Hydroamination Reactions of Alkynes with ortho‐Substituted Anilines in Ball Mills: Synthesis of Benzannulated N‐Heterocycles by a Cascade Reaction

Abstract

It was demonstrated that ortho-substituted anilines are prone to undergo hydroamination reactions with diethyl acetylenedicarboxylate in a planetary ball mill. A sequential coupling of the intermolecular hydroamination reaction with intramolecular ring closure was utilized for the syntheses of benzooxazines, quinoxalines, and benzothiazines from readily available building blocks, that is, electrophilic alkynes and anilines with OH, NH, or SH groups in the ortho position. For the heterocycle formation, it was shown that several stress conditions were able to initiate the reaction in the solid state. Processing in a ball mill seemed to be advantageous over comminution with mortar and pestle with respect to process control. In the latter case, significant postreaction modification occurred during solid-state analysis. Cryogenic milling proved to have an adverse effect on the molecular transformation of the reagents.

Solvent‐Free Mechanochemical Synthesis of Nitrogen‐Doped Nanoporous Carbon for Electrochemical Energy Storage

“…Before mechanochemistry was established for advanced materials syntheses, ball milling was primarily used for particle‐size reduction and alloying . Recent studies in the fields of pharmaceutical chemistry, organic chemistry, and inorganic chemistry, especially in the field of metal–organic frameworks (MOFs), have demonstrated the tremendous potential of mechanochemistry. However, the mechanochemical synthesis of porous carbon has remained largely unexplored.…”

Mechanochemistry for Organic Chemists: An Update

“…An alternative, readily accessible technique for ex situ reaction monitoring is Fourier‐transform infrared (FTIR) spectroscopy, which complements PXRD by providing information on chemical composition rather than crystallinity. The use of FTIR for mechanochemical reaction monitoring was illustrated by the Stolle group, who reported the hydroamination of alkynes with o ‐substituted anilines to form a benzannulated N‐heterocyclic benzoxazine (Figure a) in a planetary mill . Fast FTIR‐ATR monitoring was achieved by focusing on characteristic absorption bands of the reactant ester at 1720 cm –1 , the product lactone band at 1762 cm –1 , and changes in the aromatic bending region at 750 cm –1 (Figure b).…”

Synthesis of monosubstituted thioureas by vapour digestion and mechanochemical amination of thiocarbamoyl benzotriazoles

“…A sample of 2a was exposed to NH 3 (g) in a closed vial containing aqueous ammonia solution at room temperature and the progress of the reaction was monitored by an ex situ FTIR-ATR analysis. Recently, FTIR has been utilized as a fast and reliable spectroscopic technique for probing the course of mechanochemical reactions such as hydroamination of alkynes 16 and amide synthesis. 17 The absorption bands positioned at 1617 and 509 cm −1 characteristic for N-(4-bromophenyl)thiourea (3a) appeared after 90 minutes, whereas 6 hours into aging resulted in significant transformation to 3a as evidenced by reduction of 2a band intensities at 1588, 1520, 1157, 1143, 968, 924 and 494 cm −1 .…”