Synthesis of highly functionalized 2,2′-bipyridines by cyclocondensation of β-ketoenamides - Scope and limitations

Article abstract of DOI:10.3762/bjoc.12.112

The scope of a flexible route to unsymmetrically functionalized bipyridines is described. Starting from 1,3-diketones 1a-e, the corresponding β-ketoenamines 2a-e were converted into different β-ketoenamides 3a-g by N-acylation with 2-pyridinecarboxylic acid derivatives. These β-ketoenamides were treated with a mixture of TMSOTf and Hünig's base to promote the cyclocondensation to 4-hydroxypyridine derivatives. Their immediate O-nonaflation employing nonafluorobutanesulfonyl fluoride provided the expected 4-nonafloxy-substituted bipyridine derivatives 5a-g in moderate to good overall yields. The bipyridyl nonaflates are excellent precursors for palladium-catalyzed reactions as demonstrated by representative Suzuki and Sonogashira couplings. Thus, a library of specifically substituted bipyridine derivatives was generated, showing the versatility of the simple 1,3-diketone-based approach to this important class of ligands.

Full text of DOI:10.3762/bjoc.12.112

Synthesis of highly functionalized 2,2'-bipyridines by
cyclocondensation of β-ketoenamides – scope and limitations
Paul Hommes and Hans-Ulrich Reissig*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1170–1177.
Freie Universität Berlin, Institut für Chemie und Biochemie,
Takustrasse 3, D-14195 Berlin, Germany
doi:10.3762/bjoc.12.112
Received: 26 February 2016
Accepted: 17 May 2016
Published: 09 June 2016
Email:
Hans-Ulrich Reissig* - hreissig@chemie.fu-berlin.de
* Corresponding author
Associate Editor: B. Stoltz
Keywords:
© 2016 Hommes and Reissig; licensee Beilstein-Institut.
License and terms: see end of document.
2,2-bipyridines; cross couplings; cyclocondensation; β-ketoenamides;
nonaflates
Abstract
The scope of a flexible route to unsymmetrically functionalized bipyridines is described. Starting from 1,3-diketones 1a–e, the cor-
responding β-ketoenamines 2a–e were converted into different β-ketoenamides 3a–g by N-acylation with 2-pyridinecarboxylic acid
derivatives. These β-ketoenamides were treated with a mixture of TMSOTf and Hünig’s base to promote the cyclocondensation to
4-hydroxypyridine derivatives. Their immediate O-nonaflation employing nonafluorobutanesulfonyl fluoride provided the ex-
pected 4-nonafloxy-substituted bipyridine derivatives 5a–g in moderate to good overall yields. The bipyridyl nonaflates are excel-
lent precursors for palladium-catalyzed reactions as demonstrated by representative Suzuki and Sonogashira couplings. Thus, a
library of specifically substituted bipyridine derivatives was generated, showing the versatility of the simple 1,3-diketone-based ap-
proach to this important class of ligands.
Introduction
In 2009 we reported on a new pyridine synthesis starting from nonafluorobutanesulfonyl fluoride (NfF) as sulfonylating
symmetrically substituted 1,3-diketones 1a and 1b, respective- reagent. In subsequent publications we could demonstrate that
ly, that were converted into the corresponding β-ketoenamines this method can be used to synthesize a variety of functionali-
and subsequently by N-acylation into β-ketoenamides such as zed pyridine derivatives, 2,2´-bipyridines or terpyridines [2-4].
3a or 3b [1]. Their cyclocondensation followed by O-alkylation An alternative entry to the crucial β-ketoenamides via a Blaise
or O-nonaflation led to functionalized pyridine derivatives. reaction was also developed [5], however, only a limited num-
Scheme 1 illustrates this sequence with the synthesis of two ber of examples for the preparation of bipyridine derivatives
2,2´-bipyridine derivatives 4a or 5b that were obtained by em- have been reported so far. Hence we describe here additional
ploying picolinic acid chloride for the N-acylations followed by experiments leading to this important class of compounds, in
O-methylation with methyl iodide or by O-nonaflation using order to define scope and limitations of this reaction sequence.
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Scheme 1: Synthesis of highly functionalized 2,2'-bipyridines 4a and 5b from symmetrical 1,3-diketones 1a and 1b; TMSOTf = trimethylsilyl trifluoro-
methanesulfonate; DIPEA = N,N-diisopropylethylamine; NfF = nonafluorobutanesulfonyl fluoride.
