Highly Active Ethene Polymerization Catalysts with Unusual Imine Ligands - Late Transition Metal Polymerization Catalysis - page 64

The N-aminopyrroles 9 - 16 were obtained from these amides in 80-95% yield
by alkaline hydrolysis (1-36 h) with a KOH saturated refluxing ethylene glycol so-
lution. This convenient but rather drastic deprotection protocol shows that N-ami-
nopyrroles are quite stable compounds. The high stability of aminopyrroles 9 - 16
presumably results from the fact that both -positions are blocked in these 2,5-di-

substituted pyrroles. The parent member of this family of compounds, unsubsti-
tuted N-aminopyrrole, is the only commercially available N-amino-azole. Ironi-
cally, it is also the least stable compound in the series.
In summary, N-amino-2,5-disubstituted pyrroles with a range of substituents
can be conveniently prepared on a large scale. The only limitation of the method
is the commercial availability of the starting 1,4-diketones. They in turn are most
easily prepared from the Stetter condensation.

N-Amino-indoles and -carbazoles
To further extend the substitution pattern of these building blocks, benzannelated
N-amino-N-heterocycles were prepared: Electrophilic amination of indole, 2-
methylindole, or carbazole using hydroxylamine- O -sulfonic acid [25] yielded the
corresponding N-amino-indole 17 , -2-methylindole 18 , and -carbazole 19 , respec-
tively. However, attempting the same reaction with 2,7-dimethylindole or dibenza-
zepine (iminostilbene) failed, most likely due to steric hindrance.
Thus eleven N-amino-N-heterocycles were synthesized. Together with the com-
mercially available unsubstituted N-amino-pyrrole, twelve different N-amino-azoles
became available as ligand precursor. Their properties - with regard of their in-
tended use as steering groups in the ligand backbone of the olefin polymerization
catalysts - differ in terms of steric bulk of the substituents, symmetry, and elec-
tronic properties, as can be seen from inspecting Chart 3.1.

3.2.1.2 1,2-Diimine Ligands with N-Hetaryl Substituents
Two methods of imine formation were employed (Scheme 3.3) to condense the N-
hetaryl building blocks with 1,2-diketones.
Either a standard procedure in methanol with formic acid as catalyst or a tri-
methylaluminum promoted [26] condensation was found effective. The latter
method is an extremely efficient protocol that effects the imination through se-
quential hemiaminal and aminal formation followed by elimination of tetra-
methylaluminoxane. This method is of great value for this chemistry because
sterically or electronically unfavorable dicarbonyl substrates may be converted
with ease into the corresponding 1,2-diimine ligands in good yields (approxi-
mately 70%) and in a short reaction period (a few minutes to hours, depending
on the steric bulk of the starting dione and/or amine component). As noted in
the introduction, steric control or axial shielding is one of the most important as-
pects in the ligand design of these 1,2-diimine catalysts, therefore a reliable gener-
al ligand synthesis route is crucial for optimizing the performance of the catalyst
constitution.