Preparation of pyridine-3,4-diols, their crystal packing and their use as precursors for palladium-catalyzed cross-coupling reactions

Article abstract of DOI:10.3762/bjoc.6.42

A series of trifluoromethyl-substituted 3-alkoxypyridinol derivatives has been deprotected to furnish pyridine-3,4-diol derivatives in good yields. The X-ray crystal structure analysis proved that a 1:1 mixture of pyridine-3,4-diols and their pyridin-4-one tautomers exist in the solid state. Subsequent conversion into bis(perfluoroalkanesulfonate)s were smoothly achieved. The obtained compounds were used as substrates for palladium-catalyzed coupling reactions. Fluorescence measurements of the biscoupled products showed a maximum of emission in the violet region of the spectrum.

Full text of DOI:10.3762/bjoc.6.42

Preparation of pyridine-3,4-diols, their crystal
packing and their use as precursors for
palladium-catalyzed cross-coupling reactions
Tilman Lechel, Irene Brüdgam and Hans-Ulrich Reissig*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, No. 42.
Freie Universität Berlin, Institut für Chemie und Biochemie,
Takustrasse 3, D-14195 Berlin, Germany
doi:10.3762/bjoc.6.42
Received: 23 February 2010
Accepted: 09 April 2010
Published: 29 April 2010
Email:
Hans-Ulrich Reissig* - hreissig@chemie.fu-berlin.de
* Corresponding author
Editor-in-Chief: J. Clayden
Keywords:
© 2010 Lechel et al; licensee Beilstein-Institut.
License and terms: see end of document.
bisnonaflates; fluorescence; palladium catalysis; pyridine-3,4-diols;
pyridines
Abstract
A series of trifluoromethyl-substituted 3-alkoxypyridinol derivatives has been deprotected to furnish pyridine-3,4-diol derivatives
in good yields. The X-ray crystal structure analysis proved that a 1:1 mixture of pyridine-3,4-diols and their pyridin-4-one
tautomers exist in the solid state. Subsequent conversion into bis(perfluoroalkanesulfonate)s were smoothly achieved. The obtained
compounds were used as substrates for palladium-catalyzed coupling reactions. Fluorescence measurements of the biscoupled
products showed a maximum of emission in the violet region of the spectrum.
Introduction
Pyridine scaffolds have been found in numerous naturally methyl-substituted pyridine derivatives [18-23]. Herein, we
occurring compounds and are also frequently used in functional report different methods for the deprotection of a range of
materials [1-4]. Pyridindiol derivatives are of particular interest 3-alkoxypyridinols 1 to give pyridine-3,4-diols 2 and the
as building blocks for the construction of dendritic nano- corresponding tautomers 2′. This equilibrium between
structures in supramolecular chemistry [5], whereas N-protected pyridindiols and hydroxypyridinones will be thoroughly
pyridine-3,4-diols find applications as potent chelating agents in investigated in the solid state as well as in solution.
medicinal chemistry [6]. Furthermore, perfluorinated Furthermore, subsequent transformations into bistriflate or
heteroaromatic compounds are interesting synthetic intermedi- bisnonaflate derivatives will be described, followed by
ates for the development of novel pharmaceuticals [7]. palladium-catalyzed coupling reactions. The resulting biscoup-
Continuing our research on heterocyclic chemistry based on ling products are analysed with regard to their photophysical
alkoxyallenes [8-17], we focused on the synthesis of trifluoro- properties.
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Beilstein J. Org. Chem. 2010, 6, No. 42.
Scheme 1: Deprotection of 3-alkoxypyridinols 1 to pyridine-3,4-diols 2. aMethod a: Pd/C, H2, MeOH, rt, 1 d; bMethod b: BBr3, CH2Cl2, 0 °C to rt, 1 d;
cMethod c: TFA:CH2Cl2 (1:2), rt, 1 h.
