Redox-neutral α-C-H bond functionalization of secondary amines with concurrent C-P bond formation/N-alkylation

Redox-neutral α-functionalization of pyrrolidines: facile access to α-aryl-substituted pyrrolidines

α-Aryl-substituted pyrrolidine moiety is found in many natural alkaloids. Starting from pyrrolidine, we were able to synthesize α-aryl-substituted pyrrolidines in one step using quinone monoacetal as the oxidizing agent and DABCO as the base. We also discovered the reaction condition needed to efficiently remove the N-aryl moiety from the α-arylated product. When the above reaction was carried out without the addition of an aryl nucleophile, the reaction of pyrrolidine and quinone monoacetal in 2,2,2-trifluoroethanol afforded octahydro-dipyrroloquinoline in high yield, which has the same skeleton as that of natural product incargranine B.

Facile synthesis of α-aminophosphine oxides from diarylphosphine oxides, arynes and formamides

The straightforward synthesis of α-amino phosphine oxides via three-component reactions involving arynes, formamides and diarylphosphine oxides is disclosed. This method employs the aryne to activate formamide, without an external activating reagent, which is operationally simple under mild conditions with high efficiency. Furthermore, mechanistic perception suggests a cascade sequence including formal [2 + 2] cycloaddition of the aryne with a CO bond, and a 1,4-addition of the H-P(O) compounds to the enamine intermediates.

Condensation-Based Methods for the C–H Bond Functionalization of Amines

This review aims to provide a comprehensive overview of condensation-based methods for the C–H bond functionalization of amines that feature azomethine ylides as key intermediates. These transformations are typically redox-neutral and share common attributes with classic name reactions such as the Strecker, Mannich, ­Friedel–Crafts, Pictet–Spengler, and Kabachnik–Fields reactions, while incorporating a redox-isomerization step. This approach provides an ideal platform to rapidly transform simple starting materials into complex amines.

1 Introduction

1.1 General Remarks

1.2 Overview

1.3 Scope of This Review

2 Aromatization of Cyclic Amines

2.1 Pyridines from Piperidine

2.2 Isoquinolines from Tetrahydroisoquinolines and Quinolines from Tetrahydroquinolines

2.3 Pyrroles from 3-Pyrroline or Pyrrolidine

2.4 Indoles from Indolines

3 Pericyclic Reactions

3.1 (3+2)-Dipolar Cycloadditions

3.2 6π-Electrocyclizations

3.3 1,5-Proton Shifts

4 Redox-Variants of Classic Transformations Incorporating a C–H Bond Functionalization Step

4.1 α-Cyanation

4.2 α-Alkynylation

4.3 α-Phosphonation

4.4 α-Arylation

4.5 α-Alkylation with Ketones

4.6 Redox-Ugi Reaction

4.7 Miscellaneous Intermolecular Reactions

5 Redox-Annulations

6 Reactions Involving β-C–H Bond Functionalization

7 Outlook

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

Copper-Catalyzed Enaminone C(sp2)-N Bond Phosphonation for Stereoselective Synthesis of Alkenylphosphonates

A direct enaminone C-N bond coupling functionalization that generates a new C-P bond using dialkyl phosphonate for the efficient and stereoselective synthesis of (E)-alkenylphosphonates is reported. The reactions toward target products proceed well with a broad scope, disclosing a valuable new synthetic application of enaminones by the interesting C(sp²)-N bond elaboration.

FeCl2 Catalyzed Three-Component Reactions of Phospholes, Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of 1-Phosphafulvenes

We have developed an unprecedented approach for the synthesis of transient 1-phosphafulvenes through three component reactions of phospholes. The generation of 1-phosphafulvenes was demonstrated by in situ [6 + 4] cycloaddition with 2H-phospholes and [6 + 6] self-dimerization. The [6 + 4] and [6 + 6] reaction pathway could be modulated by the starting ketones and aldehydes. The construction of 1-phosphafulvenes is illustrated by a proposed mechanism combining nucleophilic addition of phospholide to the iminium or isomerized azomethine ylide and a [1,5]-shift of phosphole.

A three-component, general and practical route for diastereoselective synthesis of aza-spirocyclic pyrazolones via a decarboxylative annulation process

An efficient, general, and practical route for highly diastereoselective synthesis of aza-spirocyclic pyrazolones from easily available α-amino acids, aldehydes, and alkylidene pyrazolones by means of a decarboxylative annulation process is reported.

