Microwave-assisted reduction of F-BODIPYs and dipyrrins to generate dipyrromethanes

Article abstract of DOI:10.1139/cjc-2013-0341

The reduction of BODIPYs and dipyrrins to dipyrromethanes, via a reaction involving ethylene glycol and sodium methoxide, is reported. When benzyl alcohol is used in place of ethylene glycol, the addition of 2,4-dinitrophenylhydrazine to the reaction mixture after microwave irradiation results in the production of 1-benzylidene-2-(2,4-dinitrophenyl)hydrazone, indicating concomitant production of aldehyde alongside the dipyrromethane.

Full text of DOI:10.1139/cjc-2013-0341

688
ARTICLE
Microwave-assisted reduction of F-BODIPYs and dipyrrins to
generate dipyrromethanes
Jennifer A. Melanson, Deborah A. Smithen, T. Stanley Cameron, and Alison Thompson
Abstract: The reduction of BODIPYs and dipyrrins to dipyrromethanes, via a reaction involving ethylene glycol and sodium
methoxide, is reported. When benzyl alcohol is used in place of ethylene glycol, the addition of 2,4-dinitrophenylhydrazine to
the reaction mixture after microwave irradiation results in the production of 1-benzylidene-2-(2,4-dinitrophenyl)hydrazone,
indicating concomitant production of aldehyde alongside the dipyrromethane.
Key words: F-BODIPY, dipyrrin reduction, dipyrromethane formation, microwave-assisted.
Résumé : On décrit la réduction de BODIPYs et de dipyrrines en dipyrrométhanes par une réaction faisant intervenir
de l’éthylène glycol et du méthanolate de sodium. Si l’on remplace l’éthylène glycol par l’alcool benzylique, l’ajout de 2,4-
dinitrophénylhydrazine au mélange réactionnel après irradiation par micro-ondes donne de la 1-benzylidène-2-(2,4-dinitrophényl)
hydrazone, ce qui indique la production concomitante d’un aldéhyde et du dipyrrométhane. [Traduit par la Rédaction]
Mots-clés : réduction des dipyrrines, F-BODIPY, formation de dipyrrométhane, assistée par mico-ondes.
this reaction to gain an understanding of its scope, limitations,
and REDOX partner.
Introduction
Dipyrrins present a useful ligand scaffold, with significant in-
terest being devoted to dipyrrinato complexes of transition met-
Generation of dipyrromethanes
To the best of our knowledge, there are no reports of dipyrro-
methanes being produced via the reduction of F-BODIPYs. However,
als and boron.1,2 There are limited reports regarding the chemical
manipulation of dipyrrins, and indeed, one strategy that is often
used to overcome the problems associated with the relative
there is reference to the removal of other metal ions from dipyr-
instability of free-base dipyrrins involves protection by means of
rinato complexes, followed by reduction of the resulting dipyrrin,
complexation, particularly with –BF2, thus giving rise to F-BODIPYs
using NaBH4.16 Indeed, dipyrrins may be reduced to dipyrrometh-
anes via hydrogenolysis (H2/Pd) or through the use of NaBH4,
although most reported examples feature a stabilizing group ei-
(4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes).3–5 F-BODIPYs have been
a popular research focus in recent times due to the many applica-
tions of these compounds courtesy of their stability and intense flu-
ther in the way of a meso-aryl substituent or an alpha carbonyl
orescence.6 For example, F-BODIPYs have been used as dyes to label
group.17–19 Related systems such as biliverdins20–23 are routinely
proteins7–10 and DNA.11–13 We herein report upon the reduction of
reduced to bilirubins upon reaction with NaBH4: similarly, tripyr-
F-BODIPYs and dipyrrins to the corresponding dipyrromethanes us-
renes may be reduced to tripyrranes,24 and conjugated tetrapyr-
ing an alcohol as the REDOX partner.
rolic systems may be reduced to the corresponding phlorins.25,26
There is one example of a dipyrrin reacting with BH3·SMe2 to form
a dipyrrinato–BH2 adduct that isomerizes to effectively deliver
Results and discussion
Preliminary observations
hydride to the meso-position, thus generating a –BH complex of
a dipyrromethane.27 Although shaking an organic solution of a
dipyrrin with an aqueous solution of sodium dithionite removes
the colour of the dipyrrin (and hence, dipyrromethane formation
is assumed), we report herein that the strategy does not give a
reasonable yield of the anticipated dipyrromethane. When 6 was
exposed to an aqueous/THF solution10 of sodium dithionite, the
respective dipyrromethane 5 was isolated in only 10% yield, along-
side significant decomposition products including an unstable
tetrapyrrolic material. When 6 was shaken with dithionite (phos-
phate buffered, aqueous Na2S2O4 pH 7.0/CH2Cl2), comparable re-
sults were obtained. Complete conversion of the dipyrrin to
baseline material was observed within 20 min (TLC analysis) with
dilute solutions of aqueous dithionite (0.2 mol/L), and the use of
saturated aqueous dithionite solution resulted in complete de-
composition within 5 min.
