Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed Cross-Coupling of N-Fluorinated Sulfonimides with Simple Alkynes

Article abstract of DOI:10.1002/ejoc.202001673

The facile synthesis of aminoallenes, accomplished by a selenium-π-acid-catalyzed cross-coupling of an N-fluorinated sulfonimide with simple, non-activated alkynes, is reported. Until now, aminoallenes were difficult to be accessed by customary means, inasmuch as pre-activated and, in part, intricate starting materials were necessary for their synthesis. In sharp contrast, the current study shows that ordinary internal alkynes can serve as simple and readily available precursors for the construction of the aminoallene motif. The operating reaction conditions tolerate numerous functional groups such as esters, nitriles, (silyl)ethers, acetals, and halogen substituents, furnishing the target compounds in up to 86 % yield.

Full text of DOI:10.1002/ejoc.202001673

Communications
doi.org/10.1002/ejoc.202001673
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Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed
Cross-Coupling of N-Fluorinated Sulfonimides with Simple
Alkynes
Katharina Rode,[a] Poorva Ramadas Narasimhamurthy,[b] Rene Rieger,[b] Felix Krätzschmar,[b]
and Alexander Breder*[b]
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In memory of Professor Kilian Muñiz.
cally differentiated allenes display,[5,6] tremendous efforts have
The facile synthesis of aminoallenes, accomplished by
a
been devoted to their synthesis from easily available
resources.[1,4,7] With regard to the subclass of aminoallenes, the
most common strategies involve SN2’ reactions with nitro-
genous nucleophiles or sigmatropic rearrangements of prop-
argylic substrates (e.g., propargylic bromides, alcohols, and
sulfides, respectively),[8] as well as prototropic isomerizations
and electrophilic functionalizations of propargyl amines and
enynes.[9–11] While each of these strategies has led to great
success and was found to be compatible with many functional
groups, an intrinsic drawback arises from the redox and step
economy of these transformations (Scheme 1).[8a,d–e,9e,h,12] More
precisely, in either case, the adjustment of the final oxidation
state within the target molecule and the establishment of the
desired skeletal connectivity takes place in more than one
chemical operation. Against this background, a catalytic process
that unifies an intermolecular CÀ N bond formation with the
desired oxidation state in the target compound appears to be a
strategically sound complement to the portfolio of classical
selenium-π-acid-catalyzed cross-coupling of an N-fluorinated
sulfonimide with simple, non-activated alkynes, is reported.
Until now, aminoallenes were difficult to be accessed by
customary means, inasmuch as pre-activated and, in part,
intricate starting materials were necessary for their synthesis. In
sharp contrast, the current study shows that ordinary internal
alkynes can serve as simple and readily available precursors for
the construction of the aminoallene motif. The operating
reaction conditions tolerate numerous functional groups such
as esters, nitriles, (silyl)ethers, acetals, and halogen substituents,
furnishing the target compounds in up to 86% yield.
Allenes have been the subject of numerous scientific inves-
tigations over the past few decades, owing largely to their
unique structural features and reactivity profiles.[1] For example,
unlike internal alkynes, 1,3-multisubstituted allenes can exhibit
axial chirality, which can be readily exploited in asymmetric
synthesis.[2–4] In addition, allenes can differ significantly in the
electronic constitution of their two cumulated π-bonds,
depending on the substitution pattern at the termini of the
propadiene subunit.[5] This specific feature is particularly
prominent in allenes possessing heteroatomic residues such as
oxygen or nitrogen moieties in their periphery. Given the
inherent chemo- and regioselectivity that reactions of electroni-
[a] Dr. K. Rode
Institut für Organische und Biomolekulare Chemie
Universität Göttingen
Tammannstr. 2, 37077 Göttingen, Germany
[b] P. Ramadas Narasimhamurthy, R. Rieger, Dr. F. Krätzschmar,
Prof. Dr. A. Breder
Faculty of Chemistry and Pharmacy
University of Regensburg
Universitätsstraße 31, 93053 Regensburg, Germany
E-mail: alexander.breder@ur.de
http://www-oc.chemie.uni-regensburg.de/breder/index.php
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202001673
Part of the “YourJOC Talents” Special Collection.
© 2021 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial License, which permits use,
distribution and reproduction in any medium, provided the original work is
properly cited and is not used for commercial purposes.
Scheme 1. Exemplary overview on synthetic approaches toward heterosub-
stituted allenes.