Results and Discussion
NMR-measurements in CDCl3 showed that the obtained
The preparation of pyridine-3,4-diol derivatives as depicted in pyridine-3,4-diols 2 are in equilibrium with their pyridin-4-one
Scheme 1 succeeded by using highly substituted trifluoro- tautomers 2′. For instance, in case of 2c the equilibrium is
methyl-substituted 4-hydroxypyridine precursors 1 that have strongly shifted to the pyridin-4-one 2c′ (ratio 2c:2c′ = 30:70).
been prepared in two steps from lithiated alkoxyallenes, nitriles This ratio could be completely shifted to the pyridine-3,4-diol
and carboxylic acids [21]. It is noteworthy, that the respective side by a polar protic solvent such as methanol. Surprisingly,
protecting group at C-3 of the pyridine core was originally the X-ray crystal structure measurement of compound 2c [24]
incorporated with the alkoxyallene moiety. The mild cleavage revealed that in the solid state a 1:1 ratio of diol and its
of the benzyl-protected pyridine 1a to diol 2a was achieved by pyridinone tautomer 2c′ is preferred. Figure 1 shows that two
hydrogenolysis in the presence of catalytic amounts of pyridine-3,4-diol molecules are in one plane with two
palladium on charcoal. Methyl ethers such as 1b or 1d were pyridinone molecules in a perpendicular plane. The two altern-
cleaved by Lewis-acids. The (2-trimethylsilyl)ethyl-protected ating planes are connected by hydrogen bridges.
pyridine 1c was easily deprotected to diol 2c by a Brønsted acid
such as TFA. In most cases, the corresponding pyridindiols As a consequence of our ongoing interest in perfluoroalkyl
2a–d were obtained in good yields (63%–quant.).
sulfonate chemistry [25], we converted the two hydroxyl groups
Figure 1: X-ray crystal structure of compound 2c/2c′.
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Beilstein J. Org. Chem. 2010, 6, No. 42.
Pd(OAc)2/PPh3 as catalyst and CuI as co-catalyst in the pres-
ence of a 1:2 mixture of iPr2NH and DMF. Only the
corresponding biscoupled products 4, which were isolated in
moderate yields (30–44%) have been observed. Comparing
entries 2 and 3, the coupling of an alkyne with a bisnonaflate
gave a slightly lower yield than that with the corresponding
bistriflate. A subsequent cleavage of the triisopropylsilyl group
with a fluoride source provided product 5 in 58% yield.
Scheme 2: Conversion of pyridine-3,4-diols 2 into pyridinediyl bistri-
flates or -nonaflates 3. a) Et3N, Rf2O, CH2Cl2, 0 °C to rt, 1 d.
into triflates or nonaflates, respectively (Scheme 2). These
substituents represent very good leaving groups for subsequent
functionalisations such as palladium-catalyzed C–C cross-coup-
ling reactions [26,27]. At first, the pyridindiols 2a and 2c–d
were treated with Et3N in dichloromethane and an excess of
Tf2O or Nf2O, respectively. This provided bistriflates or
bisnonaflates 3a–d as the only products in moderate to very
good yields (47–90%). A direct comparison showed that the
treatment of 2c with Tf2O led to a higher yield than that with
Nf2O. This may be due to lower steric hindrance in the case of
the triflating reagent.
Scheme 3: Sonogashira couplings of pyridinediyl
bis(perfluoroalkanesulfonates) 3. a) Pd(PPh3)4 [or Pd(OAc)2/PPh3],
CuI, iPr2NH, DMF, 70 °C, 4 h. b) TBAF, THF, rt, 1 h.
The biscoupling reaction led to extended π-systems which
As typical examples of possible palladium-catalyzed cross- might have interesting photophysical properties [31-33]. Hence,
couplings we performed several Sonogashira-reactions [28-30]. absorption and emission of 4b and 4c were studied. The results
As described in Scheme 3, the pyridinyl-bistriflates or -nona- are depicted in Figure 2 and show absorption maxima in the
flates 3 were coupled with alkynes like phenylacetylene or range of 275–295 nm whereas the emission maxima are located
(triisopropylsilyl)acetylene using Pd(PPh3)4 or alternatively, between 385–400 nm.
Figure 2: Absorption and fluorescence spectra of compounds 4b and 4c.
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Beilstein J. Org. Chem. 2010, 6, No. 42.
Both products are fluorescent, emitting light in the violet region combined organic phases were dried over Na2SO4 and concen-
and show similar Stokes shifts. Owing to the additional phenyl trated to dryness. Column chromatography on silica gel
substituent at C-2 of pyridine 4c, the π-system is slightly more (hexane/ethyl acetate = 4:1) afforded 222 mg (63%) of 2b as a
extended and obviously influences the absorption and emission colorless solid, mp 192 °C.
maxima with a bathochromic shift of 10 nm.
