Formation of vinylic dipyrroles by the deprotonation of meso-alkyl and meso-benzyl dipyrrin HCl salts

Full text of DOI:10.1021/jo900064y

enhanced stability,14 cf. their meso-H counterparts,1 and free-
bases of meso-aryl dipyrrins may be isolated and stored in air.1
As part of ongoing investigations to identify the scope and
limitations of dipyrrinato ligands we herein report the synthesis
and reactivity of meso-alkyl and meso-benzyl dipyrrins, classes
of compounds that until now have been known largely as their
BODIPY complexes.15-20
Formation of Vinylic Dipyrroles by the
Deprotonation of meso-Alkyl and meso-Benzyl
Dipyrrin HCl Salts
Adeeb Al-Sheikh Ali, Judy Cipot-Wechsler,
T. Stanley Cameron, and Alison Thompson*
Department of Chemistry, Dalhousie UniVersity, Halifax,
NoVa Scotia, B3H 4J3, Canada
alison.thompson@dal.ca
FIGURE 1. Dipyrrin skeleton.
ReceiVed January 12, 2009
SCHEME 1. Deprotonation of meso-Methyl Dipyrrin 2
Free-base dipyrrins are typically prepared by treating the
corresponding hydrobromide or hydrochloride salts with bases such
as LiH or ammonium hydroxide.21 In an attempt to obtain a meso-
alkyl dipyrrin free-base, 2HCl was reacted with LiH. In this
instance, the use of LiH did not provide access to the desired free-
base, with an intractable mixture resulting. The slow addition of
1.1 equiv of nBuLi to an anhydrous THF solution of 2HCl under
an inert atmosphere afforded a product that was purified via
extraction (Scheme 1). Surprisingly, the 1H NMR spectrum of the
isolated compound dissolved in CD3OD showed that the meso-
methyl signal, previously at δ 2.75 ppm for the 2HCl salt, was no
longer present and instead a signal at δ 5.10 ppm had appeared:
such a chemical shift is more in line with an sp2 hybridized carbon
atom, rather than an sp3 hybridized carbon atom.
Further NMR data (DEPTQ-135) collected on the isolated
material revealed only three types of methyl group to be present
whereas four would be expected for the free-base 2. Moreover,
the absence of signals for the fourth methyl group in the aliphatic
region was accompanied by the presence of a vinylic resonance
at δ 107.5 ppm, correlating with the signal at δ 5.10 ppm in
the 1H NMR spectrum. Together, these data are consistent with
a formulation such as 3, in which deprotonation at the meso-
methyl group had occurred. Moreover, 15N NMR spectroscopic
analysis revealed a signal at δ -233.5 ppm after treatment with
BuLi, compared to δ -213.7 ppm for 2HCl: δ -233.5 ppm is
consistent with typical 15N chemical shifts for pyrrolic com-
pounds.22 A crystal of the isolated compound suitable for X-ray
Under basic conditions, dipyrrin salts bearing alkyl and benzyl
groups at the meso-position undergo deprotonation to give
vinylic dipyrroles, rather than the corresponding free-base
dipyrrins. The deprotonation is reversible and quantitatively
returns the dipyrrinato framework under acidic conditions.
Dipyrrins (dipyrromethenes, 1)1 are fully conjugated, planar,
12-π-electron molecules that formally consist of a pyrrole
attached to an azafulvene (Figure 1). Conversion into monoan-
ionic species upon deprotonation of the pyrrolic N-H gives
dipyrrinato units that can serve as bidentate ligands for the
synthesis of supramolecular assemblies,2,3 helicates,4-7 metal-
organic frameworks,8 boron difluoride (BODIPY) dyes,9,10 and
other complexes.11-13
For convenience, the 5-position is termed the meso-position.
Dipyrrins bearing aryl substituents at the meso-position display
(1) Wood, T. E.; Thompson, A. Chem. ReV. 2007, 107, 1831–1861.
(2) Salazar-Mendoza, D.; Baudron, S. A.; Hosseini, M. W. Chem. Commun.
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(4) Al-Sheikh Ali, A.; Benson, R. E.; Blumentrit, S.; Cameron, T. S.; Linden,
A.; Wolstenholme, D.; Thompson, A. J. Org. Chem. 2007, 72, 4947–4952.
