Intramolecular Aminoazidation of Unactivated Terminal Alkenes by Palladium-Catalyzed Reactions with Hydrogen Peroxide as the Oxidant

Article abstract of DOI:10.1021/acs.orglett.0c00010

The palladium-catalyzed aminoazidation of aminoalkenes yielding azidomethyl-substituted nitrogen-containing heterocycles was developed. The procedure requires oxidative conditions and occurs at room temperature in the presence of hydrogen peroxide and NaN₃ as the azide source. These conditions provide selective exo-cyclization/azidation of the carbon-carbon double bond, furnishing a versatile approach toward five-, six-, and seven-membered heterocyclic rings.

Full text of DOI:10.1021/acs.orglett.0c00010

pubs.acs.org/OrgLett
Letter
Intramolecular Aminoazidation of Unactivated Terminal Alkenes by
Palladium-Catalyzed Reactions with Hydrogen Peroxide as the
Oxidant
Francesca Foschi, Camilla Loro, Roberto Sala, Julie Oble, Leonardo Lo Presti, Egle M. Beccalli,
Giovanni Poli, and Gianluigi Broggini*
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ABSTRACT: The palladium-catalyzed aminoazidation of aminoalkenes
yielding azidomethyl-substituted nitrogen-containing heterocycles was
developed. The procedure requires oxidative conditions and occurs at
room temperature in the presence of hydrogen peroxide and NaN3 as the
azide source. These conditions provide selective exo-cyclization/azidation
of the carbon−carbon double bond, furnishing a versatile approach
toward five-, six-, and seven-membered heterocyclic rings.
Scheme 1. Intramolecular Aminoazidation of Olefins from
Previous Works and This Work
icinal difunctionalization of unactivated alkenes has
1
Vrecently become a powerful tool. Various combinations
of functional groups have been installed on hydrocarbons in
intra- and intermolecular procedures, resulting in bicyclic
structures or differently functionalized heterocyclic systems.2
In this context, despite the fact that organoazides are versatile
intermediates in organic synthesis,3 reactions involving the
introduction of an azido group are rather rare, and this is true
for aminoazidation procedures, as well. In one isolated
example, Chemler and co-workers reported that treatment of
an allylurea with [NaN3/excess Cu(2-ethylhexanoate)2] allows
the formation of the corresponding azidomethyl-imidazolidin-
2-one resulting from an intramolecular amination/intermolec-
ular azidation sequence.4 Another aminoazidation procedure
starting from β,γ-unsaturated hydrazones in the presence of a
stoichiometric amount of Cu(OAc)2 and NaN3 was reported
by Wang, Li, and co-workers.5 More recently, Wang and a co-
worker developed a copper-catalyzed aminoazidation of alkenyl
N-methoxy amides using an azidoiodinane reagent (Scheme 1,
eq 1).6 A dual [Cu(I)/chiral phosphoric acid]-based catalyst,
in the presence of the same azide source that was used in the
previous example, has been used by Liu and co-workers to
provide the enantioselective aminoazidation of alkenyl ureas
(Scheme 1, eq 2).7 Very recently, Berhal and Prestat reported
the aminocyclization/azidation of alkenyl oxy-ureas, oxy-
carbamates, and oxy-amides with an [Fe(OAc)2/phenanthro-
line] catalytic system and TMSN3, giving azido-containing five-
membered heterocyclic rings (Scheme 1, eq 3).8 The NCS-
promoted one-pot conversion of a series of unsaturated amines
into the corresponding 3-azidopiperidines has been performed
by Kang and co-workers in the presence of NaI and NaN3.9
Studer and a co-worker and Li and co-workers performed the
intermolecular aminoazidations of olefins by copper and iron
Received: January 1, 2020
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catalysis, respectively, using TMSN3 in the presence of N-
fluorobenzenesulfonimide.10 Overall, only a few other
cyclization/azidation synthetic protocols have been reported
in the literature, specifically concerning Cu-catalyzed oxy-
azidations.11
Scheme 3. Aminoazidation Reaction of O-Alkenyl-N-
Substituted Carbamates 1b−k
a
Although oxidative palladium catalysis plays a relevant role
in alkene 1,2-difunctionalization protocols,12 to the best of our
knowledge, no such catalysis involving the inter- or intra-
molecular installation of an azido group on an alkene has been
reported.
In the frame of a long-standing collaboration between our
groups,13 and as part of our ongoing studies of the synthesis of
nitrogen-containing heterocyclic as well as intramolecular
oxidative palladium-catalyzed functionalization of alkenes,14
we report herein a novel palladium-catalyzed aminoazidation
of alkenylamines as a straightforward procedure to access
azidomethyl-substituted aza-heterocycles (Scheme 1, eq 4).
