Conversion of F -BODIPYs to Cl -BODIPYs: Enhancing the reactivity of F -BODIPYs

Article abstract of DOI:10.1021/jo302277d

A new method for the synthesis of Cl-BODIPYs from F-BODIPYs is reported, merely requiring treatment of the F-BODIPY with boron trichloride. Cl-BODIPYs are exploited as synthetic intermediates generated in situ for the overall conversion of F-BODIPYs to O-

Full text of DOI:10.1021/jo302277d

Note
pubs.acs.org/joc
Conversion of F‑BODIPYs to Cl-BODIPYs: Enhancing the Reactivity of
F‑BODIPYs
Travis Lundrigan and Alison Thompson*
Department of Chemistry, Dalhousie University, P.O. Box 15000, Halifax, NS, B3H 4R2, Canada
S
* Supporting Information
ABSTRACT: A new method for the synthesis of Cl-
BODIPYs from F-BODIPYs is reported, merely requiring
treatment of the F-BODIPY with boron trichloride. Cl-
BODIPYs are exploited as synthetic intermediates generated
in situ for the overall conversion of F-BODIPYs to O- and C-
BODIPYs in high overall yields using a mild one-pot
procedure. This route enables F-BODIPYs to be transformed
into derivatives that are not accessible via the direct route, as
demonstrated via the use of 1,3-propanediol.
ompounds containing the 4,4-difluoro-4-bora-3a,4a-diaza-
s-indacene (F-BODIPY) framework1−3 are known for
BCl3. Alternative strategies to the dipyrrinato construct obviate
the need to isolate dipyrrin free-bases, and it generates F-
BODIPYs by trapping the free-base dipyrrins in situ with
BF3•OEt2.2 In these examples, an excess of BF3•OEt2 and NEt3
is required for F-BODIPY formation,17 and this can result in
the formation of a BF3•NEt3 adduct that complicates
purification, particularly on larger scales. These dipyrrins
cannot be trapped as their Cl-BODIPYs since the overall
procedure generally requires aqueous extractions and purifica-
tion via chromatography, conditions that the Cl-BODIPY does
not survive. Cognizant of these synthetic challenges, we sought
an alternate route for the synthesis of Cl-BODIPYs. Our
ultimate goal was to determine facile substitution at the boron
center of this structural unit so as to reveal a simple, mild route
to a range of R-BODIPYs.
As the F-BODIPY construct is readily available both
synthetically and commercially,1,17 we investigated this
construct as a source of dipyrrinato units for the synthesis of
Cl-BODIPYs. In this work we demonstrate a new trans-
formation involving the boron atom of F-BODIPYs. Indeed, the
F-BODIPY 1 was stirred in anhydrous CH2Cl2 at room
temperature and treated with 1 equiv of BCl3 (Scheme 1). The
initial solution was bright orange with a green fluorescent hue:
C
their high thermal and photochemical stability, chemical
robustness, and chemically tunable fluorescence properties:
they have wide application including as dyes, as probes or
sensitizers in biological systems, and as materials for
incorporation into devices.4−6 However, studies in recent
years have shown that the F-BODIPY unit is more modifiable,
and not as chemically robust, as traditionally believed. Indeed,
reaction under basic conditions leads to the removal of the BF2
moiety, liberating the corresponding free-base dipyrrins.7,8
Furthermore, treatment with acids of various strengths results
in decomposition of F-BODIPYs.9 Nucleophilic substitution of
F-BODIPYs, usually under forcing conditions, has provided
compounds bearing substituents other than fluorine at the
boron center of the BODIPY core: this topic is under wide
exploration,10−13 with target compounds including aryl-, alkyl-,
alkynyl-, alkoxy-, and aryloxy-BODIPYs with exotic spectro-
scopic properties.
