Photoorganocatalysis in Organic Synthesis

Maurizio Fagnoni, Stefano Protti;Davide Ravelli

• Publisher: WSPC (EUROPE)
• eBook ISBN: 9781786346063
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

The use of organocatalysts able to photocatalyze an organic reaction is a rapidly growing field. These photocatalyzed transformations are more environmentally sustainable with respect to the use of expensive/toxic metal-based (photo)catalysts.

Based on the authors' extensive experience in photogenerated intermediates, this book presents an overview on photocatalyzed organic processes having a synthetic significance, where an organic molecule functions as the photocatalyst.

After a brief introduction defining the nature and the characteristics of a photoorganocatalyst (POC), the chapters are organized according to the class of POC used, as detailed below.

Each chapter begins with a summary of the photophysical characteristics of the POCs and is followed by selected examples of synthetic applications. The last two chapters are devoted to the adoption of photoorganocatalysis in polymerization and to flow photoorganocatalysis. These in-depth explanations and practical applications make this title an essential reading for any chemistry student interested in organic (sustainable) synthesis.

Contents:

• Ketones and Aldehydes (Shin Kamijo)
• Quinones (Akichika Itoh)
• Aromatics and Cyanoaromatics (Stefano Protti and Davide Ravelli)
• Sulfur Heterocycles (Katarzyna Goliszewska, Katarzyna Orłowska and Dorota Gryko)
• Oxygen Heterocycles: Pyrylium Salts (Sergio M Bonesi and Al Postigo)
• Oxygen Heterocycles: Eosin Derivatives (Wen-Jing Xiao, Xiao-Qiang Hu, and Jia-Rong Chen)
• Oxygen Heterocycles: Fluorescein, Rhodamine, Rose Bengal (Daniele Leonori and Fabio Juliá)
• Nitrogen Heterocycles: Porphyrins (Hermenegildo García and Sonia Remiro-Buenamañana)
• Nitrogen Heterocycles: Acridinium Salts (Peter Fodran, Leticia Monjas and Carl-Johan Wallentin)
• Other Nitrogen Heterocycles: Carbazoles, Imides and PDI, mpg-C₃N₄, Tetrazines, Riboflavin, and BODIPY (Pier Giorgio Cozzi, Paola Ceroni, Andrea Gualandi, and Marianna Marchini)
• Photoorganocatalysts for Polymerization (Didier Gigmes, Frédéric Dumur, Nicolas Zivic, Ségolène Villotte, Jason C Morris, Jacques Lalevée and Aude Héloïse Bonardi)
• Photoorganocatalysis in Flow (Manuel Anselmo, Andrea Basso and Lisa Moni)

Readership: Any chemistry student interested in organic (sustainable) synthesis.Organic Chemistry;Photochemistry;Photocatalysis;Organocatalysis;Reactive Intermediates;Radicals00

Chapter 1

Ketones and Aldehydes

Shin Kamijo

Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi, Japan

[email protected]

1.1Introduction

Extensive studies have been carried out in the photochemistry of carbonyl compounds, especially ketones, since photoexcited ketones are known to induce various types of transformations [1–4]. Among them, one of the most significant features of aryl ketones, such as benzophenone (Ph₂CO) and its derivatives, is their ability to act as photoorganocatalysts (POCs) (Scheme 1.1).

For instance, Ph₂CO is readily excited to a singlet state with light irradiation [>350 nm for n–π*] and is then rapidly converted to a triplet state through intersystem crossing. The relatively longer lifetimes of the triplet state of aryl ketones [10 ns–1 μs for Ph₂CO depending on the solvent used] have allowed the promotion of photochemical transformations via energy transfer (EnT), hydrogen-atom transfer, and electron transfer (ET) reactions [redox potentials for triplet Ph₂CO:

+ 1.28 V vs SCE,

− 0.61 V vs SCE; and energy for triplet Ph₂CO:

3.0 eV]. This chapter mainly covers recent representative catalytic transformations involving carbonyl compounds, including ketones and aldehydes, under photoirradiation conditions.

Scheme 1.1.Representative ketone and aldehyde POCs.