2-tert-Butyl-5-phenyl-6-(trifluoromethyl)pyridine-3,4-diol (2b):
Conclusion
1H NMR (CD3OD, 500 MHz): δ = 1.45 (s, 9H, t-Bu), 7.24–7.47
In conclusion, we have successfully demonstrated that (m, 5H, Ph) ppm. OH-signals could not be detected. 13C NMR
3-alkoxypyridines are ideal precursors for the synthesis of (CD3OD, 101 MHz): δ = 28.8, 38.6 (q, s, t-Bu), 124.7 (s, C-5),
pyridine-3,4-diol derivatives. The coexistence of pyridindiol 126.3 (q, 1JCF = 274 Hz, CF3), 129.3, 129.4, 131.5, 133.6 (3 d,
and pyridinone tautomers in the solid state was discovered by s, Ph), 138.6 (q, 2JCF = 32.3 Hz, C-6), 144.3, 151.1, 154.3 (3 s,
an X-ray structure analysis. It was shown that pyridine-3,4-diols C-2, C-3, C-4) ppm. IR (KBr): 3450–3260 (O-H, N-H),
could easily be converted into bis(perfluoroalkanesulfonates) 3085–3060 (=C-H), 3040–2880 (C-H), 1655–1585 (C=O, C=C)
which represent substrates for the construction of extended cm−1. HRMS (80 eV, 90 °C) m/z calcd. for C16H16F3NO2:
π-systems using palladium-catalyzed coupling reactions. 311.11331; found: 311.11266.
Moreover, compounds 4b–c show interesting photophysical
properties that might be thoroughly investigated in the future. Cleavage of the (2-trimethylsilyl)ethoxy group with
The 3,4-dialkynyl-pyridine derivatives 4 or 5 are also TFA
candidates for Bergman cyclizations [34,35] which may Pyridine derivative 1c (90 mg, 0.268 mmol) was dissolved in a
establish a route to isoquinoline derivatives.
1:5 mixture of trifluoroacetic acid and dichloromethane (3 mL)
and stirred for 1 h at room temperature. After the addition of
water and dichloromethane (5 mL) the layers were separated
and the aqueous layer was extracted twice with dichloro-
methane (8 mL). The combined organic layers were dried over
Na2SO4 and concentrated to dryness. Column chromatography
(silica gel, hexane/ethyl acetate = 2:1) provided 63 mg (quant.)
of 2c and 2c′ in a ratio of 30:70 as a colorless solid,
mp 102–103 °C.
Experimental
Deprotection of 3-alkoxypyridin-4-ols 1,
typical procedures
Cleavage of the benzyloxy group by hydrogenolysis
A mixture of 1a (970 mg, 3.43 mmol) and palladium
(365 mg, 10% on charcoal, 0.34 mmol) in methanol (6 mL) was
stirred for one day under an atmosphere of hydrogen.
Filtration of the reaction mixture through celite with
methanol afforded 489 mg (74%) of 2a as a colorless solid,
mp 216 °C.
2-tert-Butyl-6-(trifluoromethyl)pyridine-3,4-diol (2c): 1H NMR
(CDCl3, 500 MHz): δ = 1.43 (s, 9H, t-Bu), 7.11 (s, 1H, 5-H)
ppm. OH-signals could not be detected. 13C NMR (CDCl3, 126
2-Methyl-6-(trifluoromethyl)pyridine-3,4-diol (2a): 1H NMR MHz): δ = 28.2, 37.6 (q, s, t-Bu), 106.5 (d, C-5), 115.6 (q, 1JCF
(CD3OD, 500 MHz): δ = 2.41 (s, 3H, Me), 7.00 (s, 1H, 5-H) = 294 Hz, CF3), 132.2 (q, 2JCF = 37.0 Hz, C-6), 142.2, 150.7,
ppm. OH-signals could not be detected. 13C NMR (CD3OD, 154.6 (3 s, C-2, C-3, C-4) ppm.
