Triflamides: Highly Reactive, Electronically Activated N-Sulfonyl Amides in Catalytic N-C(O) Amide Cross-Coupling

Article abstract of DOI:10.1021/acs.orglett.8b03901

The direct, highly chemoselective Suzuki-Miyaura cross-coupling of trifluoromethanesulfonamides (triflamides) by selective N-C(O) amide bond cleavage is reported. This operationally simple, mild, and user-friendly method accomplishes the direct synthesis of ketones from amides by a catalytic manifold as a powerful alternative to Weinreb amides. Mechanistic studies support rotational inversion and electronic activation, favoring selective insertion under mild conditions. Our data strongly suggest that triflamides should be routinely considered as precursors in amide bond cross-coupling.

Full text of DOI:10.1021/acs.orglett.8b03901

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Triflamides: Highly Reactive, Electronically Activated N‑Sulfonyl
Amides in Catalytic N−C(O) Amide Cross-Coupling
Shicheng Shi,† Roger Lalancette,† Roman Szostak,‡ and Michal Szostak*,†
†Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
‡Department of Chemistry, Wroclaw University, F. Joliot-Curie 14, Wroclaw 50-383, Poland
S
* Supporting Information
ABSTRACT: The direct, highly chemoselective Suzuki−
Miyaura cross-coupling of trifluoromethanesulfonamides
(triflamides) by selective N−C(O) amide bond cleavage is
reported. This operationally simple, mild, and user-friendly
method accomplishes the direct synthesis of ketones from
amides by a catalytic manifold as a powerful alternative to
Weinreb amides. Mechanistic studies support rotational
inversion and electronic activation, favoring selective insertion
under mild conditions. Our data strongly suggest that triflamides should be routinely considered as precursors in amide bond
cross-coupling.
he recent years have witnessed extraordinary progress in
metal-catalyzed cross-couplings of amides by selective
resonance destabilization of the acyl amide bond (Ar = Ph, R =
Ph, RE = 9.7 kcal/mol; τ = 18.8°; χN = 18.9°, RE = resonance
energy).5a In this context, we were attracted by the
trifluoromethanesulfonyl group as one of the most powerful
electron-withdrawing groups in organic chemistry.15 Drawing
from our experience in amide bond destabilization,5 we
envisioned that N-Tf (Tf = triflyl) could be employed as a
viable activating group for amides, wherein the electron-
withdrawing effect would have a two-fold positive effect by (i)
facilitating metal insertion16 and (ii) enhancing the leaving
group potential,15a permitting cross-coupling under mild
conditions.
T
metal insertion into the inert N−C(O) bond (Figure 1A).1−3
We report the realization of this hypothesis and present the
direct, highly chemoselective Suzuki−Miyaura cross-coupling of
trifluoromethanesulfonamides (triflamides) by selective N−
C(O) amide bond cleavage (Figure 1B). Most importantly, this
study introduces triflamides as highly reactive, bench-stable,
inexpensive N-sulfonyl amides for catalytic cross-couplings by
N−C(O) bond activation with high selectivity. Our data
strongly suggest that triflamides should be routinely considered
as amide bond precursors in the burgeoning manifold of amide
bond cross-coupling.2,6−13
Our investigations began with the examination of the Suzuki−
Miyaura cross-coupling of a model N-Tf amide 1a (N-phenyl-N-
((trifluoromethyl)sulfonyl)benzamide) with 4-tolyl boronic
acid using various Pd(II)-NHC (NHC = N-heterocyclic
carbene) precatalysts (eq 1). There are two main challenges in
the catalytic activation of amides: (1) selective metal insertion
into the N-acyl bond and (2) undesired cleavage of the N-
sulfonyl group, deactivating the amide toward insertion.2a,b In
addition, the stability of the acyl-metal intermediate contributes
Figure 1. (A) Activation of amides and derivatives. (B) This work.
Triflamides, a new class of highly reactive amides for cross-coupling.