We chose O-allyl-N-tosylcarbamate (1a) as our initial model
substrate, selecting hydrogen peroxide as the stoichiometric
oxidant.15 After some preliminary experimentation (see the
Supporting Information), we found that the reaction
conditions [PdCl2(MeCN)2 (5 mol %), H2O2 (1.1 equiv),
and NaN3 (1.0 equiv), in THF, rt, 24 h] afforded the
azidomethyl oxazolidinone 2a in 81% yield, according to a 5-
exo-amination/azidation sequence (Scheme 2). A change in
Scheme 2. exo-Amination/Azidation of O-Allyl-N-
a
tosylcarbamate 1a
a
Reaction conditions: 1a (1.00 mmol), PdCl2(MeCN)2 (0.05 mmol),
H2O2 (1.10 mmol), NaN3 (1.00 mmol), THF (5 mL).
the catalyst [Pd(OAc)2 and Pd(O2CCF3)2] or of the
stoichiometry of H2O2 (1.5, 2.0, or 4.0 equiv) or of the
solvent (MeCN, dioxane, DMF) decreased the purity of the
crude mixture.16 The use of TMSN3 as the azide source did
not furnish the desired product in a detectable amount.
First, we examined the reactivity of O-allyl-N-arylcarbamates
1b−d bearing substituents with different electronic properties
on the aryl group. Working with the reaction system
[PdCl2(MeCN)2 cat., H2O2, NaN3] in THF, at room
temperature, we obtained the sole recovery of unreacted
substrates. Only the 4-nitrophenyl derivative 1b gave the
expected oxazolidin-2-one 2b (53% yield) upon treatment with
the catalytic system described above under microwave
irradiation at 60 °C for 1 h (Scheme 3, entries 1−3).
Furthermore, the palladium-catalyzed aminoazidation reaction
was successful also on N-(2-nitrophenyl)sulfonyl carbamate 1e,
although in moderate yield, indicating that the high acidity of
an N-sulfonyl carbamate was essential for promoting the
process.17
a
Reaction conditions: 1a (1.00 mmol), PdCl2(MeCN)2 (0.05 mmol),
H2O2 (1.10 mmol), NaN3 (1.00 mmol), THF (5 mL), r.t., 24 h.
b
Reaction temperature of 60 °C, MW irradiation, reaction time of 1 h.
c
d
Unreacted substrate (39%) was recovered beside 2b. For 2h, the
molecular structure as determined by single-crystal X-ray diffraction at
room temperature is also given. Thermal ellipsoids are drawn at the
50% probability level. See the Supporting Information for more
details. CCDC 1970338 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/structures.
Here again, products derived from a 6-endo-amination/
azidation process were not isolated, even in traces.
Furthermore, in the case of entries 5−7, the reactions were
totally diastereoselective, affording exclusively one azidated
oxazolidinone, although the Pr-substituted substrate 1h gave
additionally a small amount of the hydroxymethyl-substituted
oxazolidinone 3h.18 The single-crystal X-ray structure analysis
of 2h in combination with the analogy of its 1H NMR
spectrum with those of 2f and 2g allowed assignment of the
trans 4R*,5R* relative configuration of the oxazolidinone ring.
On the other hand, allyl substituents incorporating a 1,2-
disubstituted alkene impede the reaction from taking place
We thus decided to focus on N-sulfonyl-protected alkenyl
substrates for further investigations. Tests on functionalized O-
allyl-N-Ts carbamates indicated that the success of this tandem
sequence depends on the location of the substituents. On one
hand, substrates 1f−i, carrying substituents at the α-allylic
position, afforded oxazolidinones 2f−i, respectively, in 65−
81% isolated yields (Scheme 3, entries 5−8, respectively).
B
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(Scheme 3, entries 9 and 10). Reactions on homoallyl
carbamates furnished only complex mixtures of degradation
compounds.
4R absolute configuration was determined by single-crystal X-
ray structure analysis.
