Intermolecular Hydroaminoalkylation of Alkynes

Article abstract of DOI:10.1002/chem.202100238

Intermolecular hydroaminoalkylation reactions of alkynes with secondary amines, which selectively give access to allylic amines with E configuration of the alkene unit, are achieved in the presence of titanium catalysts. Successful reactions of symmetrically substituted diaryl- and dialkylalkynes as well as a terminal alkyne take place with N-benzylanilines, N-alkylanilines, and N-alkylbenzylamines.

Full text of DOI:10.1002/chem.202100238

Communication
doi.org/10.1002/chem.202100238
Chemistry—A European Journal
&
Catalysis
Intermolecular Hydroaminoalkylation of Alkynes
Tobias Kaper,[a] Malte Fischer,[b] Hermann Thye,[a] Dennis Geik,[a] Marc Schmidtmann,[a]
Ruediger Beckhaus,*[a] and Sven Doye*[a]
Abstract: Intermolecular hydroaminoalkylation reactions
of alkynes with secondary amines, which selectively give
access to allylic amines with E configuration of the alkene
unit, are achieved in the presence of titanium catalysts.
Successful reactions of symmetrically substituted diaryl-
and dialkylalkynes as well as a terminal alkyne take place
with N-benzylanilines, N-alkylanilines, and N-alkylbenzyla-
mines.
Allylamines are synthetically very important molecules given
their immense potential as building blocks in organic chemis-
try and the main structural motif [R2NÀC(R)=CR2] is part of for
example, biological active ingredients.[1] Furthermore, the allyl-
amine fragment itself can be found in many natural products
and is easily functionalized at the pendant double bond. Allyla-
mines can be synthesized by several different routes including
the Overman rearrangement,[2] the Tsuji–Trost reaction as an
exemplarily substitution reaction,[3] alkenylation of imines,[4] or
Scheme 1. Selected routes for the synthesis of allylamines: allylic CÀH ami-
the reductive amination of enones and enals.[5] In the field of
nation via either p-allyl intermediates or metal-nitrenoids (top); hydroami-
noalkylation of propadiene (middle); hydroaminoalkylation of alkynes
(bottom).
direct CÀH activation, the allylic CÀH amination does not only
lead to derivatized allylamines at the N-terminus (e.g., as
amides or carbamates)[6] but has very recently been expanded
to directly access N-alkyl-substituted allylamines by a photoca-
talyzed variant.[7] The two major strategies utilize either h3-allyl-
metal species followed by nucleophilic attack of the N-com-
pound[8] or CÀH insertion of metal nitrenoids (Scheme 1, top).[9]
The hydroamination of substrates with two double bonds is
another established method.[6] For this reaction dienes, allenes,
and alkynes (which are transformed into allenes in situ) are
employed together with late transition metals to regioselec-
tively give allylic amines by CÀN bond formation. It is worth to
mention that the alkyne variant is only possible with alkyl-sub-
stituted alkynes.[11] An attractive alternative to synthesize allyla-
mines would be the catalytic intermolecular hydroaminoalkyla-
tion of alkynes with amines, which would yield the corre-
sponding allylamines by CÀC bond formation at the carbon
atom in position a to the amine starting material. The corre-
sponding hydroaminoalkylation of alkenes is well established
to date and is possible with late transition metals,[12] following
a photocatalytic approach,[13] and even with early transition
metals.[14] Especially early transition metals are attractive for
this purpose given their overall low cost and abundance com-
pared to late transition metals.[15] Our research is specifically fo-
cused on titanium catalysts as titanium is the second most
abundant metal in the Earth’s crust.[15,16] Since titanaaziridines
are considered to be key intermediates in the Ti-catalyzed hy-
droaminoalkylation of alkenes, stoichiometric reactions be-
tween group 4 aziridines and alkynes already foreshadow the
potential to realize the corresponding catalytic variant and are
well understood.[17,18] In a recent work, our group has reported
the synthesis of allylamines by reacting the parent allene prop-
adiene with various secondary amines employing a titanium
[a] T. Kaper, H. Thye, D. Geik, Dr. M. Schmidtmann, Prof. Dr. R. Beckhaus,
Prof. Dr. S. Doye
Institut für Chemie
Universität Oldenburg
Carl-von-Ossietzky-Strasse 9–11, 26129 Oldenburg (Germany)
E-mail: ruediger.beckhaus@uni-oldenburg.de
doye@uni-oldenburg.de
[b] Dr. M. Fischer
Leibniz-Institut für Katalyse e.V. (LIKAT)
Albert-Einstein-Straße 29A, 18059 Rostock (Germany)
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
https://doi.org/10.1002/chem.202100238.