We have recently reported the synthesis of Cl-BODIPYs14 in
high yields through the treatment of free-base dipyrrins with
BCl3. Importantly, Cl-BODIPYs offer significant advantages
over F-BODIPYs in terms of their utility as synthetic
intermediates, courtesy of facile substitution at the boron
center. Indeed, the weaker B−Cl bond strength makes the B−
Cl bonds of Cl-BODIPYs substantially more labile than the
corresponding B−F bonds of F-BODIPYs. Compared to F-
BODIPYs, substitutions at the boron center of Cl-BODIPYs
proceed under milder conditions and require shorter reaction
times to give high yielding BODIPY analogues that have been
previously somewhat challenging to prepare from F-BODIPYs.7
Our previous work with Cl-BODIPYs was grounded upon its
reliance on the dipyrrin free-base15,16 as starting material.14
Many dipyrrins are not generally isolated in this free-base form
due to instability, or as a consequence of protracted strategies
used to prepare them. Consequently dipyrrins are often isolated
as their HBr salts which are, unfortunately, unreactive with
Scheme 1. Synthesis of Cl-BODIPY (2) from F-BODIPY (1)
Received: October 24, 2012
Published: December 13, 2012
© 2012 American Chemical Society
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The Journal of Organic Chemistry
Note
upon treatment with BCl3 the solution became deep purple in
color. The reaction mixture was stirred for 1 h and then filtered
through Celite. The resulting solution was concentrated in
vacuo to give a quantitative yield of complex 2 as a pure solid,
as identified by comparison of characterization data with the
known compound.14
Having successfully shown that the Cl-BODIPY 2 can easily
be generated from its analogous F-BODIPY, we examined the
utility of the Cl-BODIPY as a synthetic intermediate within a
one-pot procedure. Knowing that boron substitutions of the Cl-
BODIPY 2 occur under mild conditions, F-BODIPY 1 was
treated with 1.0 equiv of BCl3 in CH2Cl2 (Scheme 2). After 1 h
The reaction occurred very rapidly, and attempts to monitor
the reaction via 11B NMR spectroscopy revealed that as soon as
the BCl3 was introduced to the solution of F-BODIPY, the Cl-
BODIPY formed immediately (<2 min, the time it took to
insert the sample into the NMR spectrometer and begin
acquiring data). Consequently, the reaction was performed in
an NMR tube using various stoichiometries of BCl3 to enable
the conversion of the F-BODIPY to the Cl-BODIPY to be
monitored using 11B resonances. As such, under an inert
atmosphere a solution of F-BODIPY 1 in CD2Cl2 was placed in
an NMR tube, which was then sealed with a septum. Aliquots
of BCl3 were added through the septum, as a 1.0 M solution in
hexanes, and the sample was quickly shaken to ensure efficient
mixing before NMR spectra were acquired. In each case, 11B
NMR data were acquired within 2 min of the BCl3 addition.
The results are compiled as Figure 1. The starting material F-
Scheme 2. Synthesis of O-BODIPY (3) and C-BODIPY (4)
from F-BODIPY (1) Using a Cl-BODIPY Intermediate
Generated in Situ From F-BODIPY 1
solid NaOMe was added to the solution under inert conditions
and the reaction mixture was stirred for 1 h. The solution was
then washed with water and extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4 and
concentrated in vacuo. The O-BODIPY 3 was thus isolated
in a 98% isolated yield from 1. It should be noted that to
convert the F-BODIPY 1 directly to the O-BODIPY 3, the
reaction requires elevated temperatures, typically reflux, and
reaction times reaching 18 h:7,12 when the F-BODIPY is treated
with NaOMe at room temperature, only trace product is
observed, supporting our discovery that the Cl-BODIPY
intermediate is crucial to allow substitution at the boron center
to proceed at room temperature within a short time frame. The
C-BODIPY 4 was also synthesized from F-BODIPY 1, via the
Cl-BODIPY intermediate. The initial step involving Cl-
BODIPY formation was carried out in the same fashion, and
the solution was then treated with EtMgBr, upon which the
reaction mixture became bright orange rather than the dark
purple color of the Cl-BODIPY that had been formed in situ.
