Gold-Catalyzed Cyclization Reactions of Allenol and Alkynol Derivatives

Abstract

A lthough gold is chemically inert as a bulk metal, the landmark discovery that gold nanoparticles can be effective catalysts has opened up new and exciting research opportunities in the field. In recent years, there has been growth in the number of reactions catalyzed by gold complexes [gold(I) and gold(III)], usually as homogeneous catalysts, because they are soft Lewis acids. In addition, alkynes and allenes have interesting reactivities and selectivities, notably their ability to produce complex structures in very few steps. In this Account, we describe our work in gold catalysis with a focus on the formation of CÀC and CÀO bonds using allenes and alkynes as starting materials. Of these, oxa-and carbo-cyclizations are perhaps the best known and most frequently studied. We have divided those contributions into sections arranged according to the nature of the starting material (allene versus alkyne).Gold-catalyzed carbocyclizations in allenyl C2-linked indoles, allenyl-β-lactams, and allenyl sugars follow different mechanistic pathways. The cyclization of indole-tethered allenols results in the efficient synthesis of carbazole derivatives, for example. However, the compound produced from gold-catalyzed 9-endo carbocyclization of (aryloxy)allenyl-tethered 2-azetidinones is in noticeable contrast to the 5-exo hydroalkylation product that results from allenyl sugars.We have illustrated the unusual preference for the 4-exo-dig cyclization in allene chemistry, as well as the rare β-hydride elimination reaction, in gold catalysis from readily available R-allenols. We have also observed in γ-allenols that a (methoxymethyl)oxy protecting group not only masks a hydroxyl functionality but also exerts directing effects as a controlling unit in a gold-catalyzed regioselectivity reversal. Our recent work has also led to a combined experimental and computational study on regioselective gold-catalyzed synthetic routes to 1,3-oxazinan-2-ones (kinetically controlled products) and 1,3-oxazin-2-one derivatives (thermodynamically favored) from easily accessible allenic carbamates.In addition, we discuss the direct gold-catalyzed cycloketalization of alkynyldioxolanes, as well as aminoketalization of alkynyloxazolidines. We performed labeling studies and density functional calculations to gain insight into the mechanisms of the bis-heterocyclization reactions. We also describe the controlled gold-catalyzed reactions of primary and secondary propargylic hydroperoxides with a variety of nucleophiles including alcohols and phenols, allowing the direct synthesis of β-functionalized ketones. Through computations and 18 O-labeling experiments, we discovered various aspects of the controlled reactivity of propargylic hydroperoxides with external nucleophiles under gold catalysis. The mechanism resembles a MeyerÀSchuster rearrangement, but notably, the presence and geometry characteristics of the OOH functional group allow a new pathway to happen, which cannot apply to propargylic alcohols.

Gold‐Catalyzed Cross‐Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

“…[11,12,[32][33][34][35][36][37][38][39] Sub-nanometer-sized unsupported gold clusters can accelerate specific reactions with extremely large turnover numbers( TONs) and turnover frequencies (TOFs). [69,70,[79][80][81][82][71][72][73][74][75][76][77][78] In the majority of homogeneous Au catalysts, Au exists in the + 1 oxidation state. [43][44][45] Owing to the localizeds urface plasmon resonance (LSPR)e ffects, [46][47][48] gold nanoparticles (AuNPs) can be used as photocatalysts.…”

Catalytic Double Cyclization Process for Antitumor Agents against Breast Cancer Cell Lines

“…Catalytic cyclization processes have recently attracted great interest in the chemistry scientific community owing to the possibility to synthesize in one step high-value-added molecules starting from simple building blocks, by their ordered sequential activation by the catalytic center ( Alcaide and Almendros, 2014 , Debrouwer et al., 2015 , Guo et al., 2011 , Krause and Winter, 2011 , Tanaka and Tajima, 2012 , Valera and Saa, 2016 , Vlaar et al., 2011 , Ye and Ma, 2014 ). Among the raw starting materials, carbon monoxide represents a simple and largely available C-1 unit, and its catalytic activation may allow to directly introduce a carbonyl functional group into an organic substrate in a highly atom-economical and efficient manner.…”

The Outer‐Sphere Mechanism of Nitrene Transfer onto Gold(I) Alkyne Complexes

“…Gold metal is as oft Lewis acid that presentsp otent reactivity and au nique ability to coordinate to unsaturated bonds. For the last two decades, the chemistry of homogeneous gold catalysis has experienced as ignificant increase in attentiona nd has become the subject of ag reat number of experimental [1][2][3][4][5][6][7][8] and theoretical [9][10][11][12] studies. In addition, gold-catalyzed reactions of alkynes with nucleophiles are recognized as one of the most powerful and useful tools in organic synthesis.…”