Introduction
<P>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]
</P><P>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 iodide
2 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]
</P>
