Iminium Salts by Meerwein Alkylation of Ehrlich’s Aldehyde

Carmen Froschauer,Gerhard Laus,Herwig Schottenberger,Hedda Weber,Volker Kahlenberg

doi:10.3390/cryst3010248

Abstract

4-(Dimethylamino)benzaldehyde is alkylated at the N atom by dialkyl sulfates, MeI, or Me3O BF4. In contrast, ethylation by Et3O BF4 occurs selectively at the O atom yielding a quinoid iminium ion. 4-(Diethylamino)benzaldehyde is alkylated only at O by either Et or Me oxonium reagent. The iminium salts are prone to hydrolysis giving the corresponding hydrotetrafluoroborates. Five crystal structures were determined.

Highlights

During unrelated work, the need arose for low-melting quaternary ammonium salts (“ionic liquids”)bearing an aldehyde functional group
The crystal structures of the new compounds and byproducts were determined by single crystal X-ray diffraction (Table 1)
For most of the different methods of alkylating 4-(dialkylamino)benzaldehydes (Figure 1a), including the use of trimethyloxonium tetrafluoroborate, the electrophilic attack occurs at the aniline N atom as expected, whereas Meerwein ethylation takes place at the carbonyl O atom, creating the quinoid iminium ion 1 in high yield and purity

Summary

Introduction

The need arose for low-melting quaternary ammonium salts (“ionic liquids”). Our obvious choice was 4-(dimethylamino)benzaldehyde (Ehrlich’s aldehyde) as inexpensive starting material which was to be N-alkylated. Common alkylating reagents such as dialkyl sulfates or methyl iodide readily gave the desired products [1,2]. A surprising selectivity was observed when trialkyloxonium tetrafluoroborates (Meerwein reagents) were employed. Methylation occurred at the N atom as expected, whereas ethylation took place at the Crystals 2013, 3. The crystal structures of the new compounds and byproducts were determined by single crystal X-ray diffraction (Table 1)

Results and Discussion

Experimental Section

Conclusions