For this purpose we prepared other β-ketoenamides, studied toluene [24]. The reaction of 1e with ammonium formate
their cyclocondensation reactions and used the present function- merely provided 2e in 42% yield.
alities for subsequent reactions, in particular palladium-cata-
lyzed coupling reactions.
The corresponding β-ketoenamides containing pyridine
moieties at the acylated nitrogen were prepared by standard
2,2´-Bipyridines are a very important class of ligands employed methods with the respective acid chlorides that furnished com-
in catalysis, supramolecular chemistry or materials science and pounds 3c and 3d in good yields. For the synthesis of com-
hence a large number of specifically substituted derivatives is pound 3e it was advantageous to employ the benzotriazole de-
known (for reviews and selected recent syntheses or applica- rivative 2-PyCOBt [25,26]. During an attempt to prepare
tions see [6-20]). Due to the flexible and modular nature of our β-ketoenamide 3a by in situ generation of picolinic acid chlo-
approach to this compound class the design of new derivatives ride with thionyl chloride under heating and by amide forma-
is possible thus supplementing the existing collection of avail- tion of the resulting product with 2a we isolated not only the ex-
able 2,2´-bipyridines.
pected compound 3a but as major component the meta-chlori-
nated compound 3f. This subsequent chlorination of picolinic
acid chloride is known from the literature to proceed under
Results and Discussion
First we prepared the corresponding β-ketoenamines 2 required enforced conditions [27,28]. On the other hand, a selective syn-
for the N-acylation step (Scheme 2). The known compounds 2a thesis of 3a is possible in 73% yield by coupling of β-enamino-
and 2b are available in excellent yields from the 1,3-diketones ketone 2a with picolinic acid under assistance of BOP as
1a and 1b employing aqueous ammonia in the presence of silica reagent (see Supporting Information File 1). The in situ genera-
gel [21]. Compound 2c was prepared from 1c in 87% yield with tion of the acid chloride with thionyl chloride was possible
ammonium formate [22,23]. Interestingly, treatment of 1c with without the undesired chlorination with the more electron-defi-
aqueous ammonia/silica gel failed to give 2c, deacetylation to cient 6-methoxycarbonyl-substituted 2-pyridine carboxylic acid
1-phenylpropan-2-one (76% yield) was observed instead. For delivering with 2a the desired β-ketoenamide 3g with excellent
the amination of benzyl-substituted diketone 1d both of the efficacy.
above utilized methods failed to provide the desired compound
2d. While treatment with ammonium formate exclusively leads We also examined an alternative approach to β-ketoenamides
to deacetylation of the starting material to 4-phenylbutan-2-one, by the addition of carboxylic acid amides to alkynones. This ap-
the reaction with aqueous ammonia/silica gel provided a mix- proach can potentially lead to products that are formally derived
ture of the desired compound 2d and the deacetylation product. from unsymmetrically substituted 1,3-dicarbonyl compounds.
In both methods slightly acidic reagents are employed, in The best result was obtained with picolinic acid amide and but-
contrast strictly basic conditions provided by the reaction of 1d 3-yn-2-one (as an equivalent of 3-oxobutanal) using methanol
with a 7 M solution of ammonia in methanol allowed preparing as solvent in a sealed tube [29]. Disappointingly, the maximum
the desired β-ketoenamine 2d in a reasonable yield. The reac- yield for compound 3h was only 22%. Many attempts to
tion of compound 1e with NH3/SiO2 also mainly induced de- improve this result – including gold catalysis [30] – either led to
composition by deacetylation but the desired β-ketoenamine 2e lower yields or no conversion at all [31]. We have to conclude
could be prepared in 70% yield using ammonium acetate in that this approach to β-ketoenamides is not efficient.
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Scheme 2: Synthesis of β-ketoenamines 2c–e and of β-ketoenamides 3c–h.
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With N-pyridyl-substituted β-ketoenamides 3a–h in hand, we cursor 3h that should lead to compound 5h with all substituents
investigated their cyclocondensations to the corresponding 2,2´- R1–R5 being hydrogen could not be isolated. We assume that
bipyridine derivatives 5a–h (Table 1). As in our preliminary ex- the missing bulk of substituents led to undesired side reactions,
periments the use of trimethylsilyl trifluoromethanesulfonate e.g., deacylation or oligomerization processes.