For an ongoing project involving the dipyrrinato framework,
we required an F-BODIPY featuring an ester to react with an
alcohol. Given a report that tris[5-(4-methoxycarbonylphenyl)-
dipyrrinato]Co(III) was hydrolyzed to its carboxylate and then
esterified via the acyl chloride,14 we utilized the analogous F-BODIPY
1 that features an ester at the 4-position of the meso-phenyl group.
To replace the electrophilic methyl carboxylate functionality with
a nucleophilic hydroxyl group, 1 was reacted with excess ethylene
glycol with the expectation that the corresponding ester 2 would
be produced. Instead, under microwave-assisted transesterifica-
tion conditions,15 and using NaOMe as the base, dipyrromethane
3 was produced with no traces of the anticipated F-BODIPY 2 or the
corresponding dipyrrin (Scheme 1). Given that the dipyrrometh-
ane 3 is likely produced via reduction of the corresponding dipyr-
rin, following removal of the –BF2 moiety, we further investigated
Received 26 July 2013. Accepted 9 October 2013.
J.A. Melanson, D.A. Smithen, T.S. Cameron, and A. Thompson. Department of Chemistry, Dalhousie University, P.O. Box 15000, Halifax, NS B3H 4R2,
Canada.
Corresponding author: Alison Thompson (e-mail: alison.thompson@dal.ca).
This article is part of a Special Issue dedicated to Professor Barry Lever in recognition of his contributions to inorganic chemistry across Canada and beyond.
Can. J. Chem. 92: 688–694 (2014) dx.doi.org/10.1139/cjc-2013-0341
Published at www.nrcresearchpress.com/cjc on 17 October 2013.

Melanson et al.
689
Scheme 1. Reaction of 1 with ethylene glycol.
We first wanted to assess the general scope of the production of
dipyrromethanes via reduction using our new methodology.
Working with the unfunctionalized system, F-BODIPY 4 was found
to generate 5-phenyldipyrromethane (5) upon treatment with ethyl-
ene glycol and NaOMe (Scheme 2), indicating that the ester function-
ality of 1 is unnecessary for the curious reduction of the dipyrrinato
unit to the dipyrromethane. More interestingly, the free-base dipyr-
rin (6) evidently also serves as a substrate for the reduction. Given
our recent work using alkoxides to remove the –BF2 moiety from
F-BODIPYs,28,29 presumably the initial transformation of 4 involves
decomplexation followed by reduction of the resultant dipyrrin.
To verify that the product was stable under the reaction condi-
tions, dipyrromethane 5 was submitted to the microwave-assisted
conditions in ethylene glycol, with and without NaOMe. These reac-
tions yielded essentially quantitative return of the dipyrromethane.
cluded that water does not play a significant role in the reaction
and that precautions to exclude water from the reaction mixture
are unnecessary.
Effects of temperature and time
In an effort to reduce both the temperature and time required
for reduction of the dipyrrin to the dipyrromethane, we evaluated
the effects of varying these reactions conditions (Table 2). The
time was successfully reduced to 10 min, with a slightly elevated
yield of 5 compared with that seen after 30 min (compare entries 1
and 3). Further reducing the reaction time returned a lower yield
(entry 4). Employing the optimized 10 min reaction time and re-
ducing the temperature from 215 °C resulted in significantly
lower yields despite consumption of the dipyrrin (entries 5 and 6):
this implies that decomposition competes with the production of
the desired dipyrromethane. As such, the optimized reaction con-
ditions emerged as 215 °C for 10 min.
Solvent effects
With the goal being to identify both the optimum reaction
conditions as well as the REDOX partner for the reduction of 6,
we explored the use of several solvents and compared their
effectiveness with that of ethylene glycol (Table 1). The use of
2-methoxyethanol, under identical conditions to those used pre-
viously, returned a comparable yield of the dipyrromethane 5
(compare entries 1 and 2). However, the use of 1,2-dimethoxyethane
(DME) (entry 3) resulted in only decomposition products: clearly
the solvent choice is crucial for the success of the reaction, with a
hydroxyl functionality being essential for reduction of the dipyr-
rin to the dipyrromethane. The use of simple alcohols was largely
unsuccessful (entries 4 and 5), perhaps due to the fact that sol-
vents with lower boiling points tended to reach elevated system
pressures during the reactions, promoting decomposition. Pleas-
ingly, the use of benzyl alcohol (entry 6) resulted in the produc-
tion of dipyrromethane 5 in acceptable yield, as did the use of
4-methoxybenzyl alcohol (entry 7).
Maintaining the use of ethylene glycol, we also explored the
effects of reducing the amount of NaOMe. Using 1 and 2 equiv. of
NaOMe resulted in slightly lower yields of 5 compared with the
use of 3 equiv.; furthermore, significantly lower yields were ob-
tained if substoichiometric amounts of NaOMe were employed.
Consequently, we maintained our use of 3 equiv. of NaOMe for the
remainder of our investigations.
