Anti-Markovnikov hydroimination of terminal alkynes in gold-catalyzed pyridine construction from ammonia

Article abstract of DOI:10.1039/c5cc04091d

Gold-catalyzed hydroimination of terminal alkynes, giving rise to anti-Markovnikov adducts concomitant with unstable Markovnikov adducts is described. The elementary step can be applied for the construction of pyridine derivatives from ammonia and alkynes

Full text of DOI:10.1039/c5cc04091d

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DOI: 10.1039/C5CC04091D
Journal Name
ARTICLE
Anti-Markovnikov Hydroimination of Terminal Alkynes in Gold-
Catalyzed Pyridine Construction using Ammonia†
Received 00th January 20xx,
Accepted 00th January 20xx
a
Liliang Wang,‡ Lingbing Kong,‡a Yongxin Li,b Rakesh Gangulyb and Rei Kinjo*a
A gold-catalyzed hydroimination of terminal alkynes, giving rise to anti-Markovnikov adducts concomitant with unstable
Markovnikov adducts is described. The elementary step can be applied for construction of pyridine derivatives from
ammonia and alkynes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Acyclic and heterocyclic nitrogen-containing skeletons are
ubiquitous in a myriad of naturally occurring compounds as well as
in industrial products involving agrochemicals, pharmaceuticals,
cosmetics, and fine chemicals.1 Hence, efficient methodology for
the formation of carbon-nitrogen bonds has been a subject of
considerable interest in synthetic chemistry. In recent years,
metal-catalyzed hydroamination, which forms C-N bonds by direct
addition of a nitrogen-hydrogen bond across carbon-carbon
multiple bonds, has represented a powerful atom-economic tool
for the synthesis of nitrogen-containing compounds.2 However,
control of the regioselectivity is a major problem to be addressed.
Thus, as classical textbooks state that the proton forms a bond
with the carbon bearing fewer substituents in accordance with
Markovnikov’s rule, archetypal hydroamination products are also
dominated by Markovnikov adducts with branched skeleton.3
Accordingly, preparation of nitrogen-containing compounds with
linear skeleton by direct anti-Markovnikov hydroamination
remains highly challenging.4
Since Beller and co-workers’ pioneering work on the first
metal-catalyzed anti-Markovnikov hydroamination of olefins in
1999,5 various methodologies for intermolecular anti-Markovnikov
hydroamination with metal catalysts involving alkali metals,6
alkaline earth metals,7 organolanthanide,8 Ti,9 Re,10 Ru,11 Rh,12
Pd,13 Cu,14 Au15 have been reported to this date. Although such
seminal approaches have led to solid progresses, these strategies
often involve disadvantages such as limiting the substrate scope,
harsh reaction condition, the use of strong bases, or stepwise
indirect processes.
been achieved thus far despite the significance of the predicted
products, 2-aza-1,3-dienes, as synthetic intermediates for N-
heterocycles.16 The use of primary imines as nucleophilic
substrates is hampered by several difficulties, mainly because of
the propensity to behave as electrophiles rather than nucleophiles
due to the polarized C=N bonds, especially in the presence of
Brønsted/Lewis acids. Primary imines are known to act as directing
groups by coordinating to metals which induces intramolecular C-
H activation.17 In addition, oxidative homo coupling of primary
imines also undergoes to form an N-N bond in the presence of
metals.18 To prevent such undesired processes, hydroimination
has mainly been limited to intramolecular reaction with the
geometrically pre-organized substrates (Scheme 1).19 Very
recently, Zhao et al. first reported nickel-catalyzed intermolecular
coupling between internal alkynes and aromatic N−H ketimines.20
However, employment of terminal alkynes in their system led to
alkyne oligomerization instead of the desired hydroimination.
Thus, to the best of our knowledge, hydroimination of terminal
alkynes is still unknown to date.
