262 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 262^263y
Preparation and Characterization of
Tetraphenylborate Salts of 2-Aminopyrrole and
1-Alkyl-2-aminopyrrolesy
Michael De Rosa,*a Ian Sellitto,a Roy P. Issac,a John Ralpha and
Mark D. Timken*b
a The Pennsylvania State University, Delaware County, Department of Chemistry,
25 Yearsley Mill Road, Media, PA 19063, USA
b Widener University, Science Division, 1 University Place, Chester, PA 19013, USA
Tetraphenylborate salts of 2-aminopyrrole and 1-alkyl-2-aminopyrroles are prepared and found to be stable.
Recently we reported the ¢rst successful synthesis of
2-aminopyrrole and several 1-substituted-2-aminopyrroles
in which the pyrrole ring did not contain additional elec-
tron-withdrawing groups.1 In the absence of these elec-
tron-withdrawing groups the newly synthesized 2-amino
derivatives were stable only in solution and were not isolable.2
It is of interest, for mechanistic and synthetic studies, to have
a readily available source of these 2-aminopyrroles. In this
paper we describe the isolation of stable tetraphenylborate
salts of the 2-aminopyrroles and the subsequent regeneration
of the parent amino derivatives in several solvents.
During the course of previous work it was noted that
2-aminopyrroles appeared to be much more stable in glacial
acetic acid than in chloroform.1 NMR studies indicated that
in acetic acid the 2-aminopyrrole was present as the conju-
gate acid, and this appeared to indicate that the conjugate
acid 3 was more stable than the 2-aminopyrrole 4. Previous
workers have isolated tri£uoroacetate3 and perchlorate4 salts
of highly substituted 2-aminopyrroles. With these facts in
mind, we sought to isolate the 2-aminopyrroles as their more
stable protonated salts. One method of isolating a cation is to
precipitate it as a tetraphenylborate salt.5 This method is par-
ticularly appropriate for precipitating monocationic
protonated amines and tertiary ammonium ions, as their
tetraphenylborate salts are often very insoluble in water.
Recently this method was also used to obtain crystalline
N-acylammonium salts.6
of the 2-aminopyrrole and phthalide.1 Addition of an
aqueous solution of sodium tetraphenylborate gave an
instantaneous precipitate of the tetraphenylborate salt.
The solid was ®ltered off, dried under vacuum and then
triturated with small amounts of cold CHCl3 to remove
neutral impurities. Tetraphenylborate salts 3a^d were
isolated in this manner and characterized by 1H and
13C NMR spectroscopy (Table 1) and positive-ion FAB mass
spectroscopy. 1H and 13C NMR spectra clearly showed the
presence of the CH2 group expected for the salt 3. The
presence of a CH2 group was con¢rmed by DEPT spectra.
These results ruled out protonation of the exo amino group.
The CH2 group, in some of the 1H NMR spectra (Table
1), did not have the expected couplings to C3H and C4H.
Singlets or broad triplets were observed. This is attributed
to proton exchange at the CH2 that causes partial or com-
plete collapse of the expected multiplet.1 In the case of
the 1-phenyl derivative 3e a black deliquescent material
was obtained that could not be further characterized. Salts
3a^d are stable enough to store for reasonably long periods
of time. After 10 months at 0 8C, washing the salt with
a small amount of CHCl3 yielded material with the same
NMR spectral properties and decomposition point as the
originally isolated product.9
The 2-aminopyrrole can be regenerated by dissolving the
salt in DMSO or acetonitrile and adding one or more equiv-
alents of triethylamine. Upon addition of the amine the
1
CH2 signal of the salt disappeared in the H NMR spectrum
and three multiplets1 diagnostic for a 2-substituted pyrrole
appeared. Addition of water to the DMSO solution following
O
O
C
NaBH4
PriOH
N
N
H
N
R
N
R
1
the addition of triethylamine, gave a precipitate the H NMR
spectrum of which was consistent with triethylammonium
tetraphenylborate.
O
HOCH2
R
1a H
2
The results of this work demonstrate that simple
2-aminopyrroles can be isolated and stored as their respective
tetraphenylborate salts and that they are readily converted
back to the amino form. Tetraphenylborate salts can therefore
serve as convenient sources of unstable 2-aminopyrrole
derivatives, eliminating the need to isolate the
2-aminopyrrole from acetic acid each time that a sample
is needed.
1. AcOH, 80 ˚C
2. NaBPh4
1b Me
1c Et
1d But
1e Ph
H
H
Et3N, MeCN
+
NH2
NH2
N
N
–
R
R
BPh4
3a–d
4a–d
Scheme 1
Experimental
General Procedure for Preparation of Tetraphenlborate Salts.öThe
Partial reduction7 of an N-(1-substituent-1H-pyrrol-2-yl)-
phthalimide8 1 with NaBH4 gave the o-hydroxymethyl benz-
amide 2 used to generate the 2-aminopyrrole (Scheme 1).
Heating the o-hydroxymethylbenzamide in glacial acetic acid
for 2 h at 80 8C under nitrogen gave the conjugate acid
o-hydroxymethyl benzamide 2 was dissolved in glacial acetic acid
1
(1mmol ml
) and heated at 80 8C for 2 h under a nitrogen
atmosphere.1 A concentrated aqueous solution of sodium tetra-
phenylborate (4 equiv.) was added to the reaction mixture and the
resulting precipitate was ®ltered and then dried in a vacuum
desiccator overnight. The salt was washed with cold chloroform
and characterized. Prior to any subsequent use the salt was washed
again with a small volume of cold chloroform. Material puri®ed in
this way was essentially pure (NMR). Salts can also be by puri®ed by
recrystallization from methanol, though with a large loss of material.
Melting points are of recrystallized materials and yields, of salts that
* To receive any correspondence.
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).