Enantioselective N-Heterocyclic Carbene Catalyzed Synthesis of Functionalized Indenes

Enantioselective Synthesis of Cyclopentanes by Phosphine-Catalyzed β,γ-Annulation of Allenoates

Herein, we report the enantioselective phosphine-catalyzed β,γ-annulation of electron-poor allenes with bifunctional malonates. The reaction exploits a 2C phosphonium synthon that when accessed using (R)-SITCP gives 23 cyclopentanes with high stereoselectivity (most >95:5 er and >9:1 dr) and yield. In addition to the (3+2) annulation, a one-pot three-component variant to give the same cyclopentanes and a (3+2) annulation/Dieckmann cyclization cascade, along with mechanistic studies, are reported.

Rhodium-Catalyzed Asymmetric Annulation of Unactivated Alkynes with 3-(ortho-Boronated Aryl) Conjugated Enones: Enantioselective Synthesis of 2,3-Disubstituted Indenes

A rhodium-catalyzed tandem arylation/cyclization reaction of 3-(ortho-boronated aryl) conjugated enones with unactivated alkynes is reported. By using a rhodium(I)/chiral-diene complex as the catalyst, the protocol was processed smoothly to provide various 2,3-disubstituted indene compounds in high yields with excellent regioselectivities and enantioselectivities. The approach outlined herein is appealing, as simple diarylalkynes, diakylalkynes, and alkyl(aryl)alkynes are the starting materials.

Recent Advances in C–F Bond Activation of Acyl Fluorides Directed toward Catalytic Transformation by Transition Metals, N-Heterocyclic Carbenes, or Phosphines

Numerous studies on the activation of carbon–fluorine bonds have been reported in recent years. For example, acyl fluorides have been utilized as versatile reagents for acylation, arylation, and even fluorination. In this review, we focus on acyl fluorides as compounds with carbon–fluorine bonds, and highlight recent advances in strategies for the activation of their C–F bonds via transition-metal catalysis, N-heterocyclic carbene (NHCs) catalysis, organophosphine catalysis, and classical nucleophilic substitution reactions.

1 Introduction

2 Transition-Metal-Mediated C–F Bond Activation

2.1 Acylation (Carbonyl-Retentive) Coupling Reactions

2.2 Decarbonylative Reactions

2.3 C–F Bond Activation by Other Transition Metals

3 C–F Bond Activation by N-Heterocyclic Carbenes (NHCs)

3.1 NHC-Catalyzed Cycloaddition of Acyl Fluorides

3.2 NHC-Catalyzed Radical Functionalization of Acyl Fluorides

3.3 NHC-Catalyzed Nucleophilic Fluorination of (Hetero)aromatics

4 C–F Bond Activation by Phosphines

4.1 Phosphine-Catalyzed Direct Activation of the C–F Bond of Acyl Fluorides

4.2 Phosphine-Catalyzed Indirect Activation of the C–F Bond of Acyl Fluorides

5 C–F Bond Activation by Classical Nucleophilic Substitution

6 Miscellaneous Examples

7 Summary and Perspective

N-Heterocyclic Carbene-Catalyzed Enantioselective Synthesis of Carbocycle-Fused Uracils via Formal [4 + 2] Annulation/Lactone Formation/Decarboxylation Cascade

An unprecedented organocatalytic asymmetric construction of novel six-membered carbocycle fused uracils has been demonstrated by the reaction of the remotely enolizable 6-methyluracil-5-carbaldehydes with 2-bromoenals. The reaction involves an N-heterocyclic carbene-catalyzed formal [4 + 2] annulation of o-quinodimethane (oQDM) dienolates with α,β-unsaturated acylazoliums, followed by a cascade lactone formation/decarboxylation process. This protocol unlocks a new platform to access enantioenriched carbocycle-fused uracils.

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity

In recent years, N-heterocyclic carbenes (NHCs) have established themselves as a masterful and promising type of organocatalyst for the speedy construction of medicinally and biologically significant molecules from common and accessible small molecules. In particular, various cyclic scaffolds, including carbocycles and heterocycles, have been synthesized using NHCs via cycloaddition reaction. An exhaustive review focused on the chemistry of NHCs in these cyclic molecules has yet to be reported. In this contribution, a general picture of the utilization of NHCs in constructing twelve kinds of bioactive cyclic skeletons is firstly presented. We provide a systematic and comprehensive overview from the perspective of cycloaddition reactions; moreover, the limitations, challenges, and future prospects were discussed.

Chiral Aluminum Complex Controls Enantioselective Nickel-Catalyzed Synthesis of Indenes: C-CN Bond Activation

A chiral aluminum complex controlled, enantioselective nickel-catalyzed domino reaction of aryl nitriles and alkynes proceeding by C-CN bond activation was developed. The reaction provides various indenes, bearing chiral all-carbon quaternary centers, under mild reaction conditions in yields of 32 to 91 % and ee values within the 73-98 % range. The reaction mechanism and aspects of stereocontrol were investigated by DFT calculations.

Oxidative NHC catalysis: direct activation of β sp3 carbons of saturated acid chlorides

The first activation of saturated acid chlorides by an oxidative N-heterocyclic carbene catalyst has been utilized to synthesize enantio-enriched spirooxindole lactones and δ-lactones.

H-bonded reusable template assisted para-selective ketonisation using soft electrophilic vinyl ethers

In nature, enzymatic pathways generate C_aryl−C(O) bonds in a site-selective fashion. Synthetically, C_aryl−C(O) bonds are synthesised in organometallic reactions using prefunctionalized substrate materials. Electrophilic routes are largely limited to electron-rich systems, non-polar medium, and multiple product formations with a limited scope of general application. Herein we disclose a directed para-selective ketonisation technique of arenes, overriding electronic bias and structural congestion, in the presence of a polar protic solvent. The concept of hard–soft interaction along with in situ activation techniques is utilised to suppress the competitive routes. Mechanistic pathways are investigated both experimentally and computationally to establish the hypothesis. Synthetic utility of the protocol is highlighted in formal synthesis of drugs, drug cores, and bioactive molecules.

Electrophilic acylation of arenes is largely limited to electron rich systems, non-polar medium and often displays moderate selectivity. Here, the authors show a directed para-selective ketonisation of arenes, overriding electronic bias and structural congestion, and apply it to the synthesis of bioactive compounds.

N-Heterocyclic-Carbene-Catalyzed Domino Reactions via Two or More Activation Modes

Organocatalytic domino processes have become a rapidly growing area of research. N-heterocyclic carbenes (NHCs) have emerged as powerful organocatalysts for various transformations and continue to have widespread application. In the last decade, domino reactions catalyzed by NHCs have seen significant progress since the different activation modes could be successfully combined in one process. The most attractive features of these domino sequences include the readily available catalysts and substrates, the simple operational procedures, and the rapid assembly of complex molecular scaffolds with excellent levels of stereocontrol under mild reaction conditions. This review covers the advances in NHC-catalyzed domino reactions by focusing on the reaction scope, limitations, and mechanism with a close attention to the features of the reaction substrates.