Given the importance of C-N bonds in synthetic chemistry, the
development of general means for the selective formation is
imperative. Here, we report gold-catalyzed hydroimination of
In marked contrast to recent advances in the field of
hydroamination, addition of an N-H bond of primary imines
HN=CR2 to alkenes and alkynes, namely hydroimination, has rarely
a.Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Nanyang Link
21,Singapore 637371 (Singapore). E-mail rkinjo@ntu.edu.sg
b.NTU-CBC Crystallography Facility, Nanyang Technological University Nanyang
Link 21, Singapore 637371 (Singapore).
‡ These authors contributed equally.
†Electronic supplementary information (ESI) available: Experimental and
calculation details, and crystallographic information for LAuCl, 3h, 3i, 5b, 5g and
6d. CCDC 1051367, 1051368, 1051369, 1051370, 1051371 and 1051372. For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x.
Scheme
hydroimination.
1 Metal-catalyzed intramolecular and intermolecular
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terminal alkynes, which afforded both anti-Markovnikov and
Markovnikov products. We also describe the first example for
construction of pyridine derivatives from ammonia and alkynes.
Our protocol for intermolecular hydroimination of terminal
alkynes is based on gold catalysis employing primary ketimines
HN=CR2. We envisaged that (a) soft -acidity of gold allows
interacting with alkynes effectively prior to ketimines, and (b) the
presence of two R-substituents at the imine carbon of ketimines
suppresses the attack by nucleophiles kinetically. We also
reasoned that incorporation of a bulky ligand into gold might
minimise the interaction with ketimines by steric repulsion, which
will induce selective activation of terminal alkynes rather than
undesired reaction pathway. Among various ligands available,
pyrid-2-ylidenes ligands are considered as good candidates
because substitutions at 1- and 3-positions maximize the steric
impact at the gold center due to the six-membered ring
skeleton.21 Pyrid-2-ylidenes are also recognized as strong -donor
ligands, which will contribute to promote substrate exchange,
necessary for high turnover in catalysis, as well as the stability of
the complex.22 Hence, to commence our studies, we designed a
novel gold chloride complex supported by a pyrid-2-ylidene ligand
and monitored by NMR spectroscopy. Among the screening
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reaction conditions, the highest productiDoOnI:o1f0.a10n3ti9-/MC5aCrkCo0v4n0i9k1oDv
adduct 3a (51%) was confirmed after 6 hours (Table 1-entry 1).
To further probe the formation of anti-Markovnikov product,
we performed a 13C-labeling experiment with a 13C-labeled
phenylacetylene 1a*, which decisively afforded 3a* (Fig. 2a).
Control reactions revealed the innocence of potassium
tetrakis(pentafluorophenyl)borate KB(C6F5)4, demonstrating the
essential role of a gold complex LAuCl in this reaction. In fact, the
reaction was shut down in the absence of the Au precatalyst. With
AgOTf instead of KB(C6F5)4 under similar reaction conditions, the
product 3a was obtained in lower yield (20%). When 1-bromo-4-
ethynylbenezene 1b was used (entry 2), the formation of anti-
Markovnikov product 3b was prior to Markovnikov product 4b
even at the early stage of the reaction (after 1 h; 3b:4b = 12:1),
and the highest yield (54%) of 3b was obtained after 6 hours when
4b decomposed completely. Treatment of 1a and 4-
ethynyltoluene 1c afforded the similar result in which anti-
Markovnikov adduct 3c was formed in 43% after 6 hours (entry 3).
These results indicate that the apparent anti-Markovnikov
regioselectivity is concomitant with the decomposition of
Markovnikov product in addition to the formation of a mixture of
other unidentified products, whereas the stability of anti-
Markovnikov products encouraged us for further exploration.