ꢀ 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Commer-
cial NoDerivs License, which permits use and distribution in any medium,
provided the original work is properly cited, the use is non-commercial and
no modifications or adaptations are made.
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catalyst (Scheme 1, middle).[19] Key finding of our study was
that successful hydroaminoalkylation reactions of propadiene
can only be achieved with amines possessing an a-methylene
group; as a consequence, N-methyl anilines, which are known
to be among the best substrates for alkene hydroaminoalkyla-
tion, do not deliver the desired allylamines. During the prepa-
ration of this manuscript Schafer and co-workers reported the
targeted goal of the hydroaminoalkylation of internal alkynes
with benzylamines to yield a,b,g-substituted allylic amines
with a zirconium catalyst and a bis(ureate) ligand precursor at
1458C and 48 h reaction time (Scheme 1, bottom).[20] Herein
we report on the employment of a highly active titanium cata-
lyst (1408C, 4 h reaction time) for the hydroaminoalkylation of
internal and terminal alkynes with various secondary amines.[21]
Inspired by the good reactivity of N-benzyl anilines in hydro-
aminoalkylation reactions of propadiene,[19] our investigation
commenced with an evaluation of a range of titanium com-
plexes as catalysts for the desired hydroaminoalkylation of di-
phenylacetylene (2) with N-benzylaniline (1, Scheme 2). For
Figure 1. Molecular structure of catalyst II. Hydrogen atoms have been omit-
ted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Se-
lected bond lengths [Š]: Ti1ÀN1 2.0958(13), Ti1ÀN2 2.2229(14), Ti1ÀN4
2.0958(13), N1ÀC2 1.368(2), N1ÀC7 1.411(2), N2ÀC2 1.363(2), N2ÀC6 1.338(2),
N3ÀC6 1.376(2).
larger amounts of the desired product for characterization pur-
poses. According to GC analysis, after 4 h the experiment
showed full conversion and after subsequent workup and
chromatographic purification, the desired pure allylic amine E-
3 could be isolated in 82% yield as the single product of the
reaction. The corresponding stereoisomeric allylamine Z-3
could neither be isolated nor detected by GC, suggesting that
the hydroaminoalkylation reaction takes place with complete
stereoselectivity (Figures S64 and S65). To unambiguously
assign the structure of E-3, comprehensive NMR studies [heter-
onuclear multiple bond correlation (HMBC), heteronuclear mul-
tiple quantum correlation (HMQC), nuclear Overhauser effect
(NOE), Figures S5–S7][27] were carried out and finally, it was pos-
sible to obtain crystals suitable for single-crystal X-ray diffrac-
tion. As can be seen from Figure 2, the two phenyl substitu-
ents at the CÀC double bond of E-3 are oriented cis to each
other, which overall results in the mentioned E configuration.
The exclusive formation of the allylamine product with an E
configuration at the double bond can easily be explained by
the mechanism of the reaction (Scheme 3). Key step of the
generally accepted catalytic cycle of hydroaminoalkylation re-
actions is the insertion of the unsaturated substrate into the
TiÀC bond of a catalytically active titanaaziridine (A).[16,17a] In
Scheme 2. Investigated intermolecular hydroaminoalkylation of diphenylace-
tylene (2) with N-benzylaniline (1, top) and part of the investigated catalyst
systems I–IV (bottom).
that purpose, several corresponding catalytic reactions were in-
itially run on a 0.1 mmol-scale in toluene at 1408C for 2 h with
a catalyst loading of 10 mol% in sealed ampoules (V=1 mL).[22]
Conversion of the starting materials as well as product forma-
tion was detected by GC analysis using p-cymene as an inter-
nal standard. During these experiments, it was found that both
aminopyridinato titanium catalysts I[23] and II[24] showed con-
version of N-benzylaniline (1) while catalyst III[25] and the for-
mamidinato titanium catalyst IV[26] as well as additionally inves-
tigated homoleptic group 4 or 5 metal catalysts turned out to
be inactive (Figures S59–S62).[27] As observed before,[24] among
the two aminopyridinato titanium catalysts I and II, the meta-
tert-butyl-substituted derivative II turned out to be catalytically
more active (Figure S60). In this context, it should be men-
tioned that due to the importance of catalyst II for our study,
we further improved the synthesis of its aminopyridinato
ligand, and we additionally confirmed the molecular structure
of II by X-ray crystallography (Figure 1).
