Harnessing Stereospecific Z-Enamides through Silver-Free Cp?Rh(III) Catalysis by Using Isoxazoles as Masked Electrophiles

Article abstract of DOI:10.1021/acs.orglett.8b04130

The stereospecific synthesis of Z-enamides is described in this paper. For the first time, isoxazoles have been employed as electrophiles in C-H functionalization to afford thermodynamically less stable Z-enamides utilizing salicylaldehydes in an atom- and step-economic fashion. The stereochemistry of enamides might originate from the relative disposition of atoms present in isoxazole and the intramolecular hydrogen bonding. The reaction showed excellent scope as several structurally and electronically diverse salicylaldehydes and isoxazoles reacted efficiently.

Full text of DOI:10.1021/acs.orglett.8b04130

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Harnessing Stereospecific Z‑Enamides through Silver-Free Cp*Rh(III)
Catalysis by Using Isoxazoles as Masked Electrophiles
Suvankar Debbarma, Sourav Sekhar Bera, and Modhu Sudan Maji*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
S
* Supporting Information
ABSTRACT: The stereospecific synthesis of Z-enamides is
described in this paper. For the first time, isoxazoles have been
employed as electrophiles in C−H functionalization to afford
thermodynamically less stable Z-enamides utilizing salicylalde-
hydes in an atom- and step-economic fashion. The stereo-
chemistry of enamides might originate from the relative
disposition of atoms present in isoxazole and the intramolecular
hydrogen bonding. The reaction showed excellent scope as
several structurally and electronically diverse salicylaldehydes
and isoxazoles reacted efficiently.
he secondary enamides have received immense attention
in synthetic organic chemistry due to their pervasiveness
T
in various bioactive natural products1 and a range of marine
metabolites.2 Moreover, assessing the stability and polar-
izability of the enamides, they have been used as versatile
synthetic intermediates for the synthesis of heterocycles,3 in
cross-couplings/Heck reactions,4 in asymmetric hydrogena-
tion/halogenation,5 and recently in C−H functionalization.6
The inevitability of the enamide motif and its unambiguous
geometry, for excellent potency, have been demonstrated by
structure−activity relationship (SAR) studies. Earlier reports of
enamide synthesis comprise condensation of carbonyl groups
with amides,7 acylations of imines,8 Curtius rearrangement,1b,9
and elimination of β-hydroxy-α-silylamides (Peterson reac-
tion).10 Subsequently, transition-metal (Pd, Cu)-catalyzed11
cross-coupling of substituted vinyl derivatives with amides have
been found to be a surrogate approach to access a wide range
of enamides. Unfortunately, these methods often lead to poor
yields and scarce of E/Z selectivity and often require
anhydrous as well as harsh reaction conditions (eminent
temperature, strong base).12 Although the synthetic strategy
involving the metalation/functionalization of ynamides13 and
the hydroamidation of alkynes14 had attracted chemists, these
methods lacked broad substrate scope. Gold-catalyzed isomer-
ization of propargylic alcohol15 (Meyer−Schuster rearrange-
ment) and the isomerization of N-allyl amides under
transition-metal16 catalysis could implement a diverse alter-
native to the coupling reactions. Stoichiometric oxidation of N-
alkyl amide using a metal-free approach17 also offered a new
synthetic route. However, this method failed to furnish
stereospecific products, particularly the thermodynamically
less feasible Z-enamides (Figure 1).18
Figure 1. Bioactive natural products containing a Z-enamide.
electrophilic C−H functionalization (Scheme 1). To the best
of our knowledge, to serve our purpose of obtaining Z-
enamides, for the first time, we have employed isoxazole as an
electrophile in C−H functionalization which can be easily
accessed from the corresponding ketone derivatives.18 We
envisioned that (a) isoxazole can be used as a masked form of
1,3-imino carbonyl electrophile and can act as an enamine
source and (b) the structural rigidity of isoxazole might be
transferred to achieve our desired stereospecific formation of
Z-enamides. Herein, we disclose a novel, silver-free Rh-
catalyzed C−H functionalization approach for the synthesis
of acyclic enamides with an exclusive Z-selectivity. Our
synthetic strategy comprises a benign catalytic system, high
functional group tolerance, and appreciable yields. The
conserved geometry of enamides is confirmed by the 1H
NMR analysis and is likely to be aroused from the relative
To anticipate this intricacy in the synthesis of enamide
functionality, we have found an advanced strategy, with
potentially a broader synthetic application, using the activation
of an aldehyde C−H bond through a metal-catalyzed selective
Received: December 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b04130
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
indicates a pivotal role of a CsOAc base in this amination
reaction. The product 3a was not detected upon performing
this reaction either in the absence of any additive or catalyst
(entries 9 and 10). On conducting the reaction at lower
temperature yield of 3a was deteriorated, and the reaction was
almost stopped at room temperature (entries 11 and 12).
Switching the catalyst from Rh(III) to its cobalt or iridium
congeners failed to provide any product 3a (entries 13 and
14).
