Size-Selective Molecular Flasks

Abstract

Molecular flasks are compounds that are able to mediate or catalyze chemical transformations inside their cavities. The development of such compounds is often inspired by nature. Enzymes, nature's catalysts, are able to convert a certain substrate with very high turnover number and selectivity. In addition to their very high chemo-, regio-, and stereoselectivity, enzymes are also able to distinguish their substrates on the basis of size, resulting in size selectivity. To date, many synthetic materials such as metal− organic frameworks are used to accomplish size-selective transformations. However, also the number of molecular flasks known to mediate or catalyze size-selective transformations is increasing. In this perspective an overview on classic and the most recent examples of size-selective molecular flasks is given. In addition, an outlook on promising developments in cavity chemistry that may lead to the development of additional sizeselective molecular flasks is given.

Gold Catalysis in (Supra)Molecular Cages to Control Reactivity and Selectivity

“…Importantly, the use of the holey ligands resulted in an important rate enhancement in the 6-exo-dig cyclization reaction compared to [PPh 3 AuNTf 2 ] and [P(OPh) 3 AuNTf 2 ], the latter complex having comparable electronic properties. [15] The influence of the cavity was evident by using similar ethynyl phosphine ligands without the bulky substituents, which all gave lower conversion. [15] This demonstrated the importance of the steric effect from the cavity formed by the bulky substituents in the rate enhancement of the reaction.…”

“…[15] The influence of the cavity was evident by using similar ethynyl phosphine ligands without the bulky substituents, which all gave lower conversion. [15] This demonstrated the importance of the steric effect from the cavity formed by the bulky substituents in the rate enhancement of the reaction. The substrate scope could be extended to a variety of 7-membered rings [16][17][18] and to 8-membered rings for which the carbonanalogue of the ligand was required (see Figure 2).…”

“…In these systems the transition metal complex is (partly) isolated from the bulk phase which may already lead to changed reactivity. In addition, substrate preorganization by bringing it in close proximity to the catalyst, sometimes in a specific conformation induced by the steric constraints of the cage, also may lead to different selectivity and reactivity compared to reactions that take place in bulk solution . Furthermore, isolation of the complex from the bulk enhances the stability of the catalyst providing catalyst systems that can reach high turn‐over.…”

Positive Allosteric Control of Guests Encapsulation by Metal Binding to Covalent Porphyrin Cages

“…Many more complex multidomain regulatory proteins and biological machines that integrate multiple signaling events and long‐range coupled conformational modifications through homotropic or heterotropic binding events have shown to proceed also through allosteric mechanisms . Although the design of 3D hollow structures has led to selective receptors for guest encapsulation and promising molecular nanoreactors, their development as biomimetic allosteric receptors able to control guest complexation through large conformational changes remains a significant challenge . Such a control implies to integrate in the framework of the structure several components with defined regulation and guest binding functions.…”

Crystallographic investigation into the self-assembly, guest binding, and flexibility of urea functionalised metal-organic frameworks

“…Many examples contain arrays of chemically addressable pores, with functionalised derivatives acting as 'crystalline molecular flasks' (4,5) and finding application in guest binding and activation for heterogeneous catalysis (6)(7)(8)(9)(10). 30,No.…”