Atom‐Economic Alk(en)ylations of Esters, Amides, and Methyl Heteroarenes Utilizing Alcohols Following Dehydrogenative Strategies

Quinoline-derived NNP-manganese complex catalyzed α-alkylation of ketones with primary alcohols

An air-stable quinoline-derived NNP ligand chelated Mn catalyst was developed for the efficient α-alkylation of ketones with primary alcohols via a hydrogen auto-transfer methodology. The sole by-product formed is water, rendering the protocol atom efficient. A wide range of ketone and alcohol substrates were employed, providing the α-alkylated ketones with isolated yields up to 94%. This system was also efficient for the green synthesis of quinoline derivatives while using (2-aminophenyl)methanol as an alkylating reagent.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Nickel-Catalyzed Deoxygenative Amidation of Alcohols with Carbamoyl Chlorides

We report a deoxygenative amidation reaction of alcohols with carbamoyl chlorides to afford amides through nickel-photoredox dual catalysis. Good to excellent yields can be obtained even for diverse complex sugar and steroid derivatives. The reaction is scalable, and the synthetic utility of the reaction was demonstrated by the homologation of alcohols to deliver several important γ-amino alcohols and a synthetically challenging bioactive compound intermediate.

NNN manganese complex-catalyzed α-alkylation of methyl ketones using alcohols: an experimental and computational study

We present here a phosphine-free, quinoline-based pincer Mn catalyst for α-alkylation of methyl ketones using primary alcohols as alkyl surrogates. The C-C bond formation reaction proceeds via a hydrogen auto-transfer methodology. The sole by-product formed is water, rendering the protocol atom efficient. Electronic structure theory studies corroborated the proposed mechanism.

Heteroditopic NHC Ligand Supported Manganese(I) Complexes: Synthesis, Characterization, and Activity as Non-bifunctional Phosphine-Free Catalyst for the α-Alkylation of Nitriles

In the present work, several manganese(I) complexes of chelating heteroditopic ligands Mn1-3, featuring ImNHC (imidazol-2-ylidene) connected to a 1,2,3-triazole-N or tzNHC (1,2,3-triazol-5-ylidene) donors via a methylene spacer, with possible modifications at the triazole backbone have been synthesized and completely characterized. Notably, the CO stretching frequencies, electrochemical analysis, and frontier orbital analysis certainly suggest that the chelating ImNHC-tzNHC ligands have stronger donation capabilities than the related ImNHC-Ntz ligand in the synthesized complexes. Moreover, these well-defined phosphine-free Mn(I)-NHC complexes have been found to be effective non-bifunctional catalysts for the α-alkylation of nitriles using alcohols and importantly, the catalyst Mn1 containing ImNHC connected to a weaker triazole-N donor displayed higher activity compared to Mn2/Mn3 containing an unsymmetrical bis-carbene donors (ImNHC and tzNHC). A wide range of aryl nitriles were coupled with diverse (hetero)aromatic as well as aliphatic alcohols to get the corresponding products in good to excellent yields (32 examples, up to 95 % yield). The detailed mechanistic studies including deuterium labelling experiments reveal that the reaction follows a Borrowing Hydrogen pathway.

N-Heterocyclic Carbene-Supported Nickel-Catalyzed Selective (Un)Symmetrical N-Alkylation of Aromatic Diamines with Alcohols

The "borrowing hydrogen" (BH) approach for the N-alkylation of phenylenediamines using alcohols as coupling partners is highly challenging due to the selectivity issue of the generated products. Furthermore, the development of base-metal systems that can potentially substitute precious metals with competitive activity is a major challenge in BH catalysis. We present herein an efficient protocol for the N,N'-di-alkylation of aromatic diamines using an in situ-generated Ni-NHC complex from NiCl₂ and the ligand L1, which gave access to a wide range of N,N'-di-alkylated orthophenylene diamines (rather than the generally observed benzimidazole derivatives), meta- and para-phenylene diamines along with 2,6-diamino pyridine derivatives in good to excellent yields. Moreover, the catalyst system was also successful in the derivatization of a clinically important drug molecule, Dapsone. Notably, the present protocol could be applied effectively to synthesize unsymmetrically substituted N,N'-di-alkylated diamines via sequential alkylation and is the first report in the base-metal system to the best of our knowledge. Diverse control experiments including the deuterium incorporation studies suggest that the present protocol proceeds via a BH sequence.

