Electrochemical Conversion of Phthaldianilides to Phthalazin-1,4-diones by Dehydrogenative N?N Bond Formation

Article abstract of DOI:10.1002/chem.201705578

The electrochemical synthesis of 6-membered rings via anodic dianilide N?N coupling is challenging due to concurring benzoxazole co-formation. The rigidity of the a phthalic acid backbone allows a novel access to phthalazin-1,4-diones by N?N bond formation using anodically generated amidyl radicals. Since conventional synthetic routes to phthalazin-1,4-diones require the use of toxic N,N′-diarylhydrazines and generate reagent waste, a safer and more sustainable approach is required. Easy accessible starting materials, a broad scope of applicable functional groups, promising yields, and a very simple set-up elevate this sustainable method.

Full text of DOI:10.1002/chem.201705578

A Journal of
Accepted Article
Title: Electrochemical Conversion of Phthaldianilides to Phthalazin-1,4-
diones by Dehydrogenative N-N Bond Formation
Authors: Siegfried R Waldvogel, Tile Gieshoff, Anton Kehl, and Dieter
Schollmeyer
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To be cited as: Chem. Eur. J. 10.1002/chem.201705578
Link to VoR: http://dx.doi.org/10.1002/chem.201705578
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Electrochemical Conversion of Phthaldianilides to Phthalazin-1,4-
diones by Dehydrogenative N-N Bond Formation
Anton Kehl,[a] Tile Gieshoff,[a],[b] Dieter Schollmeyer,[a] and Siegfried R. Waldvogel[a],[b],*
Abstract: The electrochemical synthesis of 6-membered rings via
anodic dianilide N-N coupling is challenging due to concurring
benzoxazole co-formation. The rigidity of the a phthalic acid backbone
allows a novel access to phthalazin-1,4-diones by N-N bond formation
using anodically generated amidyl radicals. Since conventional
synthetic routes to phthalazin-1,4-diones require the use of toxic N,N’-
diarylhydrazines and generate reagent waste, a safer and more
sustainable approach is required. Easy accessible starting materials,
a broad scope of applicable functional groups, promising yields, and
a very simple set-up elevate this sustainable method.
is electro-organic chemistry as a ‘green’ alternative.[7] By
employing electric current as inexpensive and sustainable
reducing or oxidizing agent, the amount of waste is tremendously
reduced and the employment of toxic starting materials can be
avoided. Besides, electrochemistry as synthetic tool is inherently
safe and extraordinary reaction pathways are accessible.[8]
Until now, only few electrochemical N-N coupling reactions are
reported.[9] Baran and co-workers developed a potentiostatic
electrolysis protocol in an undivided cell for the
dehydrodimerization of carbazole derivatives.[10] Recently, we
reported an electrolytic synthesis to pyrazolidin-3,5-diones.[11]
The formation of 6-membered rings is challenging compared to 5-
membered rings. In initial studies, the co-formation of
benzoxazoles due to a slower N-N bond formation is the major
obstacle (scheme 1, middle part).[11b,12] The targeted 6-membered
ring was only isolated in a low yield in a single example. Due to
the orientation of the anilide moieties bound to a phthalic acid
backbone, which capitalizes on the rigidity due to its sp2-character,
the requirements for a successful N-N bond formation are present.
Based on these findings, we report a new synthetic route to
phthalazin-1,4-diones (scheme 1, lower part), combining a new
access to 6-membered rings via N-N bond formation with the
installation of an unsaturated backbone.
Phthalazin-1,4-dione derivatives were investigated by Kaufmann
during his pharmaceutical studies in 1927.[1] During the 1950’s,
industrial research confirmed the anesthetic properties of these
derivatives and developed more efficient access to medicinal
chemistry applications based on phthalazin-1,4-diones with a
functionalized phthalic acid backbone in order to replace the
barbiturate based anesthetics with their side effects.[2] Nowadays,
phthalazin-1,4-diones are not used as anesthetic drugs. However,
the heterocyclic motif is apparent in various compounds with anti-
inflammatory, analgesic, antipyretic, anti-hypoxant,[3a] anti-
convulsant,[3b] and anti-bacterial[3c] properties. Furthermore, these
derivatives can also be used as precursors for the synthesis of
pharmaceutically relevant structures.[4]
The general synthetic approach for phthalazin-1,4-diones usually
includes a condensation reaction of activated phthalic acid
derivatives and N,N’-diarylhydrazines (scheme 1, upper part).
General strategies for N-N bond formation involve Cu,[5a]
NiII/NiIII,[5b] RhIII/CuII,[5c] and Pd complexes[5d] or hypervalent iodine
reagents (e.g. PIFA[5e]). Major drawbacks accompany these two
synthetic strategies: Hydrazine derivatives are highly toxic and
carcinogenic.[6] Furthermore, the diversity of commercially
available starting materials is limited and thus upstream reactions
are necessary to achieve more complex substitution patterns.
