Cobalt-Catalyzed Cross-Coupling of α-Bromo Amides with Grignard Reagents

Article abstract of DOI:10.1021/acs.orglett.7b02848

A cobalt-catalyzed cross-coupling between α-bromo amides and Grignard reagents is disclosed. The reaction is general and allows access to a large variety of α-aryl and β,γ-unsaturated amides. Some mechanistic investigations have been undertaken to determine the nature of the intermediate species.

Full text of DOI:10.1021/acs.orglett.7b02848

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Cobalt-Catalyzed Cross-Coupling of α‑Bromo Amides with Grignard
Reagents
E. Barde, A. Guerinot,* and J. Cossy*
́
Laboratoire de Chimie Organique, Institute of Chemistry, Biology and Innovation (CBI)-UMR 8231, ESPCI Paris/CNRS/PSL
Research University, 10 rue Vauquelin 75231 Paris Cedex 05, France
S
* Supporting Information
ABSTRACT: A cobalt-catalyzed cross-coupling between
α‑bromo amides and Grignard reagents is disclosed. The
reaction is general and allows access to a large variety of α‑aryl
and β,γ‑unsaturated amides. Some mechanistic investigations
have been undertaken to determine the nature of the intermediate species.
mides are ubiquitous in both chemical and biological
entities. They are present in numerous natural products,
Scheme 1. Access to α-Aryl and β,γ-Unsaturated Amides
A
pharmaceuticals, synthetic polymers, and functional materials,
and they constitute the backbone of all natural peptides.1 As such,
the formation or the functionalization of amides remains a
significant research area in organic chemistry. Particularly, α-aryl
amides are important pharmacophores present in various
biologically active compounds such as atenolol,2 which is used
in the treatment of cardiovascular diseases, or almorexant,3 which
helps to overcome insomnia. In addition, α‑aryl amides are
precursors of β‑aryl amines4 and of α‑aryl carboxylic acids, these
latter being a key motif in several nonsteroidal anti-inflammatory
drugs.5 Currently, the most widespread method to access α‑aryl
amides is the metal-catalyzed arylation of amide enolates.6,7
However, these reactions generally require the use of strong
bases to generate the enolate, thus reducing the functional group
tolerance.8,9 An alternative strategy avoiding the detrimental
deprotonation step is a metal-catalyzed cross-coupling between
α‑halo amides and organometallics. Palladium-catalyzed Suzuki
cross-couplings were first developed on α-bromo amides lacking
β-hydrogen.10 Suzuki, Hiyama, and Negishi cross-couplings were
then performed on substituted α‑halo amides using nickel
complexes, which are less prone to β-H elimination.11,12
β,γ‑Unsaturated amides are also useful synthetic intermediates
that can be notably transformed into homoallyl amines.13 They
are generally accessed through carbonylation using carbon
monoxide,14,15 and few examples of metal-catalyzed alkenylation
of α-halo amides have been reported to date (Scheme 1).16,17 All
the reported arylations or alkenylations of α‑halo amides require
cost-effective and/or toxic Ni- and Pd-catalysts. In addition, to
the best of our knowledge, there is no general method for the
synthesis of either α‑aryl or α‑alkenyl amides from common
α‑halo amide precursors. In the course of our ongoing studies on
metal-catalyzed cross-couplings,18 we decided to investigate the
arylation and alkenylation of α‑bromo amides focusing on earth-
abundant and cheap first row metal catalysts. Herein, we report a
cobalt-catalyzed cross-coupling between α‑bromo amides and
Grignard reagents.19 This simple and general method allows
access to both α‑aryl and β,γ‑unsaturated amides using similar
conditions and precursors.
The arylation of N,N-dibenzyl-2-bromopentanamide 1a using
phenylmagnesium bromide was examined to determine the
appropriate catalytic system. In the absence of any metal catalyst
or ligand, the coupling product 2a was formed but the
dehalogenated amide 3a was obtained as the major product
(2a/3a = 23:77) (Table 1, entry 1). The dehalogenation may
proceed through a bromine−magnesium exchange leading to a
magnesium enolate. To prevent this undesired process, metal
catalysts were introduced in the reaction mixture. Based on our
previous results in Fe- and Co-catalyzed cross-couplings,18c,d
(R,R)-tetramethylcyclohexan-1,2-diamine (TMCD) was se-
lected as a ligand and a panel of metal complexes were screened.
