N-Heterocyclic Carbene-Catalyzed Truce-Smiles Rearrangement of N-Arylacrylamides via the Cleavage of Unactivated C(aryl)-N Bonds

Article abstract of DOI:10.1021/acs.orglett.0c04281

We report on the N-heterocyclic carbene (NHC)-catalyzed Truce-Smiles rearrangement of aniline derivatives, in which an unactivated C(aryl)-N bond is cleaved, leading to the formation of a new C(aryl)-C bond. The key to the success of this reaction is the

Full text of DOI:10.1021/acs.orglett.0c04281

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Letter
N‑Heterocyclic Carbene-Catalyzed Truce−Smiles Rearrangement of
N‑Arylacrylamides via the Cleavage of Unactivated C(aryl)−N Bonds
Kosuke Yasui, Miharu Kamitani, Hayato Fujimoto, and Mamoru Tobisu*
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ABSTRACT: We report on the N-heterocyclic carbene (NHC)-
catalyzed Truce−Smiles rearrangement of aniline derivatives, in which
an unactivated C(aryl)−N bond is cleaved, leading to the formation of
a new C(aryl)−C bond. The key to the success of this reaction is the
utilization of a highly nucleophilic NHC, which enables the formation
of a highly nucleophilic ylide intermediate that is generated from an
α,β-unsaturated amide.
type of catalytic transformation of C(aryl)−N bonds involves
he catalytic transformation of C(aryl)−N bonds of
Taniline derivatives represents a formidable challenge the use of transition metal catalysts, which allows for cross-
coupling type reactions, although the scope of both the aniline
due, in part, to their inertness (the BDE of Ph−NH2: ca.
103 kcal/mol).1 In fact, only a limited number of reports on
derivatives and the nucleophiles is significantly limited
compared with that for the cross-coupling using aryl halides
the catalytic transformation of unactivated aniline derivatives
(Scheme 1A).4,5 The Truce−Smiles rearrangement of aniline
derivatives is a unique alternative approach for converting an
unactivated C(aryl)−N bond to a C(aryl)−C bond via an
intramolecular SNAr reaction.6,7 Although the Truce−Smiles
rearrangement enables an otherwise difficult bond disconnec-
tion, the need for the use of a stoichiometric amount of a
strong base (e.g., BuLi, LDA etc.) diminishes its synthetic
utility, and therefore catalytic variants would be highly
desirable. To date, only two reports have appeared on the
catalytic Truce−Smiles rearrangement of aniline derivatives,
both of which are based on photoredox catalysis (Scheme 1B).
Stephenson reported on the photocatalytic Truce−Smiles
rearrangement of aminothiophene derivatives, in which an
alkyl radical generated from a pendant alkyl halide moiety
mediates the C−N bond cleavage.8 Clayden reported on a
photocatalytic two component coupling that involves the
Truce−Smiles rearrangement of aniline derivatives.9 We report
herein that a simple nucleophilic catalysis by an N-heterocyclic
carbene can be used to promote the Truce−Smiles rearrange-
ment of readily available aniline derivatives (Scheme 1C).
Unlike the radical-based approach, photoirradiation is not
required, and no radical precursors are involved, thus allowing
for 100% atom economy.10
have been appeared to date.2,3 One major approach to this
Scheme 1. Catalytic Transformation of C(aryl)−N Bonds
Received: December 29, 2020
Published: February 12, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04281
© 2021 American Chemical Society
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a
sterically demanding NHCs, such as N8, N9, and N10, were
less effective for promoting the catalytic Truce−Smiles
rearrangement of 1a (entries 8−10). Finally, using N7 with a
prolonged reaction time permitted the complete consumption
of 1a with the quantitative generation of 2a (entry 11). These
conditions were applicable to the reaction on a gram scale
(entry 11).