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(18) Goze, C.; Ulrich, G.; Ziessel, R. J. Org. Chem. 2007, 72, 313–322.
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2866 J. Org. Chem. 2009, 74, 2866–2869
10.1021/jo900064y CCC: $40.75 2009 American Chemical Society
Published on Web 03/02/2009

TABLE 1. pH and pKa for the Titration of 2HCl with NaOD in
CD3OD
ratio of
2HCl:3 triplet
integrals below
δ 1.25 ppm
equiv of
NaOD
spectrum
pH
pKa
a
b
c
d
e
f
0.00
0.18
0.36
0.54
0.72
0.90
1.26
1.44
1:0
5.5
6.6
6.8
7.0
7.2
7.4
7.8
10.4
1:0.41
1:0.48
1:0.72
1:1.24
1:1.96
1:4.37
0:1
7.22
7.11
7.14
7.11
7.11
7.16
FIGURE 2. ORTEP representation of dipyrrole 3, shown with prob-
ability ellipsoids of 50%.
g
h
FIGURE 3. 1H NMR titration of 2HCl with NaOD in CD3OD. Spectra
b-h were recorded upon the addition of aliquots containing NaOD, as
per Table 1.
FIGURE 4. 1H NMR titration of 3 with DCl in CD3OD. Spectra b-g
were recorded upon subsequent additions of aliquots each containing
0.18 equiv of DCl.
diffraction analysis was grown via the evaporation of solvent
from a pentane solution, and the structure was thus confirmed
to be that of 3, a vinylic dipyrrole (Figure 2).
The X-ray structure of 3 shows the loss of the planarity
of the molecule (cf. a dipyrrin) where the two nitrogen atoms
adopt an almost anti conformation to presumably minimize
the polarity of the molecule and steric interactions. The bond
angles of C4-C5-C6 117.6(2)°, C4-C5-C14 120.8(2)°,
and C6-C5-C14 121.6°(2) support the formation of a
trigonal planar-sp2 carbon atom at the meso-position. Im-
portantly, the C5-C14 bond was found to be 1.338(3) Å,
the shortest C-C bond within the molecule, again supporting
the formation of a double bond between C5 and C14.
In response to these interesting observations, dipyrrin salt
2HCl was titrated with a solution of deuterated sodium
hydroxide NaOD (in CD3OD) (Figure 3). As expected, the
deprotonation could be monitored by the disappearance of the
meso-methyl resonance at δ 2.75 ppm in the 1H NMR spectrum.
FIGURE 5. Absorption spectra of 2HCl titrated with 0.01 M NaOH.
5) showed the disappearance of the band at 496 nm, corre-
sponding to the fully conjugated dipyrrinato unit, concomitant
with the appearance of a new band at 295 nm, corresponding
to the cross-conjugated dipyrrolic ethene 3. Furthermore, back-
titration of the solution of 3 with 0.01 M HCl (Figure S1,
Supporting Information) resulted in total recovery of the dipyrrin
salt 2HCl, also supporting the reversibility of the deprotonation.
Michael addition to the meso-position of dipyrrins gives meso-
substituted dipyrromethanes;23-27 however, the chemistry of
meso-alkyl dipyrrins is under investigated. A meso-methyl
BODIPY complex underwent hydroxide addition at the meso-
position, followed by the elimination of water to give a vinylic
dipyrrole BF2- potassium salt.28 Vinylic dipyrroles were isolated
as minor products upon reacting 2-unsubstituted pyrroles with
1-acyl-2-bromoacetylenes.29,30 Furthermore, a 2,2′-dicarboxylate
analogue of 3 was isolated in quantitative yield from the reaction
1
Additionally, the H NMR spectra showed a change in the
chemical shift of all other alkyl substituents. Taking advantage
of these changes, the ratio of the product dipyrrole to the reactant
dipyrrin was calculated based on the integration of the triplet
methyl groups below δ 1.25 ppm. Determining the pH of the
reaction mixture during the titration gave enough data to
calculate the apparent acidity constant, pKa, of 2HCl by using
the Henderson-HasselBalch equation (Table 1). After the
addition of each aliquot of NaOD solution, the pH was measured
using a stainless steel probe inserted into the NMR tube.