Scheme 1. C−H Activation Strategy for Enamide Synthesis
Once suitable reaction conditions were obtained, the scope
of the salicylaldehydes was studied by keeping isoxazole 2a as a
model electrophile (Scheme 2). Salicylaldehyde 1b bearing an
a,b
Scheme 2. Scope of Salicylaldehydes
disposition of nitrogen and oxygen atoms present in 2, and Z-
enamide is further stabilized by the intramolecular hydrogen
bonding.11d,f
To test the feasibility of our hypothesis, we initiated our
study on the identification of appropriate reaction conditions
by screening reaction parameters for the coupling of
salicylaldehyde 1a and isoxazole 2a (Table 1). By using
a
Table 1. Optimizations of the Reaction Conditions
b
temp
(°C)
yield
(%)
entry solvent
catalyst
additive
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
DCE
TCE
DCM
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
AgSbF6
AgSbF6
AgSbF6
AgSbF6
CsOAc
NaOAc
KOAc
80
80
80
80
80
80
80
80
80
80
rt
45
30
38
trace
76
25
30
trace
nd
nd
trace
62
CHCl3 [Cp*RhCl2]2
c
c
c
c
a
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
Reaction conditions: salicylaldehydes 1 (0.2 mmol), isoxazole 2a
(0.3 mmol), catalyst (6.2 mg, 5 mol %), CsOAc (0.2 mmol, 1.0
equiv), DCE (1.5 mL). Isolated yields.
b
Cu(OAc)2
electron-donating methyl group at the para position to the
aldehyde reacted smoothly with 2a to form enamide 3b in 70%
yield. Similar reactivity was recognized in the case of electron-
deficient aldehyde 1c furnishing enamide 3c in 63% yield.
Notably, salicylaldehydes bearing electron-donating methoxy,
diethylamine, and tert-butyl functional groups at the C5-
position of 1 also responded well under the optimized
conditions to provide 3d−f in excellent yields (66−81%). 5-
Bromosalicylaldehyde delivered enamide 3g in 54% yield. On
the contrary, salicylaldehyde 1h bearing an electron-deficient
ester group provided 3h in lower yield. Salicylaldehydes 1i,j
bearing either electron-donating methoxy or -withdrawing
fluoride functional groups at the C3-position provided 3i and
3j in 71−75% yields. Similarly, 1-hydroxy-2-naphthaldehyde
also took part in the catalysis to furnish enamide 3k in 78%
yield. Considering that the enamide functional groups are
widespread in medicines and natural products, late-stage
modification of tyrosine 1l was also tested, and the desired
product 3l was isolated in 77% yield, displaying a broader
synthetic utility of this catalytic system.
c
c
c
c
c
c
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
[Cp*RhCl2]2
[Cp*RhCl2]2
Cp*Co(CO)I2
[Cp*IrCI2]2
[Ru(p-cymene)
Cl2]2
60
80
80
80
nd
nd
nd
a
Reaction conditions: salicylaldehydes 1a (0.2 mmol), isoxazole 2a
(0.3 mmol), catalyst (6.2 mg, 5 mol %), additive (0.04 mmol, 20 mol
%), DCE (1.5 mL), 80 °C, 16 h. Isolated yield. 1.0 equiv of additive
was used.
b
c
[Cp*RhCl2]2 (5.0 mol %) as a catalyst and AgSbF6 (20 mol %)
as an additive in DCE solvent, only 45% of our desired product
3a was isolated (entry 1). Switching to other chlorinated
solvents such as tetrachloroethane, dichloromethane, and
chloroform under identical reaction conditions did not provide
any better results (entries 2−4). Upon changing the additive
from AgSbF6 to different acetate sources, a dramatic change in
the yield of 3a was observed. Among different acetate salts
studied, while 1.0 equiv of CsOAc provided 3a in 76% yield
(entry 5), other acetate sources under the identical reaction
conditions were not very effective (entries 6−8). This result
After the scope of the salicylaldehydes was studied, we next
investigated the reactivity of a range of isoxazole derivatives 2
with salicylaldehyde 1a (Scheme 3). Isoxazoles bearing 4-
fluoro-, 4-chloro-, and 4-bromo-substituted phenyl rings at the
B
DOI: 10.1021/acs.orglett.8b04130
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
executed. Along this line, the enamide 3f was reduced to N-
alkylamide 5 by using Pd/C catalyst and molecular hydrogen
in excellent yield (Scheme 4a). Our attempt to convert (Z)-3a
Scheme 3. Scope of Isoxazoles
Scheme 4. Synthetic Utility of Enamide
to (E)-3a only provided 38% of the isomerized enamide
(Scheme 4b).15 The free phenol present in 3a was also readily
protected as benzyl ether 6 (86% yield) by using benzyl
bromide (Scheme 4c).
To understand the mechanistic pathway of the enamide
formation, the deuterium-labeling experiments were executed
with substrate 1e in the absence of isoxazole by using
isotopically labeled solvents (Scheme 5a). We found for the
a
Reaction conditions: salicylaldehydes 1a (0.2 mmol), isoxazole 2
(0.3 mmol), catalyst (6.2 mg, 5 mol %), CsOAc (0.2 mmol, 1.0
equiv), DCE (1.5 mL). Isolated yields.