Table 1. Optimization of the Catalyst
entry
NHC
GC yield of 2a (%)
With the optimized reaction conditions in hand, we next
examined the scope of substrates (Scheme 2). A range of
functional groups, including cyano (1b), iodo (1c), methoxy
(1d), and ester (1e) groups, were tolerated with the
corresponding rearranged products being successfully pro-
duced. When para-substituted anilides were used, this
organocatalytic rearrangement proceeded efficiently with
substrates bearing a CH2Mes group (i.e., 1f−1i), possibly
because this bulkier N-protecting group suppressed the
undesired C(acyl)−N bond cleavage and the associated side
reactions. Notably, since this method does not require the use
of a strong, nucleophilic base, substrates bearing an enolizable
ketone moiety, as in 1i, were applicable. Sterically demanding
ortho-substituted anilides 1j−1m also participated in this
organocatalytic C−N cleavage reaction. Interestingly, ortho-
chloro and ortho-bromo substituents did not undergo
nucleophilic aromatic substitution by the postulated ylide
intermediate11,13 under these conditions, but rather C−N
bond substitution occurred exclusively to form the correspond-
ing cinnamamides 2j and 2k, respectively. One of the
advantages of this organocatalytic method over transition
metal and photoredox catalysis is the tolerance of halogen
groups, as evidenced in the reactions of 1c, 1j, and 1k, which
should serve as a synthetic handle for further functionaliza-
tion.15 The heteroaromatic compound 1n and π-extended
arenes 1o also participated in this catalytic reaction. The
electronic properties of the benzyl protecting groups in anilides
had no apparent effect on the yield of the products, as
evidenced by the reactions of anilides bearing 4-methoxybenzyl
(1p) and 4-trifluoromethylbenzyl (1q) groups.
1
2
3
4
5
6
7
8
N1
N2
N3
N4
N5
N6
N7
N8
N9
N10
N7
0
0
0
0
16
9
26
0
0
0
9
10
11
b
c
>95 (79)
a
Reaction conditions: amide 1a (0.20 mmol), NHC·HCl (0.060
mmol), CsF (0.40 mmol), and toluene (1.0 mL) in sealed tube at 140
°C for 12 h. Reaction conducted for 40 h in toluene (3.0 mL).
Isolated yield of 2a when conducted on 8 mmol scale (40 h).
b
c
This organocatalytic C−N bond cleavage is applicable to the
transformation of biologically active molecules containing an
aniline moiety (Scheme 3, top). For example, the C(aryl)−N
bonds in the antimicrobial sulfamethazine 1r and anesthetic
dimethocaine 1s can be transformed into C(aryl)−C bonds via
this NHC-catalyzed rearrangement to provide new amide
derivatives 2r and 2s, respectively. As mentioned above, an
ortho bromo group is tolerated in the N7-catalyzed Truce−
Smiles rearrangement of 1k to afford a cinnamamide 2k
exclusively. Interestingly, the use of N9, instead of N7, as a
catalyst led to the formation of a cyclized product 3 as a major
product, which was produced by catalytic nucleophilic
aromatic substution (Scheme 3, bottom).11,13 These results
demonstrate that a catalyst-controlled selectivity between C−
N and C−Br bond cleavage is possible.
To gain additional insights into the reaction mechanism, we
next investigated the reaction pathway by DFT calculations
using N-methyl-N-(naphthalene-1-yl)methacrylamide as a
model substrate. The energy changes at the M06-2X/6-
311+G*//M06-2X/6-31+G* level of theory [SCRF (pcm,
solvent = toluene)] are shown in kcal/mol (Scheme 4).
Because the route from an α,β-unsaturated ester and an NHC
catalyst to the ylide intermediate similar to INT1 was
previously calculated,16 our calculations focused on the critical
C−N bond cleavage process via intramolecular nucleophilic
substitution. The calculations revealed that this catalytic C−N
We previously reported on the NHC-catalyzed intra-
molecular concerted nucleophilic aromatic substitution
(CSNAr) of aryl halides bearing an α,β-unsaturated amide
moiety,11 in which a highly nucleophilic ylide species is
involved (Scheme 1D, attack a).12 In these reactions, oxygen-
based poor leaving groups, such as a OPh and even an a OMe
group, can also participate,13 although C(aryl)−O bonds are
generally assumed to be inert.14 The broad scope of leaving
groups led us to envision that α,β-unsaturated amides without
any leaving groups at the ortho position would direct the attack
of the ylide nucleophile to the most electrophilic ipso carbon of
the anilide substrate to undergo the Truce−Smiles rearrange-
ment (Scheme 1D, attack b). The key challenges to realizing
the nucleophile-initiated Truce−Smiles rearrangement are (i)
the low electrophilicity of anilides compared to aryl halides and
(ii) difficulties associated with suppressing the competing
cleavage of a more reactive C(acyl)−N bond in anilides.
