The cross-dehydrogenative coupling of C(sp3)-H bonds: a versatile strategy for C-C bond formations

A metal free domino synthesis of 3-aroylindoles via two sp³ C-H activation

A metal free synthesis of 3-aroylindole involving two sp(3) C-H activation has been achieved starting from o-alkynyl-N,N-dialkylamines using catalyst TBAI and oxidant TBHP.

The Rügheimer-Burrows reaction revisited: Facile preparation of 4-alkylisoquinolines and 3,5-dialkylpyridines from (partially) saturated amines

A long-known class of cyclic amine/aldehyde condensation reactions was reinvestigated. Benzoic acid was found to efficiently promote condensations of amines such as piperidine or 1,2,3,4-tetrahydroisoquinoline with aromatic aldehydes, resulting in amine β-functionalization and aromatization. These redox-neutral transformations provide 3,5-dialkylpyridines and 4-alkylisoquinolines in moderate to good yields, following short reaction times under microwave conditions.

Acceleration Effects of Phosphine Ligands on the Rhodium-Catalyzed Dehydrogenative Silylation and Germylation of Unactivated C(sp(3))-H Bonds

The current work describes the marked rate of acceleration caused by phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of unactivated C(sp(3))-H bonds. The reactivity was affected by the steric and electronic nature of the phosphine ligands. The use of the bulky and electron-rich diphosphine ligand (R)-DTBM-SEGPHOS was highly effective to yield the dehydrogenative silylation products selectively in the presence of a hydrogen acceptor. An appropriate choice of C2-symmetric chiral diphosphine ligand enables the asymmetric dehydrogenative silylation via the enantioselective desymmetrization of the C(sp(3))-H bond. The unprecedented catalytic germylation of C(sp(3))-H bonds with dehydrogenation was also examined with the combination of the rhodium complex and a wide bite angle diphosphine ligand to provide the corresponding 2,3-dihydrobenzo[b]germoles in good yield.

Organocatalytic amination of alkyl ethers via n-Bu4NI/t-BuOOH-mediated intermolecular oxidative C(sp(3))-N bond formation: novel synthesis of hemiaminal ethers

A novel method for constructing the hemiaminal ether framework under metal-free conditions has been developed. It involves direct organocatalytic amination of alkyl ethers through intermolecular oxidative C(sp(3))-N bond formation, with t-BuOOH being the oxidant and n-Bu4NI as the catalyst.

Iodine-catalyzed radical oxidative annulation for the construction of dihydrofurans and indolizines

Through iodine catalysis, the direct oxidative coupling/annulation of β-keto esters or 2-pyridinyl-β-esters with alkenes was achieved. This reaction procedure provides a simple and selective way for the synthesis of dihydrofurans and indolizines in one step.

The electron is a catalyst

The electron is an efficient catalyst for conducting various types of radical cascade reaction that proceed by way of radical and radical ion intermediates. But because electrons are omnipresent, catalysis by electrons often passes unnoticed. In this Review, a simple analogy between acid/base catalysis and redox catalysis is presented. Conceptually, the electron is a catalyst in much the same way that a proton is a catalyst. The 'electron is a catalyst' paradigm unifies mechanistically an assortment of synthetic transformations that otherwise have little or no apparent relationship. Diverse radical cascades, including unimolecular radical substitution reactions (SRN1-type chemistry), base-promoted homolytic aromatic substitutions (BHAS), radical Heck-type reactions, radical cross-dehydrogenative couplings (CDC), direct arene trifluoromethylations and radical alkoxycarbonylations, can all be viewed as electron-catalysed reactions.

Palladium-catalyzed R2(O)P directed C(sp2)-H acetoxylation

A novel and efficient Pd-catalyzed C-H acetoxylation is described. The approach uses R2(O)P as a directing group to synthesize various substituted 2'-phosphorylbiphenyl-2-OAc compounds. Notably, the reaction exhibits smooth operation under mild conditions and shows good functional group tolerance. Products are obtained with high selectivity and yields.

C-H alkenylations with alkenyl acetates, phosphates, carbonates, and carbamates by cobalt catalysis at 23 °C

Inexpensive cobalt catalysts with N-heterocyclic carbene ligands enable direct arene alkenylations with easily accessible alkenyl acetates through regioselective C-H/C-O functionalizations in a stereoconvergent fashion. The versatile cobalt catalyst was broadly applicable and thus also allowed for the efficient conversion of alkenyl phosphates, carbonates, and carbamates at ambient temperature.

Palladium(II)-catalyzed directed trifluoromethylthiolation of unactivated C(sp³)-H bonds

The synthesis of trifluoromethylthiolated aliphatic acid derivatives by Pd-catalyzed C(sp(3))-H bond functionalization was developed. Using a bidentate directing group, the direct and selective introduction of a SCF3 moiety was possible on a range of amides with remarkable selectivity for C(sp(3))-centers with an electrophilic SCF3 source and pivalic acid as an additive. This work constitutes an example of the unactivated C(sp(3))-SCF3 bond formation by C-H activation offering a new access to relevant molecules.