When metal complexes or stoichiometric amounts of oxidizers or
leaving groups are necessary, this has negative effects onto atom
economy and ecological aspects due to the generation of reagent
waste. Hence, improvements with regards to these distinct
drawbacks are highly desired. A way to meet those requirements
[a]
[b]
A. Kehl, T. Gieshoff, Dr. D. Schollmeyer, Prof. Dr. S. R. Waldvogel
Institut für Organische Chemie
Duesbergweg 10-14, 55128 Mainz (Germany)
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Waldvogel/
E-mail: waldvogel@uni-mainz.de
Scheme 1. Strategies for the synthesis of N-N bonds in 6-membered rings and
our novel approach for phthalazin-1,4-diones.
T. Gieshoff, Prof. Dr. S. R. Waldvogel
Graduate School Materials Science in Mainz
Staudingerweg 9, 55128 Mainz (Germany)
Supporting information for this article is given via a link at the end of
the document.
Easily accessible phthaldianilides as starting materials, an
inexpensive anode material, and a constant current protocol
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demonstrate the general and simple applicability of this approach.
The established methodology is applied to a scope of different
phthaldianilides in order to demonstrate its generality on the one
hand, and to get access to various phthalazin-1,4-dione
derivatives on the other hand. This concept relies on the formation
of amidyl radicals as intermediates, which subsequently form the
N-N bond.[11] The application of amidyl radicals is well-studied in
the context of hydroaminations,[13a] rearrangements,[13b] and
cyclization reactions. [13b,14]
screening.[18] Since we observed variations in the isolated yield in
the course of our scale-up studies, we decided to optimize these
parameters in a 25 mL beaker-type preparative cell as well (Table
1).
Table 1. Influence of the current density onto the yield of 2d.
5 mL Teflon cell[a]
25 mL beaker-type cell[b]
The symmetric precursors can be easily prepared in good yields
by treatment of phthalic dichloride with a corresponding aniline
derivative.[15] Compared to N,N’-diarylhydrazines, much more
aniline derivatives are commercially available. When aniline
derivatives with strong electron withdrawing or releasing
properties are used, phthalic imide can be the major product. This
issue can be effectively circumvented by using a synthetic route,
which also enables access to non-symmetric phthalic dianilides
(Scheme 2; further details in supporting information).[16] Starting
with commercially available phthalic anhydride, the aniline
induced ring opening proceeds smoothly. Subsequent formation
of the isoimide and ring opening with the second aniline enables
a facile access to phthalic dianilides in good yields without further
purification steps.
Current density
Current density
Entry
Yield [%]
Yield [%]
[mA/cm2]
[mA/cm2]
1
2
3
4
5
0.5
1
56
62
60
63
59
1
2
3
4
5
39
55
65
33
35
1.5
2
3
[a] 0.2 mmol substrate 1d, 0.01 M NBu4PF6, anode: graphite, cathode:
platinum, solvent: HFIP, 2.2 F, undivided Teflon cell (5 mL); [b] 1.0 mmol
substrate 1d, 0.01 M NBu4PF6, anode: graphite, cathode: platinum, solvent:
HFIP, 2.2 F, undivided beaker-type cell (25 mL).
The best results were achieved by applying a current density of
3 mA/cm2 in a beaker-type set-up. Compared to the N-N bond
formation in pyrazolidin-3,5-dione synthesis, whereby 0.5 mA/cm2
had to be applied, this results in a shorter reaction time.[11b,13]
Applying the optimized parameters to various phthaldianilide
substrates, we were able to establish a scope of phthalazin-1,4-
dione derivatives with various functional groups and substitution
patterns at the aromatic ring and the phthalic acid backbone
(Scheme 3).
Scheme 2. Synthetic route to non-symmetric precursors. *TFAA = trifluoroacetic
anhydride.
The majority of compounds were synthesized in moderate to good
yields, tolerating chloro- (2d, 2i, 2j, 2l, 2m, 2n, 2o), bromo- (2j,
2k, 2n), triflate- (2i), or nitrile (2l) functionalities, which allow
subsequent reactions. The highest yield of 89% was achieved at
the generation of compound 2o, which contains a methyl
substitution of the phthalic acid backbone. The formation of the N-
N coupled product was confirmed via X-ray structure analysis of
a suitable single crystal of 2d (scheme 3). Electron releasing (2g)
as well as electron withdrawing moieties (2e, 2f, 2i, 2l) are also
compatible and give promising yields. The possibility to convert
non-symmetric dianilides (1h, 1i, 1j, 1k, 1l) displays the significant
value of this methodology. Moreover, substitutions like bromo
(2n) or methyl (2o) are tolerated at the phthalic acid backbone,
whereby further functionalizations are possible. With regards to
heterocyclic moieties, the use of pyridine as backbone function is
tolerated.
The electrolysis conditions for the N-N cyclization were
determined by screening studies, whereby the protocol for the
synthesis of pyrazolidin-3,5-diones served as a starting point.
1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) exclusively enables an
effective conversion. This key function is due to its radical
stabilizing properties.[17] HFIP is recovered almost entirely by
distillation, thus the fluorine footprint can be maintained low.
