NHC‐Capped Cyclodextrins (ICyDs): Insulated Metal Complexes, Commutable Multicoordination Sphere, and Cavity‐Dependent Catalysis

Abstract

Metal centers associated with cavities have attracted much attention, mainly because of their resemblance to metalloenzymes.[1] Among concave molecules with a cavity, cyclodextrins (CDs) are unique owing to their natural occurrence, their hydrosolubility, and the structure of their cavity. Unlike any other cavity, in particular those based on aromatic rings, their interior is carpeted with hydrogen atoms, which confer hydrophobicity and introduce additional van der Waals interactions. Therefore, CDs are widely used to host hydrophobic molecules in polar solvents. The possibility of converting cyclodextrins into enzyme mimics very soon attracted the interest of scientists.[2] More specifically, for CDs to be used to mimic metalloenzymes, a metal must be attached to the CD scaffold. [3] Owing to the size of the cavity, two different ways to append the metal can be considered for the study of two different phenomena. First, the metal can be positioned at the entrance of the cavity to exploit the inclusion ability of the cavity in its interaction with a substrate and mimic the binding pocket of an enzyme (Figure 1 a).Second, the metal can be encapsulated inside the cavity to study the effect of confinement on its coordination sphere and chemical properties; this arrangement mimics the environment of a metal buried deeply within a folded protein (Figure 1 b).The first kind of design has been widely studied, often with the attachment of a metal-ligand unit through a single linkage.[3] Such structures can be used in a multitude of applications, for example, in catalysis.[4] However, for the metal center to be fixed directly above the cavity, double linkage of the metal was necessary. In the resulting so-called metal-capped CDs, [5,6] the metal ion is located right on top of the CD cavity.[7] The deepest position in which the metal has been placed so far intercepts the plane defined by the C-6 atoms of the sugar units. [8,9] This metal position leaves the cavity available for the inclusion of guests. The cavity can thus serve as a host for substrates (the interaction of which with the metal center can lead to an acceleration of the reaction rate [10] in analogy with an enzymatic reaction), as a probe for ligand exchange, [8] or as a second coordination sphere through C À H···X À M interactions. [11] For metallocyclodextrins of the second kind, in which the metal center occupies the middle of the cavity like an included guest, typically at the level of the H-5 atoms, only noncovalent inclusion complexes of metal ions have been described so far. Their electrochemical properties have been studied thoroughly, and electron transfer is thought not to involve the included complex, but the free portion of nonincluded metallic guest ions.[12] In other words, no studies on cyclodextrin complexes in which the metal ion is forced through covalent bonding to be included deep inside the Figure 1. In a metal-capped cyclodextrin, the cavity interacts either a) with the substrate or b) with the metal, depending on the depth of inclusion....

Supramolecular Control of Biomimetic Coordination – ZnII Cavity Complexes Presenting Two Differentiated Labile Sites in cis Positions

“…The corresponding metal centers are under the supramolecular control of the cavity, and -in view of this specific feature -they will be here designated as "cavity complexes". Cavity complexes are based on various macrocycles such as calixarenes, cyclodextrins, [4] and glycoluril. [1] The first examples of such cavity complexes with an embedded labile ligand evidenced both in the solid state and in solution were reported in 1998 and are based on calix [6]arenes [5] and calix [4]arenes.…”

“…Cavity complexes are based on various macrocycles such as calixarenes, cyclodextrins, [4] and glycoluril. [1] The first examples of such cavity complexes with an embedded labile ligand evidenced both in the solid state and in solution were reported in 1998 and are based on calix [6]arenes [5] and calix [4]arenes. [6] These two systems display common and different features ( Figure 1).…”

Newly synthesized furanoside-based NHC ligands for the arylation of aldehydes

“…26−35 However, just a few of them were used as catalysts in Suzuki-Miyaura, 31,32 olefin metathesis, 33 gold-catalyzed cyclopropanation reaction, 34 or hydrosilylation of ketones. 35 On the other hand, the synthesis of alkoxy-substituted NHC-Rh(I) complexes and their applications in the arylation reaction were also reported.…”

Cyclodextrin Cavity‐Induced Mechanistic Switch in Copper‐Catalyzed Hydroboration

“…[13] The position of an appended metal inside this cavity forces external ligands to be influenced by its shape to interact with the metal center. Thus, these particular structures were found to induce shapedependent enantio-and regioselectivities in gold-catalyzed cycloisomerisation reactions.…”

“…NMR analyses of all (L)CuCl, (L)Cu-OtBu, and (L)Cu-Bpin intermediates for both α-ICyD and -ICyD ligands (see SI), showed deshielding of H-5s characteristic of the metal inside the cavity. [13,14] This demonstrates that the syn borylcupration step, which controls regioselectivity, is forced to take place inside the cavity with (ICyD)CuBpin intermediates.…”