Visible-Light-Induced Difluoropropargylation Reaction with Benzothiazoline as a Reductant

Pro-aromatic Dihydroquinazolinones - From Multigram Synthesis to Reagents for Gram-scale Metallaphotoredox Reactions

Dihydroquinazolinone (DHQZ) has recently been harnessed as a ketone-derived pro-aromatic reagent extensively employed in (metalla)photoredox reactions as versatile group transfer agents. In this work, we outline a column chromatography-free protocol for the multigram-scale synthesis of pro-aromatic DHQZs as well as its use in a gram-scale nickel/photoredox dual-catalyzed cross-coupling in single-batch, photoflow, and simultaneous multiple smaller batches. While the single-batch approach leveraged moderate yields, a simple plug-flow photoreactor also exhibited amenable productivity (up to 45 % yield) despite the use of a heterogeneous base. Meanwhile, performing the metallaphotoredox-catalyzed reaction in multiple smaller batches in an improvised photoreactor facilitated high yields of up to 59 % and good reproducibility, implying a convenient alternative in the absence of photoflow setups.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Catalyst-free electrosynthesis of benzothiophenes from 2-alkenylaryl disulfides

The synthesis of benzothiophenes through electrosynthesis under oxidant- and metal-free conditions has been discovered. Electrolysis of symmetrical 2-alkenylaryl disulfides using an undivided cell leads to the formation of the corresponding benzothiophenes in good to moderate yields with good functional group tolerance. The usefulness of this methodology was further investigated with a scale-up experiment, which delivered a similar result to that of the small scale reaction. Several mechanistic investigations including DFT calculations were carried out to elucidate the reaction mechanism.

Kinetic Studies of Hantzsch Ester and Dihydrogen Donors Releasing Two Hydrogen Atoms in Acetonitrile

In this work, kinetic studies on HEH₂, 2-benzylmalononitrile, 2-benzyl-1H-indene-1,3(2H)-dione, 5-benzyl-2,2-dimethyl-1,3-dioxane-4,6-dione, 5-benzyl-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione, 2-(9H-fluoren-9-yl)malononitrile, ethyl 2-cyano-2-(9H-fluoren-9-yl)acetate, diethyl 2-(9H-fluoren-9-yl)malonate, and the derivatives (28 XH₂) releasing two hydrogen atoms were carried out. The thermokinetic parameters ΔG ^⧧° of 28 dihydrogen donors (XH₂) and the corresponding hydrogen atom acceptors (XH^•) in acetonitrile at 298 K were determined. The abilities of releasing two hydrogen atoms for these organic dihydrogen donors were researched using their thermokinetic parameters ΔG ^⧧°(XH₂), which can be used not only to compare the H-donating ability of different XH₂ qualitatively and quantitatively but also to predict the rates of HAT reactions. Predictions of rate constants for 12 HAT reactions using thermokinetic parameters were determined, and the reliabilities of the predicted results were also examined.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Direct Photoexcitation of Benzothiazolines: Acyl Radical Generation and Application to Access Heterocycles

An acyl radical generation and functionalization strategy through direct photoexcitation of benzothiazolines has been developed. The formed acyl radical species can either be trapped by quinoxalin-2-ones to realize their C(3)-H functionalization or trigger a cascade radical cyclization with isonitriles to synthesise biologically important phenanthridines. The synthetic value of this protocol can be further illustrated by the modification of quinoxalin-2-ones, containing important natural products and drug-based complex molecules.

Modular Enantioselective Synthesis of cis-Cyclopropanes through Self-Sensitized Stereoselective Photodecarboxylation with Benzothiazolines

Chiral cis-cyclopropanes are strained rigid analogues of alkyl chains, whose study and application are limited by their difficult synthesis. A modular approach from olefin materials is enabled by the discovery of the electron donor-acceptor (EDA) interaction between 2-substituted benzothiazolines and N-hydroxyphthalimide esters. These complexes are activated by visible light without photocatalysts, and the benzothiazoline reagent plays a triple role as a photoreductant, a stereoselective hydrogen-atom donor, and a Brønsted acid. Beyond the enantioselective synthesis of cis-cyclopropanes, these results introduce benzothiazolines as accessible and easily tunable self-sensitized photoreductants.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Thermodynamic Network Cards of Hantzsch Ester, Benzothiazoline, and Dihydrophenanthridine Releasing Two Hydrogen Atoms or Ions on 20 Elementary Steps

In this work, thermodynamic driving forces on 20 possible elementary steps of Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydrophenanthridine (PDH₂) releasing two hydrogen atoms or ions were measured or derived from the related thermodynamic data using Hess' law in acetonitrile. Furthermore, thermodynamic network cards of HEH₂, BTH₂, and PDH₂ releasing two hydrogen atoms or ions on 20 elementary steps were first established. Based on the thermodynamic network cards, hydride-donating, hydrogen-atom-donating, and electron-donating abilities of XH₂ and XH^-, and two hydrogen-atom(ion)-donating abilities of XH₂ are discussed in detail. Obviously, the thermodynamic network cards of HEH₂, BTH₂, and PDH₂ not only offer rational data guidance for organic synthetic chemists to properly choose an appropriate reducer among the three reducing agents to hydrogenate various unsaturated compounds but also strongly promote elucidatation of the detailed hydrogenation mechanisms.

Highly Chemoselective gem-Difluoropropargylation of Aliphatic Alcohols

Despite the potential of α-fluoroethers in medicinal chemistry, their synthetic methods, especially etherification of aliphatic alcohols, have been limited. Herein, we developed two- and three-step gem-difluoropropargylation of aliphatic alcohols including amino acid derivatives and naturally occurring bioactive molecules. Highly chemoselective etherification proceeded by using the gem-difluoropropargyl bromide dicobalt complex in the presence of silver triflate and triethylamine. Decomplexation of dicobalt complexes was achieved by using cerium ammonium nitrate or N,N,N'-trimethylethylenediamine. The thus obtained gem-difluoropropargyl ethers were converted to various α-difluoroethers which are expected to be useful for medicinal chemistry.