1.2Functionalizations of Unsaturated Bonds

1.2.1Alkylation of olefins

One of the pioneering investigations on the alkylation of oxygen containing compounds, such as alcohols and acetals, via the photoinduced conjugate addition to carbohydrate-derived enones in the presence of a catalytic amount of benzophenone (Ph₂CO) was carried out by Fraser-Reid and co-workers [5–7]. As an example, irradiation of the enone derived from acetylated glucose in MeOH containing a catalytic amount of Ph₂CO (16 mol%) resulted in the regioselective formation of the 1,4-adduct in 50% yield (Scheme 1.2) [5]. The reaction was proposed to proceed with hydrogen-atom abstraction from MeOH by the photoexcited Ph₂CO. The derived hydroxymethyl radical then added to the enone in 1,4-fashion from the less-hindered face. This addition step should be favored due to the nucleophilic nature of the hydroxymethyl radical. Subsequently, the hydrogen-atom transfer between the generated radical B1 and the ketyl radical A1 or MeOH took place to form the adduct [6]. In addition to MeOH, the applicable substances were expanded to include diols, acetals, and aldehydes (Scheme 1.3) [7]. A similar type of diastereoselective alkylation of isopropanol, 1,3-dioxolane, and tetrahydrofuran (THF) was also investigated by Mattay and co-workers [8].

Scheme 1.2.Addition of alcohols to cyclic enones.

Scheme 1.3.Addition of acetals or aldehydes to cyclic enones.

The research group of Fagnoni and Albini achieved the catalytic photoinduced alkylation of 1,3-dioxolane with α,β-unsaturated ketones (aliphatic, both acyclic and cyclic, as well as aryl-substituted) in the presence of a ketone as POC [9]. The irradiation of the alkyl ketone in dioxolane with Ph₂CO (40 mol%) led to the formation of the addition product with a yield of 92% (Scheme 1.4(a)). On the other hand, the reaction employing the aryl ketone substrate was efficiently catalyzed by anthraquinone (AQ, 2 mol%) rather than Ph₂CO, furnishing the expected adduct in 85% yield (Scheme 1.4(b)). Further investigations enabled the alkylation of dioxolane using α,β-unsaturated aldehydes in aqueous solution [10], as well as by designing the water-soluble photocatalyst disodium benzophenonedisulfonate (BPSS) (Scheme 1.5) [11].

Scheme 1.4.Addition of 1,3-dioxolane to enones.

Scheme 1.5.Addition of 1,3-dioxolane to enals.

Hoffmann and co-workers investigated intensively the diastereoselective alkylation of amine derivatives by a chiral furanone via a photoinduced conjugated addition in the presence of a catalytic amount of aryl ketones [12–16]. In the initial stage of the investigation, the alkylation of N-methylpyrrolidine was attained (Scheme 1.6) [12].

The treatment of the pyrrolidine (20 equiv) and the chiral furanone bearing a menthyloxy group in MeCN with a catalytic amount of 4,4′-dimethoxybenzophenone (10 mol%) under irradiation produced the addition products in 85% combined yield. A complete facial selectivity at the furanone ring was observed; however, the other asymmetric center adjacent to the nitrogen atom could not be controlled. The current strategy for diastereoselective alkylation of pyrrolidine derivatives was applied to the total synthesis of two pyrrolizidines alkaloids, viz. (–)-isoretronecanol and (+)-lauburine [13]. In the proposed catalytic cycle, the reaction was initiated by photoinduced electron transfer (PET) between the photoexcited aryl ketone and pyrrolidine followed by proton transfer to generate the α-aminoalkyl radical A2 and the ketyl radical B2 [14]. The derived radical A2 was added to the electron-deficient double bond of the furanone from the less-hindered face to form the oxoally radical C2. A hydrogen-atom transfer from the ketyl radical B2 to the radical C2 accounted for the recovery of the aryl ketone after the reaction. When dimethylaniline was employed as a starting amine in the presence of 4,4′-bis(N,N-dimethylamino)benzophenone (8 mol%), a radical tandem reaction took place to form tetrahydroquinolines in a highly diastereoselective manner (Scheme 1.7) [15]. Further investigations enabled the alkylation of aliphatic amines, including Et₃N and six-membered azacycles with electron-deficient olefins [16].

Scheme 1.6.Diastereoselective addition of pyrrolidine to a furanone.

Scheme 1.7.D...

Table of contents

• Cover Page
• Title
• Copyright Page
• Preface
• About the Editors
• About the Contributors
• Contents
• Chapter 1 Ketones and Aldehydes
• Chapter 2 Quinones
• Chapter 3 Aromatics and Cyanoaromatics
• Chapter 4 Sulfur Heterocycles
• Chapter 5 Oxygen Heterocycles: Pyrylium Salts
• Chapter 6 Oxygen Heterocycles: Eosin Derivatives
• Chapter 7 Oxygen Heterocycles: Fluorescein, Rhodamines, Rose Bengal
• Chapter 8 Nitrogen Heterocycles: Porphyrins
• Chapter 9 Nitrogen Heterocycles: Acridinium Salts
• Chapter 10 Other Nitrogen Heterocycles: Carbazoles, Imides and PDI, mpg-C3N4, Tetrazines, Riboflavin, and BODIPY
• Chapter 11 Photoorganocatalysts for Polymerization
• Chapter 12 Photoorganocatalysis in Flow
• Index