126 MHz): δ = 17.7 (q, Me), 108.0 (d, C-5), 123.1 (q, 1JCF =
273 Hz, CF3), 139.1 (q, 2JCF = 35.3 Hz, C-6), 139.0, 144.4, 2-tert-Butyl-3-hydroxy-6-(trifluoromethyl)pyridin-4(1H)-one
154.1 (3 s, C-2, C-3, C-4) ppm. IR (KBr): ν = 3350–3240 (O-H, (2c′): 1H NMR (CDCl3, 500 MHz): δ = 1.51 (s, 9H, t-Bu), 6.82
N-H), 3110–3040 (=C-H), 3000–2670 (C-H), 1650–1550 (C=O, (s, 1H, 5-H), 8.43 (sbr, 1H, NH) ppm. OH-signal could not be
C=C) cm−1. HRMS (ESI-TOF) calcd. for C12H9F3NO2 detected. 13C NMR (CDCl3, 126 MHz): δ = 26.7, 34.9 (q, s,
[M+H]+: 194.0423, found: 194.0419. C7H6F3NO2 (193.1): t-Bu), 120.1 (q, 1JCF = 273 Hz, CF3), 108.9 (d, C-5), 138.4 (q,
calcd. C, 43.53; H, 3.13; N, 7.25; found: C, 43.94; H, 3.10; N, 2JCF = 36.4 Hz, C-6), 136.7, 146.5, 170.7 (3 s, C-2, C-3, C-4)
6.95.
ppm. IR (KBr): 3490–3330 (O-H, N-H), 3100–3060 (=C-H),
2960–2870 (C-H), 1710–1580 (C=O, C=C) cm−1.
C10H12F3NO2 (235.2): calcd. C, 51.07; H, 5.14; N, 5.96; found:
Cleavage of the methoxy group by BBr3
To a solution of 1b (370 mg, 1.14 mmol) in dichloromethane C, 50.87; H, 5.04; N, 5.81.
(4 mL) under an argon atmosphere, BBr3 (1 M in CH2Cl2,
Conversion into pyridinediyl bis(perfluoro-
3.42 mL, 3.42 mmol) was added dropwise at 0 °C and allowed
to warm to room temperature. The reaction was monitored by alkanesulfonates), typical procedure
TLC; upon completion, ice-water was added and the mixture Pyridine-3,4-diol 2c (100 mg, 0.425 mmol) was dissolved in
was extracted three times with dichloromethane (5 mL). The dichloromethane (4 mL) and Et3N (0.24 mL, 1.70 mmol) was
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Beilstein J. Org. Chem. 2010, 6, No. 42.
added. The solution was cooled to 0 °C and Tf2O (0.29 mL, tion mixture was diluted with water (3 mL) and extracted three
1.70 mmol) was added dropwise. After stirring for 1 d at room times with ethyl acetate (3 mL). The combined organic phases
temperature the reaction mixture was diluted with water (5 mL) were dried over Na2SO4 and concentrated to dryness. Column
and extracted three times with dichloromethane (5 mL). The chromatography on silica gel (hexane/ethyl acetate = 40:1)
combined organic phases were dried over Na2SO4 and concen- afforded 13 mg (58%) of 5 as a colorless solid, mp 79–81 °C.
trated to dryness. Column chromatography on silica gel
(hexane) afforded 190 mg (90%) of 3b as a colorless oil 2-tert-Butyl-3,4-diethynyl-6-(trifluoromethyl)pyridine (5): 1H
(volatile under high vacuum).
