Allylation reactions of aldehydes with allylboronates in aqueous media: unique reactivity and selectivity that are only observed in the presence of water

Allylation of Lactol in Water

We developed a water-promoted cross-coupling of 3-hydroxyisobenzofuran-1(3H)-ones and allylic boronates under biocompatible conditions. This approach facilitates the synthesis of diverse 3-allylic phthalides with high yields and exceptional selectivity in a metal-free manner. The versatility and practicality of this protocol underscore its significant potential for drug development and applications in medicinal chemistry.

Site-Isolated Main-Group Tris(2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization Catalysis

Tris(2-pyridyl)borates are an emerging class of scorpionate ligands, distinguished as exceptionally robust and electron-donating. However, the rapid formation of inert homoleptic complexes with divalent metals has so far limited their catalytic utility. We report site-isolating tris(2-pyridyl)borate ligands, bearing isopropyl, tert-butyl, and mesityl substituents at the pyridine 6-position to suppress the formation of inert homoleptic complexes. These ligands form the first 1:1 complexes between tris(2-pyridyl)borates and Mg²⁺, Zn²⁺, or Ca²⁺, with isopropyl-substituted Tpy^iPrH showing the most generality. Single-crystal X-ray diffraction analysis of the resulting complexes and comparison to density functional theory (DFT) models showed geometric distortions driven by steric repulsion between the pyridine 6-substituents and the hexamethyldisilazide (HMDS^-, ^-N(SiMe₃)₂) anion. We show that this steric profile is a feature of the six-membered pyridine ring and contrasts with more established tris(pyrazolyl)borate and tris(imidazoline)borate scorpionate complexes. Tpy^iPrMg(HMDS) (1) and its zinc analogue are moderately active for the controlled polymerization of l-lactide, ε-caprolactone, and trimethylene carbonate. Furthermore, 1 gives controlled polymerization under more demanding melt-phase polymerization conditions at 100 °C, and block copolymerization of ε-caprolactone and trimethylene carbonate. These results will enable useful catalysis and coordination chemistry studies with tris(2-pyridyl)borates, and characterizes their structural complementarity to more familiar scorpionate ligands.

Biocatalytic Production of Aldehydes: Exploring the Potential of Lathyrus cicera Amine Oxidase

Aldehydes are a class of carbonyl compounds widely used as intermediates in the pharmaceutical, cosmetic and food industries. To date, there are few fully enzymatic methods for synthesizing these highly reactive chemicals. In the present work, we explore the biocatalytic potential of an amino oxidase extracted from the etiolated shoots of Lathyrus cicera for the synthesis of value-added aldehydes, starting from the corresponding primary amines. In this frame, we have developed a completely chromatography-free purification protocol based on crossflow ultrafiltration, which makes the production of this enzyme easily scalable. Furthermore, we determined the kinetic parameters of the amine oxidase toward 20 differently substituted aliphatic and aromatic primary amines, and we developed a biocatalytic process for their conversion into the corresponding aldehydes. The reaction occurs in aqueous media at neutral pH in the presence of catalase, which removes the hydrogen peroxide produced during the reaction itself, contributing to the recycling of oxygen. A high conversion (>95%) was achieved within 3 h for all the tested compounds.

Hydrogen-Bonding-Assisted Cationic Aqua Palladium(II) Complex Enables Highly Efficient Asymmetric Reactions in Water

Metal-bound water molecules have recently been recognized as a new facet of soft Lewis acid catalysis. Herein, a chiral palladium aqua complex was constructed that enables carbon-hydrogen bonds of indoles to be functionalized efficiently. We embraced a chiral 2,2'-bipyridine as both ligand and hydrogen-bond donor to configure a robust, yet highly Lewis acidic, chiral aqua complex in water. Whereas the enantioselectivity could not be controlled in organic solvents or under solvent-free conditions, the use of aqueous environments allowed the σ-indolylpalladium intermediates to react efficiently in a highly enantioselective manner. This work thus describes a potentially powerful new approach to the transformation of organometallic intermediates in a highly enantioselective manner under mild reaction conditions.

BINOL derivatives-catalysed enantioselective allylboration of isatins: application to the synthesis of (R)-chimonamidine

The asymmetric synthesis of the 3-allyl-3-hydroxyoxindole skeleton was accomplished in yields up to 99% via a metal-free and enantioselective allylation of isatins (90-96% ee) using BINOL derivatives as catalysts and an optimized allylboronate. This methodology was applied at a gram-scale to the synthesis of the natural product (R)-chimonamidine.

Applications of Bolm's Ligand in Enantioselective Synthesis

One pathway for the preparation of enantiomerically pure compounds from prochiral substrates is the use of metal complex catalysis with chiral ligands. Compared to the other frequently used chiral ligands, chiral 2,2'-bipyridines have been underexploited, despite the data indicating that such ligands have considerable potential in synthetic chemistry. One of those is the so-called Bolm's ligand, a compound possessing chiral alcohol moieties in the side chains attached to the 2,2'-bipyridine scaffold. Various metal salts have been used in combination with Bolm's ligand as potent catalysts able to bring about enantioselective alkylations, allylations, conjugate additions, desymmetrization of meso-epoxides, aldol reactions, etc. This review aims to summarize Bolm's ligand applications in the area of enantioselective synthesis over the last three decades since its preparation.

An Expeditious Synthesis of Sialic Acid Derivatives by Copper(I)-Catalyzed Stereodivergent Propargylation of Unprotected Aldoses

We developed a copper(I)-catalyzed stereodivergent anomeric propargylation of unprotected aldoses as a facile synthetic pathway to a broad variety of sialic acid derivatives. The soft allenylcopper(I) species, catalytically generated from stable allenylboronic acid pinacolate (2), is unusually inert to protonolysis by the multiple hydroxy groups of the substrates and thereby functions as a carbon nucleophile. The key additive B(OMe)3 facilitated ring-opening of the nonelectrophilic cyclic hemiacetal forms of aldoses to the reactive aldehyde forms. The chirality of the catalyst, and not the internal stereogenic centers of substrates, predominantly controlled the stereochemistry of the propargylation step; i.e., the diastereoselectivity was switched simply by changing the catalyst chirality. This is the first nonenzyme catalyst-controlled stereodivergent C-C bond elongation at the anomeric center of unprotected aldoses, which contain multiple protic functional groups and stereogenic centers. The propargylation products can be expeditiously transformed into naturally occurring and synthetic sialic acid derivatives in a simple three-step sequence. This synthetic method, which requires no protecting groups, can be performed on a gram-scale and thus offers general and practical access to various sialic acid derivatives from unprotected aldoses.