Suzuki-Miyaura cross-coupling of amides and esters at room temperature: Correlation with barriers to rotation around C-N and C-O bonds

Article abstract of DOI:10.1039/c7sc02692g

The Suzuki-Miyaura cross-coupling has been widely recognized as one of the most important methods for the construction of C-C bonds. However, in contrast to traditional aryl halide or pseudohalide electrophiles, coupling reactions with unactivated C-N and C-O electrophiles have proven significantly more challenging. Here we report the first general palladium-catalyzed Suzuki-Miyaura cross-coupling of both common amides and aryl esters through the selective cleavage of the C-N and C-O bonds under exceedingly mild conditions. Notably, for the first time we demonstrate selective C(acyl)-N and C(acyl)-O cleavage/cross-coupling under the same reaction conditions. The reaction uses a commercially available, bench-stable and operationally-convenient (η³-1-t-Bu-indenyl)Pd(IPr)(Cl) precatalyst. Furthermore, we demonstrate that the reactivity of generic amides and aryl esters can be correlated with barriers to isomerization around the C(acyl)-X (X = N, O) bond, thus providing a blueprint for the development of a broad range of novel coupling reactions of ester and amide electrophiles by the selective activation of C-O and C-N bonds.

Full text of DOI:10.1039/c7sc02692g

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Suzuki–Miyaura cross-coupling of amides and
esters at room temperature: correlation with
barriers to rotation around C–N and C–O bonds†
Cite this: DOI: 10.1039/c7sc02692g
b
b
Peng Lei,ab Guangrong Meng,
Roman Szostak
Shicheng Shi,
Yun Ling,a Jie An,a
c
b
*
and Michal Szostak
The Suzuki–Miyaura cross-coupling has been widely recognized as one of the most important methods for
the construction of C–C bonds. However, in contrast to traditional aryl halide or pseudohalide
electrophiles, coupling reactions with unactivated C–N and C–O electrophiles have proven significantly
more challenging. Here we report the first general palladium-catalyzed Suzuki–Miyaura cross-coupling
of both common amides and aryl esters through the selective cleavage of the C–N and C–O bonds
under exceedingly mild conditions. Notably, for the first time we demonstrate selective C(acyl)–N and
C(acyl)–O cleavage/cross-coupling under the same reaction conditions. The reaction uses
a
commercially available, bench-stable and operationally-convenient (h3-1-t-Bu-indenyl)Pd(IPr)(Cl)
Received 16th June 2017
Accepted 20th July 2017
precatalyst. Furthermore, we demonstrate that the reactivity of generic amides and aryl esters can be
correlated with barriers to isomerization around the C(acyl)–X (X ¼ N, O) bond, thus providing
a blueprint for the development of a broad range of novel coupling reactions of ester and amide
electrophiles by the selective activation of C–O and C–N bonds.
DOI: 10.1039/c7sc02692g
rsc.li/chemical-science
intermediates through direct coupling and decarbonylative
pathways (Fig. 1A).4–7
Introduction
Here we demonstrate the selective C(acyl)–N and C(acyl)–O
cleavage/cross-coupling of common amides and esters by
operationally-convenient Pd–NHC precatalyst at room temper-
ature. The following features of our study are notable:
(1) We describe the rst room temperature Suzuki coupling
of both common esters and amides by O–C/N–C bond
Transition-metal-catalyzed cross-coupling reactions have
become
a
central transformation in organic synthesis.1
Although cross-coupling reactions of aryl halides and pseudo-
halides have been extensively developed,2 chemical methods
using unconventional electrophiles have been more challenging
to develop.3 Over the past ve years, strategies for the cross-
coupling of unactivated aryl ester and amide electrophiles
using Ni-, Pd- and Rh-catalysis have been reported.4–7 However,
the development of practical methods by selective C(acyl)–O
and C(acyl)–N scission in common esters and amides remains
an elusive goal in organic synthesis.
