Dimethyl(methylene)ammonium Salts: Mannich Reagents

Introduction

These reagents comprise a family of preformed iminium salts which have been extensively utilized in condensation reactions with carbonyl and aromatic compounds in a variant of the Mannich reaction. Böhme prepared the first dimethyl(methylene)ammonium chloride (1), [1] and the ­iodide salt 2 was later developed by Eschenmoser. [2] Preformed iminium salts are sufficiently soluble in many aprotic solvents, enabling the use of highly reactive nucleophiles which ordinarily would decompose under the protic conditions of the classical Mannich reaction. A comparison study of these salts favors the trifluoroacetate salt (3) because it is the most soluble and can be transferred by syringe, although it is more tedious to prepare. [3]

Salt 1 is prepared by cleavage of N,N,N¢,N¢-tetramethylmethanediamine by AcCl [4] or cleavage of methyl dimethylaminomethyl ether by TMSCl. [5] Salt 2 is prepared by thermolysis of (iodomethyl)trimethylammonium iodide² or cleavage of N,N,N¢,N¢-tetramethylmethanediamine by TMSI. [6] Salt 3 is prepared by Polonovski reaction of tri­methylamine oxide with TFAA or cleavage of N,N,N¢,N¢-tetramethylmethanediamine with TFA. [7]

References

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