Subsequent optimization of the reaction conditions
(Table S2) showed that the best results are obtained with
10 mol% catalyst loading (II), 1408C, 4 h, and using 1.2 equiva-
lents of diphenylacetylene (2). With these optimized conditions
in hand, a 1 mmol-scale experiment was performed to isolate
Figure 2. Molecular structure of E-3. Hydrogen atoms (except H1, H1A, and
H3) have been omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Š] and angles [8]: N1ÀC1 1.4566(9),
C1ÀC2 1.5308(9), C2ÀC3 1.3438(9), Æ(angles)(C2) 359.9, Æ(angles)(C3) 360.0.
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2 took place smoothly at 808C to give the expected titanapyr-
roline 5 in good isolated yield of 60%.
With the optimized conditions in hand, the scope of N-
benzyl anilines was investigated, and the results are summar-
ized in Scheme 5. While experiments with substrates that con-
tain para- and meta-substituents on the N-phenyl ring or the
benzyl group usually delivered the expected E-configurated
hydroaminoalkylation products within 4 h at 1408C, it was
found that ortho-substituted substrates did not show any reac-
tivity, which can be explained by steric hindrance.[23] With
regard to functional groups, the reaction tolerates methyl (E-6,
E-7), bromo (E-8, E-9, E-14, E-16), and chloro (E-10, E-15, E-19)
substitution as well as other pharmacologically promising sub-
stituents such as trifluoromethyl (E-12, E-17, E-18), trifluorome-
thoxy (E-11), and trifluoromethylthio groups (E-13). While in
most cases, the corresponding E-allylamines were formed in
good to very good isolated yields (65–87%), the presence of a
bromo- or a trifluoromethylthio-group led to reduced yields
(31–41%). However, in this context, it should be noted that
even in the latter cases, full conversion of the starting materials
was observed by GC analysis (Figures S66 and S67). Full con-
version was also observed with a series of para- and meta-me-
thoxy substituted benzylamines but in these cases, we were
not able to purify the corresponding allylamine products,
Scheme 3. Proposed mechanism of the Ti-catalyzed hydroaminoalkylation of
alkynes.
the case of diphenylacetylene (2) as the substrate, the corre-
sponding reaction delivers a titanapyrroline (B) in which the
two phenyl substituents bonded to the C=C double bond
must be cis-oriented. Subsequent protonation of the TiÀC
bond by the amine substrate 1 and regeneration of the cata-
lytically active titanaaziridine A then closes the catalytic cycle
and liberates the E-configurated allylic amine E-3.
To confirm the possibility of a migratory insertion reaction of
the sterically demanding alkyne diphenylacetylene (2) into the
TiÀC bond of an a-phenyl-substituted titanaaziridine, the reac-
tion of the model titanaaziridine 4[17b] with 2 was performed
(Scheme 4). In this context, it should be mentioned that corre-
sponding insertion reactions of sterically less demanding al-
kynes such as phenylacetylene, 2-butyne, 1-phenylpropyne,
and 1-trimethysilylpropyne have already been described with
a-unsubstituted titanaaziridines.[16,17a] It should also be noted
that titanaaziridines of type 4 possessing adamantyl-substitut-
ed cyclopentadienyl ligands are known to lack any catalytic ac-
tivity in hydroaminoalkylation reactions and are therefore per-
fectly suited for the isolation and characterization of stable
five-membered intermediates of the expected catalytic cycle.
For that reason, it is not surprising that the reaction of 4 with
Scheme 5. Ti-catalyzed synthesis of allylamines from diphenylacetylene (2)
and N-benzyl anilines. Reaction conditions: amine (1.0 mmol), diphenylacety-
lene (2, 1.2 mmol), II (0.10 mmol, 10 mol%), 1408C, 4 h, sealed ampoule
(V=5 mL).
Scheme 4. Migratory insertion of diphenylacetylene (2) into the TiÀC bond
of titanaaziridine 4.
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which are undoubtedly formed under the reaction conditions
(Figure S68), by distillation or column chromatography. Particu-
larly promising are the good yields in which the chloro-substi-
tuted allylamines E-10, E-15, and E-19 were formed because
the presence of a chloro-substituent offers various possibilities
for further functionalization (e.g., Pd-catalyzed cross coupling).
