Linear free energy correlation analysis on the electronic effects of Rh(II) carbene O-H insertion

Palladium/Iron-Catalyzed Wacker-Type Oxidation of Aliphatic Terminal and Internal Alkenes Using O2

The Wacker-type oxidation of aliphatic terminal alkenes proceeds using a Pd/Fe catalyst system under mild reaction conditions using 1 atm O2 without other additives. The use of 1,2-dimethoxyethane/H2O as a mixed solvent was effective. The slow addition of alkenes is also important for improving product yields. Fe(III) citrate was the most efficient cocatalyst among the iron complexes examined, whereas other complexes such as FeSO4, Fe2(SO4)3, Fe(NO3)3, and Fe2O3 were also operative. This method is also applicable to aliphatic internal alkenes, which are generally difficult to oxidize using conventional Pd/Cu catalyst systems. The gram-scale synthesis and reuse of the Pd catalysts were also demonstrated.

B(C6F5)3-catalyzed O-H insertion reactions of diazoalkanes with phosphinic acids

A highly efficient base-, metal-, and oxidant-free catalytic O-H insertion reaction of diazoalkanes and phosphinic acids in the presence of B(C6F5)3 has been developed. This powerful methodology provides a green approach towards the synthesis of a broad spectrum of α-phosphoryloxy carbonyl compounds with good to excellent yields (up to 99% yield). The protocol features the advantages of operational simplicity, high atom economy, practicality, easy scalability and environmental friendliness.

Brønsted acid-catalyzed homogeneous O–H and S–H insertion reactions under metal- and ligand-free conditions

The economical and accessible CF₃SO₃H successfully catalyzed homogeneous O–H and S–H bond insertion reactions between hydroxyl compounds, thiols and diazo compounds under metal- and ligand-free conditions.

Catalyst-free synthesis of α,α-disubstituted carboxylic acid derivatives under ambient conditions via a Wolff rearrangement reaction

Herein, a hexafluoroisopropanol (HFIP)-promoted Wolff rearrangement reaction was developed, delivering various α,α-disubstituted carboxylic acid derivatives in good to excellent yields.

Synthesis of benzofurans from the cyclodehydration of α-phenoxy ketones mediated by Eaton’s reagent

Cyclodehydration of α-phenoxy ketones promoted by Eaton’s reagent (phosphorus pentoxide–methanesulfonic acid) is used to prepare 3-substituted or 2,3-disubstituted benzofurans with moderate to excellent yields under mild conditions. The method provides a facile access to benzofurans from readily available starting materials such as phenols and α-bromo ketones. The reaction is highly efficient, which is attributed to the good reactivity and fluidity of Eaton’s reagent. The reaction can be applied to prepare naphthofurans, furanocoumarins, benzothiophenes, and benzopyrans.

Silver-Catalyzed Asymmetric Insertion into Phenolic O-H Bonds using Aryl Diazoacetates and Theoretical Mechanistic Studies

An enantioselective insertion reaction of silver carbenes generated from donor-acceptor-substituted diazo compounds into the O-H bond of phenols was developed. A homobinuclear silver complex with a chiral phosphorous ligand was created in situ from AgNTf₂ and (S)-XylylBINAP (in a 2:1 mole ratio). Detailed mechanistic studies using combined experimental and computational techniques revealed that one silver atom center of the catalyst forms a silver carbene and another one works as a Lewis acid for the nucleophilic addition of a phenol. Two counter-anions, two water molecules, and two silver atoms cooperatively mediate the subsequent protonation event to lower the activation energy and control enantioselectivity, affording an array of valuable α-aryl-α-aryloxy esters.

Synthesis of azasilacyclopentenes and silanols via Huisgen cycloaddition-initiated C-H bond insertion cascades

An unusual transition metal-free cascade reaction of alkynyl carbonazidates was discovered to form azasilacyclopentenes. Mild thermolysis afforded the products via a series of cyclizations, rearrangements, and an α-silyl C-H bond insertion (rather than the more common Wolff rearrangement, 1,2-shift, or β-silyl C-H insertion) to form silacyclopropanes. A mechanistic proposal for the sequence was informed by control experiments and the characterization of reaction intermediates. The substrate scope and post-cascade transformations were also explored.

Origins of unique gold-catalysed chemo- and site-selective C-H functionalization of phenols with diazo compounds

In past decade, gold revealed more and more unique properties in carbene chemistry. It was disclosed in our recent communication (J. Am. Chem. Soc. 2014, 136, 6904) that gold carbenes have unprecedented chemo- and site-selectivity and ligand effect toward the functionalization of C-H bonds in phenols. In this full article, we report a comprehensively combined theoretical and experimental study on the mechanism of the insertion of gold carbenes into C-H and O-H bonds in phenol. It significantly revealed that the ligands have an important effect on C-H insertion and the reaction proceeds through a pathway involving the formation of an enolate-like intermediate. Moreover, two water molecules serving as a proton shuttle are believed to be the key issue for achieving chemoselective C-H functionalization, which is strongly supported by the DFT calculations and control experiments. It is the first time that a clear explanation is given about the prominent catalysis of gold carbenes toward C-H functionalization based on a theoretical and experimental study.

