Ethanol as a Hydrogenating Agent: Palladium-Catalyzed Stereoselective Hydrogenation of Ynamides To Give Enamides

Article abstract of DOI:10.1002/anie.201702277

Ethanol is shown to act as a hydrogenating agent for ynamides under palladium catalysis. This behavior is different from the normally expected reaction of ethanol addition to alkynes. The reaction shows stereoselectivity for E enamides, which is in contrast to reports using other hydrogenating sources. The method was also extended to ynamines. Alternatively, the use of ethanol and ammonium formate as the hydrogenating source gives Z enamides. The role of ethanol in hydrogenation was demonstrated by means deuterium labeling experiment.

Full text of DOI:10.1002/anie.201702277

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702277
German Edition: DOI: 10.1002/ange.201702277
Homogeneous Catalysis
Ethanol as a Hydrogenating Agent: Palladium-Catalyzed Stereoselec-
tive Hydrogenation of Ynamides to Give Enamides
Alla Siva Reddy and K. C. Kumara Swamy*
Abstract: Ethanol is shown to act as a hydrogenating agent for
ynamides under palladium catalysis. This behavior is different
from the normally expected reaction of ethanol addition to
alkynes. The reaction shows stereoselectivity for E enamides,
which is in contrast to reports using other hydrogenating
sources. The method was also extended to ynamines. Alter-
natively, the use of ethanol and ammonium formate as the
hydrogenating source gives Z enamides. The role of ethanol in
hydrogenation was demonstrated by means deuterium labeling
experiment.
T
ransition-metal-catalyzed hydrogenation of alkynes to give
alkenes is an important reaction of vast synthetic utility.[1]
Either a heterogeneous catalyst (Raney Ni, Lindlar catalyst,
Pd/C)[2] or a homogeneous catalyst consisting of a Rh, Ru, or
Ir complex[3] can be utilized to achieve this transformation
using hydrogen gas as the hydrogenating agent. Two problems
associated with many of these methods are 1) a lack of
chemo- and stereoselectivity for the alkenes and 2) over-
reduction of the resulting alkenes to alkanes.[4] While much of
the literature is devoted to diaryl/dialkyl-substituted alkynes,
ynamides have also emerged as important synthons.[5,6]
Selected recent examples of hydrogenation of alkynes
including ynamides are shown in Scheme 1. We are aware of
only two reports on the catalytic hydrogenation of ynamides
to give Z alkenes by using hydrogen gas and either Lindlar or
a [Pd] catalyst.[7] With regard to general alkynes, Szymczak
and co-workers recently described hydrogenation to give
Z alkenes by using a borane-appended [Ru] catalyst, and
Tokmic and Fout reported E-specific hydrogenation using
a [Co] catalyst.[8] Apart from these, several other interesting
reports on the semihydrogenation of alkynes are known.[9]
Among the methods shown in Scheme 1, reaction (a) utilizes
formic acid as the hydrogen source, and those in (b)–(d)
involve hydrogen gas. In reaction (e), a H3B:NH3 adduct is
utilized in methanol, which acts only as a solvent.
Scheme 1. Selected examples of hydrogenation of alkynes or ynamides.
group (Scheme 2).[11] Numerous transformations involving
ynamides have been explored in recent years for the
generation of carbocycles, heterocycles, and enamides,
among others.[12–14] The enamides thus obtained are versatile
Scheme 2. Reactivity specificity of ynamides.
The unusual reactivity of ynamides is presumably the
result of formation of a reactive keteniminium ionic species[10]
or coordination of the metal catalyst to the alkyne, and in
some cases, to the heteroatom of the electron-withdrawing
building blocks that are present in several natural products.[15]
Our idea was to use inexpensive ethanol as the hydrogenating
agent, since it can be oxidized to aldehyde with the
elimination of hydrogen, thereby avoiding the more diffi-
cult-to-handle hydrogen gas. To the best of our knowledge,
the use of ethanol as a hydrogenating agent in the catalytic
reduction of alkynes is rather unknown.[4c] Alcohol/phenol
addition to alkynes (hydroalkoxylation/aryloxyaltion) is lot
more common.[16] Herein, we report easy-to-handle and
inexpensive ethanol as the hydrogenating source. The use of
either ethanol or an ethanol/ammonium formate system with
an ynamide precursor is presented as an elegant stereoselec-
tive approach to either E or Z enamides through [Pd]-
catalyzed hydrogenation.