Additionally, isostatic graphite as inexpensive anode material and
platinum as long-lasting cathode material show best results. We
also tested glassy carbon and boron-doped diamond as anode
materials (supporting information), but their performance was
inferior to the established graphite anode. Remarkably, low
concentrations of supporting electrolyte (0.01 M NBu4PF6) are
sufficient. For a general and easy applicability, a simple set-up
employing an undivided cell rounds the methodology off. Critical
parameters in electro-organic conversions, which demand further
optimization, are current density and the amount of applied
charge. N,N’-Di(4-chlorophenyl)phthaldiamide (1d) was chosen
as test substrate, since this particular anilide moiety is already
proven to achieve high yields in N-N cyclization reactions.[11a] The
optimization reactions were carried out in Teflon cells (reaction
volume:
5 mL), which enable a time-efficient parameter
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exhibits a free para-position, which is also likely to contribute to
the lower yield due to side reactions (see SI, compound 2p).
Interestingly, 1a, a substrate with an exposed benzylic position,
achieved comparably high yields within this scope. Although one
would expect that radicals can be stabilized in the benzylic
position and thus facilitate side reactions. This substrate also
gave similar yields on an almost gram scale procedure, proving
the general applicability of the method in different set-ups and at
different scales. Alkyl amides are generally not convertible due to
a lack of radical stabilization and high oxidation potentials. We
tested this upon our studies of the electrochemical pyrazolidin-
3,5-dione synthesis.
In order to further improve the yields, we tested another
commonly used set-up which contains a three-necked flask
employing a RVC anode and a platinum wire cathode oriented
angular to each other for some substrates (1a, 1b, 1g).[12] For 2g,
an increased yield of 53% was observed. Derivatives 2a and 2b,
on the other hand, are less suitable for this electrode arrangement.
The difference in the influence of this set-up to the reaction
outcome reveals a significant optimization potential (further
details in supporting information). If a specific conversion should
be addressed, changes of the set-up can further improve yields
and performance. Moreover, these results imply that first
investigations can be conducted in very simple electrode and cell
arrangements. In terms of the mechanism, it is likely that the
cyclization derives from a diradical intermediate. We recently
demonstrated that this possibility is most likely for the
electrochemical formation of pyrazolidin-3,5-diones.[11b]
Interestingly, the liberation of the substituted hydrazobenzene is
possible. With an excess of hydrazine (3 eq.), this is an easy and
mild access to azobenzene derivatives as well as to 2,3-
dihydrophthalazin-1,4-dione in excellent yields (Scheme 4). The
nucleophilic nature of hydrazine is sufficient in order to facilitate a
substitution reaction at ambient conditions. The emerging
hydrazobenzene is unstable and undergoes oxidation to yield the
azobenzene derivative. This methodology might be an alternative
to conventional azobenzene synthesis, if functional groups are
neither tolerated nor accessible.
Scheme 3. Optimized parameters for the electrochemical N-N cyclization and
scope of the cyclization. [a] 25 mL beaker-type cell, 1 mmol substrate; [b] 5 mL
Teflon cell, 0.2 mmol substrate; [c] 50 mL three-necked flask with RVC anode
in an angular orientation, 0.6 mmol substrate; [d] 80 mL beaker-type cell,
3.9 mmol substrate.
Scheme 4. Liberation of 4,4’-dichloroazobenzene from 2d.
We established a new and sustainable access to phthalazin-1,4-
diones with avoiding highly toxic and carcinogenic hydrazine
compounds by employing electrochemical synthesis. This
methodology is a valuable alternative to the conventional
synthetic route having the advantage of easily accessible and
inexpensive starting materials. A very simple set-up, metal
catalyst- and organic oxidizer-free conditions as well as scalable
and long-lasting electrode materials provide a straightforward and
Compound 2m was synthesized in 57% yield. When employing
anilides with an electron-rich moiety (1b, 1c, 1g), no complete
conversion was observed during the electrolysis. Increasing the
amount of charge did not improve the yield. An over-oxidation of
the product is unlikely due to the high potential difference of
approximately 300 mV compared to the precursors (cyclic
voltammograms in supporting information). Furthermore, 2c
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Acknowledgements
T. G. is a recipient of a DFG fellowship by the Excellence Initiative
by the Graduate School Materials Science in Mainz (GSC 266).
The support by The Advanced Lab for Electrochemistry and
Electrosynthesis – ELYSION (Carl Zeiss Stiftung) and the DFG
(WA 1276/17-1) is highly appreciated.
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Keywords: electrochemistry • N,N coupling • heterocyclic
chemistry • green chemistry • phthalazin-1,4-diones
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Entry for the Table of Contents (Please choose one layout)
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A. Kehl, T. Gieshoff, D. Schollmeyer, S.
R. Waldvogel*
Page No. – Page No.
Electrochemical Conversion of
Phthaldianilides to Phthalazin-1,4-
diones by Dehydrogenative N-N Bond
Formation
Direct electrochemical synthesis of phthalazin-1,4-diones was realized by anodic
N,N bond generation. Easy accessible phthalic dianilides and a simple electrolysis
set-up ensure a straightforward access to 6-membered ring systems avoiding the
use of highly carcinogenic N,N’-diarylhydrazines as building blocks.
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