and Hünig’s base (DIPEA) gave the best results if the mixture
was heated for 3 d in 1,2-dichloroethane at 90 °C under fairly The development of the discussed pyridine syntheses by cyclo-
high dilution (ca. 0.04 M). Higher concentrations as reported condensation reactions of β-ketoenamides was inspired by a
earlier [1] gave inferior results, probably due to increased serendipitously discovered three-component reaction of lithi-
amounts of intermolecular reactions leading to oligomeric side ated alkoxyallenes, nitriles and carboxylic acids [33-40]. These
products. The resulting primary condensation products contain a components form α-alkoxy-substituted β-ketoenamides as pre-
4-hydroxy group in the newly formed pyridine ring that also cursors of highly substituted pyridine derivatives. We therefore
exists as 4-pyridone tautomer making the identification and tried to independently prepare β-ketoenamides with this substi-
purification at this stage inconvenient. We therefore removed tution pattern starting from simple 1,3-diketones such as 1a. An
all volatile components from the crude products of the cyclo- oxidative coupling of acetylacetone 1a employing iodosylben-
condensation step and subjected the unpurified compounds to zene in the presence of boron trifluoride in methanol [41] fol-
the O-nonaflation procedure. Treatment with an excess of sodi- lowed by the standard amination procedure indeed furnished
um hydride in THF followed by reaction with nonafluorobu- β-enaminoketone 2i in moderate yield (Scheme 3). This inter-
tanesulfonyl fluoride (NfF) afforded the desired 2,2´-bipyridine mediate could be N-acylated via its potassium salt with acti-
derivatives 5a–g containing the 4-nonafloxy substituent in mod- vated picolinic acid to furnish the desired α-methoxy-β-ketoen-
erate to good overall yields. This functional group fixes the amide 3i in 50% yield. This sequence demonstrates that this
tautomeric structure and makes the products less polar which type of β-ketoenamide precursor is also available by this oxida-
allowed smooth isolation, purification and identification. An ad- tive method; in this case the direct preparation of 3i by the
ditional advantage of this substituent is its ability to allow tran- multicomponent route employing methoxyallene was similarly
sition-metal-catalyzed coupling reactions [32]. Entries 1–7 of efficient (22% yield, [39]). It has been earlier demonstrated that
Table 1 demonstrate that quite differently substituted 2,2´- 3i can be cyclized and O-methylated to the corresponding
bipyridine derivatives are accessible by this method. Only pre- bipyridine derivative 4i in 53% yield (see Scheme 4) [39].
Table 1: Attempted TMSOTf/DIPEA-promoted cyclocondensations of β-ketoenamides 3a–h and subsequent O-nonaflation leading to 2,2'-bipyridines
5a–h.
Entry
β-Keto-
enamide
R1
R2
R3
R4
R5
2,2'-Bipyrid-4-yl
nonaflate
(yield)a
1
2
3a
3b
3c
3d
3e
3f
H
H
H
H
H
H
Cl
H
H
Me
Et
H
H
H
Me
H
5a (75%)
5b (51%)b
5c (86%)
5d (54%)
5e (52%)
5f (66%)
5g (51%)
5h –d
H
3
CO2Me
Me
Ph
Bn
H
4
H
Me
H
5c
6
H
H
CO2Et
Me
H
H
H
7
3g
3h
CO2Me
H
Me
H
H
8
H
H
H
aYield over two steps. bExample taken from ref. [1]. cNf2O was used instead of NfF. dNo defined products isolated.
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Scheme 3: Synthesis of α-methoxy-β-ketoenamine 2i, its N-acylation to 3i and the reaction of β-ketoenamide 3a with iodosylbenzene to 5-acetyloxa-
zole 6.
When we tried to employ the oxidative methoxylation to As mentioned above β-ketoenamide 3i is a good precursor for
β-ketoenamide 3a the expected methoxy-substituted product 3i 2,2´-bipyridine derivative 4i, but Scheme 4 also reveals that this
was not formed. Instead, we isolated the trisubstituted oxazole intermediate could be converted into the expected pyrimidine
derivative 6 in 55% yield. The scope of this reaction has not derivative 7 by treatment with ammonium acetate [48-50] or
been studied by us, however it is known in the literature that into pyrimidine N-oxide 8 by employing hydroxylamine hydro-
enamides can undergo related oxidative cyclizations to oxazoles chloride [51,52]. These products of simple condensation reac-
in the presence of the reagent cocktail employed or under tions may be regarded as azabipyridine derivatives and their
related reaction conditions [41-45]. In this context we want to synthesis demonstrates the versatility of β-ketoenamide such as
notice that our approach to alkoxy-substituted β-ketoenamides 3i for the synthesis of highly functionalized heterocycles.
via alkoxyallenes also allowed the synthesis of 5-acetyloxa-
zoles with high flexibility, if the acid-labile trimethylsilylethyl The 2,2´-bipyridyl nonaflates prepared by the methods de-
group was introduced via the alkoxyallene [46,47].
scribed above are excellent and versatile precursors for transi-
Scheme 4: Cyclizations of β-ketoenamide 3i leading to 2,2´-bipyridine derivative 4i or to the related 2-(2-pyridyl)pyrimidine 7 or the corresponding
N-oxide 8.
