Eschenmoser's salt explained

In organic chemistry, Eschenmoser's salt (named for Albert Eschenmoser) is the ionic, organic compound . It is the iodide salt of the dimethylaminomethylene cation .

The dimethylaminomethylene cation is a strong dimethylaminomethylating agent, used to prepare derivatives of the type .^{[1] [2]} Enolates, silyl enol ethers, and even more acidic ketones undergo efficient dimethylaminomethylation. Once prepared, such tertiary amines can be further methylated and then subjected to base-induced elimination to afford methylidenated ketones. The salt was first prepared by the group of Albert Eschenmoser^[3] after whom the reagent is named.

Structure and bonding

Dimethylaminomethylene cation is described as a resonance hybrid of the carbocation and an iminium cation:

(CH3)2N-CH2+ <=> (CH3)2N+=CH2The atoms are coplanar. The cation is isoelectronic with isobutene.

Preparation

Pyrolysis of iodomethyltrimethylammonium iodide affords the desired salt:^[3]

[(CH3)3N-CH2I]I -> [(CH3)2NCH2]I + CH3IAn alternative route starts with bis(dimethylamino)methane:

[(CH3)2N]2CH2 + (CH3)3SiI -> [(CH3)2NCH2]I + (CH3)3SiN(CH3)2

Related salts

Other salts of the dimethylaminomethylene cation:

• Dimethyl(methylidene)ammonium trifluoroacetate.^[4]
• Dimethyl(methylidene)ammonium chloride (Böhme's salt, after Horst Böhme)

See also

• Vilsmeier reagent, .

Notes and References

1. E. F. Kleinman in "Dimethylmethyleneammonium Iodide and Chloride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.
2. Horst Böhme . Eberhard Mundlos . Otto-Erich Herboth . Über Darstellung und Eigenschaften α-Halogenierter Amine. Chemische Berichte. 90. 9. 1957. 2003–2008. 10.1002/cber.19570900942.
3. Jakob Schreiber . Hans Maag . Naoto Hashimoto . Albert Eschenmoser . Dimethyl(methylene)ammonium Iodide. Angewandte Chemie International Edition in English. 10. 5. 1971. 330–331. 10.1002/anie.197103301.
4. 10.15227/orgsyn.059.0153. Regioselective Mannich Condensation with Dimethyl(Methylene)Ammonium Trifluoroacetate: 1-(Dimethylamino)-4-Methyl-3-Pentanone. Organic Syntheses. 1979. 59. 153. Michel. Gaudry. Yves. Jasor. Trung Bui. Khac.