Switchable C-H Functionalization of N-Tosyl Acrylamides with Acryloylsilanes

Article abstract of DOI:10.1021/acs.orglett.7b01107

A controllable Rh-catalyzed protocol to access alkylation and alkenylation-annulation of N-tosyl acrylamide with acryloyl silane is reported. In contrast to the directing group or catalyst-dependent divergent sp² C-H alkylation/alkenylation, the intrinsic property of acryloylsilane allows the switchable reaction manifold, thereby affording either alkylation or annulation products with slight modification of the reaction conditions.

Full text of DOI:10.1021/acs.orglett.7b01107

Letter
pubs.acs.org/OrgLett
Switchable C−H Functionalization of N‑Tosyl Acrylamides with
Acryloylsilanes
Shengjin Song,† Ping Lu,† Huan Liu,† Sai-Hu Cai,†,‡ Chao Feng,*,‡ and Teck-Peng Loh*,†,§
†Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China
‡Institute of Advanced Synthesis, College of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for
Advanced Materials, Nanjing Tech University, Nanjing, Jiangsu 211816, P. R. China
§Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
637371 Singapore
S
* Supporting Information
ABSTRACT: A controllable Rh-catalyzed protocol to access
alkylation and alkenylation−annulation of N-tosyl acrylamide
with acryloyl silane is reported. In contrast to the directing
group or catalyst-dependent divergent sp2 C−H alkylation/
alkenylation, the intrinsic property of acryloylsilane allows the
switchable reaction manifold, thereby affording either
alkylation or annulation products with slight modification of the reaction conditions.
ransition-metal-catalyzed alkenylation/alkylation of an sp2
C−H bond with olefins has attracted considerable interest
from the synthetic community because it represents a
straightforward C−C cross-coupling protocol using readily
available starting materials.1 In general, those metalacyclic
intermediates generated through olefin incorporation could
potentially afford alkenylation products via β-hydride elimi-
nation2 or give alkylation products by protonation3 (Scheme 1).
divergent outcome, the reaction conditions or DGs must be
extensively screened, and usually the substrate scope is limited.
We recently reported that a variety of acryloylsilanes acted as
coupling partners in the selective C−H bond functionalization of
indole derivatives.9 Particularly noteworthy is the fact that the
acylsilane functional group plays a vital role in achieving the
divergent functionalizations (our strategy). In contrast to the
corresponding acrylate10/α,β-unsaturated ketone11 that tends to
afford only alkenylation or alkylation products, acryloylsilane
successfully delivers either product, respectively, by slightly
modifying the reaction conditions. The electronic inherent in
acylsilane leads to a reactivity profile that is distinct from other
carbonyl compounds.12 In the past decades, the synthetic use of
acylsilane has received much attention.13 Acylsilane intermedi-
ates have proven to be useful in complex molecule synthesis.14
The conventional utilization of acylsilane is dominated by the
cleavage of the C−Si bond due to their ability to undergo Brook
rearrangement to generate a siloxyl intermediate.15 Additionally,
transition-metal-catalyzed cross coupling of acylsilane has been
postulated to proceed via transmetalation.16 Moreover, it was
reported that acylsilane could be employed as a directing group
for the C−H bond functionalization.17 Herein, we would report a
switchable functionalization of N-tosyl acrylamide with acryl-
oylsilane which proceeds under mild reaction conditions. It is
worth mentioning that the weakly coordinating directing group
N-tosyl amide is of vital importance for the success of this
reaction.18
T
Scheme 1. Controlling Elements Affecting the Reaction
Pathway: β-Hydride Elimination vs Protonation
Despite impressive advances in either direction, the development
of a divergent as well as controllable alkenylation/alkylation
process is still quite rare and highly desirable. Among the
established methods, catalysts are often responsible for the
