Palladium-catalyzed intramolecular alpha-arylation of alpha-amino acid esters

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

An umpolung-enabled copper-catalysed regioselective hydroamination approach to α-amino acids

A copper-catalysed regio- and stereoselective hydroamination of acrylates with hydrosilanes and hydroxylamines has been developed to afford the corresponding α-amino acids in good yields. The key to regioselectivity control is the use of hydroxylamine as an umpolung, electrophilic amination reagent. Additionally, a judicious choice of conditions involving the CsOPiv base and DTBM-dppbz ligand of remote steric hindrance enables the otherwise challenging C-N bond formation at the α position to the carbonyl. The point chirality at the β-position is successfully controlled by the Xyl-BINAP or DTBM-SEGPHOS chiral ligand with similarly remote steric bulkiness. The combination with the chiral auxiliary, (-)-8-phenylmenthol, also induces stereoselectivity at the α-position to form the optically active unnatural α-amino acids with two adjacent stereocentres.

A Short Approach to N-Aryl-1,2,3,4-tetrahydroisoquinolines from N-(2-Bromobenzyl)anilines via a Reductive Amination/Palladium-Catalyzed Ethoxyvinylation/Reductive N-Alkylation Sequence

N-Aryl-1,2,3,4-tetrahydroisoquinolines are obtained via a convenient and short protocol with a broad range of substituents on both aromatic rings and high functional group tolerance. Starting from readily available ortho-brominated aromatic aldehydes and primary aromatic amines, condensation of these building blocks under reductive conditions gives N-aryl 2-bromobenzylamines. The C-3/C-4-unit of the tetrahydroisoquinoline is introduced using commercially available 2-ethoxyvinyl pinacolboronate under Suzuki conditions. Finally, the obtained crude ortho-ethoxyvinyl benzylamines are cyclized via an intramolecular reductive amination using the combination of triethylsilane/TFA to give the desired N-aryl-1,2,3,4-tetrahydroisoquinolines.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

Planar Chiral [2.2]Paracyclophane-Based Bisoxazoline Ligands and Their Applications in Cu-Mediated N-H Insertion Reaction

New catalysts for important C–N bond formation are highly sought after. In this work, we demonstrate the synthesis and viability of a new class of planar chiral [2.2]paracyclophane-based bisoxazoline (BOX) ligands for the copper-catalyzed N–H insertion of α-diazocarbonyls into anilines. The reaction features a wide substrate scope and moderate to excellent yields, and delivers the valuable products at ambient conditions.

Palladium-catalyzed α-arylation for the addition of small rings to aromatic compounds

Small, strained rings have rigid, defined conformations and unique electronic properties. For these reasons, many groups seek to use these subunits to form biologically active molecules. We report a generally applicable approach to attach small rings to a wide range of aromatic compounds by palladium-catalyzed α-arylation of cyclopropyl, cyclobutyl and azetidinyl esters. The direct α-arylation of cyclopropyl esters and cyclobutyl esters is achieved in high yield by ensuring that the rate of coupling exceeds the rate of Claisen condensation. The α-arylation of azetidines is achieved without ring opening of the strained saturated heterocycle by conducting the reactions with an azetidine derivative bearing a benzyl protecting group on nitrogen. Mechanistic studies show that the α-arylation of small rings is challenging because of the weak acidity of α C-H bond (cyclopropanes), strong sensitivity of the strained esters to Claisen condensation (cyclobutatanes), or facile decomposition of the enolates (azetidinyl esters).

Methods to prepare small, strained rings are sought after due to the importance of such scaffolds in medicinal chemistry. Here, the authors report a palladium-catalyzed α-arylation of cyclopropyl, cyclobutyl and azetidinyl esters.

CBr4-mediated cross-coupling reactions of α-amino carbonyl compounds with alcohols and thiols to build C–O and C–S bonds, respectively

The direct C(sp³)–H bond functionalization of α-amino carbonyl compounds with CBr₄ to construct C–O and C–S bonds.

Iron-catalyzed direct α-arylation of α-amino carbonyl compounds with indoles

A mild and general α-arylation of α-amino carbonyls with indoles catalyzed by Fe(ClO4)3 has been developed. C-H activation is smoothly fulfilled by using TBHP as the oxidant with good yields. Two hydrogen dissociations make this transformation more environmentally benign because of high atom efficiency.

Synthesis of Unsymmetrical α,α-Diarylacetates

Ethyl bromofluoroacetate has been developed as a precursor for the convenient synthesis of unsymmetrical α,α-diarylacetates featuring indoles, anilines, and other electron-rich aromatics. In conjunction with a mild Lewis acid catalyzed C-N → C-C exchange, intermediate arylglycines can be synthesized and transformed into α,α-diarylacetates in a one-pot protocol, resulting in a net diarylation reaction exhibiting a wide scope. In the context of diarylacetates, the synthetic equivalence of the fluorinated reagent with α-nitro-α-diazo carbonyls was established.

