[3 + 2] Cycloaddition with photogenerated azomethine ylides in β-cyclodextrin

Abstract

Stability constants for the inclusion complexes of cyclohexylphthalimide 2 and adamantylphthalimide 3 with β-cyclodextrin (β-CD) were determined by ¹H NMR titration, K = 190 ± 50 M^-1, and K = 2600 ± 600 M^-1, respectively. Photochemical reactivity of the inclusion complexes 2@β-CD and 3@β-CD was investigated, and we found out that β-CD does not affect the decarboxylation efficiency, while it affects the subsequent photochemical H-abstraction, resulting in different product distribution upon irradiation in the presence of β-CD. The formation of ternary complexes with acrylonitrile (AN) and 2@β-CD or 3@β-CD was also essayed by ¹H NMR. Although the formation of such complexes was suggested, stability constants could not be determined. Irradiation of 2@β-CD in the presence of AN in aqueous solution where cycloadduct 7 was formed highly suggests that decarboxylation and [3 + 2] cycloaddition take place in the ternary complex, whereas such a reactivity from bulky adamantane 3 is less likely. This proof of principle that decarboxylation and cycloaddition can be performed in the β-CD cavity has a significant importance for the design of new supramolecular systems for the control of photoreactivity.

Keywords: [3 + 2] cycloadditions; inclusion complexes; photochemistry; phthalimides; β-cyclodextrin.

Figure 1

Phthalimide derivatives 1–3 and the corresponding azomethine ylides 1AMY-3AMY.

Scheme 1

Irradiation of 1 in the presence of acrylonitrile (AN).

Figure 2

Dependence of the chemical shift of the H-atom at the cyclohexane 2 position in compound 2 on the β-CD concentration. Dots are experimental values and the red line corresponds to the calculated values by the WINEQNMR program [51] according to the model for the formation of 1:1 stoichiometry of the inclusion complex 2@β-CD.

Scheme 2

Complexation of 2 with β-CD, and formation of a ternary complex AN@2@β-CD.

Scheme 3

Photochemistry of 2 in the presence of AN, with or without β-CD.

Scheme 4

Photochemistry of 3 in the presence of AN, with or without β-CD.