Chemistry Reference
In-Depth Information
Scheme 2.2
C4
0
carbon, the dimethylamino group then transferred the proton to the
-carbon of
the developing amino acid. Thus this preference for increased chain length seems to
argue for such a dual catalytic role of the amine. As expected from this, compound
6
with yet an additional methylene group, which is no longer needed to permit the
a
a
-
carbon to be reached, is now less active.
Related but even more striking effects are seen with the imidazole catalysts (
8
and
9
).
This might simply reflect the rigidity in an imidazole ring, with a resulting entropy
advantage for these systems. However, imidazole is a considerably weaker base than
an alkyl amine, and weaker basicity should be an advantage in this catalytic process
since the base is not fully protonated at the operating pH. In addition, ethylenediamine
systems
13
-
15
are remarkably good catalysts. They improve the transamination rate by
100-200-fold. This is because the diamine is acting as a mono-protonated bifunctional
catalyst, with one amino group removing the proton from the C4
0
carbon while the
other puts a proton on the
carbon of the forming amino acid. Molecular models show
that this bifunctional mechanism is possible for
13
-
15
but not for
12
, which gives a
much slower rate.
a
