An Enantioselective Cpx Rh(III)-Catalyzed C−H Functionalization/Ring-Opening Route to Chiral Cyclopentenylamines

Asymmetric cascades of the π-allyl complex: a journey from transition-metal catalysis to metallaphotocatalysis

The enantioselective catalytic cascade involving Tsuji-Trost allylation has provided a viable strategy for the construction of multiple asymmetric C-C and C-X centres and numerous methods have been developed around it for the synthesis of various vital scaffolds. The synthetic utility of this strategy was enhanced by replacing the customary allyl acetates with ethylene diacetates/dicarbonates, vinyl epoxides, vinyl oxetanes, vinyl ethylene carbonates, vinyl cyclopropanes, enynes, and dienes using transition-metal catalysis. One more milestone was achieved when metallaphotocatalysis provided the necessary platform for these cascades by using a cheaper metal. This review will provide a summary of these enantioselective catalytic cascades from 2015.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Oxygen-Linked Cyclopentadienyl Rhodium(III) Complexes-Catalyzed Asymmetric C-H Arylation of Benzo[h]quinolines with 1-Diazonaphthoquinones

Chiral cyclopentadienyl rhodium (CpRh) complex-catalyzed asymmetric C-H functionalization reactions have witnessed a significant progress in organic synthesis. In sharp contrast, the reported chiral Cp ligands are limited to C-linked Cp and are often synthetically challenging. To address these issues, we have developed a novel class of tunable chiral cyclopentadienyl ligands bearing oxygen linkers, which were efficient catalysts for C-H arylation of benzo[h]quinolines with 1-diazonaphthoquinones, affording axially chiral heterobiaryls in excellent yields and enantioselectivity (up to 99 % yield, 98.5:1.5 er). Mechanistic studies suggest that the reaction is likely to proceed by electrophilic C-H activation, and followed by coupling of the cyclometalated rhodium(III) complex with 1-diazonaphthoquinones.

A New Class of C2 -Symmetric Chiral Cyclopentadienyl Ligand Derived from Ferrocene Scaffold: Design, Synthesis and Application

A new class of C₂ -symmetric, chiral cyclopentadienyl ligand based on planar chiral ferrocene backbone was developed. A series of its corresponding rhodium(I), iridium(I), and ruthenium(II) complexes were prepared as well. In addition, the rhodium(I) complexes were evaluated in the asymmetric catalytic intramolecular amidoarylation of olefin-tethered benzamides via C-H activation.

Chiral Bicyclo[2.2.2]octane-Fused CpRh Complexes: Synthesis and Potential Use in Asymmetric C-H Activation

A new class of chiral cyclopentadienyl rhodium(I) complexes (CpRh^I ) bearing C₂ -symmetric chiral bridged-ring-fused Cp ligands was prepared. The complexes were successfully applied to the asymmetric C-H activation reaction of N-methoxybenzamides with quinones, affording a series of chiral hydrophenanthridinones in up to 82 % yield with up to 99 % ee. Interestingly, structure analysis reveals that the side wall of the optimal chiral CpRh^I catalyst is vertically more extended, horizontally less extended, and closer to the metal center in comparison with the classic binaphthyl and spirobiindanyl CpRh^I complexes, and may thus account for its superior catalytic performance.

Enantioselective Cpx RhIII -Catalyzed Carboaminations of Acrylates

Enantioselective carboaminations of olefins constitute an attractive strategy for a rapid increase in molecular complexity from readily available starting materials. Reported here is an intermolecular asymmetric carboamination of acrylates using rhodium(III)-catalyzed alkenyl C-H activations of N-enoxysuccinimides to generate the nitrogen and carbon portion for the transfer. A rhodium complex equipped with a tailored bulky trisubstituted chiral Cp^x ligand ensures carboamination chemoselectivity as well high levels of enantioinduction. The transformation operates under mild reaction conditions at ambient temperatures and provides access to a variety of α-amino esters in good yields and excellent enantiomeric ratios of >99.5:0.5.

Diverse Approaches for Enantioselective C-H Functionalization Reactions Using Group 9 Cpx MIII Catalysts

Transition-metal-catalyzed C-H functionalization reactions with Cp*M^III catalysts (M=Co, Rh, Ir) have found a wide variety of applications in organic synthesis. Albeit the intrinsic difficulties in achieving catalytic stereocontrol using these catalysts due to their lack of additional coordination sites for external chiral ligands and the conformational flexibility of the Cp ligand, catalytic enantioselective C-H functionalization reactions using the Group 9 metal triad with Cp-type ligands have been intensively studied since 2012. In this minireview, the progress in these reactions according to the type of the chiral catalyst used are summarized and discussed. The development of chiral Cp^x ligands the metal complexes thereof, artificial metalloenzymes, chiral carboxylate-assisted enantioselective C-H activations, enantioselective alkylations assisted by chiral carboxylic acids or chiral sulfonates, and chiral transient directing groups are discussed.

Formal Lossen Rearrangement/Alkenylation or Annulation Cascade of Heterole Carboxamides with Alkynes Catalyzed by CpRhIII Complexes with Pendant Amides

It has been established that a cyclopentadienyl (Cp) Rh^III complex with two aryl groups and a pendant amide moiety catalyzes the formal Lossen rearrangement/alkenylation cascade of N-pivaloyl heterole carboxamides with internal alkynes, leading to alkenylheteroles. Interestingly, the use of sterically demanding internal alkynes afforded not the alkenylation but the [3+2] annulation products ([5,5]-fused heteroles). In these reactions, the pendant amide moiety of the CpRh^III complex may accelerate the formal Lossen rearrangement. The use of five-membered heteroles may deter reductive elimination to form strained [5,5]-fused heteroles; instead, protonation proceeds to give the alkenylation products. Bulky alkyne substituents accelerate the reductive elimination to allow the formation of the [5,5]-fused heteroles.