Scheme 5. Deuterium-Labeling Study and Competition
Experiment
b
C5-position reacted efficiently to provide enamides 4a−c in
79−86% yields. To show the applicability of the method, a
gram-scale synthesis of 4b was also executed. A similar
reactivity was recorded when the phenyl ring of isoxazoles was
substituted with thiomethyl or phenyl groups (4d,e, 66−83%
yields). Upon changing the substitution position from para to
ortho or meta, the enamide formation still worked very
efficiently. Several isoxazoles containing either electron-
donating (methoxy and phenyl) or withdrawing (chloro and
bromo) groups substituted on the phenyl ring underwent
smooth reaction to provide enamides 4f−j in good yields.
However, in the case of O-methoxy substitution, 4i was
isolated in lower yield. Isoxazole 2k bearing an electron-rich
phenyl substitution also reacted smoothly to form enamide 4k
in 73% yield. On replacing the R group of isoxazoles 2 with
aromatic hydrocarbons naphthalene 2l and phenanthrene 2m,
the potency of isoxazoles toward catalysis remained consistent
and delivered enamides 4l and 4m in 80 and 70% yields,
respectively. The same propensity of the isoxazoles toward
catalysis was obtained even in the case of heterocycle
substituents and afforded enamides 4n,o in 53−62% yields.
After the aryl-substituted isoxazoles were screened, the
reactivity of the corresponding alkenyl-substituted isoxazoles
was investigated. Isoxazoles bearing styrene 2p or stilbene 2q
substituents underwent smooth reaction to afford enamides 4p
and 4q in 43−55% yields. These results clearly demonstrate
the stability of alkenes under the reaction conditions. A similar
result was obtained in the case of substrates 2r and 2s.
Pleasingly, even 1,3-diene-substituted isoxazoles 2t,u were also
very effective, providing highly conjugated enamides 4t,u in
modest yields.
sufficient C−H/D exchange of the aldehyde C−H bond that
the correct combination of additives and catalysts is essential.
The reversibility of the C−H activation step is supported by
this sufficient amount of H/D scrambling. Eventually, a
sufficient amount of H/D scrambling has also been observed
at the ortho position of the aromatic C−H bond; however, no
amination product was detected. The methyl-protected
salicylaldehyde did not provide any enamide, indicating the
necessity of free hydroxyl group for the functionalization of
aldehyde C−H bond (Scheme 5b). Interestingly, when a
competition reaction between two electronically different
salicylaldehydes 1d and 1h was carried out, the electron-
deficient aldehyde coupled to form enamide 3h at a higher rate
Considering various functional groups present in our
synthesized enamides, different chemical transformations are
C
DOI: 10.1021/acs.orglett.8b04130
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ORCID
which may be attributed due to higher acidity of the phenolic
group of 1h (Scheme 5c).
Suvankar Debbarma: 0000-0002-6467-8764
Modhu Sudan Maji: 0000-0001-9647-2683
Notes
On the basis of previous literature reports19 and our
mechanistic studies, a plausible catalyst cycle is proposed in
Scheme 6. In the presence of CsOAc, an active catalyst A is
The authors declare no competing financial interest.
Scheme 6. Plausible Mechanism
ACKNOWLEDGMENTS
■
M.S.M. gratefully acknowledges Council of Scientific &
Industrial Research, India (Sanction No. 02(0322)/17/EMR-
II) and SERB, Department of Science and Technology, New
Delhi, India (Sanction No. EMR/2015/000994) for funding.
S.D. sincerely thanks IIT Kharagpur and SSB thanks CSIR,
India, for fellowships.
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generated which undergoes C−H metalation with salicylalde-
hyde 1a to form a five-membered rhodacycle B through the
directed C−H functionalization. The coordination of 2a with
the intermediate B generates intermediate C, which experi-
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formation of intermediate 7 (Scheme 5d),18 which upon
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enamide product 3a, stabilized by the intramolecular hydrogen
bonding.
In conclusion, we have developed a phenol-directed
umpolung reactivity of aldehydes using Cp*Rh(III)-catalyzed
C−H functionalization to achieve stereospecific acyclic Z-
enamides. The polarity of N−O bond of isoxazoles have been
utilized as a masked electrophile under our reaction conditions
to mark this a complete step- and atom-economic strategy.
Silver-free reaction conditions leads our method toward a
greener approach. A variety of salicylaldehydes and isoxazoles
have successfully responded to make this method versatile. The
existence of intermediate, detected by LC−MS analysis,
justifies the final step of the catalytic cycle. We hope that
our method of stereospecific Z-enamide synthesis will be
instrumental for further applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b04130.
Experimental procedures, spectroscopic data, and NMR
spectra of compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: msm@chem.iitkgp.ac.in.
D
DOI: 10.1021/acs.orglett.8b04130
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Organic Letters
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DOI: 10.1021/acs.orglett.8b04130
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