We next investigated the role of water in the reaction. Using
1H NMR spectroscopy and a relaxation time of 15 s, we confirmed
the <0.2% water content of commercial anhydrous ethylene glycol
by spiking with a known quantity of water and comparing the
quantitative integrals of the –OH signals.30 Given the hygroscopic
properties of ethylene glycol, we similarly determined that our
reagent-grade ethylene glycol contained 1.3% water. Anhydrous
ethylene glycol and anhydrous reaction conditions were subse-
quently employed for the transformation of 6 to 5. Furthermore,
two parallel reactions were run whereby reactions using anhy-
drous ethylene glycol were doped with 10 and 100 equiv. of water,
respectively: the yields remained consistent throughout all three
experiments and matched the yield obtained when reagent-grade
(i.e., 1.3% water, wet) ethylene glycol was used. As such, we con-
Exploring substrate scope
Satisfied that we had identified a reasonable set of reaction
conditions, we then explored the scope of the substrate. In partic-
ular, we were interested in whether other meso-aryl functional-
ities would be tolerated and also whether meso-unsubstituted (i.e.,
meso-H) F-BODIPYs and dipyrrins would undergo reduction as per
their meso-aryl-substituted counterparts. Furthermore, we were
interested in the influence of substituents about the pyrrolic
units.
First, we investigated F-BODIPYs bearing a variety of substi-
tuted phenyl groups at the meso-position (Table 3). Both electron-
withdrawing and electron-donating moieties were tolerated
(entries 2–7). The requisite dipyrromethanes were obtained in
moderate yields in all cases except for the 4-nitro derivative (en-
try 8), which presumably undergoes decomposition facilitated by
the nitro functionality. Nevertheless, the use of aryl halides, aryl
amines, and aryl ethers all returned good yields of the correspond-
ing meso-aryl dipyrromethanes.
A series of meso-unsubstituted substrates (13–17, Fig. 1) were also
explored using the optimized reaction conditions (10 min, 215 °C,
microwaves, 3 equiv. of NaOMe, ethylene glycol): these substrates
all featured alkyl chains and functional groups on the pyrrolic
units. These meso-H F-BODIPYs yielded only decomposition prod-
ucts under the reaction conditions: dipyrromethane was not ob-
tained. As the meso-phenyl substituent seems to offer stability to
the dipyrrolic unit, substrates 18 and 19 were also investigated:
these F-BODIPYs feature a meso-phenyl group as well as substitu-
ents about the pyrrolic framework. These two substrates also
yielded decomposition products under the reaction conditions, as
did the analogue 20 that is unsubstituted adjacent to the meso-
position. Of course, alkyl-substituted dipyrromethanes are prone
to auto-oxidation (to the dipyrrins), but nevertheless, we saw no
sign of these compounds.
The scope of this reduction of F-BODIPYs and dipyrrins to the
corresponding dipyrromethanes is thus limited to dipyrrinato
units that are unsubstituted about the dipyrrolic framework and
that feature an aryl group at the meso-position.
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Can. J. Chem. Vol. 92, 2014
Scheme 2. Production of 5-phenyldipyrromethane (5).
Table 1. The role of the solvent in the reduction of 6.
Fig. 1. Further investigation into substrate tolerance.
Entry
Solvent
Isolated yield (%)
1
Ethylene glycol
2-Methoxyethanol
DME
Ethanol
n-Butanol
68
59
0a
0a
0a
59
47
2
3
4
5
6
7
Benzyl alcohol
4-Methoxybenzyl alcohol
Note: Reaction conditions: 30 min, 215 °C, microwaves, 3 equiv. of NaOMe,
solvent.
aOnly decomposition products observed.
Table 2. The effects of temperature and time on consumption and
yield.
be produced. As such, we anticipated that the addition of 2,4-
dinitrophenylhydrazine (2,4-DNPH)32 would lead to formation of
the yellow 2,4-DNP hydrazone. After our optimized microwave
irradiation of 6 and NaOMe in ethylene glycol, solutions of 2,4-
DNPH (2.5 and 80 equiv.)33 in acidic methanol were added directly
to the reaction mixture: the respective hydrazone was not de-
tected, even after heating to 65 °C.34 Likewise, in trying to identify
the REDOX partner for the reduction in ethylene glycol, matters
are complicated by the fact that glycolaldehyde has been reported
to be a product of the air-oxidation of ethylene glycol at high
temperatures.35 Furthermore, glycolaldehyde is a better reducing
agent than ethylene glycol, as demonstrated by its use in the
polyol synthesis of metal colloids.36 However, our reaction pro-
tocol incorporates a degassing step to remove oxygen from the
reaction mixture, and so we are confident that significant concen-
trations of glycolaldehyde are not generated via this alternative
mechanism.
Isolated
yield of
5 (%)
Time
(min)
Temperature
(°C)
Consumption
Entry
of 6 (%)a
1
30
20
10
5
10
10
215
215
215
215
200
150
100
100
100
89
80
5
61
63
71
54
34
2
2
3
4
5
6
aObtained from the ratio of 6 to 5 via integration of NMR signals.
Table 3. Deprotection and reduction of meso-aryl F-BODIPYs.
Regardless, when benzyl alcohol was used as solvent, the
addition of 2,4-DNPH to the reaction mixture post-microwave
irradiation of 6 resulted in the formation of 1-benzylidene–2-
(2,4-dinitrophenyl)hydrazone in 66% isolated yield, alongside an
equivalent yield of the requisite dipyrromethane 5. In a parallel
reaction, without 2,4-DNPH, column chromatography enabled
the isolation of benzaldehyde in a comparable yield. As such, we
hypothesize that the alcohol effectively transfers hydrogen to the
dipyrrinato core under the reaction conditions. Of course, given
the high temperatures and the presence of base, interference via
a Cannizzaro-type hydride transfer cannot be ruled out once some
benzaldehyde has formed.
