Maurizio Fagnoni

Carbon–Carbon Bond Forming Reactions via Photogenerated Intermediates

The present review offers an overview of the current approaches for the photochemical and photocatalytic generation of reactive intermediates and their application in the formation of carbon-carbon bonds. Valuable synthetic targets are accessible, including arylation processes, formation of both carbo- and heterocycles, alpha- and beta-functionalization of carbonyls, and addition reactions onto double and triple bonds. According to the recent advancements in the field of visible/solar light catalysis, a significant part of the literature reported herein involves radical ions and radicals as key intermediates, with particular attention to the most recent examples. Synthetic application of carbenes, biradicals/radical pairs and carbocations have been also reported.

Dyes as Visible Light Photoredox Organocatalysts

Light-driven reactions are of crucial importance for the existence of mankind on Earth. In the aim to exploit photons for synthetic purposes, photocatalysis has recently proven to be a powerful method to perform several organic reactions including oxidations, reductions, valuable C C bond formation reactions, and many others. The photocatalyst (PC, Scheme 1) in the excited state (PC*) is able to activate one of the reagents by means of a chemical reaction, such as an atom or electron transfer process.

Green chemistry and photochemistry were born at the same time

Where to look for really ‘green’ synthetic methods, under conditions as mild as those nature uses? A hundred years ago, a great scientist, Giacomo Ciamician, confronted the problem. He had no doubt of the answer: it was solar light. The approach and the discoveries by Ciamician are illustrated in connection with present-day green chemistry.

The Aromatic Carbon–Carbon ipso‐Substitution Reaction

The aim of this review is to illustrate what we have dubbed an aromatic carbon-carbon ipso-substitution reaction in which a new Ar-C bond is formed at the expense of another Ar-C bond. The potentiality of several C-based leaving groups including alkyl, carbinol, CN, COOH, and carbonyl groups in arylation reactions will be illustrated accordingly for the preparation of biphenyl-, vinyl-, alkynyl-, and alkyl-substituted aromatics.

Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals

An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon–carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity, radical transformations have been far less successful. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors. Here we show how the combination of photoredox and asymmetric organic catalysis enables enantioselective radical conjugate additions to β,β-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts—this method represents the application of iminium ion activation (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.

Sunlight photocatalyzed regioselective β-alkylation and acylation of cyclopentanones

Sunlight-induced direct regioselective β-alkylation of cyclopentanones with electron-deficient alkenes was accomplished by using tetrabutylammonium decatungstate (TBADT) as the catalyst. The regiochemistry can be rationalized by a polar transition state for an SH2 reaction. In the presence of CO, the reaction gave the three-component β-acylation product in good yield.

Synthesis of monoprotected 1,4-diketones by photoinduced alkylation of enones with 2-substituted-1,3-dioxolanes

Abstract Photosensitized hydrogen abstraction from 2-alkyl-1,3-dioxolanes by triplet benzophenone gives the corresponding 1,3-dioxolan-2-yl radicals and these are trapped by α,β-unsatured ketones yielding monoprotected 1,4-diketones. With open chain ketones (3-buten-2-one and 4-penten-3-one) the yields are low and competitive pathways in part consume the radicals. With cyclic enones however, yields are good as tested with cyclopentenone, cyclohexenone and 4-hydroxy-cyclopentenone. More generally, this is a viable alternative for the synthesis of 1,4-diketones via radicals while the thermal initiation gives only low yield. The reaction cannot be extended to strongly stabilized radicals, such as the 2-phenyl-1,3-dioxolanyl radical.

Photocatalytic CH Activation by Hydrogen‐Atom Transfer in Synthesis

This minireview encompasses the most recent (since 2010) advancements in the field of photocatalytic hydrogen‐atom transfer (HAT) processes for CH bond functionalization. The direct HAT process promoted from the excited state of a photocatalyst is limited to the classes of polyoxometalates and aromatic ketones, but alternative photocatalytic strategies have recently been devised to alleviate this shortcoming, making use of indirect HAT reactions. The applications of these two approaches in organic synthesis, including the formation of CC, as well as Chalogen, CO (CO), and CN bonds, are presented herein.

Efficient C–H/C–N and C–H/C–CO–N Conversion via Decatungstate-Photoinduced Alkylation of Diisopropyl Azodicarboxylate

Tetrabutylammonium decatungstate (TBADT) accelerated the addition of C-H bonds to the N═N double bond of diisopropyl azodicarboxylate (DIAD) under irradiation conditions. The photoinduced three-component coupling between cyclic alkanes, CO, and DIAD was also achieved to give the corresponding acyl hydrazides.

Environment-friendly organic synthesis. The photochemical approach

Photochemical reactions are expected to have an increasing role in organic synthesis with the drive towards environment-friendly reactions. Some examples illustrating the scope and versatility of radical alkylation of electrophilic alkenes via photoinduced electron transfer are presented. A few applications in special fields are mentioned.