We further extended the scope of the reaction. N-Tosyl-2-
allylanilines 7a and 7b reacted as expected, providing the
corresponding 2-azidomethylindolines 8a and 8b, albeit in
moderate yields (Scheme 5, entries 1 and 2, respectively). The
To confirm the totally selective 5-exo-trig cyclization
observed in the aminoazidation of O-allyl carbamates, we
tested our conditions on N-allyl-N′-tosylureas, which are
structurally analogous compounds. Accordingly, treatment of
ureas 4a−e with the system of Pd(OAc)2 (5 mol %), H2O2,
and NaN3 at room temperature gave, after an overnight
stirring, the corresponding 4-azidomethyl-imidazolidinones
5a−e in fairly satisfactory yields as the major products
(Scheme 4).
a
Scheme 5. Scope of the Aminoazidation Reaction
Scheme 4. Aminoazidation Reactions of N-Allyl-N′-
a
tosylureas 4a−e
a
Reaction conditions: substrate 7, 9, 11, or 13 (1.00 mmol),
Pd(OAc)2 (0.05 mmol), H2O2 (1.10 mmol), NaN3 (1.00 mmol),
THF (5 mL).
N-allylamides of N-tosylglycine 9a and 9b and O-allyl-N-tosyl-
2-aminophenol 11 were then considered in view of obtaining
larger rings. Accordingly, treatment of these substrates under
the reaction conditions described above gave the piperazinones
10a and 10b and the 1,4-benzoxazine 12 in satisfactory yields
(Scheme 5, entries 3−5, respectively). The reactions
successfully proceeded also on N-allyl-anthranilamides 13a−
c, yielding 1,4-benzodiazepines 14a−c, respectively, through
selective 7-exo-aminoazidation (Scheme 5, entries 6−8,
respectively), with, in the case of 13c, a small amount of the
hydroxymethyl-substituted compound 15c.
a
Reaction conditions: 4 (1.00 mmol), Pd(OAc)2 (0.05 mmol), H2O2
b
(1.10 mmol), NaN3 (1.00 mmol), THF (5 mL). Major
diastereoisomer (isolated yield); minor diastereoisomer not isolated;
diastereomeric ratio of 78:22 determined by H NMR analysis of the
crude reaction mixture. The experimental molecular structure of 5e is
given, with same specs as in Scheme 3. See the Supporting
Information for more details. CCDC 1970339 contains the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/structures.
1
c
Relevant information about the mechanism of this reaction
was obtained by inspecting the role of the oxidant and the
nature of the alkene moiety. In relation to the former point, we
found that oxidizing agents that typically work by Pd(0)/
Pd(II) cycles were unable to provide the aminoazidation
process. Indeed, when carbamate 1a was treated under
standard conditions, but using 1,4-benzoquinone or Cu(OAc)2
instead of H2O2, no azidomethyl oxazolidinone 2a was
detected in the resulting intractable reaction mixtures (Scheme
In this case, Pd(OAc)2 proved to be the best performing
palladium source. Only when starting from the N-allyl-N-
phenyl-urea 4c was a small amount of the amination/
hydroxylation byproduct 6c isolated. In contrast to some
literature precedents,19 no product arising from intramolecular
C−O bond formation was detected. The application of
standard conditions to the (R)-N-α-methylbenzyl-N-allylurea
4e afforded the optically active imidazolidinone 5e, whose αR,
C
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6, eq A). This is likely due to the competitive paths of
protonolysis and/or β-hydride elimination that become
conditions with NaN3 as the azide anion source and H2O2 as
the inexpensive oxidant agent to yield azidomethyl-substituted
heterocycles. The use of H2O2 is essential to generate a Pd(IV)
intermediate, which avoids competitive reactions such as β-
hydride elimination. Overall, this approach is the first example
of successful palladium-catalyzed aminoazidation and appears
to be a valid and useful alternative to the copper-promoted
intramolecular aminoazidation. The procedure based on
palladium catalysis runs at room temperature, while the use
of a copper catalyst requires heating at 50−60 °C. The
mechanism of the reaction does not involve radical
intermediates as in the case of copper-catalyzed reactions,
and a broader range of different heterocycles, especially six-
membered ones, could be accessed in comparison to the more
limited scope of the copper protocol. Further work will explore
the asymmetric aminoazidation of aminoalkenes as well as the
oxyazidation reactions of alkenols.
Scheme 6. Experiments for Inspecting the Role of the
Oxidant and the Nature of the Alkene Moiety
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00010.
dominant when the oxidant is incapable of bringing the Pd(II)
complex to the Pd(IV) oxidation state.20 With respect to the
latter point, reaction of carbamate 1j with the system of PIFA,
KI, and NaN3 provided the cyclization/azidation product 16 as
a single diastereoisomer, isolated in 42% yield (Scheme 6, eq
B).21 This result confirms that the failure to obtain the
aminoazidation products from substrates 1j and 1k (Scheme 3,
entries 9 and 10, respectively) is not due to the intrinsic
instability of the final product.
