Enantioselective C(sp3)‒H bond activation by chiral transition metal catalysts

Organic molecules are rich in carbon-hydrogen bonds; consequently, the transformation of C-H bonds to new functionalities (such as C-C, C-N, and C-O bonds) has garnered much attention by the synthetic chemistry community. The utility of C-H activation in organic synthesis, however, cannot be fully realized until chemists achieve stereocontrol in the modification of C-H bonds. This Review highlights recent efforts to enantioselectively functionalize C(sp³)-H bonds via transition metal catalysis, with an emphasis on key principles for both the development of chiral ligand scaffolds that can accelerate metalation of C(sp³)-H bonds and stereomodels for asymmetric metalation of prochiral C-H bonds by these catalysts.

Nickel-catalysed alkylation of C(sp3)–H bonds with alcohols: direct access to functionalised N-heteroaromatics

Base-metal catalysed C(sp³)–H bond functionalisation of methyl-N-heteroaromatics with alcohols is reported for the first time.

Enantioselective C-H Functionalization-Addition Sequence Delivers Densely Substituted 3-Azabicyclo[3.1.0]hexanes

An enantioselective C-H functionalization route to perfluoroalkyl-containing 3-azabicyclo[3.1.0]hexanes is disclosed. A modular and bench-stable diazaphospholane ligand enables highly enantioselective Pd(0)-catalyzed cyclopropane C-H functionalization using trifluoroacetimidoyl chlorides as electrophilic partners. In turn, the resulting cyclic ketimine products react smoothly with a broad variety of nucleophiles in one-pot processes enabling the rapid and modular construction of heavily substituted pyrrolidines.

Ring Construction by Palladium(0)-Catalyzed C(sp3)-H Activation

The catalytic activation and functionalization of unactivated C(sp³)-H bonds of alkyl groups has undergone intense development in recent years. In particular, a variety of directing groups as well as native functional groups have been employed in combination with palladium(II) catalysis in order to perform a variety of intermolecular, and to some extent intramolecular reactions. In parallel, inspired by precedents in C(sp²)-H arylation, our group and others have developed a different approach, which is the focus of this Account. This strategy relies on the use of oxidative addition of a carbon-leaving group bond to palladium(0) to induce intramolecular C(sp³)-H activation and the subsequent formation of a C(sp²)-C(sp³) or C(sp³)-C(sp³) bond. Since our first publication in 2003, the construction of olefins and, more interestingly, of an array of valuable monocyclic and polycyclic systems has been reported according to this principle. (Hetero)aryl bromides were initially employed as reactants, but the scope was later expanded to include (hetero)aryl chlorides and triflates, alkenyl bromides, carbamoyl chlorides and α-chloroamides. Mechanistic studies enabled a better understanding of the C-H activation step, which was proposed to occur through ambiphilic metal-ligand activation-6 (AMLA-6), also known as concerted metalation deprotonation (CMD), and a better rationalization of the observed selectivity patterns. Moreover, the wealth of accumulated experimental data indicate that the number of atoms separating the C-H bond from Pd and the type of C-H bond are the main factors controlling the site-selectivity of the C-H bond cleavage. Recent efforts have been devoted to the development of enantioselective reactions. To this purpose, two different strategies have been employed: a chiral ancillary ligand in combination with an achiral base, and a chiral base in combination with an achiral ligand, and allowed for the achievement of high enantioselectivities in the construction of both tri- and tetrasubstituted stereocenters. On the other hand, the current C-H activation-based ring-forming method was applied to the synthesis of pharmacologically active substances and agrochemicals, as well as complex natural products such as the aeruginosins, thereby demonstrating its great potential for step-economical organic synthesis.

P(O)R2-Directed Enantioselective C-H Olefination toward Chiral Atropoisomeric Phosphine-Olefin Compounds

An effective synthesis of chiral atropoisomeric biaryl phosphine-olefin compounds via palladium-catalyzed enantioselective C-H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R₂ not only acts as the directing group to direct C-H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.

Pd-Catalyzed Intramolecular Aminoalkylation of Unactivated Alkenes: Access to Diverse N-Heterocycles

A highly efficient palladium-catalyzed intramolecular aminoalkylation of unactivated alkenes in the absence of an external ligand and oxidant is described. New C-N and C(sp³)-C(sp³) bonds are formed simultaneously. This general transformation allows for construction of diverse N-heterocycles. Mechanistic studies show that the process may involve a four-membered Pd(alkyl)amido intermediate.

Palladium-Catalyzed Transformations of Alkyl C-H Bonds

This Review summarizes the advancements in Pd-catalyzed C(sp³)-H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C-H bonds, many C(sp³)-H activation/C-C and C-heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp³)-H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C-H functionalization reactions.

Palladium(0)-catalyzed asymmetric C(sp3)-H arylation using a chiral binol-derived phosphate and an achiral ligand

High enantioselectivities can be achieved using an appropriate chiral base/achiral ligand combination in Pd⁰-catalyzed C(sp³)–H activation.

The first efficient palladium(0)-catalyzed enantioselective C(sp³)–H activation reaction using a catalytic chiral base and an achiral phosphine ligand is reported. Fine-tuning the binol-derived phosphoric acid pre-catalyst and the reaction conditions was found to be crucial to achieve high levels of enantioselectivity for a variety of indoline products containing both tri- and tetrasubstituted stereocenters.

