Direct Acylation of Aryl Chlorides with Aldehydes by Palladium−Pyrrolidine Co-catalysis

Unique biomedical application of fluorescence derivatization based on palladium-catalyzed coupling reactions for HPLC analysis of pharmaceuticals and biomolecules

Palladium-catalyzed coupling reactions are versatile and powerful tools for the construction of carbon-carbon bonds in organic synthesis. Although these reactions have favorable features that proceed selectively in mild reaction conditions using aqueous organic solvents, no attention has been given to their application in the field of biomedical analysis. Therefore, we focused on these reactions and evaluated the scope and limitations of their analytical performance. In this review, we describe the pros and cons and future trends of fluorescence derivatization of pharmaceuticals and biomolecules based on palladium-catalyzed coupling reactions such as Suzuki-Miyaura coupling, Mizoroki-Heck coupling, and Sonogashira coupling reactions for HPLC analysis.

Domino Sequences Involving Stereoselective Hydrazone-Type Heck Reaction and Denitrogenative [1,5]-Sigmatropic Rearrangement

Although the Heck reactions of alkene partners with various electrophiles have achieved great success, the variant focused on carbon═heteroatom counterparts still remains elusive. Herein, we report a Pd(0)-catalyzed asymmetric intramolecular hydrazone-type Heck reaction of N-[(Z)-3-iodoallyl]-aminoacetaldehyde and hydrazine hydrate (NH₂NH₂-H₂O), wherein the required hydrazone is in situ generated via an acid-promoted condensation. A key strategic advantage of this Heck paradigm is that the resultant Heck product allylic diazene rapidly undergoes stereospecific denitrogenative [1,5]-sigmatropic rearrangement, eventually furnishing a domino sequence toward 3-substituted tetrahydropyridine (THP) with high enantioselectivity. The substrate-induced diastereoselective version has also been realized, exclusively giving cis-2,5-disubstituted THPs. The utility of this sequence is demonstrated by the formal synthesis of multiple valuable bioactive targets, including 3-ethylindoloquinolizine, preclamol, and niraparib.

Rapid Access to γ-Amino-α-aryl Alcohol Scaffolds via an Enamine-Based Heck Coupling

The γ-amino-α-aryl alcohol is a key functional group for the design of inhibitors directed toward a critical family of metabolic enzymes. Here we report the transformation of simple aryl halides to a highly functionalized benzyl (3-oxo-3-arylpropyl)carbamate intermediate that can rapidly be converted to a high value γ-amino-α-aryl alcohol. This chemistry is realized through a two-step process involving an enamine-based Heck coupling (EBHC) followed by a one-pot catalytic Cbz-deprotection and ketone reduction of EBHC products.

Mechanism of nickel-catalyzed direct carbonyl-Heck coupling reaction: the crucial role of second-sphere interactions

We present a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates and aldehydes to afford ketones. The precatalyst Ni(COD)2 is activated with the phosphine (phos) ligand, followed by coordination of the substrate PhOTf, to form [Ni(phos)(PhOTf)] for intramolecular PhOTf to Ni(0) oxidative addition. The ensuing phenyl-Ni(ii) triflate complex substitutes benzaldehyde for triflate by an interchange mechanism, leaving the triflate anion in the second coordination sphere held by Coulomb attraction. The Ni(ii) complex cation undergoes benzaldehyde C[double bond, length as m-dash]O insertion into the Ni-Ph bond, followed by β-hydride elimination, to produce Ni(ii)-bound benzophenone, which is released by interchange with triflate. The resulting neutral Ni(ii) hydride complex leads to regeneration of the active catalyst following base-mediated deprotonation/reduction. The benzaldehyde C[double bond, length as m-dash]O insertion is the rate-determining step. The triflate anion, while remaining in the second sphere, engages in electrostatic interactions with the first sphere, thereby stabilizing the intermediate/transition state and enabling the desired reactivity. This is the first time that such second-sphere interaction and its impact on cross-coupling reactivity has been elucidated. The new insights gained from this study can help better understand and improve Heck-type reactions.

Chemoselective Hydrosilylation of the α,β-Site Double Bond in α,β- and α,β,γ,δ-Unsaturated Ketones Catalyzed by Macrosteric Borane Promoted by Hexafluoro-2-propanol

The B(C₆F₅)₃-catalyzed chemoselective hydrosilylation of α,β- and α,β,γ,δ-unsaturated ketones into the corresponding non-symmetric ketones in mild reaction conditions is developed. Nearly 55 substrates including those bearing reducible functional groups such as alkynyl, alkenyl, cyano, and aromatic heterocycles are chemoselectively hydrosilylated in good to excellent yields. Isotope-labeling studies revealed that hexafluoro-2-propanol also served as a hydrogen source in the process.

The α-alkylation of ketones with alcohols in pure water catalyzed by a water-soluble Cp*Ir complex bearing a functional ligand

A water-soluble dinuclear Cp*Ir complex bearing 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine as a bridging ligand was found to be a highly effective catalyst for the α-alkylation of ketones with alcohols in water.

Nickel-Catalyzed Alkylation of Ketone Enolates: Synthesis of Monoselective Linear Ketones

Herein we have developed a Ni-catalyzed protocol for the synthesis of linear ketones. Aryl, alkyl, and heteroaryl ketones as well as alcohols yielded the monoselective ketones in up to 90% yield. The catalytic protocol was successfully applied in to a gram-scale synthesis. For a practical utility, applications of a steroid derivative, oleyl alcohol, and naproxen alcohol were employed. Preliminary catalytic investigations involving the isolation of a Ni intermediate and defined Ni-H species as well as a series of deuterium-labeling experiments were performed.