The key challenge in the cross-coupling of unactivated esters
and amides is slow oxidative addition of the C(acyl)–X (X ¼ N, O)
bond to the metal.8 Accordingly, these reactions typically
require elevated temperatures, leading to problems associated
with low reactivity and reaction selectivity of acyl-metal
aDepartment of Applied Chemistry, College of Science, China Agricultural University,
Beijing 100193, China
bDepartment of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102,
USA. E-mail: michal.szostak@rutgers.edu
cDepartment of Chemistry, Wroclaw University, F. Joliot-Curie 14, Wroclaw, 50-383,
Poland
Fig. 1 (A) Cross-coupling of carboxylic acid derivatives. (B) The first
general coupling of amides and esters at room temperature enabled
by designer Pd–NHC catalysis (this work).
† Electronic supplementary information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/c7sc02692g
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activation. This is by far the most active catalyst identied to common amides.7a With the goal of further expanding the
date. The current-state-of-the-art for ester coupling is 90 ꢀC, manifold of amide functionalization, we anticipated that s
while for amides 60 ꢀC. The practicality and generality of donor nucleophilic N-heterocyclic carbene (NHC) ligands could
method supersede the current methods.
provide rapid access to acylmetal intermediates from amides
(2) We establish the rst Suzuki coupling of both common using practical palladium catalysis.7m,12 Our studies focused on
esters and amides by selective O–C/N–C bond activation under the identication of a suitable Pd–NHC precatalyst capable of
the same catalytic reaction conditions. This concept could facilitating oxidative addition at ambient conditions. We
enable the productive engagement of the acyl and aryl coupling established that a commercially available, bench-stable and
mechanisms from both types of these unconventional electro- operationally-convenient (h3-1-t-Bu-indenyl)Pd(IPr)(Cl) (1),
philes and avoid restriction to a particular acylmetal precursor. developed by Hazari and co-workers,13 is an effective catalyst for
(3) We provide guidelines for cross-coupling of amide/ester the coupling of common amides and esters with boronic acids
electrophiles by showing that the reactivity can be correlated at room temperature (eqn (1)).
with barriers to isomerization around the C(acyl)–X bond.
These innovative concepts in the cross-coupling of amide/
ester acyl electrophiles demonstrate how simple and readily
available amides and esters can be systematically used as
coupling partners in transition metal catalysis.
(1)
There is a strong impetus to develop selective coupling
reactions of bench-stable unactivated amide and ester electro-
philes due to the prevalence of these precursors in modern
organic synthesis.9 Furthermore, amides and esters are tradi-
tionally derived from different pool of precursors than aryl
halides, phenols and anilines.10 Additionally, amides and esters
Elegant studies have shown that a major advantage of (h3-
are robust, easy to handle, and typically inert to a variety of
reaction conditions allowing for ring prefunctionalization.11
Developing a single manifold for C(acyl)–O and C(acyl)–N
cleavage could enable the productive engagement of the acyl
and aryl coupling mechanisms from both types of these
unconventional electrophiles and avoid restriction to a partic-
ular acylmetal precursor. Methods that engage common amides
and esters in coupling manifolds under mild conditions are
absent.4–7
Here we establish the cross-coupling of common amides and
aryl esters through the selective cleavage of the C–N and C–O
bonds that proceeds under exceedingly mild conditions
(Fig. 1B). These studies represent the rst example of a room
temperature Suzuki–Miyaura coupling of common amide and
ester electrophiles.4–7 As a result of these studies, we report
experimental evidence showing that (1) the selective C(acyl)–N
and C(acyl)–O cleavage/cross-coupling can proceed under the
same reaction conditions, (2) the reactivity of generic amides
and aryl esters can be correlated with barriers to isomerization
around the C(acyl)–X (X ¼ N, O) bond, (3) the present barrier to
chemoselective carbon–carbon bond forming reactivity of
unactivated esters and amides by C(acyl)–X (X ¼ N, O) bond
cleavage is located at ca. 10 kcal molꢁ1 bond isomerization.
Collectively, our studies provide a blueprint for the develop-
ment of a broad range of novel coupling reactions of uncon-
ventional ester and amide electrophiles by the selective cleavage
of C–O and C–N bonds.
indenyl)Pd(IPr)(Cl) complexes stems from preventing the
formation of the catalytically inactive Pd(I) dimer, and fast
reduction of Pd(II) to Pd(0), depending on reaction conditions.14
The high reactivity of these complexes results from maintaining
the optimal 1 : 1 Pd to ligand ratio.15 The indenyl complex (1)
was screened as a catalyst in the Suzuki–Miyaura cross-coupling
of N-Boc activated amide 2a with 4-tolylboronic acid (eqn (1)).
