Synthesis of Axially Chiral Olefin-Oxazoline Ligands via Pd-Catalyzed Multiple C-H Functionalization

Enantioselective synthesis of molecules with multiple stereogenic elements

This review explores the fascinating world of molecules featuring multiple stereogenic elements, unraveling the different strategies designed over the years for their enantioselective synthesis. Specifically, (dynamic) kinetic resolutions, desymmetrisations and simultaneous installation of stereogenic elements exploiting either metal- or organo-catalysis are the principal approaches to efficiently create and control the three-dimensional shapes of these attractive molecules. Although most molecules presented in this review possess a stereogenic carbon atom in combination with a stereogenic axis, other combinations with helices or planes of chirality have started to emerge, as well as molecules displaying more than two different stereogenic elements.

Atroposelective Amination of Indoles via Chiral Center Induced Chiral Axis Formation

The construction of an N-C chiral axis for N-aryl indole derivatives is meaningful as they widely exist in functionalized molecules. This work provides a novel method for this purpose via amination of amino acid derivatives at the C2 position of the indole and chiral center induced chiral axis formation. The protocol of this transformation is easily accessible, not requiring metal or an organic chiral catalyst, endowing this method with great potential in the construction of axis chiral N-aryl indoles.

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)₂ complexes were investigated using the density functional theory (DFT) method. The Pd(0)L₂(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (S_EAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.

Catalytic Atroposelective Dynamic Kinetic Resolution of Substituted Indoles

Advances in asymmetric catalysis have led to enormous progress in the atroposelective synthesis of axially chiral biaryls. Because of the biological importance of indoles, stereogenic axes in aryl-substituted indoles have attracted considerable research attention in recent years. Here we present a summary of recent advances in the atroposelective synthesis of aryl-substituted indoles by dynamic kinetic resolution. Although several researchers have developed enantioselective syntheses of 3-arylindoles, N-arylindoles have been much less studied. Accordingly, we have developed a Pictet–Spengler reaction with catalytic and enantioselective control of the axial chirality around the C–N bond of the product. A chiral phosphoric acid induces the cyclization smoothly and with high yields and excellent enantioselectivities. To achieve this high selectivity, an NH group at the ortho-position of the N-substituted aromatic ring that interacts favorably with the catalyst is required. Furthermore, when substituted aldehydes are used instead of paraformaldehyde, both point and axial chiralities can be controlled during the cyclization.

1 Introduction

2 Atropisomerism in Indoles

3 Atroposelective Dynamic Kinetic Resolution of 3-Arylindoles

4 Atroposelective Dynamic Kinetic Resolution of N-Arylindoles

5 Conclusions

An entry to 2-(cyclobut-1-en-1-yl)-1H-indoles through a cyclobutenylation/deprotection cascade

A transition-metal-free strategy for the synthesis of 2-(cyclobut-1-en-1-yl)-1H-indoles under mild conditions is described herein. A series of substituted 2-(cyclobut-1-en-1-yl)-1H-indoles are accessed by a one-pot cyclobutenylation/deprotection cascade from N-Boc protected indoles. Preliminary experimental and density functional theory calculations suggest that a Boc-group transfer is involved in the underlying mechanism.