Synthesis and Application of Tetrahydro-2H-fluorenes by a Pd(0)-Catalyzed Benzylic C(sp(3) )-H Functionalization

A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp(3) )-H functionalization. Importantly, the success of the cyclization step was dependent on there being substituents at the two positions ortho to the benzylic group to avoid an undesired C(sp(2) )-H functionalization. This method was subsequently used to prepare the right-hand fragment of the hexacyclic triterpenoid benzohopanes, and therefore represents a powerful tool for the construction of the related compounds.

TADDOL-based phosphorus(III)-ligands in enantioselective Pd(0)-catalysed C-H functionalisations

Monodentate TADDOL-derived phosphoramidites and phosphonites are versatile chiral ligands for enantioselective Pd(0)-catalysed C-H functionalisations. They enable highly selective cyclisations to access a wide range of chiral carbo- and heterocycles. The high attractiveness of this ligand class consists in their modular structure, allowing for a quick assembly of a library with variable steric properties. Asymmetric C-H functionalisation methods utilising catalytic systems based on Pd(0) complexes and TADDOL-type ligands are presented and aspects of selectivity are discussed.

2-(Trifluoromethyl)indoles via Pd(0)-Catalyzed C(sp(3))-H Functionalization of Trifluoroacetimidoyl Chlorides

Perfluoroalkylated indoles are valuable compounds in drug discovery. A Pd(0)-catalyzed C(sp(3))-H functionalization enables access to 2-(trifluoromethyl)indoles from trifluoroacetimidoyl chlorides. These are stable compounds, easily obtained from anilines. The cyclization operates with catalyst loadings as low as 1 mol % and accommodates a variety of substituents.

Synthesis of Strained γ-Lactams by Palladium(0)-Catalyzed C(sp(3) )-H Alkenylation and Application to Alkaloid Synthesis

A variety of strained α-alkylidene-γ-lactams were synthesized by palladium(0)-catalyzed intramolecular C(sp(3) )-H alkenylation from easily accessible acyclic and monocyclic bromoalkene precursors. These lactams are valuable intermediates for accessing various classes of mono- and bicylic alkaloids containing a pyrrolidine ring, as illustrated with the synthesis of an advanced model of the marine natural product plakoridine A and of the indolizidine alkaloid δ-coniceine.

A novel iridium/acid co-catalyzed transfer hydrogenative C(sp3)–H bond alkylation to access functionalized N-heteroaromatics

A transfer hydrogenative coupling strategy, enabling direct alkylation of C(sp³)–H bonds and atom-economic access to functionalized N-heteroaromatics, has been demonstrated.

Palladium-catalyzed Si–C bond-forming silylation of aryl iodides with hydrosilanes: an enhanced enantioselective synthesis of silicon-stereogenic silanes by desymmetrization

An enantioselective Pd-catalyzed silicon–carbon bond-forming silylation reaction of aryl iodides with hydrosilanes for the synthesis of silicon-stereogenic silanes has been developed with good enantioselectivity under mild conditions.

Enantiospecific C-H Activation Using Ruthenium Nanocatalysts

The activation of C-H bonds has revolutionized modern synthetic chemistry. However, no general strategy for enantiospecific C-H activation has been developed to date. We herein report an enantiospecific C-H activation reaction followed by deuterium incorporation at stereogenic centers. Mechanistic studies suggest that the selectivity for the α-position of the directing heteroatom results from a four-membered dimetallacycle as the key intermediate. This work paves the way to novel molecular chemistry on nanoparticles.

Enantioselective palladium(0)-catalyzed intramolecular cyclopropane functionalization: access to dihydroquinolones, dihydroisoquinolones and the BMS-791325 ring system

Taddol-based phosphoramidite ligands enable enantioselective palladium(0)-catalyzed C-H arylation of cyclopropanes. The cyclized products are obtained in high yields and enantioselectivities. The reported method provides efficient access to a broad range of synthetically attractive cyclopropyl containing dihydroquinolones and dihydroisoquinolones as well as allows for an efficient enantioselective construction of the 7-membered ring of the cyclopropyl indolobenzazepine core of BMS-791325.

Chiral γ-Lactams by Enantioselective Palladium(0)-Catalyzed Cyclopropane Functionalizations

Cyclopropanes fused to pyrrolidines are important structural features found in a number of marketed drugs and development candidates. Typically, their synthesis involves the cyclopropanation of a dihydropyrrole precursor. Reported herein is a complementary approach which employs a palladium(0)-catalyzed C-H functionalization of an achiral cyclopropane to close the pyrrolidine ring in an enantioselective manner. In contrast to aryl-aryl couplings, palladium(0)-catalyzed C-H functionalizations involving the formation of C(sp(3) )-C(sp(3) ) bonds of saturated heterocycles are very scarce. The presented strategy yields cyclopropane-fused γ-lactams from chloroacetamide substrates. A bulky Taddol phosphonite ligand in combination with adamantane-1-carboxylic acid as a cocatalyst provides the γ-lactams in excellent yields and enantioselectivities.