chemoselective C−H functionalization (strategy A). For
example, alkenylation and alkylation of 2-arylpyridine derivatives
are achieved by Rh(III)4 and Mn(I)5 catalysis, respectively; in the
ortho C−H functionalization of arylketone oximes, while Ru(II)6
affords alkenylation products, the Rh(III)7 protocol leads to
alkylation congeners. Directing group is another promising
element which, in addition to catalyst, is crucial in changing the
reaction pathway by tuning the flexibility of the metallacycle
intermediate (strategy B).8 In this context, to achieve the
We commenced our study by examining the reaction of N-
tosyl acrylamide 1a with acryloylsilane 2a. Initially, the rhodium
catalyst [RhCp*Cl2]2, in combination with a catalytic amount of
Cu(OAc)2·H2O, was investigated and found to be effective in
Received: April 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01107
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
this sp2 C−H alkylation. The product 3aa was obtained in 28%
yield in 1,4-dioxane (Table 1, entry 1). Using THF and DCE as
Scheme 2. Substrate Scope of Alkylation
a
Table 1. Optimization of Reaction Conditions
entry
additive
solvent
3aa (%)
1
2
3
4
5
6
7
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
dioxane
THF
DCE
DCE
DCE
DCE
28
42
72
0
CuOAc
52
4
NaOAc
c
Cu(OAc)2·H2O
Cu(OAc)2·H2O
dioxane/H2O/HOAc
88
b
c
d
8
b
dioxane/H2O/HOAc
98 (92 )
c
9
dioxane/H2O/HOAc
77
a
Unless otherwise noted, the reactions were carried out at 60 °C
under N2 atmosphere using 1a (0.1 mmol), 2a (0.15 mmol),
[RhCp*Cl2]2 (0.0025 mmol), additive (0.01 mmol) in solvent (1
mL) for 12 h with yields determined by 1H NMR using nitromethane
b
as internal standard. RhCp*(MeCN)3(SbF6)2 (0.005 mmol) as
c
d
catalyst. Dioxane/H2O/HOAc = 8/2/1 (1.1 mL) as solvent. Isolated
yield.
solvents for this Rh-catalyzed alkylation resulted in 42% and 72%
yields, respectively (Table 1, entries 2 and 3). As we recently
revealed, Cu(OAc)2·H2O was indispensable for the Rh-catalyzed
hydroarylation of acryloylsilane.9 This reaction ceased without
Cu(OAc)2·H2O additive, and other additives such as CuOAc and
NaOAc showed much lower efficiency (Table 1, entries 4−6).
Interestingly, addition of water and acetic acid to 1,4-dioxane
further improved the yield of 3aa to 88% (Table 1, entry 7).
Moreover, a cationic rhodium catalyst RhCp*(MeCN)3(SbF6)2
exhibited higher efficiency (Table 1, entry 8, conditions A). To
our surprise, in the absence of Cu(OAc)2·H2O, 3aa was obtained
in 77% yield in mixed solvent (Table 1, entry 9). During the
optimization of reaction conditions for the synthesis of 3aa, it
was found a trace amount of compound 4aa was formed.
Encouraged by this result, we ran the reaction with 2 equiv of
Cu(OAc)2·H2O as oxidant and 10 mol % of AgSbF6 as additive.
After the mixture was stirred in 1,4-dioxane at 60 °C for 12 h,
product 4aa was obtained in 51% yield (Table S2, entry 1).
Solvent screening showed THF was superior to acetone and 1,4-
dioxane, in which the yield of 4aa was improved to 78% (Table
S2, entry 2, conditions B). When CuF2, Cu(OTf)2, or CuCl2 was
employed instead of Cu(OAc)2·H2O, no significant 4aa was
observed, which suggests Cu(OAc)2·H2O was required for this
transformation (Table S2, entries 4−6). In addition, a lower yield
was observed by varying the additive to AgPF6 or AgN(Tf)2
(Table S2, entries 7 and 8).
a
See the Supporting Information for the detailed reaction conditions.
Reaction was carried out for 48 h.
b
were compatible, leading to the corresponding alkylated
products (3la−ra) in slightly diminished yields. Notably, in the
case of cyclopropyl-substituted acrylamide 3r, the three-
membered ring remained intact, which demonstrated the good
compatibility and chemoselectivity of this reaction. α,β-
Disubstituted compounds 2sa−wa gave a somewhat lower
yield of products than those of the monosubstituted analogues,
presumably because of steric hindrance. The effect of
substituents on the silyl group was negligible, and the products
bearing different substituents (TMS, TIPS, DMPS) were all
obtained in excellent yields.