A zinc-catalyzed oxidative reaction of ynamides with phenols and thiophenols: highly site-selective synthesis of versatile α-aryloxy amides and α-arylthio amides

A zinc-catalyzed oxidative reaction of ynamides with phenols and thiophenols under mild reaction conditions has been developed, which provides various α-aryloxy amides and α-arylthio amides in moderate to good yields, respectively. Importantly, high chemoselectivity is achieved by such a non-noble metal-catalyzed alkyne oxidation.

Intermolecular carbene S-H insertion catalysed by engineered myoglobin-based catalysts†

The first example of a biocatalytic strategy for the synthesis of thioethers via an intermolecular carbene S–H insertion reaction is reported.

The first example of a biocatalytic strategy for the synthesis of thioethers via an intermolecular carbene S–H insertion reaction is reported. Engineered variants of sperm whale myoglobin were found to efficiently catalyze this C–S bond forming transformation across a diverse set of aryl and alkyl mercaptan substrates and α-diazoester carbene donors, providing high conversions (60–99%) and high numbers of catalytic turnovers (1100–5400). Furthermore, the enantioselectivity of these biocatalysts could be tuned through mutation of amino acid residues within the distal pocket of the hemoprotein, leading to myoglobin variants capable of supporting asymmetric S–H insertions with up to 49% ee. Rearrangement experiments support a mechanism involving the formation of a sulfonium ylide generated upon attack of the thiol substrate to a heme-bound carbene intermediate.

Novel multicomponent reactions via trapping of protic onium ylides with electrophiles

Multicomponent reactions (MCRs) are one-pot processes in which three or more starting materials form a product that incorporates the structural features of each reagent. These reactions date back to the mid-19th century, when Strecker first prepared α-aminonitriles through the condensation of aldehydes with ammonia and hydrogen cyanide. In addition to affording products with structural complexity and diversity, MCRs offer the advantages of simplicity, synthetic efficiency, synthetic convergence, and atom economy. Therefore, they have played an important role in modern synthetic organic chemistry and drug-discovery research. The irreversible trapping of an active intermediate generated from two components by a third one offers an effective way to discover novel MCRs. In cases where the intermediate from the first two components is reactive enough to generate a two-component byproduct, it becomes challenging to control of the chemoselectivity of these MCRs over the side reaction. For example, researchers had expected that ammonium/oxonium ylides, high energy intermediates that have acidic protons and basic carbanions attached to adjacent carbons, would be too reactive to be intercepted by external electrophiles. Instead, a very fast 1,2-proton transfer would neutralize the charge separation, resulting in a stable N-H/O-H insertion product. In this Account, we present our efforts toward the development of novel MCRs via trapping of the active ammonium/oxonium ylide intermediates with a number of electrophiles. In these reactions, a "delayed proton transfer" that occurs after the trapping process produces novel multicomponent coupling products. Thus, transition-metal-catalyzed MCRs of diazocarbonyl compounds, anilines/alcohols, and electrophiles efficiently afford polyfunctional molecules such as α-amino-β-hydroxy acids, α-hydroxy-β-amino acids, α,β-diamino acids, and α,β-dihydroxy acid derivatives. We have also applied a cooperative catalysis strategy to some of these MCRs leading to reactions with high chemo-, diastereo-, and enantioselectivity. These MCRs also provide solid experimental evidence for the existence of the active protic onium ylides.

Competitive [2,3]- and [1,2]-oxonium ylide rearrangements. Concerted or stepwise?

The axial-equatorial conformational isomer distribution of the reactant diazoacetoacetate or its metal carbene intermediate is reflected in Rh(II) catalyzed oxonium ylide forming reactions of 3-(trans-2-arylvinyl)tetrahydropyranone-5-diazoacetoacetates that afford diastereoisomeric products for both the symmetry-allowed [2,3]- and the formally symmetry-forbidden [1,2]-oxonium ylide rearrangements.

DFT investigation on mechanism of dirhodium tetracarboxylate-catalyzed O-H insertion of diazo compounds with H2O

The mechanism of the dirhodium tetracarboxylate-catalyzed O-H insertion reaction of diazomethane and methyl diazoacetate with H2O has been studied in detail using DFT calculations. The rhodium catalyst and a diazo compound couple to form a rhodiumcarbene complex. Of two reaction pathways of the Rh(II)-carbene complex with H2O, the stepwise pathway is more preferable than the concerted one. Formation of a Rh(II) complex-associated oxonium ylide is an exothermal process, and direct decomposition of the ylide gives a very high barrier. The high barriers for the 1,2-H shift of Rh(II) complex-associated oxonium ylides make the ylides become stable intermediates in both reactions, especially for the reactions in solution. Difficulty in formation of a free oxonium ylide supports experimental results, indicating that the Rh(II) complex-catalyzed nucleophilic addition of a diazo compound proceeds via a Rh(II) complex-associated oxonium ylide rather than via a free oxonium ylide.

Three-Component Reaction of Aryl Diazoacetates, Alcohols, and Aldehydes (or Imines): Evidence of Alcoholic Oxonium Ylide Intermediates

The Rh(II)-catalyzed three-component reaction of aryl diazoacetates, alcohols and aldehydes was explored, which provided evidence of alcoholic oxonium ylide formation for O-H insertion. A new C-C bond formation reaction where alcoholic oxonium ylides were trapped by electron-deficient aryl aldehydes (or imines) was realized.