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Published January 16, 2008 | Supplemental Material
Journal Article Open

Origins of Selectivity for the [2+2] Cycloaddition of α,β-unsaturated Ketones within a Porous Self-assembled Organic Framework

Abstract

This article studies the origins of selectivity for the [2+2] cycloadditions of α,β-unsaturated ketones within a porous crystalline host. The host, formed by the self-assembly of a bis-urea macrocycle, contains accessible channels of ∼6 Å diameter and forms stable inclusion complexes with a variety of cyclic and acyclic α,β-unsaturated ketone derivatives. Host 1 crystals provide a robust confined reaction environment for the highly selective [2+2] cycloaddition of 3-methyl-2-cyclopentenone, 2-cyclohexenone, and 2-methyl-2-cyclopentenone, forming their respective exo head-to-tail dimers in high conversion. The products are readily extracted from the self-assembled host and the crystalline host can be efficiently recovered and reused. Molecular modeling studies indicate that the origin of the observed selectivity is due to the excellent match between the size and shape of these guests to dimensions of the host channel and to the preorganization of neighboring enones into favorable reaction geometries. Small substrates, such as acrylic acid and methylvinylketone, were bound by the host and were protected from photoreactions. Larger substrates, such as 4,4-dimethyl-2-cyclohexenone and mesityl oxide, do not undergo selective [2+2] cycloaddition reactions. In an effort to understand these differences in reactivity, we examined these host−guest complexes by thermogravimetric analysis (TGA), NMR, powder X-ray diffraction (PXRD) and molecular modeling.

Additional Information

© 2008 American Chemical Society. Received 9 August 2007. Published online 21 December 2007. Published in print 1 January 2008. This work was supported by the NSF (CHE-071817), by the Petroleum Research Fund (44682), and by a grant from the University of South Carolina, Office of Research and Health Sciences Research Funding Program.

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