Yttrium-Catalyzed Tandem Intermolecular Hydroalkoxylation/Claisen Rearrangement

Zinc-Catalyzed Cyclization of Alkynyl Derivatives: Substrate Scope and Mechanistic Insights

Novel alkyl zinc complexes supported by acetamidate/thioacetamidate heteroscorpionate ligands have been successfully synthesized and characterized. These complexes exhibited different coordination modes depending on the electronic and steric effects of the acetamidate/thioacetamidate moiety. Their catalytic activity has been tested toward the hydroelementation reactions of alkynyl alcohol/acid substrates, affording the corresponding enol ether/unsaturated lactone products under mild reaction conditions. Kinetic studies have been performed and confirmed that reactions are first-order in [catalyst] and zero-order in [alkynyl substrate]. DFT calculations supported a reaction mechanism through the formation of the catalytically active species, an alkoxide-zinc intermediate, by a protonolysis reaction of the Zn-alkyl bond with the alcohol group of the substrate. Based on the experimental and theoretical results, a catalytic cycle has been proposed.

Transition metal catalysed cascade C-C and C-O bond forming events of alkynes

The past few decades have witnessed the emergence of domino reactions as a powerful tool for the multi-functionalization of alkynes for the rapid and smooth construction of complex molecular architectures. In this context, employing transition metal catalysis, vicinal/geminal cascade functionalization of alkynes involving C-C and C-O bond-formation reactions, has become a preferred strategy for the synthesis of oxygenated motifs. Despite this significant progress, reviews documenting such strategies are either metal/functional group-centric or target-oriented, thus hampering further developments. Therefore, in this review, different conceptual approaches based on C-C and C-O bond-forming events of alkynes such as carboxygenation (C-C and CO bond formation), carboalkoxylation (C-C and C-OR bond formation), and carboacetoxylation (C-C and C-OAc bond formations) are discussed, and examples from the literature from the last two decades are presented. Further, we have presented detailed insights into the mechanism of different transformations.

Stereoselective Approach to Multisubstituted Enolates from Unactivated Alkynes: Oxyalkylidenation of Alkynyl Ketone Enolates with Aldehydes

The preparation of multisubstituted enolates with precise regio- and stereocontrol is a nontrivial task when conventional deprotonation methods are used on the corresponding carbonyl compounds. We describe herein an approach to preparing stereodefined enolates by leveraging the stereoselective oxyfunctionalization of unactivated alkynes, particularly in the context of the alkynylogous aldol reaction. trans-Iodo(III)acetoxylation of alkynes and subsequent Sonogashira coupling allow for the facile preparation of multisubstituted enynyl acetates, which can be deacetylated by MeLi into the corresponding enolates. The alkynyl enolates react with aldehydes to afford γ,δ-unsaturated β-diketones through a cascade of alkynylogous aldol addition, intramolecular Michael addition, and ring opening of the oxetene intermediate.

Recent Advances in Catalytic [3,3]-Sigmatropic Rearrangements

Carbon–carbon bond formation by [3,3]-sigmatropic rearrangement is a fundamental and powerful method that has been used to build organic molecules for a long time. Initially, Claisen and Cope rearrangements proceeded at high temperatures with limited scopes. By introducing catalytic systems, highly functionalized substrates have become accessible for forming complex structures under mild conditions, and asymmetric synthesis can be achieved by using chiral catalytic systems. This review describes recent breakthroughs in catalytic [3,3]-sigmatropic rearrangements since 2016. Detailed reaction mechanisms are discussed to enable an understanding of the reactivity and selectivity of the reactions. Finally, this review is inspires the development of new cascade reaction pathways employing catalytic [3,3]-sigmatropic rearrangement as related methodologies for the synthesis of complex functional molecules.

Transition Metal-Catalyzed Hydroalkoxylation of Alkynes: An Overview

Oxygen-bearing motifs, mainly the congener heterocycles are ubiquitous due to their presence in various natural products and bioactive scaffolds. Although in literature, several strategies have been developed for their synthesis, hydroalkoxylation of alkynes has come forward as a method of choice and has been used extensively. In particular, hydroalkoxylation of alkynes has gained enormous attention from the synthetic community due to the rapid access to a very useful and reactive synthetic intermediate like 'enol ether'. Furthermore, to manifold the utility of these methods, reports have been developed elaborating the generation of 'enol ether' using hydroalkoxylation and their usage in various reactions in cascade or tandem manner. This review focuses on recent development on the hydroalkoxylation of alkynes for the synthesis of oxygen-containing entities.

Zinc-Catalyzed Hydroalkoxylation/Cyclization of Alkynyl Alcohols

Despite the great interest in zinc catalysis for hydroelementation reactions, the use of zinc complexes as catalysts for the hydroalkoxylation of alkynyl alcohols has not been reported to date. Scorpionate zinc complexes have been successfully designed as precatalysts for the hydroalkoxylation reaction of alkynyl alcohols under mild reaction conditions. Zinc amide complex 8 has been shown to be an excellent precatalyst for the highly selective intramolecular hydroalkoxylation process to yield the corresponding exocyclic enol ethers. Kinetic studies have been performed and confirmed that reactions are first-order in [catalyst] and zero-order in [alkynyl alcohol]. NMR spectroscopy and X-ray diffraction analysis provided evidence for the formation of an alkynyl zinc compound which has been shown to be a key intermediate in the hydroalkoxylation process. On the basis of the experimental results, a catalytic cycle is proposed.

