NHC-CDI Betaine Adducts and Their Cationic Derivatives as Catalyst Precursors for Dichloromethane Valorization.

David Sánchez-Roa,Juan Cámpora,Marta E G Mosquera

doi:10.1021/acs.joc.1c01971

Abstract

Zwitterionic adducts of N-heterocyclic carbene and carbodiimide (NHC-CDI) are an emerging class of organic compounds with promising properties for applications in various fields. Herein, we report the use of the ICyCDI(p-Tol) betaine adduct (1a) and its cationic derivatives 2a and 3a as catalyst precursors for the dichloromethane valorization via transformation into high added value products CH2Z2 (Z = OR, SR or NR2). This process implies selective chloride substitution of dichloromethane by a range of nucleophiles Na+Z– (preformed or generated in situ from HZ and an inorganic base) to yield formaldehyde-derived acetals, dithioacetals, or aminals with full selectivity. The reactions are conducted in a multigram-scale under very mild conditions, using dichloromethane both as a reagent and solvent, and very low catalyst loading (0.01 mol %). The CH2Z2 derivatives were isolated in quantitative yields after filtration and evaporation, which facilitates recycling the dichloromethane excess. Mechanistic studies for the synthesis of methylal CH2(OMe)2 rule out organocatalysis as being responsible for the CH2 transfer, and a phase-transfer catalysis mechanism is proposed instead. Furthermore, we observed that 1a and 2a react with NaOMe to form unusual isoureate ethers, which are the actual phase-transfer catalysts, with a strong preference for sodium over other alkali metal nucleophiles.

Highlights

Dichloromethane (DCM) is a relatively inert compound that is widely applied as a solvent in organic synthesis and separation procedures
We shifted our trial to solid sodium methoxide, which would enable the synthesis of methylal through the substitution of chlorine atoms of DCM by methoxy groups (Scheme 2)
Synthesis of Methylal Using 2a as a Catalyst dimethoxymethane is an interesting target since it can be used as a green solvent23 or as a safer surrogate of formaldehyde, preventing the use of this toxic substance or its insoluble polymer, paraformaldehyde

Summary

Introduction

Dichloromethane (DCM) is a relatively inert compound that is widely applied as a solvent in organic synthesis and separation procedures. The heavier dihalomethane congeners, dibromomethane and diiodomethane, are considerably more reactive than DCM Those are rarely used as a solvent but often employed as a methylene source via nucleophilic substitution reactions.. The chemical stability of C−Cl bonds, which makes DCM such an excellent solvent, usually leads to difficult chloride substitution processes.. The chemical stability of C−Cl bonds, which makes DCM such an excellent solvent, usually leads to difficult chloride substitution processes.4 The latter are often plagued by side reactions, such as the HCl elimination whenever the nucleophiles behave as strong bases or free radical chain processes.4a,5 These unwanted reactions can entail explosion hazards in some cases.. Those have proven useful as fuels or fuel additives, and as reagents for the introduction of an alkoxy- or aryloxymethylene group. On the other hand, dithioacetals and aminals are less used in the industry than formals but have important applications in synthesis and coordination chemistry. transition metal catalysts (e.g., Ni14a or Cu14b complexes) have been occasionally used for coupling dichloro-

Methods

Results

Conclusion