To investigate the reversibility of the process, a solution
containing 3 was back-titrated with a solution of DCl in CD3OD
(Figure 4). The 1H NMR spectra for the acidic titration reveals
the reversibility of the deprotonation, and the complete recovery
of the dipyrrin 2HCl.
(23) Bamfield, P.; Johnson, A. W.; Leng, J. J. Chem. Soc. 1965, 7001–7005.
(24) Brunings, K. J.; Corwin, A. H. J. Am. Chem. Soc. 1942, 64, 593–600.
(25) Jain, A. C.; Kenner, G. W. J. Chem. Soc. 1959, 185–189.
(26) Oddo, B.; Cambieri, F. Gazz. Chim. Ital. 1940, 70, 559–566.
(27) Treibs, A.; Zimmer-Galler, R. Liebigs Ann. Chem. 1963, 664, 140–
145.
The deprotonation of the meso-methyl dipyrrin salt 2HCl was
also studied using absorption spectroscopic analysis: a solution
of 2HCl in methanol was titrated with 0.01 M aliquots of NaOH.
During the course of the titration, the absorption spectra (Figure
(28) Treibs, A.; Kreuzer, F. H. Liebigs Ann. Chem. 1968, 718, 208–223.
J. Org. Chem. Vol. 74, No. 7, 2009 2867

SCHEME 2. Synthesis of meso-Benzyl and meso-Isopropyl
Dipyrrins salts 4HCl and 6HCl
SCHEME 4. Synthesis of meso-Methyl Dipyrrinato Zinc
Complex 8
SCHEME 3. Deprotonation of meso-Benzyl and
meso-Isopropyl Dipyrrins Salts 4HCl and 6HCl
with the increasing absorbance band at 370 nm. This transfor-
mation corresponds to the loss of the dipyrrin to give the
dipyrrolic styrene cross-conjugated system. The back-titration
of the dipyrrole 5 with dilute HCl (0.01 M) did not recover
4HCl, in contrast to the meso-methyl case whereby 2HCl was
recovered easily. However, a concentrated solution of HCl
1
converted 5 to its salt 4HCl as confirmed by using both H
of ethyl 2,3-dimethyl-4-pyrrolecarboxylate with acetyl chlo-
ride.31 The latter report related the tautomerization of meso-
methyl dipyrrin free-bases to the corresponding vinylic dipyr-
roles, but did not directly acknowledge that the deprotonation
of dipyrrin HX salts would directly give the vinylic dipyrrole
if the pKa of a meso-methyl proton were lower than that of the
azafulvenium N-H atom. Two related reports32,33 describe and
characterize a series of 1,1′-dicarboxylate analogues of 3, the
first of which was isolated as a minor product upon the reaction
of benzyl 3,4-dimethylpyrrole-2-carboxylate with acetic anhy-
dride and tin(IV) chloride. Subsequently, good yields of a vinylic
dipyrrole were obtained upon treatment of a 5-(chloromethyl)
dipyrromethane with base. The vinyl group of such dipyrroles
was found to undergo electrophilic attack and hydrogenation,
and to be susceptible to photo-oxidative cleavage in the case
of a 3,3′-unsubstituted derivative.
NMR and absorption spectroscopy (Figures S4 and S6, Sup-
porting Information). This result is in contrast to the case where
only 13.5 equiv of 0.01 M HCl were needed to protonate 3 to
produce the meso-methyl dipyrrin 2HCl. The requirement to
use concentrated HCl indicates the enhanced stability of 5, cf.
3, and this may be attributed to the formation of the cross-
conjugated dipyrrolic styrene.
A similar set of titration experiments were conducted with
the meso-isopropyl substituted dipyrrin salt 6HCl (Scheme 3).
