An I2-mediated aerobic oxidative annulation of amidines with tertiary aminesviaC–H amination/C–N cleavage for the synthesis of 2,4-disubstituted 1,3,5-triazines

Visible-light photocatalyzed C-N bond activation of tertiary amines: a three-component approach to synthesize quinazolines

A metal-free three-component approach has been developed to prepare 2,4-disubstituted quinazolines from o-acylanilines, trialkylamines and ammonium chloride under visible-light using eosin Y as the photocatalyst. The notable features of this work include (i) the use of tertiary amines as an alkyl synthon and triethanolamine as a C2-OH synthon; (ii) good functional group tolerance with 52%-98% yields; (iii) proof of concept with o-amino benzaldehyde as a substrate to deliver 2-methyl quinazoline 3pa; and (iv) gram-scale synthesis of compounds 3ga, 3ja and 3ma. A reductive quenching mechanism was proposed based on the control studies and redox potential values.

Triazines: Syntheses and Inverse Electron-demand Diels-Alder Reactions

Triazines are an important class of six-membered aromatic heterocycles possessing three nitrogen atoms, resulting in three types of regio-isomers: 1,2,4-triazines (a-triazines), 1,2,3-triazines (v-triazines), and 1,3,5-triazines (s-triazines). Notably, the application of triazines as cyclic aza-dienes in inverse electron-demand Diels-Alder (IEDDA) cycloaddition reactions has been established as a unique and powerful method in N-heterocycle synthesis, natural product preparation, and bioorthogonal chemistry. In this review, we comprehensively summarize the advances in the construction of these triazines via annulation and ring-expansion reactions, especially emphasizing recent developments and challenges. The synthetic transformations of triazines are focused on IEDDA cycloaddition reactions, which have allowed access to a wide scope of heterocycles, including pyridines, carbolines, azepines, pyridazines, pyrazines, and pyrimidines. The utilization of triazine IEDDA reactions as key steps in natural product synthesis is also discussed. More importantly, a particular attention is paid on the bioorthogonal application of triazines in fast click ligation with various strained alkenes and alkynes, which opens a new opportunity for studying biomolecules in chemical biology.

Photochemical Synthesis of 1,4-Dicarbonyl Bifluorene Compounds via Oxidative Radical Coupling Using TEMPO as the Oxygen Atom Donor

A visible-light-induced metal-free synthesis of 1,4-dicarbonyl compounds from alkyne-containing aryl iodides via photochemical C-I bond cleavage, intramolecular cyclization, oxidation, and intermolecular radical coupling sequence is reported. TEMPO was employed as the oxygen atom donor in this transformation. This protocol provided a new strategy for the synthesis of 1,4-dicarbonyl bifluorene compounds.

Electrosynthesis of Quinazolines and Quinazolinones via an Anodic Direct Oxidation C(sp3)-H Amination/C-N Cleavage of Tertiary Amine in Aqueous Medium

An electrochemical synthesis for quinazolines and quinazolinones was developed via a C(sp³)-H amination/C-N cleavage by virtue of the anodic oxidation. The reaction can be carried out in aqueous media under mild conditions to afford the desired products with high yields. The reaction mechanism was proposed after detailed investigation.

Iodine-Promoted Multicomponent Synthesis of 2,4-Diamino-1,3,5-triazines

A novel and efficient multicomponent cyclization of methyl ketones, cyanamides, and arylamines for the synthesizing 2,4-diamino-1,3,5-triazines via consecutive formation of four C-N bonds is reported. This multicomponent reaction is characterized by the employment of two molecules of cyanamide for double C(sp³)-H amination of methyl ketones, avoiding complicated prepreparation of substrates and expanding the substrate scope. Furthermore, this multicomponent cyclization strategy provides a new approach for generating diverse 2,4-diamino-1,3,5-triazines with a broad substrate scope under mild conditions.

Synthesis of 1,2-dihydro-1,3,5-triazine derivatives via Cu(II)-catalyzed C(sp3)-H activation of N,N-dimethylethanolamine with amidines

1,2-Dihydro-1,3,5-triazines and symmetrical 1,3,5-triazines were obtained in up to 81% yields from amidines and N,N-dimethylethanolamine catalyzed by CuCl2. The reaction involves three C-N bond formations during the oxidative annulation process and the mechanism was proposed. This efficient synthesis of 1,2-dihydro-1,3,5-triazines was developed for the first time.

Atom-efficient synthesis of 2,4,6-trisubstituted 1,3,5-triazines via Fe-catalyzed cyclization of aldehydes with NH4I as the sole nitrogen source

An atom-efficient, straightforward method for the synthesis of 2,4,6-triaryl-1,3,5-triazines via iron-catalyzed cyclization of aldehydes with NH₄I as the sole nitrogen source is demonstrated. This strategy works smoothly under air atmosphere, and affords symmetrical 2,4,6-trisubstituted and unsymmetrical 1,3,5-triazines with yields from 18% to 72%. Compared to other methods, the present protocol provides a straightforward and atom-efficient approach to 2,4,6-trisubstituted 1,3,5-triazines using an inexpensive, easily available ammonium salt as the sole nitrogen source. Research into the preliminary mechanism indicates that N-benzylidenebenzimidamides are involved in this cyclization reaction.

An atom-efficient, straightforward method for the synthesis of 2,4,6-triaryl-1,3,5-triazines via iron-catalyzed cyclization of aldehydes with NH₄I as the sole nitrogen source is demonstrated.

Transition metal-free C-F/C-Cl/C-C cleavage of ClCF2COONa for the synthesis of heterocycles

A transition metal-free and external oxidant-free annulation of substrates having two nitrogen-nucleophilic sites with ClCF₂COONa was demonstrated, affording a series of 1,3,5-triazines and quinazolinones in up to 96% yields. Notably, ClCF₂COONa was employed as the C1 synthon for valuable heterocycles. Using this protocol, two C-N bonds were formed in one pot via the cleavage of two C-F bonds, one C-Cl bond and one C-C bond. This method avoided the use of a transition metal and an oxidant and generated low toxicity inorganic waste.

Developments towards synthesis of N-heterocycles from amidines via C–N/C–C bond formation

This review focuses on the synthesis of N-heterocycles using amidines as starting materials, with an emphasis on the mechanisms of these reactions via C–N/C–C bond formation.

Transition metal-free assembly of 1,3,5-triazines using ethyl bromodifluoroacetate as C1 source

Transition-metal and oxidant-free annulation of amidine to access 2,4-disubstituted-1,3,5-triazines using ethyl bromodifluoroacetate as C1 source via quadruple cleavage.