The Mechanism of Cucurbituril Formation

This article begins by describing the synthesis and recognition properties of the cucurbit[n]uril homologues CB[5], CB[6], CB[7], CB[8], and CB[10]. Subsequently, we describe the state‐of‐the‐art in understanding the mechanism of CB[n] formation. We describe the experiments that establish that glycoluril (1 H) undergoes condensation with formaldehyde by a combination of chain‐growth and step‐growth polymerization processes. Chain‐growth processes deliver methylene bridged glycoluril oligomers 2 C–8 C as intermediates that may undergo macrocyclization to nor‐seco‐CB[n] when the oligomer is long enough (5 C–8 C) and subsequently form CB[n]. Step‐growth processes allow oligomers to condense to give longer oligomers connected by a single CH2‐bridge that undergo macrocyclization to deliver (±)‐bis‐nor‐seco‐CB[6] and bis‐nor‐seco‐CB[10]. Lastly, we describe some of the exciting new recognition processes of the newly formed members of the CB[n] family. For example, bis‐nor‐seco‐CB[10] undergoes homotropic allostery during ternary complex formation, (±)‐bis‐nor‐seco‐CB[6] exhibits moderately diastereoselective recognition processes (d.r. up to 88 : 12) with chiral ammonium ions in water, and nor‐seco‐CB[6] functions as an aldehyde reactive CB[n] synthon that can control the folding of alkanediammonium ions into a backfolded conformation in water.