Nickel-Catalyzed Reductive Arylation of Redox Active Esters for the Synthesis of α-Aryl Nitriles: Investigation of a Chlorosilane Additive

A nickel-catalyzed reductive cross-coupling of redox active N-hydroxyphthalimide (NHP) esters and iodoarenes for the synthesis of α-aryl nitriles is described. The NHP ester substrate is derived from cyanoacetic acid, which allows for a modular synthesis of substituted α-aryl nitriles, an important scaffold in the pharmaceutical sciences. The reaction exhibits a broad scope, and many functional groups are compatible under the reaction conditions, including complex highly functionalized medicinal agents. Mechanistic studies reveal that reduction and decarboxylation of the NHP ester to the reactive radical intermediate are accomplished by a combination of a chlorosilane additive and Zn dust. We demonstrate that stoichiometric chlorosilane is essential for product formation and that chlorosilane plays a role beyond activation of the metal reductant.

A Photoinduced, Nickel-Catalyzed Reaction for the Stereoselective Assembly of C-Linked Glycosides and Glycopeptides

C-Alkyl glycosides and glycoproteins exist in natural products and are prized for their role as carbohydrate mimics in drug design. However, a practical strategy that merges glycosyl donors with readily accessible reagents, derived from abundant carboxylic acid and amine feedstocks, is yet to be conceived. Herein, we show that a nickel catalyst promotes C-C coupling between glycosyl halides and aliphatic acids or primary amines (converted into redox-active electrophiles in one step), in the presence of Hantzsch ester and LiI (or Et₃ N) under blue LED illumination to deliver C-alkyl glycosides with high diastereoselectivity. Mechanistic studies support the photoinduced formation of alkyl radicals that react with a glycosyl nickel species generated in situ to facilitate cross-coupling. Through this manifold, innate CO₂ H and NH₂ motifs embedded within amino acids and oligopeptides are selectively capped and functionalized to afford glycopeptide conjugates through late-stage glycosylation.

Control of Redox-Active Ester Reactivity Enables a General Cross-Electrophile Approach to Access Arylated Strained Rings

Strained rings are increasingly important for the design of pharmaceutical candidates, but cross-coupling of strained rings remains challenging. An attractive, but underdeveloped, approach to diverse functionalized carbocyclic and heterocyclic frameworks containing all-carbon quaternary centers is the coupling of abundant strained-ring carboxylic acids with abundant aryl halides. Herein we disclose the development of a nickel-catalyzed cross-electrophile approach that couples a variety of strained ring N-hydroxyphthalimide (NHP) esters, derived from the carboxylic acid in one step, with various aryl and heteroaryl halides under reductive conditions. The chemistry is enabled by the discovery of methods to control NHP ester reactivity, by tuning the solvent or using modified NHP esters, and the discovery that t-Bu BpyCamCN , an L2X ligand, avoids problematic side reactions. This method can be run in flow and in 96-well plates.

The Formal Cross-Coupling of Amines and Carboxylic Acids to Form sp3 -sp3 Carbon-Carbon Bonds

We have developed a deaminative-decarboxylative protocol to form new carbon(sp³ )-carbon(sp³ ) bonds from activated amines and carboxylic acids. Amines and carboxylic acids are ubiquitous building blocks, available in broad chemical diversity and at lower cost than typical C-C coupling partners. To leverage amines and acids for C-C coupling, we developed a reductive nickel-catalyzed cross-coupling utilizing building block activation as pyridinium salts and redox-active esters, respectively. Miniaturized high-throughput experimentation studies were critical to our reaction optimization, with subtle experimental changes such as order of reagent addition, composition of a binary solvent system, and ligand identity having a significant impact on reaction performance. The developed protocol is used in the late-stage diversification of pharmaceuticals while more than one thousand systematically captured and machine-readable reaction datapoints are reposited.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

N-Doped Yellow TiO2 Hollow Sphere-Mediated Visible-Light-Driven Efficient Esterification of Alcohol and N-Hydroxyimides to Active Esters

Herein we report a simple synthetic protocol for N-doped yellow TiO₂ (N-TiO₂ ) hollow spheres as an efficient visible-light-active photocatalyst using aqueous titanium peroxocarbonate complex (TPCC) solution as precursor and NH₄ OH. In the developed strategy, the ammonium ion of TPCC and NH₄ OH acts as nitrogen source and structure-directing agent. The synthesized N-TiO₂ hollow spheres are capable of promoting the synthesis of active esters of N-hydroxyimide and alcohol through simultaneous selective oxidation of alcohol to aldehyde followed by cross-dehydrogenative coupling (CDC) under ambient conditions upon irradiation of visible light. It is possible to develop a novel and cost-effective one-pot strategy for the synthesis of important esters and amides on gram scale using the developed strategy. The catalytic activity of N-TiO₂ hollow spheres is much superior to that of other reported N-TiO₂ samples as well as TiO₂ with varying morphology.

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp2 )-C(sp3 )Coupling Reactions

A number of new transition metal catalyzed methods for the formation of C(sp2 )-C(sp3 ) bonds have recently been described. These reactions often utilize bidentate polypyridyl-ligated Ni catalysts, and paramagnetic NiI halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type. Here, we report the synthesis of paramagnetic bidentate polypyridyl-ligated Ni halide and aryl complexes through elementary reactions proposed in catalytic cycles for C(sp2 )-C(sp3 ) bond formation. We investigate the ability of these complexes to undergo organometallic reactions that are relevant to C(sp2 )-C(sp3 ) coupling through stoichiometric studies and also explore their catalytic activity.

The Direct Conversion of α-Hydroxyketones to Alkynes

Alkynes are highly important functional groups in organic chemistry, both as part of target structures and as versatile synthetic intermediates. In this study, a protocol for the direct conversion of α-hydroxyketones to alkynes is reported. In combination with the variety of synthetic methods that generate the required starting materials by forming the central C-C bond, it enables a highly versatile fragment coupling approach toward alkynes. A broad scope for this novel transformation is shown alongside mechanistic insights. Furthermore, the utility of our protocol is demonstrated through its application in concert with varied α-hydroxyketone syntheses, giving access to a broad spectrum of alkynes.

Photoinduced Copper-Catalyzed Coupling of Terminal Alkynes and Alkyl Iodides

We have developed a photoinduced copper-catalyzed alkylation of terminal alkynes with primary, secondary, or tertiary alkyl iodides as electrophiles. The reaction has a broad substrate scope and can be successfully performed in the presence of ester, nitrile, aryl halide, ketone, sulfonamide, epoxide, alcohol, and amide functional groups. The alkylation is promoted by blue light (λ≈450 nm) and proceeds at room temperature in the absence of any additional metal catalysts. The use of a terpyridine ligand is essential for the success of the reaction and is shown to prevent photoinduced copper-catalyzed polymerization of the starting materials.