Unfortunately, the use of iron salts led to either incomplete
conversion of 1a or formation of the dehalogenated product 3a as
the major product.20 Using the nonhygroscopic Co(acac)3, the
arylation failed as, once again, the amide 3a was the major
compound (Table 1, entry 2). To our delight, when the simple
and cost-effective CoCl2 was utilized, the arylated compound 2a
was the main product which was isolated with an encouraging
yield of 58% (Table 1, entry 3). Thus, CoCl2 was selected and a
range of ligands were examined. Replacing TMCD by
tetramethylethylenediamine (TMEDA) increased the formation
Received: September 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02848
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of the dehalogenated compound (Table 1, entry 4). Several
diphosphine ligands were then associated with CoCl2, and
among them, Xantphos was the most promising.20,21 Using an
equimolar mixture of CoCl2 and Xantphos (10 mol % each)
allowed the isolation of 2a with a good yield of 70% (Table 1,
entry 5). Noteworthy, the reaction could be performed on a gram
scale without affecting the yield.22,23
respectively (Table 2, entries 4−5). Using p‑chlorophenyl-
magnesium bromide led to the formation of the corresponding
coupling product 2g with a good yield of 73% (Table 2, entry 6).
Interestingly, the presence of a chlorine atom offers subsequent
functionalization opportunities. A meta-substituent on the
phenyl ring of the Grignard was well tolerated (Table 2, entry
7). In contrast, the reaction was sensitive to steric hindrance, as
by using o‑tolylmagnesium bromide, the dehalogenated amide 3a
was the major product formed under the developed conditions
(Table 2, entry 8).
Table 1. Optimization of the Reaction Conditions
A panel of amide partners 1b−1d bearing various side chains
(R1) were then reacted with phenylmagnesium bromide in the
presence of the catalytic system. The corresponding coupling
products 4b−4d were isolated in moderate yields ranging from
34% to 52%. The reaction was inefficient on secondary amides
such as 4e, but a variety of tertiary amides were well tolerated
under the optimized conditions. One of the N-benzyl
substituents could be replaced by a phenyl group without
affecting the yield (4f, 76%). Different alkyl substituents (R2 = R3
= iPr, Et, Cy) could be introduced on the nitrogen atom, and the
coupling products 4g−4i were isolated with yields up to 80%.
The nitrogen atom could be part of a heterocycle, and high yields
were obtained for the synthesis of piperidine-containing
products 4j and 4k. The α‑phenyl amide 4l possessing a
morpholine moiety was obtained with a lower yield of 41%
(Scheme 2).
conversion of
a
b
c
entry
[M]
ligand
1a
2a/3a
2a (yield%)
1
2
3
4
5
−
−
100%
100%
100%
100%
100%
23:77
10:90
85:15
58:42
92:8
nd
d
Co(acac)3
TMCD
nd
d
CoCl2
TMCD
58%
nd
d
CoCl2
TMEDA
d
CoCl2
Xantphos
70%
a
1
Transformation of 1a into 2a and 3a estimated on the crude H
b
c
1
NMR. Determined on the crude H NMR spectrum. Isolated yield.
d
TMCD: (R,R)-tetramethylcyclohexan-1,2-diamine; TMEDA: tetra-
methylethylenediamine; Xantphos: 4,5-Bis(diphenylphosphino)-9,9-
dimethylxanthene.
With these optimized conditions in hand, the scope of the
arylation was evaluated by varying the aryl Grignard reagent.
Electron-rich p‑tolylphenylmagnesium bromide and p‑methoxy-
phenylmagnesium bromide were efficiently coupled to bromo
amide 1a delivering the expected products in ca. 68% yield
(Table 2, entries 1−2). Similarly, when 1a was reacted with the
Scheme 2. Variation of the Amide Partner
Table 2. Variation of the Aryl Grignard Reagent
a
,26
entry
R1
2 (yield%)
2b (68%)
b
1
2
3
4
5
6
7
8
p-Me
b
p-OMe
2c (69%)
c
d
p-NMe2
p-CF3
2d (64%, 48% )
c
d
2e (70%, 41% )
2f (58%)
b
p-F
b
p-Cl
2g (73%)
b
Pleasingly, the arylation of α-bromo lactams such as 5a and 5b
was successful and the corresponding coupling products were
isolated with moderate yields of 56% and 51% (Scheme 3).
m-OMe
2h (70%)
2i (0%)
b
o-Me
a
b
Isolated yield. Commercially available Grignard reagents were used.