A methanolic solution of 6HCl was thus titrated with 0.01 M
NaOH and the deprotonation was monitored with absorption
spectroscopy (Figure S4, Supporting Information). The forma-
tion of the dipyrrole 7 was confirmed through the decreasing
absorbance of the band at 503 nm and the increasing absorbance
of the band at 268 nm. 1H NMR spectroscopic analysis
confirmed the deprotonation of dipyrrin 6HCl to dipyrrole 7
through monitoring the disappearance of the doublet at δ 1.37
ppm (corresponding to the secondary isopropyl proton) and the
appearance of a new signal at δ 1.06 ppm (corresponding to
the allylic methyl protons). Additionally, 15N NMR spectro-
scopic analysis showed a shift in the chemical shift of 6HCl
from δ -211.0 to -231.0 ppm for the corresponding dipyrrole
7.22 Back-titration returned the salt (Figure S5, Supporting
Information).
As a result of the interesting acid/base behavior of these meso-
substituted dipyrrins, we sought to investigate the ability of
meso-alkyl dipyrrins to form metal complexes with Zn(OAc)2
in the presence of NaOAc; this general procedure usually gives
excellent yields of Zn(II) complexes (Scheme 4).22 Surprisingly,
the homoleptic Zn(II) dipyrrinato complex of 2 was not isolated
after treatment under these standard conditions, although use
of lithium hydroxide in combination with Zn(OAc)2 led to the
isolation of the Zn(II) complex 8 in low yield (Scheme 4), cf.
high yields (83%) for the meso-H analogue.22 The low com-
plexation yield was attributed to the formation of 3 under the
basic conditions. In the absence of base with either ZnCl2 or
Zn(OAc)2, no reaction occurred. These experiments shed light
upon the potential mechanism of complexation for HBr and HCl
salts of dipyrrins, i.e. initial (reversible) formation of the dipyrrin
free-base is essential, and then complexation involves a series
of equilibria. If the free-base is removed by way of dipyrrole
formation, then complexation yields for the dipyrrinato ligand
will be low although tautomerization to the free-base is
obviously under equilibrium. It is curious that treatment of meso-
methyl dipyrrinato BF2 complexes with Grignard or organo-
lithium reagents does not effect deprotonation at the meso-alkyl
Given the interesting properties of meso-methyl dipyrrin salt
2HCl, we explored the deprotonation of the meso-benzyl and
the meso-isopropyl dipyrrin salts 4HCl and 6HCl, respectively.
These were prepared by using the corresponding acetyl chloride
and 2 equiv of 3-ethyl-2,4-dimethylpyrrole, according to Scheme
2.
Deprotonation of 4HCl upon treatment with base occurred
at the meso-substituent to give the dipyrrole 5 in excellent yield
(Scheme 3). The deprotonation was monitored through the
disappearance of the signal at δ 4.69 ppm (corresponding to
the benzylic CH2 protons of 4HCl) and the appearance of a
new signal at δ 6.40 ppm (corresponding to the vinylic proton
of 5, Figure S6 in the Supporting Information). Moreover, 15
N
NMR spectroscopic analysis showed a shift from δ -212.0 for
4HCl to δ -232.5 ppm for dipyrrole 5, again consistent with
chemical shifts for pyrrolic, rather than dipyrrinato, com-
pounds.22
Titrating a solution of the meso-benzyl dipyrrin salt 4HCl in
methanol with 0.01 M NaOH also resulted in the production of
dipyrrole 5, as monitored using absorption spectroscopy (Figure
S2, Supporting Information). The deprotonation was confirmed
through the decreasing absorbance band at 506 nm, concomitant
(29) Stepanova, Z. V.; Sobenina, L. N.; Mikhaleva, A. I.; Ushakov, I. A.;
Chipanina, N. N.; Elokhina, V. N.; Voronov, V. K.; Trofimov, B. A. Synthesis
2004, 2736–2742.
(30) Trofimov, B. A.; Sobenina, L. N.; Demenev, A. P.; Mikhaleva, A. I.
Chem. ReV. 2004, 104, 2481–2506.
(31) Treibs, A.; Reitsam, F. Liebigs Ann. Chem. 1958, 611, 194–205.
(32) Xie, H.; Lee, D. A.; Senge, M. O.; Smith, K. M. J. Chem. Soc., Chem.
Commun. 1994, 791–792.
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Org. Chem. 1996, 61, 8508–8517.