After 1 h the reaction mixture was washed with water and brine
and extracted with CH2Cl2. The organic layer was dried over
anhydrous Na2SO4, and the resulting solution was concentrated
in vacuo to give the C-BODIPY 4 in a 98% isolated yield from
1. Although treatment of F-BODIPY 1 with EtMgBr at room
temperature also results in the formation of 4, the yield is 67%
compared to the 98% when using the Cl-BODIPY intermediate.
Furthermore, proceeding via the Cl-BODIPY intermediate
results in cleaner reactions: for example column chromatog-
raphy is required to isolate/purify the C-BODIPY 4 prepared
directly from the F-BODIPY at room temperature, but no such
procedure is required after proceeding via the Cl-BODIPY.
To demonstrate the utility of this one-pot procedure, we
expanded the scope of the dipyrrin ligands used (Table 1).
Using a variety of dipyrrinato units with various substitutions
around the pyrrolic rings as well as substitutions at the meso
position we obtained high yields throughout. Compounds 4−6
Figure 1. 11B NMR stack plot of Cl-BODIPY formation. Bottom = F-
BODIPY 1; middle = F-BODIPY 1 and Cl-BODIPY 2 both present
after the addition of 0.5 equiv of BCl3; top = only Cl-BODIPY 2
present after the addition of 1.0 equiv of BCl3.
BODIPY (1) is represented as a triplet due to B−F coupling.
After 0.5 equiv of BCl3 had been added, the intensity of the
triplet due to the F-BODIPY decreased and a singlet
corresponding to the Cl-BODIPY appeared: integration of
the two signals revealed the two species to be present in
essentially equal amounts, as expected after the addition of 0.5
equiv of BCl3. Upon the addition of 1.0 equiv of BCl3 complete
conversion to the Cl-BODIPY was observed, with only the
singlet apparent in the 11B NMR spectrum.
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Note
derivatization of F-BODIPYs, demonstrating the overall
efficiency of this new procedure.
Table 1. Synthesis of C-BODIPYs from F-BODIPYs Using
Cl-BODIPY Intermediates Formed in Situ
To further demonstrate the advantageous use of Cl-
BODIPYs as in situ intermediates, we selected a transformation
that is unsuccessful for F-BODIPYs. The reaction of a meso-
aryl F-BODIPY with alcohols is ameliorated in the presence of
AlCl3: the reaction is postulated to proceed via in situ
formation of a chelate involving B−F−Al coordination.19
Such Lewis acid activation of the B−F bonds allowed a range
of O-BODIPYs to be prepared in low−good yields: the
reactions did not occur in the absence of AlCl3. The reactions
proceeded well with alkyl and aryl alcohols, as well as several
diols to provide cyclic analogues. However, attempted
substitution of the fluoro substitituents with 1,3-propanediol
did not provide the corresponding cyclic derivative at boron.
We thus probed the utility of our method involving in situ
generation of Cl-BODIPYs, for the overall conversion of F-
BODIPYs using 1,3-propanediol.
We thus reacted F-BODIPY 1 with BCl3, to form 2 in situ.
The lithium dienolate of 1,3-propanediol was then added in a
stoichiometric amount, and the desired cyclic O-BODIPY 9 was
subsequently isolated in 40% yield (Scheme 3) for the two-step,
Scheme 3. Preparation of Cyclic O-BODIPY 9
one-pot transformation: reaction of Cl-BODIPY 2 with 1,3-
propanediol was unfruitful, and so the dienolate was used in the
one-pot procedure. Attempted reaction of F-BODIPY 1 with
the dienolate was unsuccessful, demonstrating the advantage of
proceeding via the Cl-BODIPY using our one-pot procedure.