With these challenges in mind, we commenced our study by
evaluating various NHC catalysts for the conversion of the
anilide 1a into the rearranged product 2a (Table 1). Although
common triazole-based (N1) and imidazole-based NHCs (N2,
N3, and N4) did not produce the desired 2a (entries 1−4), the
introduction of methyl groups at the 4,5-positions of the
imidazole ring (i.e., N5, N6, and N7) permitted the desired
C−N bond cleavage to form 2a (entries 5−7) with the
methoxy-substituted N7 being among the most effective. Less
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a
Scheme 2. Scope of Substrates
a
Reaction conditions: amide 1 (0.20 mmol), N7·HCl (0.060 mmol), CsF (0.40 mmol), and toluene (3.0 mL) in sealed tube at 140 °C for 40 h.
Isolated yield is shown. For 2f−i, the yield refers to that determined by NMR due to the formation of inseparable byproducts. Ratio in parentheses
b
c
d
e
f
g
refers to that of E/Z isomers of 2. For 72 h. At 160 °C. N9·HCl was used as a catalyst. For 144 h. For 24 h. 50 mol % catalyst was used.
Scheme 3. Synthetic Applications
bond cleavage proceeds through an SNAr pathway, including a
Meisenheimer intermediate INT2. Approaching the TS1
requires a barrier of 22.9 kcal/mol, followed by the formation
of the Meisenheimer intermediate INT2. The collapse of
INT2 to INT3 proceeds through TS2 with a higher activation
barrier of 29.6 kcal/mol, which indicates that the cleavage of
C−N bond is likely the rate-determining step of this process.
Natural population analysis (NPA) of INT2 revealed that the
negative charges were largely distributed on the aromatic ring
of an amide substrate (C2, −0.459; C3, −0.230; C4, −0.442)
while the positive charges were on the imidazolium ring (see
the Supporting Information for details). These results are
consistent with the experimental results showing that electron-
poor anilides react more efficiently in this Truce−Smiles-type
rearrangement.17
In summary, we report on the development of the
nucleophilic organocatalytic C−N bond cleavage of aniline
derivatives. The key to realizing this reaction is the utilization
of a highly nucleophilic NHC, which enables the formation of
a highly nucleophilic ylide intermediate generated from an α,β-
unsaturated amide. This reaction is applicable to substrates
bearing a variety of functional groups and biologically active
molecules having an aniline moiety, such as sulfonamide drugs.
DFT calculations suggest that this reaction is likely to proceed
via the formation of the nonstabilized Meisenheimer
intermediate INT2. Further investigations of organocatalytic
reactions involving the cleavage of strong bonds are currently
underway in our laboratory.
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Scheme 4. DFT Calculations
M.T. thanks Iketani Science and Technology Foundation for
their financial support. H.F. thanks the JSPS Research
Fellowship for Young Scientists and the Program for Leading
Graduate Schools: “Interactive Materials Science Cadet
Program.” for their support. We also thank Dr. Hiroaki
Iwamoto (Osaka University) for helpful discussions on DFT
calculations. We also thank the Instrumental Analysis Center,
Faculty of Engineering, Osaka University, for their assistance
with HRMS.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04281.
■
sı
Detailed experimental procedures, characterization of
new compounds, Cartesian coordinates, NMR spectra,
and computational details (PDF)
AUTHOR INFORMATION
Corresponding Author
■
REFERENCES
■
Mamoru Tobisu − Department of Applied Chemistry,
Graduate School of Engineering and Innovative Catalysis
Science Division, Institute for Open and Transdisciplinary
Research Initiatives (ICS-OTRI), Osaka University, Suita,
Osaka 565-0871, Japan; orcid.org/0000-0002-8415-
2225; Email: tobisu@chem.eng.osaka-u.ac.jp
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Authors
Kosuke Yasui − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0002-3906-8307
Miharu Kamitani − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan
Hayato Fujimoto − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan; orcid.org/0000-0002-2046-
4596
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04281
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a Scientific Research on
Innovative Area “Hybrid Catalysis” (20H04818) MEXT.
■
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