The capacity of the amide bond to control resonance (nN →
π*C=O donation, barrier to rotation, planar amides = 15−20
kcal/mol)4,5 and therefore facilitate metal insertion through N-
substitution has engendered significant interest as a way to
develop mild catalytic methods exploiting the activation of N−
C(O) bonds.6−13 Given the ubiquity of amide bonds in drugs,
polymers, and functional materials, new selective methods for
the functionalization of amides offer great practical oppor-
tunities for organic synthesis in both academic and industrial
settings.14
Most of the successful approaches in the cross-coupling of
amides to date utilize N-Ts (Ts = 4-toluenesulfonyl) as an
activating group enabling metal insertion into the N−C(O)
bond.2,6−13 Harnessing N-Ts amides has been possible through
Received: December 6, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03901
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Letter
to the efficiency of the cross-coupling. The use of bench-, air-,
and moisture-stable strongly σ-donating Pd(II)-NHC precata-
lysts provides an excellent avenue for the synthesis of acyl-metal
intermediates from amides.17 After extensive optimization, we
were delighted to identify that the cross-coupling of N-Tf amide
1a proceeds under exceedingly mild conditions at 40 °C (eq 1,
[Pd(IPr)(L)Cl] (3 mol %), K2CO3 (3 equiv), water (5 equiv),
THF, 15 h). Importantly, the high reactivity was observed using
Pd-NHC precatalysts bearing various throwaway ligands (Pd-
PEPPSI-IPr, 95%; Pd(IPr)(cin)Cl, 94%, Pd(IPr)(1-t-Bu-
indenyl)Cl, 95%, eq 1),18 highlighting the aptitude of the N-
Tf activating group as the acyl-metal precursor.16 The observed
reactivity of N-Tf amides compares very favorably with the
current state of the art, namely, the cross-coupling of N-Ts
amides, which proceeds at 30% conversion under the same
conditions.6g Two additional points should be noted. (1) The
cross-coupling of N-Tf amide 1a proceeds in 33, 38, and 79%
yields at 23 °C using Pd-PEPPSI-IPr, Pd(IPr)(cin)Cl, and
Pd(IPr)(1-t-Bu-indenyl)Cl. (2) Water has only a minor effect
on the cross-coupling efficiency (Pd-PEPPSI-IPr, 40 °C, 88%
yield),19 consistent with facile N−C insertion. Furthermore, it
should be noted that a 56% yield of 3b is obtained using 1.2
equiv of boronic acid versus 2.0 equiv. 21 and 8% yields of
biphenyl are formed under these standard conditions. The
reaction in the absence of boronic acid leads to the cleavage of
the N−Tf group; reductive coupling product or carboxylic acid
is not observed. The high reactivity of triflamides may also result
from sulfonyl chelation to Pd, assisting N−C cleavage. Whereas
all three catalysts (eq 1) serve as excellent throwaway ligands,
Pd-PEPPSI was selected for further studies due to the ease of
synthesis, broad accessibility, low price, and versatility of this
class of Pd-NHC precatalysts.17
Scheme 1. Pd-NHC-Catalyzed Chemoselective Suzuki−
a b
,
Miyaura Cross-Coupling of Triflamides
a
Conditions: triflamide (1.0 equiv), Ar−B(OH)2 (2.0 equiv), [Pd] (3
mol %), K2CO3 (3.0 equiv), THF (0.25 M), H2O, 40 °C, 15 h.
b
c
Isolated yields. 110 °C. See the Supporting Information (SI) for
details.
testament to the high efficiency of N-Tf as the activating group,
we demonstrated high facility in the cross-coupling to form a
notoriously difficult doubly deactivated ketone 3g, wherein both
the amide electrophile and boronic acid are electronically
disfavored toward the coupling17 as well as in the synthesis of
bis-sterically hindered di-o-tolylmethanone 3j. The use of N-Ts
amide leads to low conversion in both cases. In the present stage,
electron-withdrawing groups other than CO2Me on the amide
have not been tested. Aliphatic amides are not compatible with
the reaction conditions. N-Tf-substituted secondary amides are
recovered from the reaction conditions. N-Benzyl has been
selected as a representative N-alkyl group (vide infra). Ongoing
studies are focused on further optimization of the reaction
conditions. Collectively, the examined examples show that N-Tf
amides permit a broad scope in the direct synthesis of ketones
from amides, presenting a powerful alternative to the venerable
Weinreb amides20 with superior functional group tolerance
toward electrophilic functional groups.