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protocols. Promising candidates for the aspired title reaction
At the outset of this endeavor, we decided to use 5-decyne
(1) as the model substrate for the optimization studies (Table 1).
Initially, we obtained allene 2a in 45% yield when using
10 mol% of (o-anisyl-Se)2 and 1.0 equiv. NFSI in toluene at
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are selenium-π-acids, as they have been frequently found to
electrophilically activate olefinic and acetylenic π-bonds with
salient chemoselectivity.[13] Typical examples include allylic
alkene functionalizations such as esterifications, etherifications,
nitrogenations, and halogenations.[13a–13d,13f–13I,14] With regard to
alkyne functionalizations, Zhao et al. recently demonstrated the
Se-catalyzed oxidation of carboxylic and phosphonic esters
possessing β,γ-CÀ C triple bonds to give the respective α,β-
ynones,[13k,15] and Liu et al. reported a visible-light mediated
synthesis of oxazole acetals from N-propargylamides using
selenium-π-acid catalysts.[16] In some earlier work by Back et al.
it was shown that 1,2-selenosulfonylated alkenes, derived by
radical addition of selenosulfonates to terminal alkynes, would
undergo facile elimination under oxidative conditions to
provide access to allenic sulfones.[17] Based on our previous
mechanistic investigations on photo- and electrocatalytic allylic
°
100 C (entry 1). No reaction was observed in the absence of the
diselenide (entry 2). We then proceeded by examining different
solvents for the title reaction. Ethereal solvents such as THF and
1,4-dioxane gave the desired allene 2a in yields comparable to
that obtained with toluene, while TCE led to a diminished yield
of only 19% (entries 3–5). Having decided to proceed with
toluene as the solvent of choice, we went on to optimize the
amount of NFSI required for the reaction. With 1.2 equivalents
of NFSI, an increased yield of 53% was observed. However,
using 3.0 equivalents led to a reduced yield of 41% (entries 6
and 7). Additionally, we hypothesized that a base could further
enhance the elimination of the selenium moiety, facilitating the
formation of the desired aminoallene. To substantiate our
hypothesis, we individually added 1.0 equivalent of NaOAc,
K2HPO4, or Li2CO3 to the reaction. The acetate was observed to
impede the reaction, giving the desired product in a very low
yield of 9% (entry 8). The hydrogen phosphate increased the
yield to 65% and the carbonate led to an even higher yield of
68% (entries 9 and 10). Occasionally, we also observed the
formation of a side-product upon full conversion of alkyne 1,
which we identified as adduct 4 (Scheme 2). To minimize the
formation of this selenofunctionalized product, we decreased
the catalyst loading to 2.5%, resulting in the formation of the
desired product with a 77% yield (entries 11 and 12).
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functionalizations
of
alkenes
using
selenium-π-acid
catalysts,[14,18] we wondered whether the stepwise addition-
elimination sequence described by Back et al. could be
translated into a corresponding allenic amination protocol of
alkynes that is merged into a single catalytic cycle. As a result of
these considerations, we disclose herein the first example of a
selenium-π-acid-catalyzed cross-coupling between N-fluoroben-
zenesulfonimide (NFSI) and a broad series of both functional-
ized and non-functionalized, electronically unbiased alkynes to
furnish an extended set of aminoallenes, with NFSI acting as
both the nucleophile and terminal oxidant.
Prof. Alexander Breder studied chemistry at
the University of Bielefeld, Germany, and
received his diploma degree in 2005. Sub-
sequently, he moved to the Swiss Federal
Institute of Technology Zurich (ETH), Switzer-
land, where he joined the group of Prof. Erick
M. Carreira. During his doctoral studies, he
was working on the synthesis of marine
natural products. Upon completion of his
Ph.D. in 2009, he joined the group of Prof.
Barry M. Trost for a postdoctorate at Stanford
University, USA, where he investigated ruthe-
nium-catalyzed domino- and consecutive re-
actions. In late 2011, he started his independ-
ent research career at the Georg-August-
University Goettingen, Germany, where he
completed his habilitation in 2017. Since April
2019, he is a Professor of Organic Chemistry at
the University of Regensburg, Germany.
Dr. Katharina Rode studied chemistry at the
Georg-August-University Göttingen, Germany.
She obtained her Master’s degree with a
thesis on the synthesis of chiral thioamides in
the group of Prof. Alexander Breder and
continued her Ph.D. studies in the same
group. In 2020, she completed her doctorate,
during which she focused on selenium-π-acid-
catalyzed functionalizations of alkenes and
alkynes.