Three-Component Reactions of α-Amino Acids, p-Quinone Monoacetals, and Diarylphosphine Oxides to Selectively Afford 3-(Diarylphosphinyl)anilides and N-Aryl-2-diarylphosphinylpyrrolidines

The three-component reactions of α-amino acids, p-quinone monoacetals (or p-quinol ethers), and diarylphosphine oxides have been developed for the synthesis of 3-(diarylphosphinyl) anilides and N-aryl-2-diarylphosphinylpyrrolidines. The transformations may involve the in situ generation of conjugated azomethine ylides or 2-azaallyl anion species from the reaction of α-amino acids and p-quinone monoacetals, which are further trapped by diarylphosphine oxides.

Direct (het)arylation of tetrahydroisoquinolines via a metal and oxidant free C(sp3)-H functionalization enabled three component reaction

An unprecedented method for the direct arylation and heteroarylation of tetrahydroisoquinolines under metal and oxidant free conditions is reported. The arylation reactions occurred via a C(sp3)-H functionalization enabled three component condensation of tetrahydroisoquinolines, 9-fluorenone imine, and arenes without involving a pre-functionalization/pre-derivatization step. A wide range of arenes and heteroarenes participated in the reaction to provide structurally diverse arylated tetrahydroisoquinolines with good to excellent yields.

Highly diastereoselective synthesis of tricyclic fused-pyrans by sequential hydride shift mediated double C(sp3)-H bond functionalization

Described herein is the Brønsted acid-catalyzed double C(sp3)-H bond functionalization of alkyl phenethyl ether derivatives. In this process, a [1,5]-[1,5]-hydride shift occurred successively to afford tricyclic fused pyran derivatives in excellent chemical yields with excellent diastereoselectivities (up to >20 : 1). The key to achieve this reaction was the introduction of two methyl groups at the benzylic position, which was effective in both hydride shift processes: (1) the Thorpe-Ingold effect for the first hydride shift and (2) conformational control in the second hydride shift.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Copper-catalyzed aerobic decarboxylative coupling between cyclic α-amino acids and diverse C–H nucleophiles with low catalyst loading

An aerobic decarboxylative cross-coupling of α-amino acids with diverse C–H nucleophiles has been realized using Cu₂(OH)₂CO₃ (1 mol%) as the catalyst under air. This protocol enables highly efficient formation of various C(sp³)–C(sp³), C(sp³)–C(sp²) and C(sp³)–C(sp) bonds under simple conditions without the use of any ligand or extra oxidant, providing a practical approach to numerous nitrogen-containing compounds in good to excellent yields. The efficiency and practicability were also demonstrated by the gram-scale experiment and three-step synthesis of a Rad51 inhibitor.

An aerobic decarboxylative cross-coupling of α-amino acids was realized using 1 mol% Cu₂(OH)₂CO₃ catalyst under ligand free conditions.

Asymmetric Synthesis of Tetrahydroisoquinoline Alkaloids Using Ellman's Chiral Auxiliary

Chiral t-butylsulfinamide has been successfully employed for the stereoselective synthesis of 1-benzyl tetrahydroisoquinoline alkaloids. This is the first report on the synthesis of chiral 1-benzyltetrahydroisoquinoline natural products using tert-butylsulfinamide through a haloamide cyclization.

Acetic Acid Promoted Redox Annulations with Dual C-H Functionalization

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-alkylquinoline-3-carbaldehydes as well as the corresponding 4-alkyl isomers and pyridine analogues. These processes involve dual C-H bond functionalization. Acetic acid is used as a cosolvent and acts as the sole promoter of these transformations.

A tandem annulation with a [1,3]-hydride transfer as the key step leading to isochromans

The first [1,3]-hydride transfer/cyclization process for oxacarbenium isomerization.

Hf(OTf)4-Catalyzed highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives via benzylic C(sp3)–H bond functionalization

A highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives was achieved via Hf(OTf)₄-catalyzed benzylic C(sp³)–H bond functionalization.