c
The Grignard reagent was prepared using classical method (Mg,
d
Scheme 3. Coupling Involving Bromo Lactams
THF). The Grignard reagent was prepared according to Knochel et
al. procedure (Mg, LiCl, THF).24
p‑dimethylaminophenylmagnesium bromide, the coupling prod-
uct was isolated with a satisfying yield of 64%. Surprisingly, in this
latter case, when the Grignard reagent was prepared according to
Knochel’s procedure (Mg, LiCl, DIBAL-H cat.),24 the yield in
the coupling product dropped to 48% (Table 2, entry 3). The
presence of LiCl seems to interfere with the reaction outcome,
favoring side reactions.25 Electron-poor Grignard reagents such
as p‑trifluoromethylmagnesium bromide and p-fluorophenyl-
magnesium bromide were reactive under the coupling
conditions, furnishing 2e and 2f in 70% and 58% yields
Following these encouraging results, the alkenylation of
α‑bromo amides was next examined. Decreasing the temperature
to −40 °C was necessary to obtain good yields in coupling
products. Reaction between 1a and vinylmagnesium bromide
afforded 7a in 63% yield. Alkenyl Grignard reagents possessing
various substitution patterns on the double bond were involved
in the process, resulting in the formation of coupling products
B
DOI: 10.1021/acs.orglett.7b02848
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
7b−7d in good yields up to 89%. Thus, the developed cross-
coupling appears as a valuable method for the synthesis of
β,γ‑unsaturated amides (Scheme 4).
with the Grignard reagent may regenerate catalyst A. The
Xantphos ligand may accelerate the reductive elimination step,
thus increasing the turnover and inhibiting the noncatalyzed
dehalogenation process.30
Scheme 4. Alkenylation of α-Bromo Amide 1a
Scheme 6. Hypothetic Mechanism
To gain some mechanistic insight, two elementary experi-
ments were carried out. The formation of radical intermediates
during cobalt-catalyzed cross-couplings between alkyl halides
and aryl Grignard reagents has already been demonstrated.18,27
These radicals would be formed through two successive single-
electron transfers composing the formal oxidative addition. A
first experiment was conducted in the presence of a radical
scavenger. When 1a was treated with phenylmagnesium bromide
in the presence of 1 equiv of 1,1-diphenylethylene under the
optimized conditions, the coupling product was obtained with a
slightly decreased yield of 61% (versus 70%) and no trace of an
adduct resulting from the addition on 1,1-diphenylethylene was
observed (Scheme 5, eq 1). In addition, the bromo amide 8a was
used as a radical clock. Under classical radical conditions
(Bu3SnH, AIBN), this compound has been shown to deliver the
corresponding lactam through a 5-endo-trig cyclization.28
However, in our case, the cobalt-catalyzed cross-coupling
conditions applied to 8a only afforded the linear coupling
product and no trace of a cyclized compound could be detected
(Scheme 5, eq 2). These two results suggest that α-bromo amides
exhibit different behavior from nonactivated alkyl halides and a
concerted oxidative addition may be hypothesized excluding the
formation of radicals. However, kinetic considerations cannot be
ignored and the absence of cyclic product starting from 8a could
also be explained by a coupling that could be faster than the
cyclization process.
In summary, a cobalt-catalyzed cross-coupling between
α‑bromo amides and Grignard reagents has been developed
giving access to both α‑aryl and β,γ‑unsaturated amides. To the
best of our knowledge, it is the first unified approach toward the
formation of these two important motifs. The reaction is general
and scalable and involves a cost-effective cobalt catalyst and easily
available Grignard reagents. Therefore, it can be considered as a
potent attractive synthetic tool for the functionalization of
amides.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02848.
Experimental procedures and spectral data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Scheme 5. Mechanistic Studies
*E-mail: janine.cossy@espci.fr.
*E-mail: amandine.guerinot@espci.fr.
ORCID
A. Guerinot: 0000-0002-7002-8215
́
J. Cossy: 0000-0001-8746-9239
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
E.B. thanks the french Minister
̀
e de l’Enseignement Super
́
ieur et
de la Recherche for a grant.
Based on our observations and on literature reports,27
a
REFERENCES
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proposed. A concerted oxidative addition may deliver
intermediate B, which may lead to the desired product together
with complex C after reductive addition. A transmetalation of C
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DOI: 10.1021/acs.orglett.7b02848
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