2868 J. Org. Chem. Vol. 74, No. 7, 2009

substituent, and substitution at boron occurs instead:10,16-18,34
presumably the nature of the N-B bonds within a dipyrrinato
BF2 complex (formally monoanionic dipyrrinato ligand) is much
more favored than that in the corresponding vinylic dipyrrole
(dianionic), and thus the meso-methyl protons are not particu-
larly acidic once complexation to boron has occurred.
To summarize, the meso-methyl dipyrrin 2HCl, the meso-
isopropyl 6HCl, and the meso-benzyl dipyrrin 4HCl exhibit
deprotonation at the meso-substituent when treated with base.
The resulting dipyrridyl ethenes were fully characterized,
including crystallographic data for 3. The reaction of the dipyrrin
2HCl with Zn(OAc)2 gave an unexpected low yield for Zn(II)
complexation involving dipyrrinato ligands, presumably as a
consequence of the formation of the dipyrrole under the reaction
conditions. Caution should be taken when working with meso-
alkyl and meso-benzyl dipyrrins under basic conditions such
that the dipyrrinato skeleton is maintained, as needed for efficient
and high-yielding complexation. This study demonstrates that
meso-alkyl and meso-benzyl dipyrrins have very different
acid-base properties compared to the meso-H and meso-aryl
analogues, and that very different reactivity profiles are exhibited.
151.8, 148.3, 139.4, 139.1, 134.7, 132.9, 130.8, 130.0, 129.1, 128.0,
18.4, 14.9, 12.5, 12.4; δN (50.7 MHz, CDCl3) -212.0; m/z (ESI+)
347.2456 (MH - HCl)+, calcd C24H31N2 347.2487.
2,2′-Di-K-(3-ethyl-2,4-dimethyl)pyrrolylstyrene (5). Sodium
hydroxide (1 M, 25 mL) was added to a magnetically stirring
solution of 4HCl (55.5 mg, 0.145 mmol) in CH2Cl2 (25 mL), and
the solution color changed from dark red to yellow-brown. After 5
min, the CH2Cl2 solution was separated and dried over Na2SO4.
The solvent was removed in Vacuo to yield the title compound (48
mg, 96%). λmax/nm 370 (ε 23 600); δH (500 MHz, CD3OD)
6.90-7.03 (5H, m), 6.35 (1H, s), 2.31 (4H, q, J ) 7.5), 2.10 (3H,
s), 2.01 (3H, s), 1.65 (3H, s), 1.50 (3H, s), 0.98 (3H, t, J ) 7.50),
0.97 (3H, t, J ) 7.50); δC (125 MHz, CD3OD) 140.8, 129.6, 128.9,
128.9, 128.0, 126.2, 125.1, 124.9, 124.6, 123.3, 123.2, 122.3, 118.5,
117.1, 18.8, 18.6, 16.6, 16.4, 11.1, 11.0, 10.2, 10.1; δN (50.7 MHz,
CD3OD) δ -232.5; m/z (ESI+) 369.2274 (M+Na)+, calcd
C24H30N2Na 369.2307.
K2-(4,4′-Diethyl-3,3′,5,5′-tetramethyl-meso-isopropyldipyr-
rin) Hydrochloride (6HCl). Isobutyryl chloride (3.68 g, 35 mmol)
was added to a solution of 3-ethyl-2,4-dimethylpyrrole (1.85 g, 15
mmol) in CH2Cl2 (15 mL), and the mixture was heated at reflux
temperature for 1 h. The pink mixture was extracted with water (2
× 30 mL), and the organic solution was dried over Na2SO4.
Removal of the organic solvent in Vacuo gave the crude product
that was triturated with petroleum ether to give the title compound
as a dark orange solid (1.97 g, 71%). λmax/nm 503 (ε 40 000); δH
(500 MHz, CDCl3) 11.9 (2H, s) 4.04 (1H, m, J ) 7.0), 2.51 (6H,
s), 2.34 (4H, q, J ) 7.5), 1.79 (6H, s), 1.37 (6H, d, J ) 7.0), 0.99
(6H, t, J ) 7.5); δC (125 MHz, CD3OD) 158.3, 149.6, 138.5, 132.4,
131.7, 35.8, 22.7, 18.1, 15.1, 13.1, 12.1; δN (50.7 MHz, CDCl3)
-211.0; m/z (ESI+) 299.2453 (MH - HCl)+, calcd C20H31N2
299.2487.