Note that N-Boc activated amides are readily prepared by
selective N-tert-butoxycarbonyl activation, which provides
a general method for the cross-coupling of common
(2)
(3)
secondary amides.4a,b We postulate that N-Boc activated amides
represent the most synthetically useful family of amide elec-
trophiles for cross-coupling.7 Under optimal conditions, the
cross-coupling of 2a proceeded in excellent yield using both
heterogeneous (THF, 96% yield) and homogeneous (i.e. the base
is either in or out of solution) (MeOH : THF, 19 : 1, 95% yield)
reaction conditions13 (K2CO3, 4.5 equiv.) at room temperature
(eqn (1)). Furthermore, coupling of aryl ester 5a with 4-tol-
ylboronic acid afforded the biaryl ketone product in 80% yield
(THF, K2CO3, 7.2 equiv.). Interestingly, the use of homogeneous
conditions with 5a was ineffective (23% yield) (eqn (1)).
Although detailed mechanistic studies are beyond the scope of
Results and discussion
Recently, our laboratory introduced a new amide bond activa-
tion mode by ground-state destabilization.4a The seminal
studies by Garg and co-workers have demonstrated that clas-
sical acyl cross-coupling pathways can be accessed from
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Table 2 Cross-coupling of esters at 23 ꢀCa,b
this report, based on the elegant studies by Hazari, Nolan and
others, the mechanism of activation involves rapid reduction
from Pd(II) to Pd(0) under both types of conditions.14b Other
bases (KOH, K3PO4, KF) and solvents (dioxane, toluene) were
screened, with THF and K2CO3 providing the optimum
results.
With the optimal reaction conditions in hand, the scope of
the process was surveyed. As shown in Table 1, this method
proceeds at room temperature with an extensive range of
sterically and electronically differentiated boronic acids. The
coupling using neutral (entry 1), sterically-hindered (entry 2),
electron-rich (entries 3–4), electron-withdrawing (entry 5) and
uoro-containing (entry 6) boronic acids proceeded in excellent
yields. Next, the scope of the amide component was examined
(Table 1). As shown, a broad range of electrophilic amide
acceptors serves as effective acyl precursors in this coupling.
Notably, sterically-demanding (entry 7), electron-rich (entry 8),
electron-decient (entries 9–10), uorine-containing (entry 11)
and electron-rich, heterocyclic amides conjugated at the 2-
position (entry 12) proved highly effective in the reaction. For
comparison, the coupling was conducted using aryl ester 5a,
providing the biaryl ketone products in high to excellent yields
at room temperature (Table 2, entries 1–6). Importantly, the
scope of the ester component is also broad, furnishing high to
Table 1 Cross-coupling of amides at 23 ꢀCa,b
a
Conditions: (1) (3 mol%), Ar-B(OH)2 (4.5 equiv.), K2CO3 (7.2 equiv.),
b
THF (0.25 M), 23 ꢀC, 15 h. Isolated yields. See ESI for full details.
excellent yields in the coupling at room temperature (Table 2,
entries 7–12). Comparison of (1) with (h3-cinnamyl)Pd(IPr)(Cl)
(Cin-IPr)7m,17 demonstrated that (1) signicantly outperforms
Cin-IPr in the coupling (see ESI†). The biaryl ketone motif is
a central component in the production of a wide range of
pharmaceuticals, natural products and organic materials.16
Boronic acid is used in excess. Transmetallation is generally
considered as the rate-determining step in Suzuki–Miyaura
couplings using Pd–NHC.15b While further optimization of
reaction conditions are ongoing in our laboratory, we note that
(1) under our optimized conditions the coupling using 2.0
equiv. of boronic acid proceeds in 96% and 49% yield for the
amide and ester, respectively; (2) we determined that in the
coupling of esters, aqueous K2CO3 could be used, leading to
93% yield with 2.0 equiv. of boronic acid.