A new synthetic method has been developed for the
quantitative conversion of F-BODIPYs into the recently
discovered Cl-BODIPYs. We have also demonstrated how the
Cl-BODIPY can be used as a synthetic intermediate in a one-
pot procedure for the formation of BODIPY derivatives via
substitution at the boron center. Taking advantage of the
increased reactivity of the Cl-BODIPY over the F-BODIPY, we
have successfully synthesized O-BODIPYs and C-BODIPYs
from the corresponding F-BODIPYs and have employed F-
BODIPY starting materials with various substitutions around
the dipyrrinato backbone. The one-pot conversion of F-
BODIPYs to other BODIPYs was accomplished in excellent
yields using mild conditions and shorter reaction times than the
traditional methods reported for boron substitutions of F-
BODIPYs. It is anticipated that this new one-pot method, via
Cl-BODIPYs, will have widespread utility in the facile synthesis
of various BODIPYs from F-BODIPYs, compounds that are
routinely available commercially and synthetically.
a
Isolated yields.
(Table 1, entries 1−3) contain alkyl substituents around the
pyrrolic rings, with both meso-H and meso-Ph substituents:
reaction of the Grignard reagent at the meso position was not
observed, and instead selective reaction at the boron center
occurred.7,18 Compound 7 (Table 1, entry 4) contains a methyl
alkanoate bound to one of the heterocycles: product(s) from
the reaction of the Grignard reagent with the ester functional
group were not isolated, although the yield for the preparation
of 7 was only moderate, and the reaction mixture required
filtration through a silica plug for pure 7 to be obtained.
Compound 8 (Table 1, entry 5) is completely unsubstituted on
the pyrrolic rings, containing only a meso-Ph group, and was
chosen to demonstrate the preferential reactivity of the
Grignard reagent for substitution at boron as opposed to
addition to the unsubstituted positions of the pyrrolic rings. C-
BODIPYs were thus prepared from F-BODIPYs in one pot via
Cl-BODIPY intermediates, and in high isolated yields. These
yields are significantly higher than those typical for the direct
EXPERIMENTAL SECTION
■
1,3,5,7-Tetramethyl-2,6-diethyl-8-H-4,4′-dichloro-bora-
3a,4a-diaza-s-indacene (2).14 F-BODIPY 117 (50 mg) was treated
with 1 equiv of BCl3 in anhydrous CH2Cl2, and the reaction mixture
was stirred for 1 h. The reaction mixture was filtered over Celite, and
the solution was concentrated in vacuo to give the title compound 2
(55 mg, >99%). δH (500 MHz, THF-d8) 7.34 (1H, s), 2.69 (6H, s),
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Note
2.43 (4H, q, J = 7.6 Hz), 2.21 (6H, s), 1.07 (6H, t, J = 7.6 Hz); δC
(125 MHz, THF-d8) 157.0, 138.3, 133.7, 132.3, 120.3, 18.0, 14.6, 14.4,
9.1; δB (160 MHz, THF-d8) 2.39 (s). NMR data match those
previously reported.14
BODIPY.17 Bright orange solid (53 mg, 99%). δH (500 MHz, CDCl3)
7.61−7.59 (2H, m), 7.57−7.45 (5H, m), 6.85 (2H, d, J = 4.3 Hz), 6.54
(2H, dd, J = 1.5, 4.3 Hz), 0.67 (4H, q, J = 7.3 Hz), 0.50 (6H, t, J = 7.3
Hz); δC (125 MHz, CDCl3) 146.6, 142.1, 135.2, 134.4, 130.5, 130.0,
128.2, 127.3, 117.2, 29.8, 8.9; δB (160 MHz, CDCl3) 1.38 (s); mp
118−120 °C. HRMS (APCI) m/z: [M + H]+ calcd for C19H22BN2,
289.1871; found, 289.1871.