We next investigated the substrate scope of the alkenylation−
annulation (Scheme 3). Similarly, the scope extends from aryl- to
alkyl-substituted acrylamides as well as the substrates bearing
alkoxyl groups. Generally, this transformation was less efficient
than alkylation. Acrylamide bearing an aromatic substituent
afforded the annulated products smoothly (4aa−ka). Various
substituents such as alkyl, methoxy, halogen, and trifluoromethyl
on different positions of the aromatic ring were tolerated to
With these optimized reactions in hand, the scope of the
alkylation was explored with respect to acrylamides and
acryloylsilanes (Scheme 2). Acrylamides 1a−k, with aromatic
substitution at the α-position of the amide, gave rise to a series of
alkylation products in good to excellent yields. The reactions
were successful with both electron-donating and electron-
withdrawing groups on the phenyl ring. Alkyl, alkoxyl, aryl,
halide, and trifluoromethyl were well tolerated. Alkyl substituents
at the Cα position, including that functionalized with CH2OEt,
B
DOI: 10.1021/acs.orglett.7b01107
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
conditions A. In light of these results, the acylsilane functionality
is proved to be superior in enabling the present divergent C−H
bond functionalization.
Scheme 3. Substrate Scope of Alkenylation−Annulation
Based on the experimental facts above, we propose that the
reaction proceeds via N-tosylamide-assisted ortho C−H bond
cleavage and C−C double-bond insertion to form a rhodacycle
intermediate II. Subsequently, the proto-demetalation19 occurs
under the acidic conditions to afford alkylated product, and the
annulated product could be generated through β-hydride
elimination followed by intramolecular aza-Michael reaction20
(Scheme 4).
Scheme 4. Plausible Mechanism for the Rh-Catalyzed
Alkylation/Alkenylation−Annulation
In conclusion, we have presented a tunable acryloylsilane-
engaged alkylation/alkenylation−annulation of sp2 C−H bonds
of N-tosyl acrylamides for the efficient construction of β-
alkylated acrylamide and dihydropyrro-2-lones. The reactions
employ Rh(III) complex and Cu(OAc)2·H2O as the catalyst and
additive. By taking advantage of the intrinsic electronic property
of acylsilane group, both the alkylation and alkenylation−
annulation could be achieved selectively. In addition, the present
method should find broader synthetic application because of the
potential transformation of acylsilane.
ASSOCIATED CONTENT
* Supporting Information
■
S
a
See the Supporting Information for the detailed reaction conditions.
Experiments were performed with 1a (0.1 mmol), 2a (0.15 mmol),
b
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01107.
[RhCp*Cl2]2 (0.0025 mmol), and Cu(OAc)2·H2O (0.2 mmol) in
toluene (1 mL) with stirring at 110 °C under N2 for 12 h.
Experimental procedures, characterization data, and NMR
spectra of new and previously reported compounds (PDF)
afford the targeted dihydropyrro-2-lones in moderate to good
yields. In accordance with the above alkylation, substrates 1l−r
with Cα-alkyl substituents were converted into the correspond-
ing annulated products in decreased yields. Moreover, the
reactivity of α,β-disubstituted acrylamides 1s−v and 1x,y were
further lowered, thus requiring higher temperature to give
reasonable yields. Uneventfully, the influence of silyl group was
found to be minimal.
To demonstrate the prominent role of acylsilane in this
divergent functionalization protocol, ethyl vinyl ketone and ethyl
acrylate were tested as a comparison under conditions A and B,
respectively (see the Supporting Information for details). As
expected, ethyl vinyl ketone mainly afforded the alkylation
product under both conditions, although a small amount of
annulation product was isolated under the alkenylation
conditions. On the other hand, when ethyl acrylate was applied
in conditions B, 26% of the desired dihydropyrrol-2-one was
formed, but no trace of the alkylated product was detected under
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: iamcfeng@njtech.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Chao Feng: 0000-0003-4494-6845
Teck-Peng Loh: 0000-0002-2936-337X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of the State Key
Program of National Natural Science Foundation of China
(21432009). We also thank the “1000-Youth Talents Plan”, a
Start-up Grant (39837110) from Nanjing Tech University, and
C
DOI: 10.1021/acs.orglett.7b01107
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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financial support from a SICAM Fellowship from the Jiangsu
National Synergetic Innovation Center for Advanced Materials.
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DOI: 10.1021/acs.orglett.7b01107
Org. Lett. XXXX, XXX, XXX−XXX