1,1-Di-K-(3-ethyl-2,4-dimethyl)pyrrolyl-2-methylpropene (7).
Sodium hydroxide (1 M, 25 mL) was added to a magnetically
stirring solution of 6HCl (50 mg, 0.145 mmol) in CH2Cl2 (25 mL).
Upon the addition, the color of the solution changed from dark red
to yellow-brown. After 5 min, the CH2Cl2 fraction was isolated
and then dried over Na2SO4. The solvent was removed in Vacuo to
yield the title compound (41 mg, 92%). λmax/nm 268 (ε 14 000); δH
(500 MHz, CD3OD) 2.31 (4H, q, J ) 7.5), 2.05 (3H, s), 1.91 (3H,
s), 1.73 (6H, s), 1.66 (6H, s), 0.97 (3H, t, J ) 7.5), 0.96 (3H, t, J
) 7.5); δC (125 MHz, CD3OD) 131.9, 128.2, 122.8, 120.8, 120.7,
115.9, 22.8, 18.8, 18.6, 16.5, 16.4, 11.2, 11.0, 10.7, 10.5; δN (50.7
MHz, CD3OD) δ -231.0; m/z (ESI+) 299.2469, calcd C20H30N2
298.2409.
Experimental Section
1,1′-Di-K-(3-ethyl-2,4-dimethyl)pyrrolylethene (3). Within a
glovebox, nBuLi (0.292 mL of a 1.6 M hexanes solution, 0.47
mmol) was added dropwise to a magnetically stirring solution of
2HCl19 (130 mg, 0.424 mmol) in THF (3 mL). Upon the addition,
the color of the solution changed from dark red to yellow-brown.
After 1 h, the solvent was removed in Vacuo and the product was
extracted with pentane. The resulting dark yellow solution was
filtered through Celite to remove any insoluble material. The solvent
was removed from the filtrate in Vacuo to yield the title compound
(89 mg, 78%). λmax/nm 295 (ε 15 000); δH (500 MHz, CD3OD) 5.10
(2H, s), 2.44 (4H, q, J ) 7.50), 2.15 (6H, s), 1.79 (6H, s), 1.14
(6H, t, J ) 7.50); δC (125 MHz, CD3OD) 133.0, 125.9, 122.2, 122.1,
116.7, 107.5, 17.8, 15.8, 10.5, 10.3; δN (50.7 MHz, CD3OD) δ
-233.5; m/z (ESI+) 271.2122 (MH+), calcd C18H26N2 270.2096.
Crystals suitable for X-ray diffraction analysis were grown via the
evaporation of a pentane solution of 3 under reduced pressure.
K2-(4,4′-Diethyl-3,3′,5,5′-tetramethyl-meso-benzyldipyrrin)
Hydrochloride (4HCl). Phenylacetyl chloride (5.41 g, 33 mmol)
was added to a solution of 3-ethyl-2,4-dimethylpyrrole (1.85 g, 15
mmol) in CH2Cl2 (15 mL), and the mixture was heated at reflux
temperature for 1 h. The pink mixture was extracted with water (2
× 30 mL), and the organic fraction was dried over Na2SO4.
Removal of the organic solvent in Vacuo gave the crude product
that was triturated with petroleum ether to give the title compound
as a dark orange solid (2.10 g, 37%). λmax/nm 506 (ε 75 000); δH
(500 MHz, CDCl3) 12.2 (2H, s), 7.20 (2H, d, J ) 7.5), 7.11 (2H,
t, J ) 7.5), 7.04 (1H, t, J ) 7.5), 2.39 (6H, s), 2.33 (4H, q, J )
7.5), 2.00 (6H, s), 1.02 (6H, t, J ) 7.5); δC (125 MHz, CD3OD)
Acknowledgement. This work was supported by the Natural
Sciences and Engineering Research Council of Canada (NSERC).
The authors thank a reviewer for bringing ref 32 to their
attention.
Supporting Information Available: Experimental proce-
dures, spectra, and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(34) Ziessel, R.; Goze, C.; Ulrich, G. Synthesis 2007, 936–949.
JO900064Y
J. Org. Chem. Vol. 74, No. 7, 2009 2869