TBHP mediated oxidation of N-2-alkynylphenyl α-amino carbonyl compounds to oxalic amides using visible light photoredox catalysis and their application in the synthesis of 2-aryl indoles

Visible light promoted the oxidation of compounds 1 to amides 2 which were easily transferred to compounds 12.

Copper-catalyzed oxidative oxyalkylation of enol ethers with α-amino carbonyl compounds and hydroperoxides

The first example of alkene oxyalkylation through C(sp³)–H functionalization is described for the synthesis of 2-amino-3,4-dioxy carbonyl compounds.

Efficient access to enantiopure 1,3-disubstituted isoindolines from selective catalytic fragmentation of an original desymmetrized rigid overbred template

Enantiopure 1,3-disubstituted isoindolines are synthesized from selective C–C bond fragmentation of a rigid overbred template, obtained through an asymmetric desymmetrization reaction.

Palladium(0)-catalyzed single and double isonitrile insertion: a facile synthesis of benzofurans, indoles, and isatins

A palladium(0)-catalyzed cascade process consisting of isonitrile insertion and α-Csp(3)-H cross-coupling can be achieved for the synthesis of benzofurans and indoles. The construction of isatins by a Pd-catalyzed cascade reaction incorporating double isonitrile insertion, amination, and hydrolysis has also been achieved. The key features of this work include diverse heterocycle synthesis, phosphine-ligand-free reaction conditions, a one-pot procedure, simple and commercially available starting materials, broad functional-group compatibility, and moderate to good reaction yields.

Ni-Catalyzed α-arylation of esters and amides with phenol derivatives

A nickel-catalyzed α-arylation of esters and amides with phenol derivatives has been accomplished. In the presence of our unique nickel catalyst, prepared in situ from Ni(cod)2, 3,4-bis(dicyclohexylphosphino)thiophene (dcypt), and K3PO4, various esters and amides undergo α-arylation with O-arylpivalates or O-arylcarbamates to afford the corresponding coupling products. The thus obtained α-aryl esters and amides are useful precursors of privileged motifs such as α-arylcarboxylic acids and β-arylamines.

Controlling the ambiphilic nature of σ-arylpalladium intermediates in intramolecular cyclization reactions

The reactivity of main group organometallics, such as organolithium compounds (RLi) and Grignard reagents (RMgX), is quite straightforward. In these species the R group usually exhibits nucleophilic reactivity without any possibility of inducing electrophilic character. In contrast, in organopalladium complexes, researchers can switch the reactivity from electrophilic to nucleophilic relatively simply. Although σ-aryl and σ-vinylpalladium complexes are commonly used as electrophiles in C-C bond-forming reactions, recent research has demonstrated that they can also react with carbon-heteroatom multiple bonds in a nucleophilic manner. Nevertheless, researchers have completely ignored the issue of controlling the ambiphilic nature of such species. This Account describes our efforts toward selectively promoting the same starting materials toward either electrophilic α-arylation or nucleophilic addition reactions to different carbonyl groups. We could tune the properties of the σ-arylpalladium intermediates derived from amino-tethered aryl halides and carbonyl compounds to achieve chemoselective transformations. Therefore, chemists can control the ambiphilic nature of such intermediates and, consequently, the competition between the alternative reaction pathways by the adequate selection of the reaction conditions and additives (base, presence/absence of phenol, bidentate phosphines). The nature of the carbonyl group (aldehydes, ketones, esters, and amides) and the length of the tether connecting it to the aniline moiety also play an important role in the outcome of these processes. Our joint computational and experimental efforts to elucidate the reaction mechanism of these palladium-catalyzed transformations suggest that beyond the formation of the four-membered azapalladacycle, two major factors help to control the dual character of the palladium(II) intermediates derived from 2-haloanilines. First, their high nucleophilicity strongly modifies the interaction of the metal center with the carbonyl group. Second, the additive phenol exchanges the iodide ligand to give an arylpalladium(II) phenoxide complex, which has a beneficial effect on the arylation. The formation of this transient intermediate not only stabilizes the arylpalladium moiety, thus preventing the nucleophilic attack at the carbonyl group, but also assists the enolization reaction, which takes place in a more favorable intramolecular manner. The azapalladacycle intermediate is, in the words of J. R. R. Tolkien, "the one ring to bring them all and in the darkness to bind them." With this intermediate, we can easily achieve the synthesis of a variety of heterocyclic systems by selectively promoting electrophilic α-arylation or nucleophilic addition reactions from the same precursors.

Direct α-arylation of α-amino carbonyl compounds with indoles using visible light photoredox catalysis

A general and mild method for the construction of functionalized 2-(1H-indol-3-yl)-2-amino-carbonyl compounds was achieved, which represents the first example of direct α-arylation of α-amino carbonyl compounds with indoles using the visible light photoredox catalysis strategy.