posted on 2017-09-13, 00:00authored byBasil
M. Ahmed, Nicholas A. Rudell, Ixtlazihuatl Soto, Gellert Mezei
The reaction of amines with aldehydes
and ketones has been exploited
for over 150 years to produce Schiff bases, one of the most popular
classes of compounds in both organic and coordination chemistry. In
certain cases, however, compounds other than Schiff bases have been
reported to result from such reactions. After conducting a representative
reaction under various different conditions and identifying several
reaction intermediates by NMR spectroscopy, mass spectrometry and
X-ray crystallography, we now report a unified picture that explains
the scattered and often inconsistent results obtained with 3(5)-aminopyrazole
derivatives and other related molecules. Acid catalysis, which is
often employed in Schiff base synthesis, radically changes the course
of reaction and leads to bis(pyrazol-4-yl)methane derivatives instead
of the expected Schiff base products. The stoichiometry of the reaction
is also found to be crucial for obtaining quantitative conversions.
A total of 31 new compounds have been isolated and characterized as
a result of this study, including a reaction intermediate, 2 Schiff
bases, and 28 bis- or tris(pyrazol-3(4)-yl)methane ligands. The latter
defines a new class of “non-scorpionate” ligands, with
three nonchelating pyrazole moieties, as opposed to the well-known
“scorpionate” tris(pyrazol-1-yl)-borate and -methane
ligands.