Rene Rieger studied chemistry at the Georg-
August-University Göttingen, Germany. In
2016, he obtained his master’s degree in
chemistry with
a thesis on the oxidative
lactonization of alkenoic acids using dual
selenium catalysis in the group of Prof.
Alexander Breder. He is currently continuing
his dissertation in the same group, working on
selenium-catalyzed oxidative functionaliza-
tions of olefins.
Dr. Felix Kraetzschmar studied chemistry at
the Georg-August-University Göttingen, Ger-
many. He obtained his Master’s degree in
2014 with the thesis ‘Synthesis of 1,3-Diary-
lpropenes for the Selenium-catalyzed Acylox-
ylation’ in the group of Prof. Alexander Breder
and continued his Ph.D. studies in the same
group. In 2020, he obtained his Ph.D. degree
with the thesis ‘Development of Regio- and
Enantioselective Transformations of Alkenes
with λ3-Iodane-Reagents and Chiral Selenium-
π-Acid Catalysts’.
Poorva Narasimhamurthy studied chemistry at
St. Joseph’s College, Bangalore, India. In 2018,
she obtained her Master’s degree with a thesis
on the functionalization of resorcinarenes in
the group of Prof. Gerald Dyker, Ruhr Univer-
sity Bochum, Germany. She started her Ph.D.
in 2018 in the group of Prof. Alexander Breder
and is currently working on selenium-π-acid-
catalyzed functionalizations of alkenes and
alkynes
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Table 1. Optimization of the reaction conditions.
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4
5
6
Yield [%][a]
7
Entry
x [mol%]
y (equiv)
Base
Solvent
8
9
1
2
3
4
5
6
7
8
10
–
1.0
1.0
1.0
1.0
1.0
1.2
3.0
1.2
1.2
1.2
1.2
1.2
–
–
–
–
–
–
–
toluene
toluene
THF
1,4-dioxane
TCE
toluene
toluene
toluene
toluene
toluene
toluene
toluene
45
–
10
10
10
10
10
10
10
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5
40
36
19
53
41
9
65
68
74
77
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NaOAc
K2HPO4
Li2CO3
Li2CO3
Li2CO3
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12
2.5
1
[a] Reactions were carried out on a 1.0 mmol scale. Yields were determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard.
Scheme 2. Mechanistic hypothesis on the oxidative selenium-π-acid-catalyzed synthesis of aminoallenes from simple alkynes.
With optimized conditions in hand, we were interested in
unveiling the scope and limitations of the reaction (Table 2 and
Table 3). We began our investigations with unsymmetrical
alkynes (Table 2) to investigate the regiochemical preference of
the title reaction. With different functional groups, such as
carbonate, ester, and (silyl)ether moieties, corresponding allenes
2 and 3 were obtained as regioisomeric mixtures (ratios
between 1:1 to 2.7:1) in moderate to good yields ranging
between 46–75% (2b–f and 3b–f). In general, we observed a
slight preference for the incorporation of the imide moiety in
the distal position of the allene relative to any of the pre-
exisiting heteroatomic functionalities. We speculate that this
trend is due to electronic effects since acyloxy and carbonate
residues led to slightly higher regiochemical differentiation
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Table 2. Scope of the selenium-π-acid-catalyzed synthesis of aminoallenes
Table 3. Scope of the selenium-π-acid-catalyzed synthesis of aminoallenes
1
2
using unsymmetric alkynes.[a]
using symmetric alkynes.[a]
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[a] Reactions were carried out on a 1.0 mmol scale. Crude yields were
determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as
internal standard; isolated yields in parentheses; rr=regioisomeric ratio.
than ether residues of similar steric demand (e.g., 2e/3e vs. 2f/
3f).
[a] Reactions were carried out on a 1.0 mmol scale. Crude yields were
determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as
internal standard; isolated yields in parentheses.
1
Alkynes 1g and 1h, which only contain a methyl group at
one terminus of the respective triple bond, gave the corre-
sponding allenes with a moderate yield of 32% (2g/3g), and
28% (2h/3h). For these products, C–N bond formation again
occurred preferentially at the distal end of the newly formed
allene moiety relative to the silyl ether (1.5:1) and pivalate
group (2.7:1), respectively. However, in the case of secondary
alkyne 1i, regioisomer 2i was the only product isolated, with a
yield of 15%. This could be due to the greater steric hindrance
from the isopropyl group, pushing the bulky selenium moiety
to the less encumbered position of the alkyne and thus
promoting the nucleophilic attack at the sterically more
demanding position.
