Four Mechanisms in the Reactions of 3-Aminopyrrole with 1,3,5-Triazines: Inverse Electron Demand Diels–Alder Cycloadditions vs SNAr Reactions via Uncatalyzed and Acid-Catalyzed Pathways

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.

Aromatic Heterocycles as Productive Dienophiles in the Inverse Electron-Demand Diels-Alder Reactions of 1,3,5-Triazines

Heterocycles are often found as the structural nucleus in natural products and biological active compounds. Consequently, research toward the discovery and development of novel and efficient synthetic methodologies is of constant interest to organic chemists. Diels-Alder reactions are powerful at forming multiple bonds simultaneously, often with stereoselectivity, and thus are one of the most widely used synthetic methodologies in organic syntheses. Inverse electron-demand Diels-Alder (IEDDA) reactions, a subclass of Diels-Alder reactions, have been developed for the efficient synthesis of various heterocycles, with 1,3,5-triazines used as azadienes. The initial 1,3,5-triazine IEDDA reactions mostly included nonaromatic, electron-rich species such as vinyl ethers, enamines, ynamines, and amidines as dienophiles, producing in high yields pyrimidine derivatives with excellent regioselectivity. We hypothesized that some electron-rich aromatic heterocycles could be studied as potential dienophiles for 1,3,5-triazine IEDDA reactions; 5-aminopyrazoles proved to be productive dienophiles leading to high yields of pyrazolopyrimidines. Subsequently, many studies were reported to investigate the mechanism and scope of this new type of IEDDA reaction. Mechanistically, this new type of IEDDA reaction is quite interesting since it entails two aromatic compounds proceeding through a perceived high energy nonaromatic transition state, leading to a new aromatic compound, a counterintuitive process. Both theoretical and experimental studies provide key insights to this reaction mechanism, with learnings from these studies possibly stimulating unconventional thinking in other areas. Theoretical calculations of these cascade reactions of amino-substituted heterocycles with 1,3,5-triazines indicate that the reactions occur in a stepwise fashion via a highly polar zwitterionic intermediate; elimination of ammonia from IEDDA adducts and subsequent retro Diels-Alder reaction drive the reaction toward the fully aromatized IEDDA products. This amino substituent is critical in determining the regioselectivity and driving the cascade reactions to completion. With regard to reaction scope, many amino-heterocycles such as pyrroles, imidazoles, furans, thiophenes, and indoles all proved to be productive dienophiles for this new IEDDA reaction, leading to various fused-pyrimidines in a single step with moderate to high yields and high regioselectivity. Application of this new IEDDA reaction with 1,3,5-triazines was reported to lead to interesting heterocyclic compounds such as nucleoside natural product nebularine and analogues, as well as fluorine-containing fused pyrimidines with potential for biological activities.Herein, the scope of various aromatic heterocycles as dienophiles in the 1,3,5-triazine IEDDA reaction is reviewed. Moreover, both experimental and theoretical studies of the mechanisms for this interesting cascade IEDDA reaction are discussed. Finally, applications of this new type (aromatic heterocycles as dienophiles with 1,3,5-triazines as azadienes) of IEDDA reaction are also covered.

Reaction of Aldehydes/Ketones with Electron-Deficient 1,3,5-Triazines Leading to Functionalized Pyrimidines as Diels-Alder/Retro-Diels-Alder Reaction Products: Reaction Development and Mechanistic Studies

Catalytic inverse electron demand Diels-Alder (IEDDA) reactions of heterocyclic aza-dienes are rarely reported since highly reactive and electron-rich dienophiles are often found not compatible with strong acids such as Lewis acids. Herein, we disclose that TFA-catalyzed reactions of electron-deficient 1,3,5-triazines and electron-deficient aldehydes/ketones can take place. These reactions led to highly functionalized pyrimidines as products in fair to good yields. The reaction mechanism was carefully studied by the combination of experimental and computational studies. The reactions involve a cascade of stepwise inverse electron demand hetero-Diels-Alder (ihDA) reactions, followed by retro-Diels-Alder (rDA) reactions and elimination of water. An acid was required for both ihDA and rDA reactions. This mechanism was further verified by comparing the relative reactivity of aldehydes/ketones and their corresponding vinyl ethers in the current reaction system.

Effect of Bronsted Acids and Bases, and Lewis Acid (Sn(2+)) on the Regiochemistry of the Reaction of Amines with Trifluoromethyl-β-diketones: Reaction of 3-Aminopyrrole to Selectively Produce Regioisomeric 1H-Pyrrolo[3,2-b]pyridines

Reaction of 3-aminopyrrole (as its salt) with trifluoromethyl-β-diketones gave γ-1H-pyrrolo[3,2-b]pyridines via reaction at the less reactive carbonyl group. The trifluoromethyl group increased the electrophilicity of the adjacent carbonyl group and decreased the basicity of the hydroxyl group of the CF3 amino alcohol formed. This amino alcohol was formed faster, but its subsequent dehydration to the β-enaminone was slow resulting in the preferential formation of the γ-regioisomer. Reaction of 4,4,4-trifluoro-1-phenyl-1,3-butadione with 3-aminopyrrole was carried out using a series of 6 amine buffers. Yields of the α-1H-pyrrolo[3,2-b]pyridine increased as the pKa of the amine buffer decreased. Surprisingly the yield went down at higher pKas. There was a change in mechanism as the reaction mixture became more basic. With strong amines trifluoromethyl-β-diketones were present mainly or completely as the enolate. Under reductive conditions (3-nitropyrrole/Sn/AcOH/trifluoromethyl-β-diketone) the α-1H-pyrrolo[3,2-b]pyridine was the major product as a result of Lewis acid catalysis by Sn(2+). Similar α-regiochemistry was observed when the reaction of the 3-aminopyrrole salt with trifluoromethyl-β-diketones was carried out in the presence of base and tin(II) acetate.

Rh(ii)-catalyzed cycloadditions of 1-tosyl 1,2,3-triazoles with 2H-azirines: switchable reactivity of Rh-azavinylcarbene as [2C]- or aza-[3C]-synthon

The Rh(ii)-catalyzed formal [3+2] and [3+3] cycloadditions of 1-tosyl 1,2,3-triazoles with 2H-azirines have been developed, which enable the efficient synthesis of polysubstituted 3-amino-pyrroles and 1,2-dihydropyrazines, respectively.