■
sı
Experimental procedures and compound characteriza-
tion data, including copies of H and 13C NMR spectra
1
(PDF)
Accession Codes
On the basis of the results presented above, we propose a
plausible mechanism as depicted in Scheme 7. Exocyclic
CCDC 1970338−1970339 (for 2g and 5e) contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Scheme 7. Plausible Reaction Mechanism
AUTHOR INFORMATION
Corresponding Author
■
Gianluigi Broggini − Dipartimento di Scienza e Alta
̀
Tecnologia, Universita degli Studi dell’Insubria, 22100 Como,
Italy; orcid.org/0000-0003-2492-5078;
Email: gianluigi.broggini@uninsubria.it
Authors
nucleopalladation on the σ-olefin complex A generates the π-
alkyl-palladium(II) intermediate B, which undergoes oxidation
by H2O2 to afford the Pd(IV) intermediate C.22 Following
azide anion direct substitution of the C−Pd(IV) bond affords
the final product with regeneration of the starting catalyst.
Hints about the mechanism of the last step are given by the
failure of substrates 1j and 1k mentioned above. Indeed, the
fact that the reaction is viable when the attacked carbon atom
is secondary but fails when it is tertiary strongly speaks in favor
of a direct substitution mechanism. On the other hand, an
alternative mechanism involving addition of an azide to the
palladium atom followed by reductive elimination is not
expected to be sensitive to the degree of substitution of the
alkene β-carbon atom. Furthermore, an analogous SN2 step
was found to be operative in the substitution of a C(sp3)−
Pd(IV) bond by acetate anion23 and should be a fortiori so in
the case of the softer azide anion.24
Francesca Foschi − Dipartimento di Scienza e Alta Tecnologia,
Universita degli Studi dell’Insubria, 22100 Como, Italy
Camilla Loro − Dipartimento di Scienza e Alta Tecnologia,
Universita degli Studi dell’Insubria, 22100 Como, Italy
Roberto Sala − Dipartimento di Scienza e Alta Tecnologia,
Universita degli Studi dell’Insubria, 22100 Como, Italy
̀
̀
̀
́
́
Julie Oble − Sorbonne Universite, Faculte des Sciences et
́
́
Ingenierie, CNRS, Institut Parisien de Chimie Moleculaire,
IPCM, 75005 Paris, France
̀
Leonardo Lo Presti − Dipartimento di Chimica, Universita
degli Studi di Milano, 20133 Milano, Italy; orcid.org/0000-
0001-6361-477X
Egle M. Beccalli − DISFARM, Sezione di Chimica Generale e
Organica “A. Marchesini”, Universita degli Studi di Milano,
̀
20133 Milano, Italy
́
́
Giovanni Poli − Sorbonne Universite, Faculte des Sciences et
́
́
In summary, we have developed a new palladium-catalyzed
procedure for the cyclization/azidation of unactivated alkenes
bearing a -NHTs group. The reaction proceeds under mild
Ingenierie, CNRS, Institut Parisien de Chimie Moleculaire,
IPCM, 75005 Paris, France; orcid.org/0000-0002-7356-
1568
D
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(d) Rigamonti, M.; Prestat, G.; Broggini, G.; Poli, G. J. Organomet.
Chem. 2014, 760, 149−155.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00010
̀
(14) (a) Giofre, S.; Beccalli, E. M.; Foschi, F.; La Rosa, C.; Lo Presti,
L.; Christodoulou, M. Synthesis 2019, 51, 3462−3470. (b) Liu, Y.;
Mao, Z.; Pradal, A.; Huang, P.-Q.; Oble, J.; Poli, G. Org. Lett. 2018,
20, 4057−4061. (c) Foschi, F.; Albanese, D.; Pecnikaj, I.; Tagliabue,
A.; Penso, M. Org. Lett. 2017, 19, 70−73. (d) Mao, Z.; Martini, E.;
Prestat, G.; Oble, J.; Huang, P.-Q.; Poli, G. Tetrahedron Lett. 2017, 58,
4174−4178. (e) Penso, M.; Foschi, F.; Pellegrino, S.; Testa, A.;
Gelmi, M. L. J. Org. Chem. 2012, 77, 3454−3461.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
̀
The authors thank Universita degli Studi dell’Insubria and
Universita degli Studi di Milano for financial support. Support
(15) Åkermark, B.; Larsson, E. M.; Oslob, J. D. J. Org. Chem. 1994,
59, 5729−5733.
̀
through CMST COST Action, CA15106 (CHAOS), is also
gratefully acknowledged.
(16) See Table S-1 for a complete list of the oxidizing agents used.
(17) Compound 1e, when treated with tiophenol in bases, to remove
the o-nosyl group, furnished complex mixtures of degradation
compounds.
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