Protic Ionic Liquid as Reagent, Catalyst, and Solvent: 1-Methylimidazolium Thiocyanate.

Ivan A Andreev,Daniel B Werz,Olga A Ivanova,Nina K Ratmanova,Irina I Levina,Igor V Trushkov,Victor N Khrustalev,André U Augustin

doi:10.1002/anie.202016593

Abstract

We propose a new concept of the triple role of protic ionic liquids with nucleophilic anions: a) a regenerable solvent, b) a Brønsted acid inducing diverse transformations via general acid catalysis, and c) a source of a nucleophile. The efficiency of this strategy was demonstrated using thiocyanate‐based protic ionic liquids for the ring‐opening of donor‐acceptor cyclopropanes. A wide variety of activated cyclopropanes were found to react with 1‐methylimidazolium thiocyanate under mild metal‐free conditions via unusual nitrogen attack of the ambident thiocyanate ion on the electrophilic center of the three‐membered ring affording pyrrolidine‐2‐thiones bearing donor and acceptor substituents at the C(5) and C(3) atoms, respectively, in a single time‐efficient step. The ability of 1‐methylimidazolium thiocyanate to serve as a triplex reagent was exemplarily illustrated by (4+2)‐annulation with 1‐acyl‐2‐(2‐hydroxyphenyl)cyclopropane, epoxide ring‐opening and other organic transformations.

Highlights

Modern demands of synthetic chemistry require the selection of reaction conditions providing both high yields of the target products due to high chemo, regio, and stereoselectivity of the processes employed and conformity with the fundamental principles of green chemistry.[1]
One of the most attractive solutions to all these problems is the use of protic ionic liquids (PILs), i.e., low-melting salts of Brønsted acid and base.[5]
To be a stable reaction medium, PIL should have a full proton transfer from an acid to a base;[5] full ionicity is desired for the highest nucleophilicity of the thiocyanate ion in the PIL

Summary

Introduction

Modern demands of synthetic chemistry require the selection of reaction conditions providing both high yields of the target products due to high chemo-, regio-, and stereoselectivity of the processes employed and conformity with the fundamental principles of green chemistry.[1] Among them, the essential ones are concepts such as atom and step economy,[2] design and application of less hazardous reagents and catalysts, as well as minimization of waste production.[3,4]. One of the most attractive solutions to all these problems is the use of protic ionic liquids (PILs), i.e., low-melting salts of Brønsted acid and base.[5] On the one hand, PILs can pursue a threefold role serving cooperatively as: a) an excellent reaction medium, dissolving both hydrophobic and hydrophilic molecules, b) process initiator via acid-base catalysis, and c) reagent, if the PIL contains highly nucleophilic species, red-ox agent, etc. To the best of our knowledge, is the absence of reports on the use of this triple role of PILs in organic synthesis (Scheme 1 a)

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Conclusion