Entry
R
Isolated yield (%)
1
−H (4)
−COOMe (1)
−CF3 (7)
−CH3 (8)
−OCH3 (9)
−Br (10)
−NMe2 (11)
−NO2 (12)
68
68
53
63
76
61
47
0a
2
3
4
5
6
7
8
Mechanistic insight
To draw some parallel between our methodology and the
reduction of dipyrrins via hydride delivery, dipyrrins 6, 21, 22,
and 22·HBr were reacted with NaBH4 (Table 4).
aOnly decomposition products observed.
Meso-phenyl substrates were successfully reduced (unoptimized,
entries 1 and 2), although the alkyl-substituted dipyrromethane
obtained from 21 readily oxidizes in air and so isolation is fraught
with the practicalities of timeliness and maintaining oxygen-free
conditions during chromatography, both of which likely contrib-
ute to the rather low yield. Reactions involving meso-H dipyrrins
(entries 3 and 4) were unsuccessful, presumably due to this posi-
tion being both less electrophilic and less able to afford resonance
Probing the REDOX partner
Since the reaction results in reduction of a dipyrrin to a
dipyrromethane, we sought to identify the concomitant REDOX
partner: we hypothesized that ethylene glycol was being oxidized
to generate 2-hydroxyacetaldehyde (glycolaldehyde).31 Similarly,
when benzyl alcohol was used as solvent, benzaldehyde would
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Melanson et al.
691
Table 4. Reduction of dipyrrins using NaBH4.
Experimental section
General experimental
All 1H and 13C NMR spectra were obtained using a 500 MHz NMR
spectrometer (operating at 500 and 125 MHz, respectively) and
CDCl3 as solvent. Chemical shifts were recorded in parts per mil-
lion (ppm) with reference to CDCl3 (1H NMR at 7.26 ppm, 13C NMR
at 77.16 ppm). 11B{1H} NMR spectra were obtained using a 500 MHz
NMR instrument (operating at 160 MHz). Coupling constants (J)
are reported in units of hertz (Hz). High- and low-resolution ESI+
mass spectra were recorded using a microTOF instrument. All
microwave-promoted reactions were performed using a Biotage
Initiator 8 laboratory microwave apparatus, 0–400 W power,
2.45 GHz. Column chromatography was performed using Silicycle
230−400 mesh ultrapure silica or Brockman (III) basic alumina,
as indicated. The following compounds were prepared acc-
ording to literature procedures: 5-(4-methoxycarbonylphenyl)
phenyldipyrrin,14 5-(4-trifluoromethyl)phenyldipyrromethane,45 1,3,
7,9-tetramethyl-2,8-dipentyl-dipyrrin·HBr,46 4,47 8,48 9,49 10,50
11,50 12,48 13,28 15,46 16,29 17,29 18,29 and 19.51 Measurements for
the crystal structure of 1 were made using a Rigaku RAXIS RAPID
imaging plate area detector with graphite monochromated Mo-K␣
radiation. The structures were solved by direct method52 and ex-
panded using Fourier techniques.53 The nonhydrogen atoms were
refined anisotropically. Hydrogen atoms were refined isotropi-
cally. Calculations were performed using the CrystalStructure54,55
crystallographic software package.
Entry
(compound)
Isolated
yield (%)
R1
R2
R3
R4
1 (6)
2 (21)
3 (22)
4 (22·HBr)
H
H
H
Ph
Ph
H
60
20a
0b
Me
Me
Me
Et
Et
Et
Me
Me
Me
H
0b
aDipyrromethane product auto-oxidizes to dipyrrin; compound isolated un-
der N2.
bComplete return of starting material.
stabilization to the consequent intermediate upon hydride attack.
Based on the similar reactivity of dipyrrin 6 and F-BODIPY 4 in our
reductive methodology ((CH2OH)2, NaOMe), we were surprised
that a very low product yield, alongside decomposition products,
was observed when 4 was reacted with NaBH4.
To further investigate the mechanism of reduction, and to
probe for an intermediate tertiary radical, 10 equiv. of 2-propanol-
2-D1 was added to a reaction mixture featuring 6, ethylene glycol,
and NaOMe. No deuterated product was observed and the 71%
yield of the undeuterated dipyrromethane (5) was consistent
with reactions conducted in the absence of 2-propanol. Our
mechanistic hypothesis thus leans towards a more reasonable
formal hydride-transfer approach. When deuterated ethylene
glycol (DOCH2CH2OD) was employed as a solvent under the opti-
mized conditions, incorporation of deuterium occurred through-
out the dipyrrolic framework and mirrored that of deuterium
incorporation via hydrogen/deuterium exchange, as previously
noted for pyrroles.37 Consequently, deuterated benzyl alcohol was
used to track the fate of deuterium incorporation from the alco-
General procedure (GP1) for the synthesis of F-BODIPYs
The appropriate dipyrrin or HBr dipyrrin salt was treated with
6 equiv. of NEt3 and 9 equiv. of BF3·OEt2 while stirring at room
temperature in anhydrous dichloromethane (ϳ0.04 mol/L). Upon
complete consumption of the dipyrrin, according to analysis us-
ing TLC, the solvent was removed from the reaction mixture in
vacuo. The crude dark red solid was dissolved in diethyl ether and
washed twice with an equal volume of 1 mol/L HCl (to remove
excess BF3·OEt2) followed by distilled water and then brine. The
organic fraction was then reduced in vacuo, and the compound
was purified using column chromatography (silica). Removal of
the organic solvent in vacuo gave the product as a red/orange
solid.