Motivated by these results, we shifted our attention to
symmetrical acetylenes (Table 3). Alkynes with varying chain
lengths, branching patterns, and substituents (1a, 1j, 1k–1l)
gave the corresponding allenes 2a, 2j, and 2k–2l in yields of
42–86%. Furthermore, functional groups such as esters and
ethers were also found to be well-tolerated under the reaction
conditions. Alkyne 1m and PMBO-substituted alkyne 1p gave
the corresponding allenes in moderate to good yields of 49 and
74%, respectively. Substrates 1n and 1s possessing aromatic
ester functionalities gave the corresponding allenes in moder-
ate yields of 49 and 64%, respectively. Additionally, substrates
with various substituents on the aromatic ring were also
tolerated well under the reaction conditions, yielding the
corresponding allenes in yields of 59 to 65% (2o, 2q, and 2r).
The efficacy of the title reaction was found to be limited to
internal alkynes, as no product formation was observed with
terminal alkynes. A potential reason for this result might be an
insufficient nucleophilicity of terminal alkynes under the title
reaction conditions.
We proceeded in our study with an investigation of the
reaction mechanism. To determine whether adduct 4 observed
during the optimization studies was an intermediate of the
catalytic cycle, we attempted to convert it into the desired
product by conducting the title reaction without additional
diselenide under otherwise unchanged conditions. With these
deviations from the standard protocol, the desired allene was
obtained in a very low yield of 8% (Equation 1). This
observation is to some degree surprising as previous inves-
tigations on the allylic functionalization of alkenes had shown
that selenofunctionalization intermediates analogous to com-
pound 4 would complete the turnover upon exposure to a
proper oxidant.[14a] Consequently, simple oxidation of the
selenium moiety followed by its elimination does not seem to
be the dominant reaction pathway. Next, we added 0.5
equivalents of (PhSe)2 to a mixture of adduct 4 and 1.2
equivalents of NFSI (Equation 2). We hypothesized that the
elimination event of the aryl selenium moiety may require
activation by another selenium electrophile generated in situ
from NFSI and (PhSe)2. Unfortunately, this idea did not meet
with success, as we did not observe any product formation.
During our optimization studies, we observed that the reaction
stagnated upon full consumption of the alkyne. Thus, we
speculated that another alkyne molecule might assist as a
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Lewis-base in the elimination of the selenium moiety from
A.B.) and the Lower Saxony Ministry for Science and Culture
(Georg-Christoph-Lichtenberg Fellowship to K.R.). Open access
funding enabled and organized by Projekt DEAL.
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adduct 4 to result in the formation of the desired allene (cf.
proposed Lewis-acid/Lewis-base pair 8, Scheme 2). Correspond-
ingly, we reacted adduct 4 with 1.0 equiv. of 2,9-dimethyldec-5-
yne in the presence of NFSI (0.95 equiv). Under these conditions
the reaction indeed displayed turnover, giving access to a Conflict of Interest
mixture of allenes 2a and 2k derived from compound 4 and
2,9-dimethyldec-5-yne. In addition, we also obtained a mixture
of the two aminoselenation adducts 4 and 5, which indicated
that the presence of an alkyne is pivotal for full turnover
(Equation 3).[19] Although the mechanistic details are not fully
understood as yet, the transfer of selenenium moieties between
alkynes seems reminiscent of the transfer of chalcogenenium
ions between olefins reported, i.a., by Denmark et al.[20]
The authors declare no conflict of interest.
Keywords: π-Acids · Amination · Homogeneous catalysis ·
Oxidations · Selenium
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of the diselenide by NFSI, followed by the addition onto alkyne
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selenoamination adduct 4.[22] After oxidation of the selenium
moiety and coordination onto another alkyne molecule, poten-
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are formed.
In summary, we reported the first metal-free, selenium-π-
acid-catalyzed synthesis of N-allyl sulfonimides using alkynes as
starting materials and NFSI as the nucleophile and terminal
oxidant. Unsymmetrical alkynes gave the corresponding regioi-
someric allenes in yields of up to 75%, while symmetric alkynes
gave the corresponding allenes in yields of up to 86%.
Furthermore, we propose a mechanism for the desired trans-
formation, accounting for the role played by each of the
participating reagents.
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Acknowledgements
This work was supported by the European Research Council (ERC
Starting Grant “ELDORADO” [grant agreement No. 803426] to
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© 2021 The Authors. European Journal of Organic Chemistry published
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