminosilicate solids consist of silicon and aluminum tetrahedra connected by bridging oxygens to form a series of supercages 13 Å in diameter which are tetrahederally connected via windows approximately 7.4 Å in diameter. A negative charge is introduced for each tetrahedral aluminum in the zeolite cage. These negative charges are compensated by counter ions which can easily be exchanged for charged sensitizers such as methylene blue (MB).⁶

The ability to incarcerate a sensitizer in the supercages of NaY has provided several workers the opportunity to examine the reactions of singlet oxygen with organic substrates. Tung and coworkers⁷ have demonstrated that the zeolite can be used to restrict interaction between the substrate and sensitizer and promote singlet oxygen rather than electron transfer chemistry. Ramamurthy and coworkers8., 9. have demonstrated that the zeolitic medium can dramatically influence the regiochemistry of the singlet oxygen ene reaction. For example during the photooxidation of 2-methyl-2-pentene in thionin dope NaY a regioselective ene reaction involving hydrogen abstraction from the gem-dimethyl end of the alkene was observed. The result was in stark contrast to the solution photooxidation which gave predominant hydrogen abstraction from the least substituted end of the double bond. (Scheme 2) This dramatically enhanced regioselectivity was tentatively attributed to complexation of the olefin to a Na⁺ compelling the sterically demanding allylic methyl group to rotate to the face approached by ¹O₂, thereby preventing access to the allylic hydrogens H_a on the methylene carbon. (Scheme 2)

Clennan and Sram^1b examined the photooxidations of a series of tetra-substituted alkenes in MB doped NaY and suggested that cation complexation to the pendant oxygen in the perepoxide intermediate was the regiochemical important interaction. (Scheme 3) This model suggested that Markovnikov directing effects, as a result of greater charge buildup on the carbon framework, and the preferential formation of the perepoxide with the pendant oxygen on the least substituted side of the double bond, were responsible for the observed regiochemistry.

We report here new results with trisubstituted alkenes that demonstrate that both cation complexation to the alkene, and the electrostatic interaction of the cation with the pendant oxygen, are important in determining the regiochemistry of the intrazeolite photooxidation. We also report that as a result of the electrostatic stabilization of the perepoxide the ‘cis-effect’ which operates in solution is diminished in importance in the intrazeolite photooxidations.

Section snippets

Preparation of NaMBY

MB doped NaY was synthesized by using a procedure similar to that reported by Ramamurthy and coworkers.⁶ A weighed sample of NaY was added to an aqueous solution of MB and allowed to stir until the blue solution turned colorless. The MB doped NaY (NaMBY) was then filtered and air dried followed by additional drying on a vacuum line at 10⁻⁴ Torr and 100–120°C for 8–10 h. It was then removed from the vacuum line and stored in a desiccator prior to use. When the doped zeolite is used directly off

Conclusion

We have provided the first experimental evidence that both cation complexation to the olefin and to the perepoxide are important features of intrazeolite photooxidations. A new model to rationalize these results has been suggested. The orientation of the alkene, the position of the sodium cation, and the direction of approach of singlet oxygen all play important roles in determining the reaction regioselectivity. The interdependence of these factors suggest that in more complex alkenes with

Experimental

Alkenes 1 and 2 were purchased from Aldrich as a mixture. They were separated and purified by preparative gas chromatography on a Gow Mac GC equipped with a thermal conductivity detector and a 30 ft by 3/8 in. column packed with 15% Se-30 on Chromsorb P. Compounds 3–6 and all the allylic hydroperoxides and allylic alcohols have previously been reported and their spectral data were consistent with the literature reports and with their assigned structures.²⁵

Acknowledgements

We thank Professor V. Ramamurthy (Tulane University) for very useful discussions during the course of this study. We also thank the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their generous support of this research.

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Photochemical Reactions in the Interior of a Zeolite. Part 5: The Origin of the Zeolite Induced Regioselectivity in the Singlet Oxygen Ene Reaction

Abstract

Introduction

Section snippets

Preparation of NaMBY

Conclusion

Experimental

Acknowledgements

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron Lett.

Acc. Chem. Res.

Acc. Chem. Res.

An Introduction to Zeolite Molecular Sieves

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.