Having identified optimal conditions for the cross-coupling of
N-Tf amides, we next investigated the scope of this protocol
(Scheme 1). As shown, a variety of N-Tf amides and aryl boronic
acids undergo successful cross-coupling under exceedingly mild
conditions. It is of note that neutral (3a,b), sterically hindered
(3c), electron-donating (3d), and electron-withdrawing (3e)
groups are well-tolerated on the boronic acid component
without any modification of the reaction conditions. Pleasingly,
the scope of the amide component is equally broad and
accommodates electron-neutral (3b′), electron-donating (3d′),
electron-withdrawing (3f′), and sterically hindered (3c′)
amides, delivering the cross-coupling products in high to
excellent yields. Furthermore, this protocol can be used to
readily assemble heterocyclic ketones (3h, 3i, 3k), albeit in some
cases higher temperature is required (3i).
Given the high catalytic activity of N-Tf amides, we became
interested in determining the turnover number in the cross-
coupling (Scheme 2).17 As shown, the cross-coupling of 1a
proceeds with TON of 550−780 at 0.10 mol % loading at 110 °C
using Pd-PEPPSI-IPr, Pd(IPr)(cin)Cl, and Pd(IPr)(1-t-Bu-
indenyl)Cl, respectively. Furthermore, TON of 1150 was
observed at 0.05 mol % loading using Pd-PEPPSI-IPr, attesting
Nevertheless, we note that the formation of 3i fails using N-Ts
amide, highlighting the benefits of triflamide activation. As a
B
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Scheme 2. Determination of TON in the Cross-Coupling of
Triflamides
to the high catalytic efficiency of triflamides as acyl-metal
precursors in amide N−C(O) cross-coupling.
Figure 2. (A) Crystal structure of 1a. (B) Newman projection along the
N−C(O) bond is shown. Bond lengths (Å) and angles (deg): N1−C1,
1.425 (2); C1−O1, 1.207 (2); C1−C2, 1.490 (2); N1−S1, 1.665 (1);
N1−C8, 1.456 (2); C2−C1−N1−S1, −165.49 (9); O1−C1−N1−C8,
−143.6 (1); O1−C1−N1−S1, 15.1 (2); C2−C1−N1−C8, 35.9 (2).
Note that the N1−C1 bond is stronger than N1−S1. Crystallographic
data have been deposited with the Cambridge Crystallographic Data
Center (1882900).
Several additional results are worth mentioning.
(1) Pleasingly, the use of N-Tf activating group could
furthermore be applied utilizing N-aliphatic amides (Scheme 3).
Scheme 3. Application of N-Alkyl Triflamides
(2) Preliminary results demonstrate that the use of Pd-
phosphane catalytic systems is also suitable for the cross-
coupling of N-Tf amides (Scheme 4), providing an alternative to
Scheme 4. Application of Pd-Phosphane Catalyst System
Pd-NHC.17 (3) Perhaps most intriguingly, N-Tf amides serve as
efficient precursors in both Pd- and Ni-catalyzed Negishi cross-
coupling (Scheme 5). To our knowledge, this is the first example
Figure 3. Rotational profile (1a, ΔE (kcal/mol) vs O−C−N−C (deg)).
DMAC (N,N-dimethylacetamide) is shown for comparison.