38
hol. When 6 was reacted with NaOMe in benzyl alcohol-OD-d1
(BnOD), incorporation of deuterium was not observed. However,
when benzyl alcohol-d2 (PhCD2OH) was used as the solvent,
39
1H NMR spectroscopy revealed that incorporation of deuterium
occurred solely at the meso-position, as expected if formal hydride
delivery occurs via nucleophilic attack at the meso-position. We
similarly observed selective meso-D incorporation when 6 was re-
acted with NaBD4 in ethylene glycol.
In all, our preliminary investigations resulted in evidence that
supports our mechanistic hypothesis. We present this work as both
a useful method to prepare select dipyrromethanes and a caution
with regard to a previously unobserved reaction of dipyrrins.
General procedure (GP2) for the synthesis of F-BODIPYs
The appropriate dipyrromethane was treated with 1.1 equiv. of
DDQ while stirring at room temperature in anhydrous dichlo-
romethane (ϳ0.12 mol/L). Upon complete consumption of the
dipyrromethane, according to analysis using TLC (i.e., conversion
to the dipyrrin, typically within 30–45 min), the reaction mixture
was treated with 6 equiv. of NEt3 and 9 equiv. of BF3·OEt2. Upon
complete consumption of the dipyrrin, according to analysis us-
ing TLC, the solvent was removed from the reaction mixture in
vacuo. The crude dark red solid was dissolved in diethyl ether and
washed twice with 1 mol/L HCl (equal parts, to remove excess
BF3·OEt2) followed by distilled water and then brine. The organic
fraction was then reduced in vacuo, and the compound was puri-
fied using column chromatography (silica). Removal of the or-
ganic solvent in vacuo gave the product as a red/orange solid.
Conclusions
In conclusion, it has been demonstrated that meso-aryl F-BODIPYs
and dipyrrins undergo reduction to produce the corresponding
dipyrromethane in basic, microwave-assisted conditions in the
presence of excess ethylene glycol, benzyl alcohol, 4-methoxybenzyl
alcohol, or 2-methoxyethanol. The REDOX partner appears to be
the hydroxyl-containing solvent, thus generating the correspond-
ing aldehyde. Although the formation of dipyrromethanes via
the condensation of pyrrole with aryl aldehydes can be high-
yielding,40 a viscous and difficult-to-manipulate reaction mixture
is often received and this is complicated by the fact that pyrrole
is often used as the reaction solvent.41–43 Alternatively, making
F-BODIPYs can be a simple one-pot process:4,44 the method de-
scribed in this article can therefore be used as a strategy by which
to produce dipyrromethanes via the respective F-BODIPY.
4,4-Difluoro-8-(4-methoxycarbonylphenyl)-4-bora-3a,4a-diaza-s-
indacene (1)
Using GP1, the title compound was synthesized from 5-(4-
methoxycarbonylphenyl)phenyldipyrrin (542 mg, 1.94 mmol) in
5.5 h. The title compound was isolated as a red solid (483 mg,
1
76% yield) after column chromatography (silica–DCM). H NMR
(500 MHz, CDCl3) ␦: 8.19 (d, 2H, J = 8), 7.96 (s, 2H), 7.64 (d, 2H, J = 8),
6.88 (d, 2H, J = 4), 6.56 (d, 2H, J = 4), 3.98 (s, 3H). 13C NMR (125 MHz,
CDCl3) ␦: 166.3, 145.9, 145.0, 138.1, 134.8, 132.3, 131.6, 130.5, 129.7,
119.1, 52.7. 11B{1H} NMR (160 MHz, CDCl3) ␦: 0.24 (t, J = 27.2). HRMS-
ESI (m/z): [M + Na]+ calcd. for C17H13B1F2N2O2Na: 349.0936; found:
Published by NRC Research Press

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Can. J. Chem. Vol. 92, 2014
349.0930. Crystal data for 1: C17H13N2BF2O2, MM = 326.11 g/mol,
dark red needle, 0.90 mm × 0.33 mm × 0.15 mm; primitive tri-
clinic, space group P1 (#2), a = 7.5380(7) Å, b = 7.8407(7) Å, c =
13.3682(7) Å, ␤ = 77.62(4)°, V = 740.0(2) Å3, Z = 2, ␳ = 1.463 g/cm3,
BF3·OEt2 followed by distilled water and then brine. The organic
solvent volume was then reduced in vacuo, and the compound
was purified using column chromatography (silica – 2% Et2O/hex-
anes). Removal of the organic solvent in vacuo gave the title com-
pound as a red/orange solid (106 mg, 16% yield). Upon NMR
analysis, it was noted that the compound was not pure. Purifica-
tion was attempted via column chromatography (silica – 10%
EtOAc/hexanes, alumina – 10% DCM/hexanes): these attempts
proved to be unsuccessful and the sample was used without fur-
␮(MoK␣) = 1.127 cm−1, 53727 reflections (20535 unique, Rint
0.029), R(F) = 0.0350, Rw(F) = 0.0487, GOF = 0.933; CCDC 949267.