An additional single-crystal X-ray diffraction of compound E-
15[27] that confirmed the E configuration of the allylamine
double bond is in good agreement with the excellent stereose-
lectivity observed in all hydroaminoalkylation reactions.
amines and terminal alkynes can principally be achieved with
the formamidinato titanium catalyst IV, which has already
shown high catalytic activity in hydroaminoalkylation reactions
of alkenes with N-methylamines.[26] Although the correspond-
ing product 27 could only be isolated in 24% yield, this result
strongly supports the idea that the hydroaminoalkylation of al-
kynes has a great potential for a wide range of further applica-
tions. Because 27 represents the first hydroaminoalkylation
product obtained from a terminal alkyne, we were delighted
to verify its structure by single-crystal X-ray diffraction of the
corresponding hydrochloride 27·HCl.[27]
In summary, we have developed a simple catalytic protocol
for the intermolecular titanium-catalyzed hydroaminoalkylation
of alkynes with secondary amines which directly gives access
to allylic amines. As a consequence of the CÀC bond forming
key step of the proposed catalytic cycle, the insertion of the
alkyne into the TiÀC bond of a catalytically active titanaaziri-
dine, disubstituted alkynes are exclusively converted into allylic
amines with an E configuration of the alkene unit. While best
results were obtained with various N-benzylanilines, reactions
of N-alkylanilines and N-alkylbenzylamines with disubstituted
alkynes such as diphenylacetylene or 3-hexyne can also be
achieved. Although unsymmetrically disubstituted alkynes
react sluggishly, a successful reaction of N-methylaniline with a
terminal alkyne regioselectively delivered the branched hydro-
aminoalkylation product. Because the latter result strongly sug-
gests that it should be possible to significantly expand the
scope of the catalytic reaction, we think that this work will be
the starting point of a synthetically useful and promising new
field in hydroaminoalkylation chemistry. Further optimization
studies are currently underway in our laboratory and will be re-
ported in due course.
Subsequently, we also investigated the behavior of a few
alkyl-substituted amines and 3-hexyne (20) as the first example
of a dialkyl substituted alkyne (Scheme 6). Although 20 turned
out to be less reactive than diphenylacetylene (2), a simple ex-
tension of the reaction time to 20 h led to a successful reaction
with N-benzylaniline (1) that gave access to the corresponding
allylic amine E-24 in an excellent yield of 84%. The products E-
21, E-22, and E-23, which were obtained from additional reac-
tions of 2 with N-alkylanilines or N-isopropylbenzylamine, clear-
ly prove that in principle, alkylamines also represent suitable
substrates for the hydroaminoalkylation of alkynes, although in
these cases, further optimization is required to increase the ef-
ficiency of the reaction.
Scheme 6. Ti-catalyzed hydroaminoalkylation reactions of diphenylacetylene
(2) or 3-hexyne (20) with N-alkylanilines, a N-alkylbenzylamine, or N-benzyla-
niline (1). Reaction conditions: amine (1.0 mmol), alkyne (1.2 mmol), II
(0.10 mmol, 10 mol%), 1408C, 4, 16, or 20 h, sealed ampoule (V=5 mL).
[a] 4 h. [b] 16 h. [c] 20 h.
Acknowledgements
We thank the Research Training Group “Chemical Bond Activa-
tion” (GRK 2226) funded by the Deutsche Forschungsgemein-
schaft and the Heinz Neumüller Stiftung for financial support
of this project and Jessica Reimer for experimental assistance.
Open access funding enabled and organized by Projekt DEAL.
Finally, we tried to expand the substrate scope of the reac-
tion to unsymmetrically disubstituted alkynes, terminal alkynes,
and N-methylaniline (25, Scheme 7). For that purpose, 1-
phenyl-1-butyne, 1-phenyl-1-hexyne, trimethylsilylacetylene,
and tert-butylacetylene (26) were initially reacted with N-ben-
zylaniline (1) in the presence of catalyst II but unfortunately,
due to poor regioselectivity (Figures S73 and S74) we were not
able to isolate pure products. On the other hand, an additional
catalyst screening for the reaction of 26 with 25 revealed that
obviously, corresponding reactions of N-methyl-substituted
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkynes
·
amines
·
CÀH
activation
·
hydroaminoalkylation · titanium
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Manuscript received: December 3, 2020
Accepted manuscript online: January 22, 2021
Version of record online: February 8, 2021
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