Cyclodextrin Cavity‐Induced Mechanistic Switch in Copper‐Catalyzed Hydroboration

Abstract

Encapsulated metal complexes show distinctive behaviors resulting from the confinement of the metal.[1] The cavity plays the role of second coordination sphere, which impacts reactivity and selectivity. [2][3][4][5][6][7] Alterations in regioselectivity were observed with encapsulated metal catalysts in hydratation or hydroformylation reactions. [8][9][10][11] An unusual coordination mode of the complex imposed by the confinement, or a controlled approach of the substrate imposed by sterics have been raised to explain the changes. Hence, regioselectivity is influenced by confinement of the reactive center but the reaction mechanism itself was not invoked. We present here a system in which not only the regioselectivity can be switched by only changing the size of a cavity but also the actual mechanism of the reaction. We also demonstrate another benefit of encapsulation in the discovery of new mechanisms.In the framework of our studies on cyclodextrin (CD)-based metal catalysts, [12] we developed N-heterocyclic carbene (NHC)-capped CD complexes, called (ICyD)M, where the metal is literally wrapped by the CD scaffold.[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. [14] Structural analyses of (-ICyD)M and (-ICyD)M complexes, derived from  and -CD, respectively, revealed significant differences in shape, in particular in the close surroundings of the metal. For instance, -and -ICyD-based complexes both present a groove leading to the metal, but for (-ICyD)M, a pit situated next to the metallic center is observed whereas the metal is tightly wrapped in (-ICyD)M (Figure 1). [14] This suggests that a lateral approach to the metal may be conceivable for (-ICyD)M complexes but precluded for

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

“…A single resorcin [4]arene is a bowl-shaped molecule and can encapsulate small cationic guests. [32,33] In 1997 Atwood discovered that when dissolved in apolar wet solvents, six of these resorcin [4]arene molecules and eight water molecules form a hexameric self-assembled structure that is held together by hydrogen bonds (Scheme 2). [34] Various guest molecules can be bound in these self-assembled cages.…”

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.…”

Preparation, Structure, Photoluminescent and Semiconductive Properties, and Theoretical Calculation of a Novel Cadmium Complex with Mixed Ligands

“…1. Complex 1 is characterized by a 1-D zigzag chain structure, consisting of [Cd(2,2'-biim)(4,4'-bipy)(H 2 O)(ClO 4 )] n n+ cationic chains and isolated ClO 4 -anions. The cadmium ion displays a slightly distorted octahedral geometry with the equatorial positions inhabited by three nitrogen atoms from one 2,2'-biim and one 4,4'-bipy ligand as well as one oxygen atom from one coordinating water molecule, and the apical sites are occupied by one oxygen atom from one coordinating perchlorate anion as well as one nitrogen atom from one 4,4'-bipy ligand (Fig.…”

“…Rentgenska strukturna analiza razkriva, da kompleks 1 kristalizira v prostorski skupini Pna2 1 ortorombskega kristalnega sistema z enodimenzionalnimi cikcak [Cd(2,2'-biim)(4,4'-bipy)(H 2 O)(ClO 4 )] n n+ kationskimi verigami in izoliranimi ClO 4 -anioni. Analiza fotoluminiscenčnih lastnosti prahu kaže, da kompleks 1 emitira zeleno svetlobo.…”