=
4,4-Difluoro-8-(4-trifluoromethylphenyl)-4-bora-3a,4a-diaza-s-
indacene (7)
Using GP2, the title compound was synthesized from 5-(4-
trifluoromethyl)phenyldipyrromethane (300 mg, 1.03 mmol). The
title compound was isolated as a red solid (166 mg, 48% yield) after
column chromatography (silica – 20% Et2O/hexanes). 1H NMR
(500 MHz, CDCl3) ␦: 7.94 (s, 2H), 7.77 (d, 2H, J = 8), 7.65 (d, 2H, J = 8),
6.83 (d, 2H, J = 4), 6.53 (d, 2H, J = 4). 13C NMR (125 MHz, CDCl3) ␦:
145.2, 137.4, 134.9, 132.9, 132.7, 131.6, 130.8, 130.1, 125.7, 119.3.
11B{1H} NMR (160 MHz, CDCl3) ␦: 1.20 (t, J = 27.5). HRMS-ESI (m/z):
[M + Na]+ calcd. for C16H10B1F5N2O2Na: 359.0755; found: 359.0749.
1
ther purification. H NMR (500 MHz, CDCl3) ␦: 7.45–7.50 (m, 5H),
6.42 (s, 2H), 3.09 (t, 4H), 2.50 (t, 4H), 1.83–1.88 (m, 4H), 1.71–1.76 (m,
4H). 13C NMR (125 MHz, CDCl3) ␦: 157.5, 141.1, 134.8, 133.9, 130.4,
129.7, 129.4, 129.3, 128.2, 126.6, 24.8, 23.3, 23.0, 22.5. 11B{1H} NMR
(160 MHz, CDCl3) ␦: 0.58 (t, J = 32.6). HRMS-ESI (m/z): [M + Na]+ calcd.
for C23H23B1F2N2Na: 399.1820; found: 399.1815.
General procedure (GP3) for the microwave-assisted
deprotection and reduction of F-BODIPYs
F-BODIPY (50 mg) was added to a solution of NaOMe (3 equiv.)
and ethylene glycol (3 mL). The reaction mixture was degassed by
bubbling with N2 for 15 mins, and then it was then heated and
stirred at 215 °C for 10 min in a Biotage microwave system. The
reaction mixture was dissolved in DCM (50 mL) and washed with
water (50 mL × 3). The organic layer was dried with anhydrous
Na2SO4, concentrated, and purified via column chromatography
(alumina) to give a semisolid.
4,4-Difluoro-1,3,5,7-tetramethyl-2,6-di-3-hydroxypropyl-4-bora-
3a,4a-diaza-s-indacene (14)
Using GP1, the title compound was synthesized from 1,3,7,9-
tetramethyl-2,8-di(hydroxypropyl)-dipyrrin·HCl (450 mg, 1.29 mmol) in
3 h. The title compound was isolated as a red solid (90 mg, 18% yield)
1
after column chromatography (silica – 5% MeOH/DCM). H NMR
(500 MHz, CDCl3) ␦: 6.92 (s, 1H), 3.62 (t, 4H), 2.46 (s, 6H), 2.43 (t, 4H),
2.14 (s, 6H), 1.64–1.70 (m, 4H), 1.24 (br s, 2H). 13C NMR (125 MHz,
CDCl3) ␦: 155.6, 137.9, 133.0, 129.8, 119.4, 62.7, 33.4, 20.7, 13.2, 10.2.
11B{1H} NMR (160 MHz, CDCl3) ␦: 0.86 (t, J = 34.2). HRMS-ESI (m/z):
[M + Na]+ calcd. for C19H27B1F2N2O2Na: 387.2031; found: 387.2026.
5-((4-(2-Hydroxy)ethoxycarboxy)phenyl)dipyrromethane (3)
Using GP3, the title compound was synthesized from 4,4-
difluoro-8-(4-methoxycarbonylphenyl)-4-bora-3a,4a-diaza-s-indacene (1).
The title compound was isolated as a dark semisolid (29 mg, 68%
yield) after column chromatography (alumina − 0%–10% MeOH/
DCM). 1H NMR (500 MHz, CDCl3) ␦: 8.12 (s, 2H), 7.99 (d, 2H, J = 8),
7.29 (d, 2H, J = 8), 6.72 (s, 2H), 6.15 (quintet, 2H), 5.88 (s, 2H), 5.53 (s,
1H), 4.45 (t, 2H), 3.95 (t, 2H). 13C NMR (125 MHz, CDCl3) ␦: 166.9,
147.9,131.7, 130.1, 128.60, 128.56, 117.7, 108.6, 107.6, 66.7, 61.4, 44.1.