[*] A. Siva Reddy, Prof. K. C. Kumara Swamy
School of Chemistry, University of Hyderabad
Hyderabad-500046 (India)
E-mail: kckssc@uohyd.ac.in
kckssc@yahoo.com
Supporting information (including experimental details; optimiza-
tion Tables S1 and S2; ORTEPS of 2a, 2e, and 2m; and 1H/13C NMR/
31P spectra)and the ORCID identification number(s) for the author(s)
of this article can be found under:
https://doi.org/10.1002/anie.201702277.
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Table 1: Scope of the hydrogenation of ynamides by ethanol to give
E enamides.[a]
Our initial examination involved the reaction of N-alkynyl
benzenesulfonamide 1a with ethanol (used also as a solvent)
in the presence of Pd(PPh3)4 (5 mol%) as the catalyst at 908C
for 12 h. We were delighted to obtain the trans-hydrogenated
product 2a in 56% yield (Scheme 3). The stereochemistry
and structure of compound 2a were confirmed by X-ray
crystallography (Figure 1).[17]
Scheme 3. [Pd]-catalyzed hydrogenation of ynamide 1a with ethanol.
Figure 1. Structures of compounds 2a (left) and 2m (right).[17]
Encouraged by the above result, we turned our attention
to improve the yield of product 2a (Table S1 in the Support-
ing Information). As expected, there was no reaction in
aprotic solvents like toluene, THF, and DMSO. There was
also no formation of the desired product in the absence of the
catalyst. The primary alcohols nPrOH and nBuOH also
i
worked well. The reaction rate was slower in PrOH and no
product was detected when using tBuOH as the solvent. Thus
only alcohols possessing at least one hydrogen substituent on
the a-carbon atom are suitable for the hydrogenation process.
Intriguingly, in the presence of K2CO3 as a base, we noticed
the formation of a cis-hydrogenated product as the major
isomer, albeit in only 36% yield (after isolation). To our
delight, the reaction proceeded smoothly when using either
Et3N or pyridine as the base, affording the desired product in
92% yield with excellent stereoselectivity. [Pd] catalysts that
do not contain phosphine failed to produce product 2a. Pd/C
and Pd2(dba)3 were less efficient as catalysts for this trans-
formation. More importantly, no product formation was seen
at room temperature. This may be due to the requirement of
[a] Conditions: 1 (0.2 mmol), Pd(PPh3)4 (5 mol%), Et3N (0.6 mmol) in
ethanol (1 mL) at 908C for 12 h. Yields of isolated product after column
chromatography are given in parenthesis. For the preparation of
compound 2q, we used Pd(PPh3)4 (10 mol%) and Et3N (1.2 mmol).
in good yields. Altering the substituents on the nitrogen atom
of the ynamide from aliphatic to aromatic was also tolerated
and delivered the enamides 2i–2k. The reaction is compatible
with cyclic sulfonamide derived ynamides to give the enam-
ides 2m,n. Chemoselectivity is demonstrated by the retention
of the carbonyl group as in product 2o (or 2s). The ynamide
1p, which has a free sulfonamide functionality, also worked to
provide the product 2p. Interestingly, hydrogenation of two
a
higher dissociation energy for hydride-ion transfer.
Although decreasing the catalyst loading to 3 mol% reduced
the product yield when keeping the reaction time of 12 h,
increasing the duration to 36–48 h afforded better yield of the
product even with 2 mol% of the catalyst (i.e., a substrate/
catalyst ratio of 50:1). The optimal conditions for this
transformation were 1a (0.2 mmol), Pd(PPh3)4 (5 mol%),
Et3N (0.6 mmol) in ethanol (1 mL) at 908C for 12 h (see
Table S1 in the Supporting Information).
Various types of ynamides were transformed into the
corresponding E enamides 2a–u in good to excellent yields
with high stereoselectivity when using ethanol as the hydro-
genating source (Table 1). Even the sterically crowded
ynamides 1d–f furnished the corresponding products 2d–f
ꢀ
different C C bonds can also be accomplished readily, as
shown by the isolation of 2q in good yield. Replacing the
sulfonyl group with a phosphoryl group does not affect the
stereoselectivity, and affords the product 2r in excellent yield.
The carbamate-derived ynamide slightly reduced the stereo-
selectivity, but with a good overall yield of 2s. The functional-
group tolerance is demonstrated by selective hydrogenation
of the ynamide functionality in the presence of non-activated
alkynyl and alkenyl groups, as shown by the isolation of
products 2t and 2u. Additionally, the enamide product does
not become over-hydrogenated, even after 48 h. The struc-
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tures of compounds 2m and 2e were confirmed by X-ray
crystallography (Figure 1, right and Figure S2 in the Support-
ing Information).