1,3,5,7-Tetramethyl-2,6-diethyl-8-H-4,4′-propane-1,3-bis-
(olate)-bora-3a,4a-diaza-s-indacene (9). F-BODIPY 117 (50 mg)
was treated with 1 equiv of BCl3 in anhydrous CH2Cl2, and the
reaction mixture was stirred for 1 h. A solution of LiO(CH2)3OLi (1
equiv) in CH2Cl2 was added to the reaction mixture, and stirring was
then continued for another 3 h. The mixture was then washed with
brine (15 mL), and the organic layer was dried over Na2SO4. The
solution was then concentrated in vacuo. The crude material was
purified over silica eluting with 50:50 hexanes/ethyl acetate. The
product was isolated as an orange solid (22 mg, 40%). δH (500 MHz,
CDCl3) 6.90 (1H, s), 4.04 (4H, t, J = 5.8 Hz), 2.54 (6H, s), 2.36 (4H,
q, J = 7.6 Hz), 2.12 (6H, s), 1.93 (2H, dt, J = 5.9, 11.9 Hz), 1.02 (6H,
t, J = 7.6 Hz); δC (125 MHz, CDCl3) 153.7, 136.2, 133.3, 131.0, 119.0
59.9, 28.2, 17.5, 14.9, 13.5, 9.6; δB (160 MHz, CDCl3) 1.55 (s);
decomposition >105 °C. LRMS (ESI+) m/z: [M + H]+ 257.2
(deprotected during ionization to give the free-base dipyrrins17).
1,3,5,7-Tetramethyl-2,6-diethyl-8-H-4,4′-dimethoxy-bora-
3a,4a-diaza-s-indacene (3).14 F-BODIPY 117 (50 mg) was treated
with 1 equiv of BCl3 in anhydrous CH2Cl2, and the reaction mixture
was stirred for 1 h. Solid NaOCH3 (2 equiv) was added to the reaction
mixture, and stirring was continued for another hour. The mixture was
then washed with brine (15 mL), and the organic layer was dried over
Na2SO4. The solution was then concentrated in vacuo to give the title
compound 3 (53 mg, 98%). δH (500 MHz, CDCl3) 6.90 (1H, s), 2.84
(6H, s), 2.47 (6H, s), 2.38 (4H, q, J = 7.5 Hz), 2.17 (6H, s), 1.06 (6H,
t, J = 7.5 Hz); δC (125 MHz, CDCl3) 154.9, 134.6, 133.8, 131.2, 118.4,
49.3, 17.5, 14.9, 12.3, 9.5; δB (160 MHz, CDCl3) 2.66 (s). NMR data
match those previously reported.14
General Procedure for the Synthesis of C-BODIPYs (GP1).
The F-BODIPY (50 mg) was dissolved in anhydrous dichloromethane
(10 mL), and 1 equiv of BCl3 was added dropwise from a 1.0 M
solution in anhydrous hexanes. The reaction was stirred for 1 h to
allow in situ formation of the Cl-BODIPY to occur. The Cl-BODIPY
was then reacted with 2 equiv of EtMgBr using a 3.0 M solution in
anhydrous diethyl ether. Stirring was continued for another hour.
Upon completion of the reaction, the mixture was washed with brine
(15 mL), and the organic layer was dried over Na2SO4. The solution
was then concentrated in vacuo to obtain the BODIPY product.
1,3,5,7-Tetramethyl-2,6-diethyl-8-H-4,4′-diethyl-bora-3a,4a-
diaza-s-indacene (4).14 Using GP1, compound 4 was synthesized
from the corresponding F-BODIPY.17 Bright orange solid (52 mg,
98%). δH (500 MHz, CDCl3) 6.99 (1H, s), 2.44−2.39 (10H, m, 2 ×
(CH3 + CH2)), 2.18 (6H, s), 1.06 (6H, t, J = 7.6 Hz), 0.82 (4H, q, J =
7.6 Hz), 0.31 (6H, t, J = 7.6 Hz); δC (125 MHz, CDCl3) 151.1, 132.6,
131.8, 131.1, 119.4, 17.9, 15.0, 13.9, 9.43, 9.40 (one signal obscured);
δB (160 MHz, CDCl3) 2.50 (s). NMR data match those previously
reported.14
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedures and copies of NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Alison.Thompson@dal.ca.