Reaction of 2a and an internal alkyne, diphenylacetylene, was also
L
bearing a 1,3,5-trimethylphenyl group and 3,5-di-tert-
butylphenyl groups. When the deprotonation of a pyridinium salt
[L-H]+[CF3SO3]‒ was performed with lithium hexamethyldisilazide
(LiHMDS)(3eq), which is followed by the addition of
chloro(tetrahydrothiophene)gold(I), a clean reaction occurred with
a characteristic signal for the carbene carbon at 187.8 ppm in the
13C{1H} NMR spectrum. After the products were worked up, LAuCl
was obtained as a white solid in 49% yield (Scheme 2). LAuCl was
fully characterized by standard spectroscopic methods, including a
single crystal X-ray diffraction study.23 LAuCl is air-stable, and can
be stored for several months without significant decomposition,
both in solution and in the solid state (m.p.; 220 oC).
tested,
which
afforded
N-(1,2-diphenylvinyl)-1,1-
diphenylmethanimine in 78% after 22h (see the ESI).
To test the scope of the hydroimination with respect to imines,
we employed various imine substrates. Each reaction was
monitored by NMR spectroscopy, and the results with anti-
Markovnikov adduct 3 in the highest yield are summarized in
Table 1. Standard functional groups are tolerated, including diaryl
imines featuring p-methylphenyl groups 2b, p-fluorophenyl groups
2c, as well as methyl benzimidate 2d. All imine substrates 2a-d
examined in this study reacted well with alkynes 1a-c, and
provided 2-aza-1,3-dienes 3a-l in moderate yields. Note that
catalytic formation of 2-aza-1,3-dienes such as 3 and 4 from
terminal alkynes and imines has never reported before. It is also
noteworthy to mention that under similar conditions, employment
of other gold catalysts such as (Ph3P)AuCl and (IPr)AuCl afforded
no and a few (< 10%) products, respectively. We also examined
the reaction of a bis-aliphatic imine, 2,2,4,4-tetramethylpentan-3-
imine (tBu2C=NH) with 1a, which gave a complex mixture.
Scheme 2 Synthesis of the gold complex LAuCl.
Reaction of 1a with 1-phenylethan-1-imine 2e also proceeded
under similar conditions. However, neither 3m nor 4m was
detected. Instead, only unidentified self-decomposed products of
2e were observed after the reaction. To our surprise, when a large
excess amount of 1a was used, we obtained 2-benzyl-4,6-
diphenylpyridine 5 in 51% yield after 5 hours indicating that two
equivalents of 1a were involved in the reaction (Table 1, entry 13).
Although we attempted to confirm the reaction intermediates by
varying reaction temperature, time, and substrate ratio, 5a was an
only detectable product under any conditions.25 Presumably,
instability of the corresponding 2-aza-1,3-diene intermediate
caused the fast cyclization with a second alkyne. High reactivity of
the intermediates may be due to the presence of the sterically less
demanding methyl group at the imine carbon, which also could
With the precatalyst LAuCl in hand, we next examined its
catalytic activity in hydroimination of terminal alkynes. Recently,
Toste and co-workers reported that gold-catalyzed reaction of
phenylacetylene 1a with N,1-diphenylmethanimine PhN=CHPh
afforded a propargyl amine.24 In marked contrast, the reaction
between two equivalents of 1a and benzophenone imine 2a with a
catalytic amount of LAuCl produced a mixture of (Z)-1,1-diphenyl-
N-styrylmethanimine 3a and 1,1-diphenyl-N-(1-phenylvinyl)
methanimine 4a after 1 hour at 150 oC, and unexpectedly, the
yield of the anti-Markovnikov adduct 3a was nearly identical to
that of Markovnikov adduct 4a (3a:4a ≈ 1:1). We also observed
that 4a gradually decomposed to unidentified mixture under the
reaction condition (see the ESI). Therefore, in order to
substantiate the apparent regioselectivity, reaction was repeated,
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induce tautomerization to transient enamine intermediates. It has
already been shown that less-hindered 2-aza-1,3-diene derivatives
react with unsaturated molecules to generate cyclic products.16,26
Bertrand and co-workers have reported that a catalytic
amount of cyclic (alkyl)(amino)carbene gold complexes effectively
promotes the addition of ammonia (NH3) across alkynes and
allenes.27 In their study, hydroamination of a terminal arylalkyne,
4-ethynyltoluene, proceeded with Markovnikov regioselectivity,
which afforded a 1-arylethan-1-imine. On the basis of these
results, we attempted the direct synthesis of pyridine skeletons,
common components of natural products and pharmaceuticals,28
from alkynes and NH3 through anti-Markovnikov hydroimination ~
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DOI: 10.1039/C5CC04091D
Table 1 Au-catalyzed hydroimination of terminal arylalkynes.a
Fig. 2 (a) Representations of 13C-labeled phenylacetylene 1a*, (Z)-
1,1-diphenyl-N-styrylmethanimine 3a*, and 2-benzyl-4,6-dipheny
lpyridine 5a*. (b) Proposed reaction pathway for construction of
pyridine 5 from alkynes 1 and ammonia.
