Evaluation of the Reactivity of Metallocatalytic Cavities in the Dimerization of Terminal Alkynes

One-pot synthesis of α,β-unsaturated ketones through sequential alkyne dimerization/hydration reactions using the Hoveyda–Grubbs catalyst

The combination of HG2/PCy3 and CCl3COOH/p-TsOH·H2O through one-pot alkyne dimerization/hydration reactions gave α,β-unsaturated ketones in quantitative yields within 2.5–4 h.

Au-Cavitands: Size governed arene-alkyne cycloisomerization

With an inwardly directed reactive center and a well-defined binding pocket, Au(I) functionalized resorcin[4]arene cavitands have been shown to catalyze molecular transformations. The reactivity profiles that emerge differ from other Au(I) catalysts. The added constraint of a binding pocket gives rise to the possibility that the substrates might have to fit into the resorcinarene pocket; our hypothesis is that substrates that match the available space have different reaction outcomes than those that do not. Herein we report on the intramolecular cyclization of alkyne-aromatic substrates with variable alkynes and aromatic composition. We see that scaffold size most drastically dictates reactivity, especially when the substrate's features are particularly designed. The results of these experiments add to the veritable goldmine of information about the selectivity in catalysis that cavitands offer.

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

Gold catalysis has experienced a tremendous development over the past decades, and is nowadays widely used in organic synthesis to perform chemical transformations of π-bond-containing molecules. Catalyst development has been based mostly on ligand development and counter-ion strategies. More recently, the encapsulation of gold catalysts in (supra)molecular cages was explored as a new way to control selectivity and reactivity of gold catalysts. In this review, we describe the cages that have been employed as hosts for gold complexes, along with their impact on the catalytic performance. Covalent and supramolecular approaches to encapsulate single metal complexes will be described and the impact on the catalytic performance will be discussed. Also, recent strategies to pre-organize multiple metal centers will be discussed.

1,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes

Metal-catalyzed dimerization reactions of terminal acetylenes are well known in the literature. However, only a few examples of the dimerization of halogen-substituted acetylenes are described. The products of the latter metal-catalyzed dimerization are the branched head-to-tail enynes. The formation of the corresponding linear head-to-head enynes has not been reported yet. Herein, we demonstrate by means of quantum chemical methods and experiments that the head-to-head dimerization of chloroarylacetylenes can be achieved via mono gold catalysis. Under the optimized conditions, a clean and complete conversion of the starting materials is observed and the dimeric products are obtained up to 75% NMR yield. A mechanistic investigation of the dimerization reaction reveals that the branched head-to-tail vinyl cation is energetically more stable than the corresponding linear head-to-head cation. However, the latter can rearrange by an unusual 1,3-chlorine shift, resulting in the highly stereoselective formation of the trans product, which corresponds to the gold complex of the head-to-head E-enyne. The activation barrier for this rearrangement is extremely low (ca. 2 kcal/mol). As the mono gold-catalyzed dimerization can be conducted in a preparative scale, this simple synthesis of trans-1,2-dichloroenynes makes the gold(I)-catalyzed head-to-head dimerization of chloroarylacetylenes an attractive method en route to more complex conjugated enyne systems and their congeners.

Calix[4]arene-fused phospholes

A calix[4]arene-based biarylphosphine has been synthesised and catalytically converted into two diastereomeric, calixarene-fused phospholes.

Aza-capped cyclodextrins for intra-cavity metal complexation

N,N-Chelators based on aza-capped cyclodextrins were designed to produce complexes with an encapsulated metal unit.