It is important to note that the carbazole-derived ynamine
substrate 3 also afforded the enamine 4 in 96% yield of
isolated product, with the E isomer still predominating
(Scheme 4).
Scheme 5. Hydrogenation (Z-specific/full) of ynamide 1s.
Scheme 4. Hydrogenation of ynamine 3 with ethanol.
Scheme 6. Control experiments.
During optimization of the above reaction, it was noted
that using Pd/C or Pd2(dba)3 as the catalyst preferentially led
to only very small quantities of the Z isomer (Table S1,
entries 19–20), with most of the starting ynamide unreacted.
We surmised that the addition of other hydrogenating source
may increase the yield of this isomer. Aqueous formic acid led
primarily to the addition product with water. Pleasingly, using
EtOH/NH4OOCH as the hydrogenating source afforded
Z enamides with high stereoselectivity. This route was then
utilized to obtain several Z enamides (2a’, 2c’, 2d’, 2 f’, 2l’,
2m’, and 2n’; Table 2). Since the yield was lower when THF
(or toluene or CH3CN) was used in place of EtOH, we believe
that the latter plays a synergistic role in hydrogenation. In
these cases, there was no over-hydrogenation even after 48 h.
The ynamide 1s was lot more reactive, and afforded a good
yield of 2s’ even at RT; at 908C, fully reduced product 5 was
obtained in high yields (Scheme 5).
catalytic cycle for the formation of compounds 2a–s, based on
control experiments and earlier reports,[18] is shown in
Scheme 7. The ynamide likely exists as the alkyne (1) and
keteniminium (1’) resonance forms. The in situ formed [Pd]
For the reaction shown in Scheme 3, replacing ethanol
with methanol led to predominantly 2a, along with small
amounts of the (competitive) methanol addition product 6
(Scheme 6a). Use of CH3OD (or C2H5OD) led to the
isolation of 2a-d1 (Scheme 6b). The reaction using CD3OD
did not proceed, probably for kinetic reasons. A plausible
Scheme 7. Proposed pathway for the formation of 2.
intermediate I undergoes addition to 1’ to give intermediate
II. It is likely that in II, the metal is coordinated to the
sulfonyl/phosphoryl oxygen atom. Steric interactions between
the nitrogen lone pair and R1/H group may be responsible for
the trans stereoselectivty, with the hydrogen preferring the
opposite side of Pd. In general, we did not observe isomer-
ization or over-hydrogenation. In the X-ray structures of 2a,
Table 2: Scope of the Z-selective hydrogenation of ynamides.[a]
À
=
2e, and 2m, the N C(H) hydrogen atom is close to one of
the sulfonyl oxygen atoms (< 2.87 ꢀ), which may explain the
stereochemistry to some degree, but more data is required to
establish this feature. Species II undergoes hydride shift with
subsequent elimination of acetaldehyde to give intermediate
III. Isolation of deuterated compound 2a-d1 is consistent with
t
the intermediacy of II. The lack of reaction with BuOH
suggests a requirement for b-hydride shift. Finally, III under-
goes reductive elimination to afford 2, thereby thus regener-
ating the active [Pd0] catalyst.
In conclusion, we have shown that ethanol can act as the
hydrogenating agent in the palladium-catalyzed reaction of
[a] Conditions: 1 (0.2 mmol), Pd/C (10 mol%), HCOONH4 (0.6 mmol)
in EtOH (1 mL) at 908C for 12 h. Yields of isolated product after column
chromatography are given in parenthesis.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: enamides · ethanol · hydrogenation · palladium ·
ynamides
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Manuscript received: March 2, 2017
Final Article published: && &&, &&&&
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Communications
Homogeneous Catalysis
A. Siva Reddy,
K. C. Kumara Swamy*
&&&& — &&&&
Ethanol as a Hydrogenating Agent:
Palladium-Catalyzed Stereoselective
Hydrogenation of Ynamides to Give
Enamides
One for the road: Ethanol acts as
mides, which is in contrast to reports
using other hydrogenating sources, and
the method was also extended to yna-
mines. Alternatively, the use of ethanol/
ammonium formate as the hydrogenating
source gives Z enamides.
a hydrogenating agent for ynamides
under palladium catalysis. This behavior
is different from the normally expected
addition of ethanol to alkynes. The reac-
tions shows stereoselectivity for E ena-
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!