Acid Catalyzed Formation of C⁻C and C⁻S Bonds via Excited State Proton Transfer

Photocatalytic systems: reactions, mechanism, and applications

The photocatalytic field revolves around the utilization of photon energy to initiate various chemical reactions using non-adsorbing substrates, through processes such as single electron transfer, energy transfer, or atom transfer. The efficiency of this field depends on the capacity of a light-absorbing metal complex, organic molecule, or substance (commonly referred to as photocatalysts or PCs) to execute these processes. Photoredox techniques utilize photocatalysts, which possess the essential characteristic of functioning as both an oxidizing and a reducing agent upon activation. In addition, it is commonly observed that photocatalysts exhibit optimal performance when irradiated with low-energy light sources, while still retaining their catalytic activity under ambient temperatures. The implementation of photoredox catalysis has resuscitated an array of synthesis realms, including but not limited to radical chemistry and photochemistry, ultimately affording prospects for the development of the reactions. Also, photoredox catalysis is utilized to resolve numerous challenges encountered in medicinal chemistry, as well as natural product synthesis. Moreover, its applications extend across diverse domains encompassing organic chemistry and catalysis. The significance of photoredox catalysts is rooted in their utilization across various fields, including biomedicine, environmental pollution management, and water purification. Of course, recently, research has evaluated photocatalysts in terms of cost, recyclability, and pollution of some photocatalysts and dyes from an environmental point of view. According to these new studies, there is a need for critical studies and reviews on photocatalysts and photocatalytic processes to provide a solution to reduce these limitations. As a future perspective for research on photocatalysts, it is necessary to put the goals of researchers on studies to overcome the limitations of the application and efficiency of photocatalysts to promote their use on a large scale for the development of industrial activities. Given the significant implications of the subject matter, this review seeks to delve into the fundamental tenets of the photocatalyst domain and its associated practical use cases. This review endeavors to demonstrate the prospective of a powerful tool known as photochemical catalysis and elucidate its underlying tenets. Additionally, another goal of this review is to expound upon the various applications of photocatalysts.

Excited state proton transfer of triplet state p-nitrophenylphenol to amine and alcohol: a spectroscopic and kinetic study

Hydroxyaromatic compounds (ArOHs) have a wide range of applications in catalytic synthesis and biological processes due to their increased acidity upon photo-excitation. The proton transfer of ArOHs via the excited singlet state has been extensively studied. However, there has still been a debate on the unique type of ArOH that can undergo an ultrafast intersystem crossing. The nitro group in p-nitrophenylphenol (NO₂-Bp-OH) enhances the spin-orbit coupling between excited singlet states and the triplet manifold, enabling ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T₁) with a high yield. In this work, we used time-resolved transient absorption to investigate the excited state proton transfer of NO₂-Bp-OH in its T₁ state to t-butylamine, methanol, and ethanol. The T₁ state of the deprotonated form NO₂-Bp-O^- was first observed and identified in the case of t-butylamine. Kinetic analysis demonstrates that the formation of the hydrogen-bonded complex with methanol and ethanol as proton acceptors involves their trimers. The alcohol oligomer size required in the excited state proton transfer process is dependent on the excited acidity of photoacid.

Photoacid-catalyzed acetalization of carbonyls with alcohols

In this report, we demonstrate that visible light photoactivation of 6-bromo-2-naphthol facilitates the photoacid-catalyzed acetalization of carbonyls with alcohols. We also demonstrate that 2-naphthol when coupled to a photosensitizer provides acetals from electron-deficient aldehydes. In addition, the S₁ excited state pK_a for 6-bromo-2-naphthol in water was determined and shown to have increased excited-state acidity relative to 2-naphthol.

Photoacid-Enabled Synthesis of Indanes via Formal [3 + 2] Cycloaddition of Benzyl Alcohols with Olefins

An environmentally friendly and highly diastereoselective method for synthesizing indanes has been developed via a metastable-state photoacid system containing catalytic protonated merocyanine (MEH). Under visible-light irradiation, MEH yields a metastable spiro structure and liberated protons, which facilitates the formation of carbocations from benzyl alcohols, thus delivering diverse molecules in the presence of various nucleophiles. Mainly, a variety of indanes could be easily obtained from benzyl alcohols and olefins, and water is the only byproduct.

Visible-light-induced photoacid catalysis: application in glycosylation with O-glycosyl trichloroacetimidates

The development of visible-light-induced photoacid catalyzed glycosylation is reported. The eosin Y and PhSSPh catalyst system is applied to realize glycosylation with different glycosyl donors upon light irradiation. The reaction shows a broad substrate scope, including both glycosyl donors and acceptors, and highlights the mild nature of the reaction conditions.

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.

Recent Advances in Photoacid Catalysis for Organic Synthesis

Photoacids are molecules that become more acidic upon the absorption of light. This short review highlights recent advances in the use of photoacids as catalysts for organic synthesis. Photoacid-catalyzed­ transformations discussed herein include: Protonation, glycosylation, acetalization, and arylation reactions.

1 Introduction

2 Protonation: Excited-State Proton Transfer (ESPT)

3 Glycosylation

4 Acetalization

5 Friedel–Crafts Arylation

6 Additional C–C and C–S Bond-Forming Reactions

7 Conclusion

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Photoacid-Catalyzed Friedel-Crafts Arylation of Carbonyls

In this report, we demonstrate that visible-light-induced thiourea photoacids catalyze C-C bond-forming reactions. Upon photoirradiation, Schreiner's thiourea [(N,N'-bis[3,5-bis(trifluoromethyl)phenyl]-thiourea] catalyzes the double Friedel-Crafts addition of indoles to aldehydes and isatins to form the corresponding triarylmethanes and 3,3'-diarylindolin-2-ones. This protocol is amenable to a wide range of aldehyde and isatin electrophiles, as well as a variety of electronically diverse indoles. Mechanistic studies show that light is required for reaction initiation.