Reasons why aldehydes do not generally participate in cucurbit[n]uril forming reactions

The reaction of substituted glycolurils or a glycoluril dimer with a dialdehyde (o-phthalaldehyde) delivers S-shaped dimers and an S-shaped tetramer selectively. A combination of X-ray crystallography, PM3 calculations, and product resubmission experiments establish that the S-shaped isomers are thermodynamically more stable than the C-shaped diastereomers which we attribute to the conformational preferences of the newly formed benzo bicyclo[3.3.2]decane ring system. The preferential formation of S-shaped subunits is one reason why o-phthalaldehyde and possibly other aldehydes do not usually participate in CB[n] forming reactions. We also present evidence that points toward an equilibrium between glycoluril monomer + phthalaldehyde and S-shaped dimer + water that responds to concentration over the 1-50 mM range. This result suggests a second reason, insufficient reactivity (e.g., low equilibrium constant), why o-phthalaldehyde and possibly other aldehydes do not participate in CB[n] forming reactions. Delineation of the reasons why some aldehydes fail in these reactions is the first step toward devising methods to overcome these limitations.