Scheme 5. Pd- and Ni-Catalyzed Negishi Cross-Coupling of
Triflamides
Scheme 6. Effect of Activating N-Sulfonyl Group: Amides
Used in Computational Studies
a
a
Activating effect of N-triflyl group (1a) and graphical model of
amide bond destabilization in triflamides (A). Note for comparison
Ph−C(O)NMePh, RE = 13.5 kcal/mol.
of an acyl−aryl Negishi coupling of simple N-sulfonyl amides.
The high reactivity of N-Tf amides bodes well for the
development of a range of synthetic methods by selective N−
C(O) cleavage that are unavailable to other amide precursors.
Next, extensive structural and computational studies were
conducted to gain insight into the origin of the high reactivity of
N-Tf amides and lay the framework for the development of
future amide precursors based on the N-sulfonyl amide
activation concept (Figures 2 and 3 and Scheme 6). The X-ray
structure of model triflamide 1a was determined (Figure 2). The
amide bond is twisted (τ = 25.5°, χN = 21.4°, χC = 0.5°, additive
Winkler−Dunitz parameter of 47°, corresponding to one-third
of the maximum theoretical amide distortion). The N−C(O),
CO, and N−S bonds are 1.425, 1.207, and 1.665 Å long.
Compared with the corresponding N-Ts amide (τ = 18.8°, χN =
19.8°, χC = 1.0°; N−C(O) = 1.410 Å; CO = 1.215 Å, N−S =
1.697 Å), these values indicate a decrease in nN → π*C=O
conjugation as a result of Nlp delocalization into the N-sulfonyl
group. The acyl CO bond in 1a is located in an antiperiplanar
arrangement to the N−Ph bond, bisecting the CF3−S−O angle.
Computations were employed to determine the energetics of
the acyl amide bond undergoing N−C cleavage (Figure 3,
Scheme 6, and Supporting Information (SI)).
C
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(1) Resonance energy (RE) in 1a was determined by the
COSNAR method.1a,5 The conjugation in 1a (RE = 8.3 kcal/
mol) is significantly lower than that in (i) N−CO planar amides
and (ii) analogous N-Ts amides (RE = 9.7 kcal/mol).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: michal.szostak@rutgers.edu
ORCID
(2) The rotational profile of 1a was determined by systematic
rotation along the O−C−N−C dihedral angle. The energy
minimum is located at ca. 140° O−C−N−C angle (τ = 31.21°;
χN = 19.08°) in a syn O−C−N−S conformation (ca. 20.9° O−
C−N−S dihedral angle). The energy maximum is at ca. 0° O−
C−N−C dihedral angle (τ = 14.07°; χN = 22.92°) in an
antiperiplanar O−C−N−S destabilizing conformation (157.1°
dihedral angle, 3.90 kcal/mol). These values can be compared
with the barrier of 7.01 kcal/mol in N-Ts amides. The less
favorable conformation at 180° O−C−N−C dihedral angle is
defined by a τ = 2.57°, χN = 6.80° geometry (6.8° O−C−N−S
angle).
(3) The difference in N-/O-protonation affinities (ΔPA)
determines that protonation at the acyl oxygen is strongly
favored in N-Tf amide 1a (ΔPA = 12.4 kcal/mol), which can be
compared with N-Ts amides (ΔPA = 9.3 kcal/mol), and is
consistent with the strong electron-withdrawing effect of the N-
Tf group. Protonation of the amide oxygen is favored over
sulfonamide oxygen atoms (ΔPA = 14.8, 17.5 kcal/mol). Thus
the activation of the N-acyl group in 1a by N-protonation is
unlikely.
Shicheng Shi: 0000-0001-8124-9235
Roger Lalancette: 0000-0002-3470-532X
Michal Szostak: 0000-0002-9650-9690
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Rutgers University and the NSF (CAREER CHE-1650766) are
gratefully acknowledged for support. We thank the Wroclaw
Center for Networking and Supercomputing (Grant Number
WCSS159).
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03901.
Experimental procedures and characterization data
(PDF)
Accession Codes
CCDC 1882900 contains 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.
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