HRMS-ESI (m/z): [M + Na]+ calcd. for C18H18N2O3Na: 333.1215; found:
333.1210.
4,4-Difluoro-1,3,5,7-tetramethyl-2,6-dipentyl-4-bora-3a,4a-diaza-s-
indacene (15)46
Using GP1, the title compound was synthesized from 1,3,7,9-
tetramethyl-2,8-di(hydroxypropyl)-dipyrrin·HBr (250 mg, 0.59 mmol) in
4 h. The title compound was isolated as a red solid (122 mg, 71% yield)
1
after column chromatography (silica–DCM). H NMR (500 MHz,
CDCl3) ␦: 6.94 (s, 1H), 2.48 (s, 6H), 2.34 (t, 4H), 2.15 (s, 6H), 1.40–1.46
(m, 4H), 1.27–1.36 (m, 8H), 0.90 (t, 6H). 13C NMR (125 MHz, CDCl3) ␦:
155.0, 137.1, 132.6, 130.4, 118.7, 31.8, 30.0, 24.2, 22.7, 14.2, 12.8, 9.7.
11B{1H} NMR (160 MHz, CDCl3) ␦: 0.76 (t, J = 34.6). HRMS-ESI (m/z):
[M + Na]+ calcd. for C23H35B1F2N2Na: 411.2759; found: 411.2754.
5-Phenyldipyrromethane (5)
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-phenyl-4-bora-3a,4a-diaza-s-indacene (4). The title compound
was isolated as a dark beige solid (28 mg, 68% yield) after column
chromatography (alumina − 10% Et2O/hexanes). 1H NMR (500 MHz,
CDCl3) ␦: 7.93 (br s, 1H), 7.20–7.35 (m, 5H), 6.70 (d, 2H, J = 6), 6.17 (d,
2H, J = 7) 5.92 (d, 2H, J = 6), 5.48 (s, 1H). Data comparable with
previously reported data.40
4,4-Difluoro-1,3,5,7-tetramethyl–2,6-diethyl-8-
(4-methoxycarbonylphenyl)-4-bora-3a,4a-diaza-s-indacene (19)51
Using GP2, the title compound was synthesized from 1,3,7,9-
tetramethyl-2,8-diethyl-5-(4-methoxycarbonylphenyl)dipyrromethane
(2.39 g, 6.10 mmol) stepwise. Following GP2, the dipyrrin formed
in 45 min and was converted to the BODIPY in an additional 2 h.
The title compound was isolated as a dark green solid (589 mg,
5-(4-Trifluoromethylphenyl)dipyrromethane
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-(4-trifluoromethylphenyl)-4-bora-3a,4a-diaza-s-indacene (7). The
title compound was isolated as a dark beige semisolid (21 mg,
53% yield) after column chromatography (alumina−DCM). 1H NMR
(500 MHz, CDCl3) ␦: 7.97 (s, 2H), 7.59 (d, 2H, J = 8), 7.35 (d, 2H, J = 8),
6.74 (s, 2H), 6.17 (s, 2H), 5.53 (s, 1H). Data comparable with previ-
ously reported data.56
1
22% yield) after column chromatography (silica–DCM). H NMR
(500 MHz, CDCl3) ␦: 8.17 (d, 2H, J = 8), 7.40 (d, 2H, J = 8), 3.98 (s, 3H),
2.53 (s, 6H), 2.29 (q, 4H), 1.25 (s, 6H), 0.98 (t, 6H). 13C NMR (125 MHz,
CDCl3) ␦: 166.7, 154.4, 140.8, 138.8, 138.2, 133.2, 130.7, 130.4, 128.8,
52.5, 17.2, 14.7, 12.7, 12.0. 11B{1H} NMR (160 MHz, CDCl3) ␦: 0.65 (t,
J = 33.3). HRMS-ESI (m/z): [M + Na]+ calcd. for C25H29B1F2N2Na:
461.2188; found: 461.2182.
5-(4-Methylphenyl)dipyrromethane
5,5-Difluoro-11-phenyl-5-bora-4b,5adiaza-s-1,2,3,4,6,7,8,9-
octahydroindeno[2,1-b]fluorene (20)
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-(4-methylphenyl)-4-bora-3a,4a-diaza-s-indacene (8). The title
compound was isolated as a dark beige solid (26 mg, 63% yield)
after column chromatography (alumina − 5%–10% EtOAc/hex-
anes). 1H NMR (500 MHz, CDCl3) ␦: 8.00 (s, 2H), 7.26 (d, 2H, J = 9),
6.80 (s, 2H), 6.30 (q, 2H), 6.06 (s, 2H), 5.56 (s, 1H), 2.49 (s, 3H). Data
comparable with previously reported data.40
Tetrahydroindole (429 mg, 3.54 mmol) and benzoyl chloride
(205 ␮L, 1.77 mmol) were heated and stirred at reflux temperature
in anhydrous dichloroethane (55 mL) for 48 h. The solution was
allowed to cool to room temperature, and then the in situ gener-
ated dipyrrin was treated with 6 equiv. of NEt3 and 9 equiv. of
BF3·OEt2. Upon consumption of the dipyrrin (monitored via TLC,
1 h), the solvent was removed from the reaction mixture in vacuo.
