Functional cavitands: Chemical reactivity in structured environments

Abstract

Container-shaped molecules provide structured environments that impart geometric bounds on the motions and conformations of smaller molecular occupants. Moreover, they provide ''solvation'' that is constrained in time and space. When inwardly directed functional groups are present, they can interact chemically with the occupants. Additionally, the potential for reactivity and catalysis is greatly enhanced. Deep cavitands, derived from resorcinarenes, nearly surround smaller molecules and have been one of the most successful platforms for elaboration with functional groups. Derivatives bearing organic and metal-binding functional groups have been shown to affect recognition properties and selectively accelerate diverse reactions. In this review, we examine recent examples of these systems with an emphasis on how and why ordered nanoenvironments impart changes in the properties and reactivity of their occupants.nanoenvironments ͉ supramolecular M olecular recognition became a popular theme in physical organic chemistry during the late 1980s, and many of those studies evaluated intermolecular forces as models for biochemical phenomena. Synthetic receptors of increasing sophistication such as clefts (1), armatures (2), tweezers (3), bowls (4), and other vehicles (5, 6) were devised for the study of reversible interactions, both in aqueous and nonaqueous media. The emergence of site-directed mutagenesis replaced much of this activity. The strength of a hydrogen bond inside, say, an enzyme can be determined in situ through substitution of an unnatural amino acid (7) with the appropriate functional groups. In addition, enzymes, aptamers, ribozymes, and catalytic antibodies can fold more or less completely around a target molecule. This creates a structured environment for the functional group interactions in question, a feature missing in the various concave structures invented by synthetic chemists. A receptor that completely and reversibly encapsulated sizable molecules was prepared in 1995 (8), but it lacked any appropriately positioned functionality. It took us another 5 years to synthesize a receptor that could, more or less, surround a target molecule and present it with a reactive functional group. This led to changes in reactivity of the sort encountered in the macromolecular complexes of biology. We do not intend here to promote these as model systems. Instead, this review is concerned with effects of physical restraints on a molecule surrounded by another; the physical organic chemistry of structured environments. More specifically, the structures are specially modified cavitands, macrocyclic compounds featuring a cavity that is sealed at one end and functionalized at the other. Cavitands have a long history in studies of molecular recognition, and several reviews are available that cover the literature up to 2002 (9-13). With rare exceptions, these cavitands offer only spaces to be filled by smaller molecules. Our departure was their functionalization with groups that are directed at the molecules held tempo...

Quinone‐Based, Redox‐Active Resorcin[4]arene Cavitands

“…[5] The most fascinating feature of top-open resorcin [4]arene cavitands is their ability to adopt two spatially well-defined conformations: an expanded "kite" form and a contracted "vase" form. While various methods have been employed to study the conformational properties of cavitands, [6] the most convenient one is 1 H NMR spectroscopy.…”

“…Since the preparation of the first quinoxaline-bridged resorcin [4]arene cavitand by Cram and co-workers in 1982, [1] numerous derivatives with various structures and functions have been prepared and employed as switches, [2] receptors and sensors, [3] catalysts, [4] and molecular hosts. [5] The most fascinating feature of top-open resorcin [4]arene cavitands is their ability to adopt two spatially well-defined conformations: an expanded "kite" form and a contracted "vase" form.…”

Capsule-controlled selectivity of a rhodium hydroformylation catalyst

“…The development in supramolecular chemistry has evolved such that several synthetic systems are now available to also study the reactivity in confined nano-spaces, such as the well-defined cavities of metal-organic frameworks, and (supra)molecular capsules. It has been demonstrated by the pioneering work of Sanders and co-workers 15 , Rebek and co-workers [16][17][18][19] , Fujita and co-workers 20 and Raymond and co-workers 21 that organic reactions that occur in such restricted environments can result in unusual selectivities and rate enhancements. These all are elegant examples in which it has been demonstrated that the activity or selectivity of organic reactions can be controlled by nano-capsules.…”

Induced‐Fit Encapsulation by a 1,3,5‐Alternate Calix[6]arene

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