C-H functionalization of cyclic amines: redox-annulations with α,β-unsaturated carbonyl compounds

Cyclic amines such as pyrrolidine and 1,2,3,4-tetrahydroisoquinoline undergo redox-annulations with α,β-unsaturated aldehydes and ketones. Carboxylic acid promoted generation of a conjugated azomethine ylide is followed by 6π-electrocylization, and, in some cases, tautomerization. The resulting ring-fused pyrrolines are readily oxidized to the corresponding pyrroles or reduced to pyrrolidines.

Copper-catalyzed regio and diastereoselective three component C–N, C–C and C–O bond forming reaction: oxidative sp3 C–H functionalization

Naphthoxazine derivatives were synthesized in good yields in the one-pot reaction of tetrahydroisoquinolines, aromatic benzaldehydes and naphthols employing CuI as a catalyst.

Intramolecular Redox-Mannich Reactions: Facile Access to the Tetrahydroprotoberberine Core

Cyclic amines such as pyrrolidine undergo redox-annulations with 2-formylaryl malonates. Concurrent oxidative amine α-CH bond functionalization and reductive N-alkylation render this transformation redox-neutral. This redox-Mannich process provides regioisomers of classic Reinhoudt reaction products as an entry to the tetrahydroprotoberberine core, enabling the synthesis of (±)-thalictricavine and its epimer. An unusually mild amine-promoted dealkoxycarbonylation was discovered in the course of these studies.

The Rügheimer-Burrows reaction revisited: Facile preparation of 4-alkylisoquinolines and 3,5-dialkylpyridines from (partially) saturated amines

A long-known class of cyclic amine/aldehyde condensation reactions was reinvestigated. Benzoic acid was found to efficiently promote condensations of amines such as piperidine or 1,2,3,4-tetrahydroisoquinoline with aromatic aldehydes, resulting in amine β-functionalization and aromatization. These redox-neutral transformations provide 3,5-dialkylpyridines and 4-alkylisoquinolines in moderate to good yields, following short reaction times under microwave conditions.

The azomethine ylide route to amine C-H functionalization: redox-versions of classic reactions and a pathway to new transformations

Redox-neutral methods for the functionalization of amine α-C–H bonds are inherently efficient because they avoid external oxidants and reductants and often do not generate unwanted byproducts. However, most of the current methods for amine α-C–H bond functionalization are oxidative in nature. While the most efficient variants utilize atmospheric oxygen as the terminal oxidant, many such transformations require the use of expensive or toxic oxidants, often coupled with the need for transition metal catalysts.

Redox-neutral amine α-functionalizations that involve intramolecular hydride transfer steps provide viable alternatives to certain oxidative reactions. These processes have been known for some time and are particularly well suited for tertiary amine substrates. A mechanistically distinct strategy for secondary amines has emerged only recently, despite sharing common features with a range of classic organic transformations. Among those are such widely used reactions as the Strecker, Mannich, Pictet–Spengler, and Kabachnik–Fields reactions, Friedel–Crafts alkylations, and iminium alkynylations. In these classic processes, condensation of a secondary amine with an aldehyde (or a ketone) typically leads to the formation of an intermediate iminium ion, which is subsequently attacked by a nucleophile. The corresponding redox-versions of these transformations utilize identical starting materials but incorporate an isomerization step that enables α-C–H bond functionalization. Intramolecular versions of these reactions include redox-neutral amine α-amination, α-oxygenation, and α-sulfenylation. In all cases, a reductive N-alkylation is effectively combined with an oxidative α-functionalization, generating water as the only byproduct.

Reactions are promoted by simple carboxylic acids and in some cases require no additives. Azomethine ylides, dipolar species whose usage is predominantly in [3 + 2] cycloadditions and other pericyclic processes, have been identified as common intermediates. Extension of this chemistry to amine α,β-difunctionalization has been shown to be possible by way of converting the intermediate azomethine ylides into transient enamines.

This Account details the evolution of this general strategy and the progress made to date. Further included is a discussion of related decarboxylative reactions and transformations that result in the redox-neutral aromatization of (partially) saturated cyclic amines. These processes also involve azomethine ylides, reactive intermediates that appear to be far more prevalent in condensation chemistry of amines and carbonyl compounds than previously considered. In contrast, as exemplified by some redox transformations that have been studied in greater detail, iminium ions are not necessarily involved in all amine/aldehyde condensation reactions.