■
1,3,5,7-Tetramethyl-2,6-diethyl-8-phenyl-4,4′-diethyl-bora-
3a,4a-diaza-s-indacene (5). Using GP1, compound 5 was
synthesized from the corresponding F-BODIPY.17 Bright orange
solid (52 mg, 97%). δH (500 MHz, CDCl3) 7.44 (3H, br app s), 7.29−
7.28 (2H, m), 2.44 (6H, s), 2.32 (4H, q, J = 7.1 Hz), 1.25 (6H, s), 0.97
(6H, t, J = 7.1 Hz), 0.87 (4H, q, J = 7.3 Hz), 0.41 (6H, t, J = 7.0 Hz);
δC (125 MHz, CDCl3) 150.2, 140.9, 137.7, 133.4, 132.4, 131.1, 129.0,
128.8, 128.3, 29.9, 17.6, 15.0, 14.1, 12.0, 9.6; δB (160 MHz, CDCl3)
1.85 (s). NMR data match those previously reported.14
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this research was provided by the Natural
Sciences and Engineering Research Council of Canada
(NSERC) and the Killam Trusts.
1,3,5,7-Tetramethyl-2-ethyl-8-H-4,4′-diethyl-bora-3a,4a-
diaza-s-indacene (6). Using GP1, compound 6 was synthesized
from the corresponding F-BODIPY.17 Bright orange solid (53 mg,
99%). δH (500 MHz, CDCl3) 7.02 (1H, s), 6.02 (1H, s), 2.42 (8H,
two s and an overlapping q, 2 × CH3 + CH2), 2.25 (3H, s), 2.19 (3H,
s), 1.06 (3H, t, J = 7.6 Hz), 0.81 (4H, qd, J = 3.2, 7.6 Hz), 0.32 (6H, t,
J = 7.6 Hz); δC (125 MHz, CDCl3) 153.0, 151.4, 135.2, 133.3, 133.1,
133.0, 132.0, 120.1, 118.3, 17.9, 16.3, 14.9, 14.0, 11.3, 9.4, 9.3 (one
signal obscured); δB (160 MHz, CDCl3) 2.85 (s); mp 128−130 °C;
HRMS (APCI) m/z: [M + H]+ calcd for C19H30BN2, 297.2497; found,
297.2491.
1,3,6-Trimethyl-2-(2-methoxy-2-oxoethyl)-7-ethyl-8-H-4,4′-
diethyl-bora-3a,4a-diaza-s-indacene (7). Using GP1, compound
7 was synthesized from the corresponding F-BODIPY.17 The crude
material was filtered through a plug of silica eluting with CH2Cl2 to
isolate the product. Bright orange solid (40 mg, 75%). δH (500 MHz,
CDCl3) 7.16 (1H, s), 7.15 (1H, s), 3.67 (3H, s), 3.45 (2H, s), 2.62
(2H, q, J = 7.6 Hz), 2.38 (3H, s), 2.24 (3H, s), 2.07 (3H, s), 1.17 (3H,
t, J = 7.6 Hz), 0.84 (2H, dq, J = 7.4, 14.5 Hz), 0.46 (2H, dq, J = 7.4,
14.6 Hz), 0.32 (6H, t, J = 7.6 Hz); δC (125 MHz, CDCl3) 171.8, 153.4,
140.8, 139.4, 135.7, 133.4, 132.0, 124.0, 121.74, 121.71, 52.2, 30.6,
18.0, 16.4, 13.6, 10.3, 9.9, 9.0 (one signal obscured); δB (160 MHz,
CDCl3) 1.83 (s); decomposition >110 °C °C; HRMS (ESI-TOF) m/
z: [M + Na]+ calcd for C21H31BN2NaO2, 377.2371; found, 377.2354.
8-Phenyl-4,4′-diethyl-bora-3a,4a-diaza-s-indacene (8). Using
GP1, compound 8 was synthesized from the corresponding F-
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