The crude dark red solid was dissolved in diethyl ether and
washed twice with 1 mol/L HCl (equal parts) to remove excess
5-(4-Methoxyphenyl)dipyrromethane
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene (9). The title
Published by NRC Research Press

Melanson et al.
693
compound was isolated as a dark beige solid (34 mg, 76% yield)
after column chromatography (alumina − 10% EtOAc/hexanes).
1H NMR (500 MHz, CDCl3) ␦: 7.89 (s, 2H), 7.14 (d, 2H, J = 9), 6.86 (d,
2H, J = 9), 6.68 (d, 2H, J = 4), 6.16 (d, 2H, J = 4), 5.92 (s, 1H), 3.80 (s, 3H).
Data comparable with previously reported data.40
Sodium dithionite-mediated reduction60
5-Phenyldipyrrin (6, 50 mg, 0.227 mmol) was added under a flow
of N2 to a stirring solution of aqueous Na2S2O4/THF (395 mg in
1 mL of H2O and 3 mL of THF). The reaction mixture was allowed
to stir at room temperature for 10 min, where the solvent was
removed azeotropically using toluene (2 × 50 mL) and the
resulting oil was separated between DCM (50 mL) and water
(50 mL). The aqueous layer was extracted using DCM (50 mL × 2)
and then the combined organic layers were dried over Na2SO4
and concentrated in vacuo. The crude mixture was then purified
over neutral Brockman III alumina − 10%–20% Et2O/hexanes.
5-Phenyldipyrromethane was isolated as a beige solid (5 mg, 10%
5-(4-Bromophenyl)dipyrromethane
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-(4-bromophenyl)-4-bora-3a,4a-diaza-s-indacene (10). The title
compound was isolated as a dark beige solid (23 mg, 61% yield)
after column chromatography (alumina − 10%–20% EtOAc/hex-
anes). 1H NMR (500 MHz, CDCl3) ␦: 7.92 (s, 2H), 7.44 (d, 2H, J = 9),
7.19 (d, 2H, J = 9), 6.70–6.72 (m, 2H), 6.17 (q, 2H), 5.87–5.91 (m, 2H),
5.43 (s, 1H). Data comparable with previously reported data.56
1
yield). H NMR (500 MHz, CDCl3) ␦: 7.93 (br s, 2H), 7.20–7.35 (m,
5H), 6.70 (d, 2H, J = 6), 6.17 (d, 2H, J = 7) 5.92 (d, 2H, J = 6), 5.48 (s, 1H).
Data comparable with previously reported data.40
5-(4-Dimethylamino)dipyrromethane
Supplementary data
Using GP3, the title compound was synthesized from 4,4-difluoro-
8-(4-dimethylaminophenyl)-4-bora–3a,4a-diaza-s-indacene (11). The
title compound was isolated as a dark beige solid (20 mg, 47%
yield) after column chromatography (alumina − 10%–20% EtOAc/
hexanes). 1H NMR (500 MHz, CDCl3) ␦: 7.90 (s, 2H), 7.09 (d, 2H, J = 9),
6.70 (d, 2H, J = 9), 6.66–6.72 (m, 2H), 6.16 (q, 2H), 5.93–5.96 (m, 2H),
5.39 (s, 1H), 2.94 (s, 6H). Data comparable with previously reported
data.57
Supplementary data (copies of NMR spectra) are available with
the article through the journal Web site at http://nrcresearchpress.
com/doi/suppl/10.1139/cjc-2013-0341. CCDC 949267 contains the
X-ray data in CIF format for this manuscript. These data can be
obtained, free of charge, via http://www.ccdc.cam.ac.uk/ products/
csd/request (or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 33603,
e-mail: deposit@ccdc.cam.ac.uk).
1-Benzylidene-2-(2,4-dinitrophenyl)hydrazone
Acknowledgements
After microwave irradiation (10 min, 215 °C) of 5-phenyldipyrrin
(6) (50 mg, 0.23 mmol) and NaOMe (37 mg, 0.68 mmol) in benzyl
alcohol (3 mL), a solution of 2,4-DNPH (2.5 equiv.) in acidic meth-
anol (Brady’s test, 25:1 MeOH/sulfuric acid) was added directly to
the reaction mixture which was then stirred at room temperature
for 3 h. The resulting yellow precipitate was isolated via suction
filtration (35 mg, 66% yield). 1H NMR (500 MHz, CDCl3) ␦: 11.32 (s,
1H), 9.16 (s, 1H), 8.37 (d, 1H, J = 10), 8.13 (d, 1H, J = 10), 8.09 (s, 1H),
7.75–7.80 (m, 2H), 7.46–7.50 (m, 3H). Data comparable with previ-
ously reported data.58
This work was supported by the Natural Sciences and Engineering
Research Council of Canada. J.A.M. is the recipient of a Walter C.
Sumner Memorial Fellowship. We thank Mr. Xiao Feng (Dalhousie
University) for recording the mass spectral data and Dr. Raymond T.
Syvitski (Institute for Marine Biosciences, National Research Council
of Canada, Halifax) for discussions regarding the use of 1H NMR spec-
troscopy to quantify residual water in protic solvents.
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