Catalyst‐free Hantzsch Ester‐mediated Organic Transformations Driven by Visible light

Abstract

In the early days of synthetic organic chemistry Hantzsch esters (HEs) were bio-inspired hydride donors and often used for the reduction of C=X (X=C, N, O) bonds. Recently, it has been proved that HEs and their derivatives (4-Alkyl-or acyl-Hantzsch esters) serve as electron donor, proton source, hydrogen atom donor, alkyl and acyl radical precursors in photoredox catalysis. Recently it was found that the excited-state of HEs has also been identified as a strong photo reductant in the absence of photocatalyst and also, the donor of HE forms EDA-complexes with organic molecules and triggers SET process to generate carbon radical entities. This minireview focuses on the recent chemistry developments where Hantzsch esters act as a photo reductant in organic synthesis, with a special focus on the reaction mechanisms. We hope that this review will help in the further development of radical-based organic transformations in absence of external photocatalysts.

Visible-Light-Induced Reductive Coupling of Arylacetylenes with Benzothiazole Sulfones

“…Meanwhile, regarding the effect of 1,4-dihydropyridine derivatives, the use of 4-substituted Hantzsch esters ( 4b – d ) resulted in no reaction, and 1a was recovered intact (Table , entries 6–8). Methyl, benzyl, and tert -butyl ester derivatives ( 4e – g ) were used in this transformation, affording 2a in 34–56% yields (Table , entries 9–11).…”

Isocyanides as Catalytic Electron Acceptors in the Visible Light Promoted Oxidative Formation of Benzyl and Acyl Radicals

“…4‐Alkyl‐1,4‐dihydropyridines (DHPs, also known as Hantzsch esters) have been conveniently exploited as alkyl radical precursors thanks to a smooth oxidative fragmentation to form a fully aromatic pyridine ( E red =+1.0 V vs SCE, on average). Furthermore, they are bench‐stable solids, easily synthesized from widely available aliphatic aldehydes [27–32] . In order to evaluate the feasibility of a PET process mediated by an aromatic isocyanide, 10 was selected as a model substrate.…”

“…Furthermore, they are bench-stable solids, easily synthesized from widely available aliphatic aldehydes. [27][28][29][30][31][32] In order to evaluate the feasibility of a PET process mediated by an aromatic isocyanide, 10 was selected as a model substrate. Following the mechanistic hypothesis in Scheme 1, the latter, once oxidized via a SET mediated by the electronically excited isocyanide, should afford the benzyl radical I, which in the presence of a H-donor such as triisopropylsilane (TIS) could give 4-phenyltoluene 12.…”

Visible-Light-Induced Synthesis of Alkylsulfonated Isoquinolinones from 2-Aryl Indoles/Benzimidazoles through Insertion of Sulfur Dioxide

“…Due to the dynamics of dehydroaromatization of the DHP structure, they tend to go through 4-position hydrogen migration to participate in the hydrogenation reduction reaction . Besides, 4-substituted HEs have been regarded as an effective alkyl radical precursor through the C–C bond cleavage at the 4-position, which can avoid the use of toxic alkyl metal reagents and is easy to obtain. Thus, there is continuous development of novel alkylation reactions via a radical process using 4-alkyl-substituted HEs as alkyl precursors …”