Asymmetric C(sp2)H Activation

A remarkable solvent effect of fluorinated alcohols on transition metal catalysed C–H functionalizations

Fluorinated solvents like 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and trifluoroethanol (TFE) have recently emerged as a remarkable synthetic hint allowing challenging C–H activation reactions.

Mild metal-catalyzed C–H activation: examples and concepts

C–H Activation reactions that proceed under mild conditions are more attractive for applications in complex molecule synthesis. Mild C–H transformations reported since 2011 are reviewed and the different concepts and strategies that have enabled their mildness are discussed.

Stereoselective synthesis of 1,3-disubstituted isoindolines via Rh(iii)-catalyzed tandem oxidative olefination–cyclization of 4-aryl cyclic sulfamidates

Rh(iii)-catalyzed tandem ortho C–H activation/olefination of 4-aryl cyclic sulfamidates and subsequent intramolecular cyclization which lead to the stereoselective synthesis of trans-1,3-disubstituted isoindolines are reported.

Enantioselective Synthesis of Spiroindenes by Enol-Directed Rhodium(III)-Catalyzed C-H Functionalization and Spiroannulation

Chiral cyclopentadienyl rhodium complexes promote highly enantioselective enol-directed C(sp²)-H functionalization and oxidative annulation with alkynes to give spiroindenes containing all-carbon quaternary stereocenters. High selectivity between two possible directing groups, as well as control of the direction of rotation in the isomerization of an O-bound rhodium enolate into the C-bound isomer, appear to be critical for high enantiomeric excesses.

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.

A combined IM-MS/DFT study on [Pd(MPAA)]-catalyzed enantioselective C-H activation: relay of chirality through a rigid framework

A combined ion-mobility mass spectrometry (IM-MS) and DFT study has been employed to investigate the mechanism and the origin of selectivity of palladium/mono-N-protected amino acid (MPAA)-catalyzed enantioselective CH activation reactions of several prochiral substrates. We captured the [Pd(MPAA)(substrate)] complex at different stages, and demonstrated that the CH bond can be activated in the absence of an external base. DFT studies lead to the establishment of a significantly modified relay mechanism invoking a key conformational effect to account for the origin of enantioselectivity. This relay mechanism successfully accounts for the enantioselectivity for all the relevant reactions reported. The enantioselectivity originates from the rigid square-planar Pd coordination in the CH activation transition state: Bidentate MPAA and substrate coordination.

Rhodium-catalyzed enantioselective silylation of arene C-H bonds: desymmetrization of diarylmethanols

We report a Rh-catalyzed, enantioselective silylation of arene C-H bonds directed by a (hydrido)silyl group. (Hydrido)silyl ethers that are formed in situ by hydrosilylation of benzophenone or its derivatives undergo asymmetric C-H silylation in high yield with excellent enantioselectivity in the presence of [Rh(cod)Cl]2 and a chiral bisphosphine ligand. The stereoselectivity of this process also allows enantioenriched diarylmethanols to react with site selectivity at one aryl group over the other. Enantioenriched benzoxasiloles from the silylation process undergo a range of transformations to form C-C, C-O, C-I, or C-Br bonds.

Asymmetric Dearomatization of Naphthols via a Rh-Catalyzed C(sp²)-H Functionalization/Annulation Reaction

A Rh-catalyzed enantioselective dearomatization of 1-aryl-2-naphthols with internal alkynes via C-H functionalization reaction was achieved. In the presence of a chiral Cp/Rh catalyst and combined oxidants of Cu(OAc)2 and air (oxygen), various highly enantioenriched spirocyclic enones bearing an all-carbon quaternary stereogenic center could be synthesized in 33-98% yields with up to 97:3 er.

Mechanistic and Kinetic Studies of the Direct Alkylation of Benzylic Amines: A Formal C(sp3)-H Activation Proceeds Actually via a C(sp2)-H Activation Pathway

Mechanistic investigations of a Rh(I)-catalyzed direct C-H alkylation of benzylic amines with alkenes, formally an C(sp3)-H activation, reveal this reaction to proceed via imine intermediates and, hence, via C(sp2)-H activation. The reaction shows a primary kinetic isotope effect of 4.3 at the benzylic C-H position together with a reversible H-D exchange at the same position, which indicates that there are at least two distinct steps in which the corresponding C-H bonds are broken. The imine intermediates are shown to be converted to the final product under the reaction conditions, and a time course analysis of the alkylated imine intermediate shows that it is formed before the final amine product in the course of the reaction.

Recent advances in transition metal-catalyzed C–H bond functionalization of ferrocene derivatives

In this Perspective, recent advances in the emerging field of transition metal-catalyzed C–H bond functionalization of ferrocene derivatives are summarized.

Construction of axial chirality by rhodium-catalyzed asymmetric dehydrogenative Heck coupling of biaryl compounds with alkenes

Enantioselective construction of axially chiral biaryls by direct C-H bond functionalization reactions has been realized. Novel axially chiral biaryls were synthesized by the direct C-H bond olefination of biaryl compounds, using a chiral [Cp*Rh(III)] catalyst, in good to excellent yields and enantioselectivities. The obtained axially chiral biaryls were found as suitable ligands for rhodium-catalyzed asymmetric conjugate additions.

Asymmetric synthesis of planar chiral ferrocenes by enantioselective intramolecular C-H arylation of N-(2-haloaryl)ferrocenecarboxamides

The palladium-catalyzed intramolecular C-H arylation reaction of N-(2-bromoaryl)ferrocenecarboxamides furnishes planar chiral ferrocene derivatives. TADDOL-derived phosphoramide ligands induce enantioselectivities ranging from 91:9 to 98:2 er.

Rhodium-catalyzed direct coupling of biaryl pyridine derivatives with internal alkynes

Axially chiral biaryls were synthesized by an isoquinoline or 2-pyridine-directed Rh(iii)-catalyzed dual C–H cleavage and coupling with internal alkynes.