To bear out this hypothesis, 2.5 equivalents of 1a were
treated with NH3 in the presence of the gold complex LAuCl (1
mol%). To our delight, after 12 hours at 150 oC 5a was obtained in
43% yield (Table 2a, entry 1). Interestingly, the spontaneous
aromatization by dehydrogenation was induced even without an
oxidant. To gain insight into the reaction pathway, we performed
further experiments. Reaction of 1a with a large excess of NH3
exclusively afforded 1-phenylethan-1-imine 2e, confirming that
the initial step is a Markovnikov hydroamination of alkyne,
affording an enamine which may subsequently tautomerize to
imine 2e. Next, a 13C-labeling experiment was conducted with
1a*. When 1a* was employed under the same reaction condition,
5a* was produced which supports the proposed reaction pathway
(Fig. 2a). The scope of the catalytic reaction was briefly examined
with a variety of alkynes 1 (Table 2a). Terminal alkynes with
electron-donating as well as electron-withdrawing aromatic
groups were well tolerated (Table 2a, entries 2-7, 9, 10).
Relatively low yield with 1h was probably due to the extremely
strong electron-withdrawing CF3-group adopted (Table 2a, entry
8). 2-ethynylthiophene and 3-ethynylthiophene also exhibited
tolerance to the reaction conditions (Table 2a, entries 11 and 12).
Finally, our preliminary test showed that this strategy can be
extended to three-component coupling reaction employing two
different terminal alkynes and NH3, which afforded rather complex
pyridine derivatives 6 (Table 2b). This result illustrates the
potential application for the preparation of various heterocycles,
although the co-products 5 assembled from the mono-component
alkyne were also formed in this reaction.
a) Reaction conditions: 1 (0.4 mmol), 2 (0.2 mmol), LAuCl (5 mol%) and KB(C6F5)4 (5
mol%), C6D6 (0.5 mL), 150 oC. b) Yields and selectivity were determined by 1H NMR
spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. c) Isolated yields
are given in parentheses.
cyclization sequence. We postulate that treatment of terminal
arylalkynes 1 and NH3 in the presence of our gold catalyst LAuCl
also would generate 1-arylethan-1-imines 2 via Markovnikov
hydroamination rather than anti-Markovnikov selectivity due to
the less bulkiness of ammonia molecule. The imines formed in situ
would further react with a second alkyne in an anti-Markovnikov
fashion to give 2-aza-1,3-diene intermediates 3 which would
isomerize to 3A and 3B followed by cyclization with an additional
alkyne. Finally, dehydrogenative aromatization from intermediates
C would afford pyridine derivatives 5 (Fig. 2b).
To investigate the reaction mechanism, we tested the reaction
of 3a and 1c in the presence of LAuCl/KBArf (5mol%) under the
similar reaction conditions. However, products corresponding to C
were not detected, and only a complex mixture was obtained. We
postulated that Me-group at the imine carbon in 3 is necessary for
the formal [4+2] cycloaddition because it could isomerize to
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In summary, we have developed the first gold-catalyzed
intermolecular hydroimination of terminal alkynes, which afforded
anti-Markovnikov adduct in moderate yields, concomitant with
unstable Markovnikov adducts. Further study on the relevant gold
ctalysis30 with LAuCl is currently under investigation.
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