Asymmetric synthesis with ynamides: unique reaction control, chemical diversity and applications

Ynamides are among the most powerful building blocks in organic synthesis and have become invaluable starting materials for the construction of multifunctional compounds and challenging architectures that would be difficult to prepare otherwise. The rapidly growing popularity originates from the unique reactivity and ease of manipulation of the polarized ynamide triple bond, the advance of practical methods for making them, and the simplicity of storage and handling. These attractive features and the demonstration of numerous synthetic applications have spurred the development of intriguing asymmetric reaction strategies during the last decade. An impressive variety of chemo-, regio- and stereoselective carbon-carbon and carbon-heteroatom bond forming reactions with ynamides have been developed and now significantly enrich the toolbox of synthetic chemists. This review provides a comprehensive overview of asymmetric ynamide chemistry since 2010 with a focus on the general scope, current limitations, stereochemical reaction control and mechanistic aspects.

Transition Metal-Catalyzed Tandem Reactions of Ynamides for Divergent N-Heterocycle Synthesis

ConspectusYnamides are electron-rich heteroatom-substituted alkynes with a C-C triple bond directly attached to the amide group. Importantly, this amide group is able to impose an electronic bias, thus resulting in the highly regioselective attack of this polarized alkyne by a large variety of nucleophiles. Over the past two decades, catalytic reactions of ynamides have experienced dramatic developments, especially those catalyzed by transition metals. As a result, ynamides have been widely applied to the rapid and efficient assembly of versatile structurally complex N-containing molecules, especially in an atom-economic and stereoselective way.On the basis of newly developed ynamide preparations and new alkyne transformations, we first developed oxidation-initiated tandem reactions of ynamides such as zinc-catalyzed ynamide oxidation/C-H functionalization and copper-catalyzed ynamide oxidation/carbene metathesis, leading to divergent synthesis of isoquinolones, β-carbolines, and pyrrolo[3,4-c]quinolin-1-ones. Importantly, this protocol represents the first non-noble-metal-catalyzed intermolecular oxidation of alkynes by N-oxide type oxidants, and the related overoxidation could be dramatically inhibited in this non-noble-metal catalysis. Then, we achieved gold-catalyzed amination-initiated tandem reactions of ynamides via α-imino gold carbenes for efficient construction of various 2-aminoindoles, 3-amino-β-carbolines, and 2-aminopyrroles, where two new types of nitrene transfer reagents (benzyl azides and isoxazoles) were discovered. In particular, the use of isoxazoles as nitrene transfer reagents for atom-economic generation of α-imino metal carbenes has also been elegantly exploited by Hashmi, Liu, and many other groups, providing ready access to a wide range of functionalized N-heterocycles. Moreover, we revealed that donor/donor copper carbenes could be generated via copper-catalyzed diyne cyclization under mild conditions. These novel copper carbenes could undergo asymmetric C-H insertion, cyclopropanation, and formal [3 + 2] cycloaddition to produce diverse chiral polycyclic pyrroles with good to excellent enantioselectivities. Thus, this strategy may open new avenues in catalytic asymmetric reaction of ynamides, which remain largely unexplored and deserve more attention. Meanwhile, we also accomplished the efficient and practical synthesis of medium-sized lactams by yttrium-catalyzed cascade cyclization of allyl alcohol-tethered ynamides and the combination of radical chemistry based on visible-light photoredox catalysis with ynamide chemistry for divergent synthesis of useful 2-benzhydrylindoles and 3-benzhydrylisoquinolines.In this Account, we describe a panoramic picture of our recent contributions since 2015 to the development and application of ynamide chemistry in organic synthesis via transition metal-catalyzed tandem reactions by focusing on the tetrafunctionalization of ynamides. These studies provide not only efficient and attractive methods for divergent synthesis of valuable N-heterocycles but also some new insights into the exploration of alkyne chemistry and metal carbene chemistry.

Au-Catalyzed tandem intermolecular hydroalkoxylation/Claisen rearrangement between allylic alcohols and chloroalkynes

An efficient protocol for the synthesis of γ,δ-unsaturated α-chloroketones has been developed via Au-catalyzed tandem intermolecular hydroalkoxylation/Claisen rearrangement.

Electrochemical Oxidation with Lewis-Acid Catalysis Leads to Trifluoromethylative Difunctionalization of Alkenes Using CF3SO2Na

The directly external oxidant-free oxytrifluoromethylation and aminotrifluoromethylation of alkenes have been developed through the constant current electrolysis synergizing with a Lewis-acid catalysis protocol. By using sodium trifluoromethanesulfinate as the trifluoromethyl source, the method enabled difunctionalization of C-C double bonds of a wide range of styrene derivatives.

Catalytic [1,3] O-to-C Rearrangement: Rapid Access to Bridged Bicyclic Systems

A catalytic [1,3] O-to-C rearrangement from enyne-ethers was developed for the rapid synthesis of diverse bridged bicyclic systems. In this reaction, a vinyl oxonium intermediate, generated in situ from enyne-ether, was the precursor for the [1,3] O-to-C rearrangement. This versatile protocol represents the first example of catalytic [1,3] O-to-C rearrangement based on ring-expansion strategy, enabling efficient access to bridged bicyclic scaffolds.