Palladium-Catalyzed Room Temperature Acylative Cross-Coupling of Activated Amides with Trialkylboranes

A highly efficient acylative cross-coupling of trialkylboranes with activated amides has been effected at room temperature to give the corresponding alkyl ketones in good to excellent yields by using 1,3-bis(2,6-diisopropyl)phenylimidazolylidene and 3-chloropyridine co-supported palladium chloride, the PEPPSI catalyst, in the presence of K₂CO₃ in methyl tert-butyl ether. The scope and limitations of the protocol were investigated, showing good tolerance of acyl, cyano, and ester functional groups in the amide counterpart while halo group competed via the classical Suzuki coupling. The trialkylboranes generated in situ by hydroboration of olefins with BH₃ or 9-BBN performed similarly to those separately prepared, making this protocol more practical.

Direct Aldehyde Csp2 -H Functionalization through Visible-Light-Mediated Photoredox Catalysis

The development of methods for carbon-carbon bond formation under benign conditions is an ongoing challenge for synthetic chemists. In recent years there has been considerable interest in using selective C-H activation as a direct route for generating reactive intermediates. Herein, the use of visible-light-mediated dual photoredox organocatalysis as a mild and effective method for Csp2 -H activation of aldehydes is reported, resulting in the generation of acyl radicals. These nucleophilic acyl radical species can undergo either addition to electrophilic alkenes or nickel-catalyzed cross-coupling reactions to provide a quick access to broad range of unsymmetrical ketones, which are abundantly found in many organic building blocks.

Microbial Transformation of Some Natural and Synthetic Aromatic Compounds by Fungi: Aspergillus and Neurospora Strains

Microbial transformation of chalcone (1), 4-hydroxychalcone (2) and 4′-hydroxychalcone (3), 1,1-diphenylmethane (4), 1,3-diphenylacetone (5), 1,3-diphenylpropane (6), bibenzyl (7), ( E)-stilbene (8a)- and ( Z)-stilbenes (8b), and phenylcyclohexane (9), (1 R,2 S)-1-phenyl-2-hydroxycyclohexane (9a) and (1 S,2 R)-1-phenyl-2-hydroxycyclohexane (9b), and a naturally occurring bis-bibenzyl, marchantin A (10) were performed by using Aspergillus niger TBUYN-2 and the other Aspergillus strains, and Neurospora crassa which were capable to hydrogenation and epoxidation of a conjugated double bond, and direct hydroxylation and hydroperoxidation on benzene ring, and hydroxylation and carbonization on cyclohexane ring. Aspergillus species converted chalcone (1) to dihydrochalcone (1a) almost quantitatively.

Palladium-Catalyzed Room-Temperature Acylative Suzuki Coupling of High-Order Aryl Borons with Carboxylic Acids

This note describes a dimethyl dicarbonate-assisted, Pd(OAc)2/PPh3-catalyzed acylative Suzuki coupling of carboxylic acids with diarylborinic acids or tetraarylboronates for practical and efficient synthesis of sterically undemanding aryl ketones at room temperature. More than just cost-effective alternatives to aryl boronic acids, diarylborinic acids and tetraarylboronates displayed higher reactivity in the acylative Suzuki coupling. A variety of alkyl aryl ketones, including those bearing a hydroxy, bromo, or carbonyl group, could be readily obtained in modest to excellent yields.

[Stereoselective Synthesis of Multi-substituted Fluoro-β-lactams and Their Conversion to Fluorinated β-Amino Acid Core]

Multi-substituted β-lactam compounds have not only attracted considerable interest as core structures of pharmaceutical compounds such as antibiotics but also have been used as building blocks for the construction of β-amino acids. Electrophilic β-lactams can be used to enhance essential biological activities. Furthermore, the ring-opening reactions of electrophilic β-lactams can be used to provide facile access to β-amino acids. The introduction of an electronegative fluorine atom to a β-lactam ring to give the corresponding fluoro-β-lactam can be used as an effective strategy for the preparation of electrophilic β-lactams. In this review, we provide a summary of our recent research towards the direct functionalization of fluoro-β-lactams. This review has been divided into four topics, including: 1) the alkylation and hydroxyalkylation of α-bromo-α-fluoro-β-lactams (1); 2) the nickel-catalyzed cross coupling reaction of 1; 3) the asymmetric synthesis of fluoro-β-lactams using chiral ligands; and 4) the utilization of fluoro-β-lactams as highly electrophilic building blocks.

Ortho C-H Acylation of Aryl Iodides by Palladium/Norbornene Catalysis

Reported herein is a palladium/norbornene-catalyzed ortho-arene acylation of aryl iodides by a Catellani-type C-H functionalization. This transformation is enabled by isopropyl carbonate anhydrides, which serve as both an acyl cation equivalent and a hydride source.

Pyracene-linked bis-imidazolylidene complexes of palladium and some catalytic benefits produced by bimetallic catalysts

Two new palladium complexes with a pyracene-linked bis-imidazolylidene (pyrabim) group have been obtained and fully characterized. The related monometallic analogues were obtained from the coordination of an acetanaphthene-supported N-heterocyclic carbene (NHC). The catalytic properties of all complexes were studied in the acylation of aryl halides with hydrocinnamaldehyde, and in the Suzuki-Miyaura coupling of aryl halides and aryl boronic acids. The results show that the presence of a second metal in the dimetallic complexes induces some benefits in the catalytic behavior of the complexes. This effect is more pronounced in the Suzuki-Miyaura coupling, for which the dimetallic complexes exhibit significantly higher activity than their monometallic counterparts.