Importantly, we have demonstrated that (1) there is no
difference in the reaction efficiency when the reactions are
performed using readily available commercial vials without pre-
treatment; (2) since we are interested in developing cross-
coupling of amides and esters of high operational simplicity,
all reactions were set up on the bench-top (cf. glove-box); (3) we
have demonstrated that adding water to the reaction (5 equiv.)
leads to no decrease in yields in the cross-coupling of amides
(95% yield) and esters (93% yield).
a
Conditions: (1) (3 mol%), Ar-B(OH)2 (3.0 equiv.), K2CO3 (4.5 equiv.),
b
THF (0.25 M), 23 ꢀC, 15 h. Isolated yields. See ESI for full details.
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To our knowledge, these results provide (1) the rst example
of efficient cross-coupling of common amide and aryl ester
electrophiles at room temperature using the versatile Pd-
catalysis manifold,1c (2) the rst example of C–C coupling of
amides and esters using a single catalyst system.4–7 Importantly,
coupling of the challenging N-alkyl amides proceeded
uneventfully (eqn (2)). It is noteworthy that one-pot N-Boc
activation/cross-coupling furnishes the coupling product in
excellent yield (eqn (3)), demonstrating the robustness of our
protocol, and the potential of Pd–NHC precatalysts in amide
coupling. To further illustrate the scope, we have systematically
investigated the reaction efficiency when both components
contain electron-rich and electron-withdrawing groups, exam-
ples when both components contain steric hindrance and
aliphatic amide/ester electrophiles (Table 3). In all these
examples good to excellent yields have been obtained. The
present method surpasses other known protocols for coupling
of amides/esters,4–7 and is performed at practical, room
temperature conditions, which is not the case with any other
coupling of these electrophiles reported to date. Note that, in
addition to the prevalence of aryl esters in organic synthesis,
aryl esters can be readily prepared from the corresponding
carboxylic acids, benzylic alcohols, aldehydes and ketones,6
providing another advantage of this coupling manifold.
Fig. 2 Kinetic profile in the Suzuki–Miyaura cross-coupling with 4-
tolylboronic acid catalyzed by (1) (3 mol%) at room temperature. 2a:
N,N-Ph,Boc; 5a: OPh; 8: N,N-Ph,Ts.
Studies were conducted to gain insight into the catalytic
reactivity of amide and ester electrophiles undergoing cross-
coupling (Fig. 2). N-Ts/Ph benzamide7 was included for
comparison. The study revealed signicantly higher catalytic
activity of 2a than 5a and N-Ts/Ph benzamide.
Furthermore, in competition studies, 2i reacts preferentially
over the less activated anilide (7) and N-Ts/Ph amide (8), giving
full selectivity for the coupling of 2i (Scheme 1A). Moreover, 2i
gives a synthetically useful selectivity over aryl ester 5a (Scheme
1B). We also note that the alkyl ester bond remains intact under
these conditions (4e).
Table 3 Cross-coupling of amides and esters at 23 ꢀCa
The reactions indicate that there is a correlation between
C–X acyl bond strength and reactivity. Classic studies by Lieb-
man and Greenberg18 demonstrated that amidic resonance (Er
¼ 15–20 kcal molꢁ1, nN/pC* ]O conjugation in planar amides)
correlates with the rotational barrier. Esters show higher reso-
nance energy than amides (e.g., MeCO2Me, Er ¼ 24 kcal molꢁ1);
however, the rotational barrier is signicantly lower as a result
of maintaining resonance during the isomerization pathway.
We have previously correlated the reactivity of amides with
resonance energy by ground-state destabilization.7m The present
study allows to include the ester bond along the C–X (X ¼ N, O)
bond rotational pathway required for the effective metal inser-
tion. Rotational barrier in PhCO2Ph has been quantied (9.3
kcal molꢁ1, see ESI†). Overall, our results provide strong
support for a generalized reactivity scale of the amide and ester
a
See, Tables 1 and 2. See ESI for full details.
Scheme 1 Selectivity in cross-coupling at 23 ꢀC.