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Beilstein J. Org. Chem. 2016, 12, 1170–1177.
tion-metal-catalyzed coupling reactions [32,53-57]. Standard
conditions for a Suzuki coupling smoothly transform com-
pound 5a and a boronic acid derivative into the expected
4-tolyl-substituted derivative 9 and the higher substituted pre-
cursor 5b was converted into compound 10 by employing an
acetyl-substituted boronic acid derivate in very good yield
(Scheme 5). Compound 10 was prepared in order to investigate
its capability to form a C3-symmetric star-shaped product by
cyclocondensation of the acetyl group. This type of aldol con-
densation that generates a new benzene ring from three acetyl
groups was successfully applied in our group in other cases
delivering compounds whose 2D self-assembly at highly
ordered pyrolytic graphite was studied by STM methods
[58,59]. However, in the case of compound 10 no full conver-
sion to the desired star-shaped compound was observed under
the applied reaction conditions (SiCl4, EtOH) [60,61], very
likely due to the insolubility of the intermediates involved in
this multistep process.
Scheme 5: Suzuki-couplings of 2,2'-bipyrid-4-yl nonaflates 5a and 5b
to compounds 9 and 10.
The chloro-substituted 2,2´-bipyridine derivative 5g – available gashira reaction with (cyclopropyl)ethyne transformed interme-
due to the accidental chlorination of the picolinic acid precur- diate 11 into the alkynyl-substituted 2,2´-bipyridine derivative
sor (Scheme 2) – offered the option to perform two subsequent 13. Again the yield of this second coupling step is only moder-
coupling reactions in a controlled fashion. In the first step, the ate since the chloro compound 11 is not very reactive; in both
more reactive nonaflate moiety of 5g was efficiently substi- cases even after two days reaction time ca. 50% of the precur-
tuted by a phenyl group under standard Suzuki reaction condi- sor was re-isolated. Nevertheless, the examples in Scheme 5
tions (Scheme 6). The untouched chloro substituent in the and Scheme 6 clearly demonstrate that by suitable subsequent
second pyridine ring was then replaced by another aryl substitu- reactions our library of unsymmetrically substituted 2,2´-bipyri-
ent leading to product 12 in 54% yield. Alternatively, a Sono- dine derivatives 5 can easily be enlarged in a flexible manner.
Scheme 6: Palladium-catalyzed couplings of chloro-substituted 2,2'-bipyrid-4-yl nonaflate 5g leading to compounds 11, 12 and 13.
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4. Hommes, P.; Fischer, C.; Lindner, C.; Zipse, H.; Reissig, H.-U.
Angew. Chem., Int. Ed. 2014, 53, 7647–7651.
doi:10.1002/anie.201403403
Conclusion
In this report we could show that the scope of the cycloconden-
sation reaction of β-ketoenamides 3 leading to unsymmetrically
substituted bipyridine derivatives 5 is fairly broad and compati-
ble with various functional groups. Only substrates such as 3h
having no “superfluous” substituents did not provide the ex-
pected bipyridine skeleton. Substrates 3 with higher substitu-
tion degree provided the expected bipyridine derivatives that
were directly converted into the corresponding nonafloxy-
substituted compounds 5 in good overall yield. These are excel-
lent precursors for palladium-catalyzed coupling reactions as
exemplified by the synthesis of compounds such as 9–13. By an
attempt to prepare the methoxy-substituted β-ketoenamide 3i by
oxidative methoxylation of compound 3a we unexpectedly ob-
served the formation of the 5-acetyl-substituted oxazole deriva-
tive 6. However, precursor 3i was smoothly available from
acetylacetone 1a. It could smoothly be transformed into the
desired bipyridine derivative 4i, but also into the related pyrimi-
dine and pyrimidine N-oxide derivatives 7 and 8. Altogether,
we could confirm that our approach to functionalized pyridine
derivatives by cyclocondensation of β-ketoenamides is also
a very versatile and flexible method for the synthesis of un-
symmetrically substituted bipyridine derivatives or related
heterocyclic compounds that are not accessible by alternative
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Acknowledgements
This work was generously supported by the Deutsche
Forschungsgemeinschaft and by Bayer HealthCare. We thank
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for the synthesis of compound 5a and Dr. Reinhold Zimmer for
his help during the preparation of this article.
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