Selective Hydrosilylation of Esters to Aldehydes Catalysed by Iridium(III) Metallacycles through Trapping of Transient Silyl Cations

Catalytic Reductive Amination and Tandem Amination-Alkylation of Esters Enabled by a Cationic Iridium Complex

Reported herein is a convenient and efficient method for one-pot, catalytic reductive amination, as well as the first multi-component tandem reductive amination-functionalization of bench-stable and readily available common carboxylic esters. This method is based on the cationic [Ir(COD)2]BArF-catalyzed chemoselective hydrosilylation of esters, followed by one-pot acid-mediated amination and nucleophilic addition. The reaction was conducted under mild conditions at a very low catalyst loading (0.1 mol % of Ir), which could be further reduced to 0.001 mol %, as demonstrated by a reaction at a 15 g scale. The method is highly versatile, allowing the use of esters with or without α-protons for the N-mono-alkylation of primary and secondary amines to produce diverse secondary and tertiary amines, as well as α-branched/functionalized amines. The method is highly chemoselective and tolerates a variety of functional groups such as bromo, trifluoromethyl, ester, and cyano groups. The value of the method was demonstrated by the one-step catalytic synthesis of two bio-relevant N-mono-methyl α-amino esters and the antiparkinsonian agent piribedil, as well as by the use of two shorter chain triglycerides as alkylating feedstock.

Direct conversion of esters to imines/enamines and applications to polyester waste upcycling

Semi-reductive transformations of esters remain an underdeveloped but valuable class of functional group interconversions. Here, we describe the development of a highly selective method for the interconversion of esters to imines, enamines, aldehydes or amines through an amine-intercepted zirconocene hydride (ZrH)-catalyzed reduction. This protocol employs an inexpensive zirconium catalyst in combination with hydrosilanes and simple unprotected amines. A variety of aryl, benzylic, and aliphatic esters are directly transformed to imines and enamines in up to 99% yield or aldehydes in up to 84% yield, with little-to-no reduction to the corresponding alcohols. The utility of this method for the direct catalytic chemical upcycling of polyester plastic waste is demonstrated through multiple unprecedented depolymerization transformations. Further, the efficient preparation of nitrogen-containing products is also presented, including single-flask multicomponent reactions and the reductive amination of esters.

Hydrogenolysis of Carboxylic Ortho Esters

It is demonstrated for the first time that carboxylic ortho esters may be reduced to acetals using hydrogen as the reducing agent. Hydrogenolysis was performed under mild conditions over commercially available hydrogenation catalysts (5% Pd/C and 5% Pt/C). The present method is especially useful for the selective transformation of carboxylic acids and lactones into acetals.

Partial reductions of carboxylic acids and their derivatives to aldehydes

Recent advances in partial reductions of inert carboxylic acids and their derivatives to active aldehydes are reviewed.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Manganese and rhenium-catalyzed selective reduction of esters to aldehydes with hydrosilanes

The selective reduction of esters to aldehydes, via the formation of stable alkyl silyl acetals, was, for the first time, achieved with both manganese, -Mn₂(CO)₁₀- and rhenium -Re₂(CO)₁₀- catalysts in the presence of triethylsilane as reductant. These two methods provide a direct access to a large variety of aliphatic and aromatic alkyl silyl acetals (30 examples) and to the corresponding aldehydes (13 examples) upon hydrolysis. The reactions proceeded in excellent yields and high selectivity at room temperature under photo-irradiation conditions (LED, 365 nm, 40 W, 9 h).

Catalytic reductive deoxygenation of esters to ethers driven by hydrosilane activation through non-covalent interactions with a fluorinated borate salt

A fluorinated borate BArF salt catalyses the reductive deoxygenation of esters to ethers by using hydrosilanes. Experimental and theoretical studies highlight the role of noncovalent interactions in the reaction mechanism.

Iridium-Catalyzed Reductive Allylation of Esters

The catalytic reductive transformation of carboxylic esters into α-branched ethers is described. The procedure pivots on the chemoselective iridium-catalyzed hydrosilylation of ester and lactone functionality to afford a silyl acetal intermediate. Upon treatment with a Lewis acid, these hemilabile intermediates dissociate to form reactive oxocarbenium ions, which can be intercepted by allyltributyltin nucleophiles, resulting in the formation of valuable α-branched alkyl-alkyl ether derivatives. This reductive allylation procedure was found to be amenable to a range of carboxylic ester starting materials, and good chemoselectivity for ethyl over tert-butyl esters was demonstrated. Furthermore, downstream synthetic manipulation of α-amino acid-derived products led to the efficient formation of pyrrolidine, piperidine, and azepane frameworks.

Size-Exclusion Borane-Catalyzed Domino 1,3-Allylic/Reductive Ireland-Claisen Rearrangements: Impact of the Electronic and Structural Parameters on the 1,3-Allylic Shift Aptitude

The reductive Ireland-Claisen rearrangement through borane-mediated hydrosilylation is reported. The method employs a borane catalyst with a special structural design and affords access to synthetically relevant products with high diastereoselectivity. Depending on electronic and structural parameters, the reaction can be coupled with a 1,3-allylic shift, thus the valence isomer of the Ireland-Claisen product is formed.

Selective ligand-free cobalt-catalysed reduction of esters to aldehydes or alcohols

Activated cobalt metal salts are effective and versatile catalysts for the chemoselective reduction of esters to aldehydes or alcohols through hydrosilylation reaction.

Revisiting the iridacycle-catalyzed hydrosilylation of enolizable imines

In situ¹H NMR spectroscopy reveals a cascade mechanism for the hydrosilylation of enolizable imines catalyzed by iridium(iii) metallacycles.