A green metal-free "one-pot" microwave assisted synthesis of 1,4-dihydrochromene triazoles

Recent advances in microwave-assisted synthesis of triazoles and their derivatives: a green approach toward sustainable development methods

Triazole, a nitrogen-containing five-membered heterocycle with two isomeric forms, 1,2,3-triazole and 1,2,4-triazole, has proven to be a valuable component in the pharmaceutical domain. Owing to its widespread utility in drug development, pharmaceutical and medicinal chemistry, several synthetic methods have been explored, such as different catalytic systems, solvents, and heating methodologies in recent years. However, some methods were associated with several limitations, such as harsh reaction conditions, high temperatures, low atom economy, and long reaction times. Conversely, the ongoing demand from the chemical industry has led to increased attention on overcoming these limitations and developing sustainable laboratory methods. In recent years, the microwave heating method in organic synthesis has evolved as a new, environmentally friendly approach with benefits such as atom economy, reduced use of hazardous chemicals, safer chemical design, few derivatives and enhanced energy efficiency. This review summarizes recent progress in microwave-assisted synthesis of triazoles (1,2,3-triazole and 1,2,4-triazole), with a comparative analysis between conventional methods and microwave-assisted methods in terms of reaction time, yield, green synthesis, sustainability and other relevant factors.

Mechanistic Aspects on [3+2] Cycloaddition (32CA) Reactions of Azides to Nitroolefins: A Computational and Kinetic Study

[3+2] cycloadditions of nitroolefins have emerged as a selective and catalyst-free alternative for the synthesis of 1,2,3-triazoles from azides. We describe mechanistic studies into the cycloaddition/rearomatization reaction sequence. DFT calculations revealed a rate-limiting cycloaddition step proceeding via an asynchronous TS with high kinetic selectivity for the 1,5-triazole. Kinetic studies reveal a second-order rate law, and ¹³ C kinetic isotopic effects at natural abundance were measured with a significant normal effect at the conjugated olefinic centers of 1.0158 and 1.0216 at the α and β-carbons of β-nitrostyrene. Distortion/interaction-activation strain and energy decomposition analyses revealed that the major regioisomeric pathway benefits from an earlier and less-distorted TS, while intermolecular interaction terms dominate the preference for 1,5- over 1,4-cycloadducts. In addition, the major regioisomer also has more favorable electrostatic and dispersion terms. Additionally, while static DFT calculations suggest a concerted but highly asynchronous Ei-type HNO₂ elimination mechanism, quasiclassical direct-dynamics calculations reveal the existence of a dynamic intermediate.

Synthesis of 5H-chromeno[3,4-b]pyridines via DABCO-catalyzed [3 + 3] annulation of 3-nitro-2H-chromenes and allenoates

The DABCO-catalyzed [3 + 3] annulation between 3-nitro-2H-chromenes and benzyl 2,3-butadienoate has been developed as a route to 5H-chromeno[3,4-b]pyridine derivatives. Under optimal reaction conditions, 5H-chromeno[3,4-b]pyridines incorporating two allenoate units were obtained in moderate to good yields (30-76%). The same type of transformation could be carried out using butynoates as allene surrogates. Mechanistic studies by mass spectrometry allowed the identification of the key intermediates involved in the reaction mechanism. The reported synthetic methodology represents an entirely new approach for the synthesis of the 5H-chromeno[3,4-b]pyridine core structure based on allene chemistry.