Highly enantioselective catalytic cross-dehydrogenative coupling of N-carbamoyl tetrahydroisoquinolines and terminal alkynes

The first catalytic asymmetric cross-dehydrogenative coupling of cyclic carbamates and terminal alkynes has been established. The reaction features high enantiocontrol and excellent functional group tolerance and displays a wide range of structurally and electronically diverse carbamates as well as terminal alkynes. N-Acyl hemiaminals were identified as the reactive intermediates through preliminary control experiments. Employing readily removable carbamates as substrates rather than traditionally adopted N-aryl amines allows applications in complex molecule synthesis and therefore advances the C-H functionalization strategy to a synthetically useful level.

Cobalt-promoted dimerization of aminoquinoline benzamides

A method for aminoquinoline-directed, cobalt-promoted dimerization of benzamides has been developed. Reactions proceed in ethanol solvent in the presence of Mn(OAc)2 cocatalyst and Na2CO3 base and use oxygen as a terminal oxidant. Bromo, iodo, nitro, ether, and ester moieties are compatible with the reaction conditions. Cross-coupling of electronically dissimilar aminoquinoline benzamides proceeds with modest yields and selectivities.

Ruthenium(II)-catalyzed C-H activation/alkyne annulation by weak coordination with O2 as the sole oxidant

Aerobic oxidative CH functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.

Direct synthesis of 2-aryl-4-quinolones via transition-metal-free intramolecular oxidative C(sp(3))-H/C(sp(3))-H coupling

A novel, metal-free oxidative intramolecular Mannich reaction was developed between secondary amines and unmodified ketones, affording a simple and direct access to a broad range of 2-arylquinolin-4(1H)-ones through C(sp(3))-H activation/C(sp(3))-C(sp(3)) bond formation from readily available N-arylmethyl-2-aminophenylketones, using TEMPO as the oxidant and KO(t)Bu as the base.

The azomethine ylide route to amine C-H functionalization: redox-versions of classic reactions and a pathway to new transformations

Redox-neutral methods for the functionalization of amine α-C–H bonds are inherently efficient because they avoid external oxidants and reductants and often do not generate unwanted byproducts. However, most of the current methods for amine α-C–H bond functionalization are oxidative in nature. While the most efficient variants utilize atmospheric oxygen as the terminal oxidant, many such transformations require the use of expensive or toxic oxidants, often coupled with the need for transition metal catalysts.

Redox-neutral amine α-functionalizations that involve intramolecular hydride transfer steps provide viable alternatives to certain oxidative reactions. These processes have been known for some time and are particularly well suited for tertiary amine substrates. A mechanistically distinct strategy for secondary amines has emerged only recently, despite sharing common features with a range of classic organic transformations. Among those are such widely used reactions as the Strecker, Mannich, Pictet–Spengler, and Kabachnik–Fields reactions, Friedel–Crafts alkylations, and iminium alkynylations. In these classic processes, condensation of a secondary amine with an aldehyde (or a ketone) typically leads to the formation of an intermediate iminium ion, which is subsequently attacked by a nucleophile. The corresponding redox-versions of these transformations utilize identical starting materials but incorporate an isomerization step that enables α-C–H bond functionalization. Intramolecular versions of these reactions include redox-neutral amine α-amination, α-oxygenation, and α-sulfenylation. In all cases, a reductive N-alkylation is effectively combined with an oxidative α-functionalization, generating water as the only byproduct.

Reactions are promoted by simple carboxylic acids and in some cases require no additives. Azomethine ylides, dipolar species whose usage is predominantly in [3 + 2] cycloadditions and other pericyclic processes, have been identified as common intermediates. Extension of this chemistry to amine α,β-difunctionalization has been shown to be possible by way of converting the intermediate azomethine ylides into transient enamines.

This Account details the evolution of this general strategy and the progress made to date. Further included is a discussion of related decarboxylative reactions and transformations that result in the redox-neutral aromatization of (partially) saturated cyclic amines. These processes also involve azomethine ylides, reactive intermediates that appear to be far more prevalent in condensation chemistry of amines and carbonyl compounds than previously considered. In contrast, as exemplified by some redox transformations that have been studied in greater detail, iminium ions are not necessarily involved in all amine/aldehyde condensation reactions.

Ru-catalysed C-H arylation of indoles and pyrroles with boronic acids: scope and mechanistic studies

The Ru-catalysed C2–H arylation of indoles and pyrroles by using boronic acids under oxidative conditions is reported. This reaction can be applied to tryptophan derivatives and tolerates a wide range of functional groups on both coupling partners, including bromides and iodides, which can be further derivatised selectively. New indole-based ruthenacyclic complexes are described and investigated as possible intermediates in the reaction. Mechanistic studies suggest the on-cycle intermediates do not possess a para-cymene ligand and that the on-cycle metalation occurs through an electrophilic attack by the Ru centre.