RuII-Complexes of heteroditopic chelating NHC ligands: effective catalysts for the β-alkylation of secondary alcohols and the synthesis of 2-alkylaminoquinoline derivatives following the dehydrogenative protocol

Ru^II-Complexes of chelating heteroditopic N-heterocyclic carbene ligands featuring imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) donors connected via a CH₂ spacer, 1a-c, were found to be very effective catalysts for the cross-coupling of secondary and primary alcohols with the elimination of H₂O. Diverse β-alkylated secondary alcohols were thus obtained by following this method in excellent yields of up to 95% by employing a very low catalyst (1a) loading of 0.01-0.001 mol% along with the inexpensive base KOH. Mechanistically, the present protocol follows the borrowing hydrogen strategy which was established by various control experiments including deuterium labelling experiments and importantly, ¹H NMR and ESI-MS analyses validated the participation of a Ru-H species in the catalytic cycle. Remarkably, the present system displayed the highest Ru-based TON of 396 000 for the β-benzylation of 1-phenylethanol with a catalyst loading of 1 ppm (0.0001 mol%). Additionally, diverse 2-alkylaminoquinoline derivatives were synthesized in a one-pot manner from 2-aminobenzyl alcohol, 2-arylacetonitrile, and various primary alcohols.

Electronically tuneable orthometalated RuII–NHC complexes as efficient catalysts for C–C and C–N bond formations via borrowing hydrogen strategy

A series of simple and electronically tuneable cyclometalated RuII–NHC complexes have been explored as efficient catalysts for various C–C/N bond forming reactions via a BH methodology.

A heteroditopic NHC and phosphine ligand supported ruthenium(ii)-complex: an effective catalyst for the N-alkylation of amides using alcohols

A heteroditopic Ru(ii)-bis-NHC complex in combination with dppe was developed as an effective catalyst system (0.2 mol% loading) for the N-alkylation of amides and selective mono-/di-alkylation of 4-aminobenzamide derivatives in excellent yields.

Nickel catalyzed sustainable synthesis of benzazoles and purines via acceptorless dehydrogenative coupling and borrowing hydrogen approach

Herein we report nickel-catalyzed sustainable synthesis of a few chosen five-membered fused nitrogen heterocycles such as benzimidazole, purine, benzothiazole, and benzoxazole via acceptorless dehydrogenative functionalization of alcohols. Using a bench stable, easy to prepare, and inexpensive Ni(ii)-catalyst, [Ni(MeTAA)] (1a), featuring a tetraaza macrocyclic ligand (tetramethyltetraaza[14]annulene (MeTAA)), a wide variety of polysubstituted benzimidazole, purine, benzothiazole, and benzoxazole derivatives were prepared via dehydrogenative coupling of alcohols with 1,2-diaminobenzene, 4,5-diaminopyrimidine, 2-aminothiphenol, and 2-aminophenol, respectively. A wide array of benzimidazoles were also prepared via a borrowing hydrogen approach involving alcohols as hydrogen donors and 2-nitroanilines as hydrogen acceptors. A few control experiments were performed to understand the reaction mechanism.

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC ^C Cyclometalated Ligands

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp² -H or Csp³ -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20^th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based C_NHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

[(PPh3)2NiCl2]-Catalyzed C-N Bond Formation Reaction via Borrowing Hydrogen Strategy: Access to Diverse Secondary Amines and Quinolines

Commercially available [(PPh₃)₂NiCl₂] was found to be an efficient catalyst for the mono-N-alkylation of (hetero)aromatic amines, employing alcohols to deliver diverse secondary amines, including the drug intermediates chloropyramine (5b) and mepyramine (5c), in excellent yields (up to 97%) via the borrowing hydrogen strategy. This method shows a superior activity (TON up to 10000) with a broad substrate scope at a low catalyst loading of 1 mol % and a short reaction time. Further, this strategy is also successful in accessing various quinoline derivatives following the acceptorless dehydrogenation pathway.