NMR (CDCl3, 500 MHz): δ = 1.54 (s, 9H, t-Bu), 3.57, 3.92 (2
s, 2H, C≡CH), 7.56 (s, 1H, 5-H) ppm. 13C NMR (CDCl3, 126
2-tert-Butyl-6-(trifluoromethyl)pyridine-3,4-diyl bistriflate MHz): δ = 28.7, 39.9 (q, s, t-Bu), 79.5, 79.7, 86.5, 92.5 (2 s, 2 d,
(3b): 1H NMR (CDCl3, 500 MHz): δ = 1.51 (s, 9H, t-Bu), 7.71 C≡CH), 120.1 (dq, 3JCF = 2.8 Hz, C-5), 121.1 (q, 1JCF = 274
(s, 1H, 5-H) ppm. 13C NMR (CDCl3, 126 MHz): δ = 29.5, 40.0 Hz, CF3), 121.3 (q, 4JCF = 1.2 Hz, C-4), 137.0 (s, C-3), 144.5
(q, s, t-Bu), 112.1 (dq, 3JCF = 3.2 Hz, C-5), 118.5, 119.8 (2 q, (q, 2JCF = 35.3 Hz, C-6), 171.0 (s, C-2) ppm. 19F NMR (CDCl3,
1JCF = 321 Hz each, OTf), 120.0 (q, 1JCF = 275 Hz, CF3), 147.2 470 MHz): δ = −68.4 (s, CF3) ppm. IR (KBr): ν = 3305 (≡C-H),
(q, 2JCF = 36.9 Hz, C-6), 136.0, 149.4, 166.5 (3 s, C-2, C-3, 3000–2850 (=C-H, C-H), 2225–2105 (C≡C), 1765–1575 (C=C)
C-4) ppm. z19F NMR (CDCl3, 470 MHz): δ = −68.3 (s, CF3), cm−1. HRMS (ESI-TOF) calcd. for C14H13F3N [M+H]+:
−71.1, −72.4 (2 s, OTf) ppm. IR (film): ν = 3110–3080 (=C-H), 252.0995; found 252.1009.
2980–2880 (C-H), 1600–1575 (C=C) cm−1. C12H10F9NO6S2
(499.3): calcd. C, 28.86; H, 2.02; N, 2.81; found: C, 28.89; H,
Supporting Information
1.68; N 2.87.
Supporting Information File 1 contains the supplementary
Sonogashira coupling reaction, typical
data for compounds 2d, 3a, 3c–d and 4b–c.
procedure
Supporting Information File 1
A mixture of pyridinediyl bistriflate 3b (245 mg, 0.491 mmol),
Pd(PPh3)4 (79 mg, 0.069 mmol), CuI (9.4 mg, 0.049 mmol),
(triisopropylsilyl)acetylene (215 mg, 1.18 mmol) in DMF (2.3
mL) and diisopropylamine (1.2 mL) was heated to 60 °C for 4 h
under an argon atmosphere. The mixture was allowed to cool to
room temperature, diluted with brine (5 mL) and extracted three
Supplementary data for compounds 2d, 3a, 3c–d and 4b–c.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-42-S1.pdf]
times with diethyl ether (5 mL). The combined organic phases Acknowledgements
were dried over Na2SO4 and concentrated to dryness. The Generous support of this work by the Deutsche Forschungsge-
residue was purified by column chromatography on silica gel meinschaft, the Fonds der Chemischen Industrie and the Bayer-
(hexane) followed by HPLC to give 98 mg (35%) of 4a as a Schering Pharma AG is most gratefully acknowledged. We
colorless oil (volatile under high vacuum).
thank Dr. R. Zimmer for his help during preparation of this
manuscript.
2-tert-Butyl-6-(trifluoromethyl)-3,4-bis[(triisopropylsilyl)ethyn-
yl] pyridine (4a): 1H NMR (CDCl3, 500 MHz): δ = 1.09–1.17
(m, 42H, TIPS), 1.56 (s, 9H, t-Bu), 7.51 (s, 1H, 5-H) ppm. 13C
NMR (CDCl3, 126 MHz): δ = 11.4, 11.6, 18.71, 18.74 (2 d, 2 q,
TIPS), 28.7, 40.0 (q, s, t-Bu), 81.6, 90.2, 102.6, 103.7 (4 s,
C≡C), 108.2 (s, C-4), 121.3 (q, 1JCF = 274 Hz, CF3), 121.4 (dq,
3JCF = 3.1 Hz, C-5), 143.7 (q, 2JCF = 34.9 Hz, C-6), 137.4 (s,
C-3), 170.9 (s, C-2) ppm. 19F NMR (CDCl3, 470 MHz): δ =
−68.4 (s, CF3) ppm. IR (film): ν = 2950–2860 (=C-H, C-H),
2145–2065 (C≡C), 1750–1575 (C=C) cm−1. HRMS (ESI-TOF)
calcd. for C32H53F3NSi2 [M+H]+: 564.3663; found 564.3690.
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