Nucleophilic Substitution on 2-Monosubstituted Quinoxalines Giving 2,3-Disubstituted Quinoxalines: Investigating the Effect of the 2-Substituent

Recent Advances in the Transition-Metal-Free Arylation of Hetero­arenes

Transition-metal-free arylation reactions have attracted considerable attention for economic and environmental reasons over the past 40 years. In recent years, much effort has been made to develop efficient transition-metal-free approaches for the arylation of heteroarenes. Covering the literature from 2015 to early 2021, this review aims to provide a thorough overview of the synthetic and mechanistic aspects of these atom-economical and environmentally benign reactions.

1 Introduction

2 Arylation of Pre-functionalized Heteroarenes

2.1 Arylation of Heteroaryl Halides

2.2 Decarboxylative Arylation of Heteroarenes

3 Direct C–H Arylation of Heteroarenes

3.1 C(sp2)–H Arylation

3.2 C(sp3)–H Arylation

4 N-Arylation of Heteroarenes

5 Summary and Outlook

A REP-FAMSEC Method as a Tool in Explaining Reaction Mechanisms: A Nucleophilic Substitution of 2-Phenylquinoxaline as a DFT Case Study

In search for the cause leading to low reaction yields, each step along the reaction energy profile computed for the assumed oxidative nucleophilic substitution of hydrogen (ONSH) reaction between 2-phenylquinoxaline and lithium phenylacetylide was modelled computationally. Intermolecular and intramolecular interaction energies and their changes between consecutive steps of ONSH were quantified for molecular fragments playing leading roles in driving the reaction to completion. This revealed that the two reactants have a strong affinity for each other, driven by the strong attractive interactions between Li and two N-atoms, leading to four possible reaction pathways (RP-C2, RP-C3, RP-C5, and RP-C10). Four comparable in energy and stabilizing molecular system adducts were formed, each well prepared for the subsequent formation of a C-C bond at either one of the four identified sites. However, as the reaction proceeded through the TS to form the intermediates (5a-d), very high energy barriers were observed for RP-C5 and RP-C10. The data obtained at the nucleophilic addition stage indicated that RP-C3 was both kinetically and thermodynamically favored over RP-C2. However, the energy barriers observed at this stage were very comparable for both RPs, indicating that they both can progress to form intermediates 5a and 5b. Interestingly, the phenyl substituent (Ph1) on the quinoxaline guided the nucleophile towards both RP-C2 and RP-C3, indicating that the preferred RP cannot be attributed to the steric hindrance caused by Ph1. Upon the introduction of H2O to the system, both RPs were nearly spontaneous towards their respective hydrolysis products (8a and 8b), although only 8b can proceed to the final oxidation stage of the ONSH reaction mechanism. The results suggest that RP-C2 competes with RP-C3, which may lead to a possible mixture of their respective products. Furthermore, an alternative, viable, and irreversible reaction path was discovered for the RP-C2 that might lead to substantial waste. Finally, the modified experimental protocol is suggested to increase the yield of the desired product.

Synthesis of novel quinoxaline-alkynyl derivatives and their anti-Mycobacterium tuberculosis activity

The study report on the synthesis of a series of novel quinoxaline-alkynyl derivatives that were evaluated for their activity against Mycobacterium tuberculosis (Mtb) H₃₇R_V strain. A total of 19 compounds bearing an alcohol, aldehyde, mesylate and ester groups on the alkynly group, and also containing a chloro and nitro groups at the 6-position, were prepared. Seven compounds (3c, 4a-b, 5a, 5c, 6c and 6i), were found to have MIC₉₀ < 10 µM, while five compounds (3b, 6a, 6b, 6d and 6h) had MIC₉₀ in the range 10-20 µM. Compounds bearing a nitro substituent in the 6-position were generally more active and demonstrated a better safety profile, when compared to the unsubstituted and 6-chloro derivatives. Of the seven most active compounds, four contained nitro group at the 6-position.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Transition Metal-Free Heteroarylation of Quinoxaline: Construction of Heteroaryl-Fused Phenazines by Oxidative Coupling

A concise method for the construction of heteroaryl-fused phenazines was developed via PIFA-BF₃·Et₂O-mediated oxidative coupling of di-heteroarylated quinoxalines for the first time. Synthesis of mono- and di-heteroarylation of quinoxaline was performed effectively using only LiTMP reagent under transition metal-free conditions and without the use of halogen-containing starting compounds. In addition, nonsymmetrical di-heteroarylated quinoxalines were synthesized through reheteroarylation of mono-heteroarylated quinoxalines in the same way. Oxidation of the saturated compounds formed after heteroarylation was easily accomplished with iodine. The UV-vis absorption and fluorescence features of some compounds were examined.