Recent Advances in Enantioselective Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes (NHCs) Containing Triazolium Motifs

Abstract

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

Remote Electronic Tuning of Chiral N‐Heterocyclic Carbenes

“…[1] A chiral N-heterocyclic carbene (NHC) is a wellknown organocatalyst used in various asymmetric reactions (Scheme 1). [2] NHCs are widely utilized for reactions in which Umpolung of aldehyde is involved, such as benzoin condensation [3] or a Stetter reaction. [4] Treatment of NHC with α-oxidized aldehyde produces the corresponding acylazolium species, which acts as a chiral acylation reagent.…”

Mechanistic Evaluation of Benzoin Catalysis Enabled by Triazolium Salt Application in the Preparation of Benzil and Heterocycles

“…In general pyrrolidinebased triazolium salt are more reactive compared to imidazole and thiazole base NHCs. [29,34,51,[63][64][65][66] The protocol starts with commercially available 2-pyrrolidone 1 and proceeds as follows.…”

“…Our synthetic protocol is developed based on previous literature reports with several modifications described below, which enabled the improvement of yields and the introduction of many different triazolium scaffolds. In general pyrrolidine‐based triazolium salt are more reactive compared to imidazole and thiazole base NHCs [29,34,51,63–66] . The protocol starts with commercially available 2‐pyrrolidone 1 and proceeds as follows.…”

“…They are typically employed in transition metal‐based catalysis, [2,7–13] since numerous imidazole and thiazole‐based NHC function as excellent nitrogen ligands in a variety of cross‐coupling reactions [3,14–16] involving carbon‐carbon (C−C), carbon‐halogen (C−X), and carbon‐heteroatom (C‐HA), as well as olefin metathesis [17–20] . However, there is growing interest in the use of pyrrolidine‐based triazolium nucleophilic carbenes in the field of organocatalytic reaction [21–29] . Since, they are more environmentally friendly, less expensive, and green than traditional organic transformation techniques, transition metal‐free organocatalytic developments are more appealing than those that use transition metals.…”

“…[17][18][19][20] However, there is growing interest in the use of pyrrolidinebased triazolium nucleophilic carbenes in the field of organocatalytic reaction. [21][22][23][24][25][26][27][28][29] Since, they are more environmentally friendly, less expensive, and green than traditional organic transformation techniques, transition metal-free organocatalytic developments are more appealing than those that use transition metals. Most of the catalytic process may recover back carbene catalyst with a good turnover rate/frequency, less sensitivity to air triazolium-salts, and non-toxic reagents.…”