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2 (a) T. J. Colacot, New Trends in Cross-Coupling, The Royal
Society of Chemistry, Cambridge, 2015; (b) M. Stradiotto
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Scheme 2 Reactivity scale of amides and esters in TM-catalyzed acyl
C–N and acyl C–O coupling.
electrophiles for the formation of acyl-metal intermediates
(Scheme 2).19 The present barrier to chemoselective carbon–
carbon bond forming reactivity of unactivated esters and
amides by C(acyl)–X (X ¼ N, O) cleavage is located at ca. 10 kcal
molꢁ1 bond isomerization.20 The development of improved
catalyst systems will lay a foundation for a general application
of amide and ester coupling in synthesis. Isomerization barrier
is an important parameter that should be considered in acyl
couplings.
Conclusions
4 Reviews: (a) G. Meng, S. Shi and M. Szostak, Synlett, 2016, 27,
2530; (b) J. E. Dander and N. K. Garg, ACS Catal., 2017, 7,
1413; (c) J. Yamaguchi, K. Muto and K. Itami, Eur. J. Org.
Chem., 2013, 19; (d) R. Takise, K. Muto and J. Yamaguchi,
Chem. Soc. Rev., 2017, DOI: 10.1039/c7cs00182g.
5 Reviews on acyl-metal intermediates: (a) L. J. Gooßen,
N. Rodriguez and K. Gooßen, Angew. Chem., Int. Ed., 2008,
47, 3100; (b) W. Dzik, P. Lange and L. Gooßen, Chem. Sci.,
2012, 3, 2671.
6 Representative examples of coupling of esters: (a) K. Amaike,
K. Muto, J. Yamaguchi and K. Itami, J. Am. Chem. Soc., 2012,
134, 13573; (b) R. Takise, K. Muto, J. Yamaguchi and K. Itami,
Angew. Chem., Int. Ed., 2014, 53, 6791; (c) K. Muto,
J. Yamaguchi, D. G. Musaev and K. Itami, Nat. Commun.,
2015, 6, 7508; (d) N. A. LaBerge and J. A. Love, Eur. J. Org.
Chem., 2015, 25, 5546; (e) R. Takise, R. Isshiki, K. Muto,
K. Itami and J. Yamaguchi, J. Am. Chem. Soc., 2017, 139,
3340; (f) T. Okita, K. Kumazawa, R. Takise, K. Muto,
K. Itami and J. Yamaguchi, Chem. Lett., 2017, 46, 218; (g)
L. Guo, A. Chatupheeraphat and M. Rueping, Angew.
Chem., Int. Ed., 2016, 55, 11810; (h) L. Guo and
M. Rueping, Chem.–Eur. J., 2016, 22, 16787; (i) X. Pu, J. Hu,
Y. Zhao and Z. Shi, ACS Catal., 2016, 6, 6692; (j)
T. B. Halima, W. Zhang, I. Yalaoui, X. Hong, Y. F. Yang,
K. N. Houk and S. G. Newman, J. Am. Chem. Soc., 2017,
139, 1311; (k) T. B. Halima, J. K. Vandavasi, M. Shkoor and
S. G. Newman, ACS Catal., 2017, 7, 2176; (l) L. Hie,
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In summary, we demonstrated that Hazari's (h3-1-t-Bu-indenyl)
Pd(IPr)(Cl) precatalyst shows unprecedented reactivity in the
Suzuki–Miyaura cross-coupling of amides and esters by selec-
tive C(acyl)–N and C(acyl)–O cleavage. The potential of this
catalyst system is illustrated by the rst example of high
yielding cross-coupling of amide and ester electrophiles at room
temperature. This study demonstrates for the rst time the
selective C(acyl)–N and C(acyl)–O cleavage/C–C coupling under
the same reaction conditions. The reactivity of generic amides
and aryl esters can be correlated with barriers to isomerization
around the C(acyl)–X bond (X ¼ N, O). This study provides
a blueprint for the development of a broad range of novel
coupling reactions by avoiding restriction to a particular acyl-
metal precursor.
Acknowledgements
P. L. thanks the China Scholarship Council (No. 201606350069)
for a fellowship. Financial support was provided by Rutgers
University. The 500 MHz spectrometer was supported by the
NSF-MRI grant (CHE-1229030). We thank the Wroclaw Center
for Networking and Supercomputing (grant number WCSS159)
and the NSF (CAREER CHE-1650766).
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