RhIII-catalyzed hydroacylation reactions between N-sulfonyl 2-aminobenzaldehydes and olefins

Article abstract of DOI:10.1002/chem.201400022

Metal-catalyzed hydroacylation of olefins represents an important atom-economic synthetic process in C-H activation. For the first time highly efficient Rh^IIICp*-catalyzed hydroacylation was realized in the coupling of N-sulfonyl 2-aminobenzald

Full text of DOI:10.1002/chem.201400022

DOI: 10.1002/chem.201400022
Communication
&
CÀH Activation
III
Rh -Catalyzed Hydroacylation Reactions between N-Sulfonyl 2-
Aminobenzaldehydes and Olefins
[a, b]
[a]
[a]
[b]
[a]
Tao Zhang,
Zisong Qi, Xueyun Zhang, Lamei Wu, and Xingwei Li*
Abstract: Metal-catalyzed hydroacylation of olefins repre-
sents an important atom-economic synthetic process in
III
CÀH activation. For the first time highly efficient Rh Cp*-
catalyzed hydroacylation was realized in the coupling of
N-sulfonyl 2-aminobenzaldehydes with both conjugated
and aliphatic olefins, leading to the synthesis of various
aryl ketones. Occasionally, oxidative coupling occurred
when a silver(I) oxidant was used.
Activation and functionalization of CÀH bonds with unsaturat-
ed molecules is a particularly attractive strategy in CÀC bond
[
1]
formation. This process is advantageous in terms of step- and
atom-economy in that no pre-activation of the CÀH bond is
necessary. Hence it represents an attractive goal in synthetic
chemistry, especially at low catalyst loading and low tempera-
[
2]
III
ture. The hydroacylation reaction of alkenes and alkynes is
highly important and fulfills these criteria with 100% atom-
economy. Two categories of hydroacylation methods have
been realized for alkenes, namely organocatalysis (the Stetter
Scheme 1. Rh -catalyzed CÀH activation of formyl groups.
III
couplings. On the other hand, Rh -catalyzed couplings using
[
3]
[7]
reaction) and metal catalysis, whereby the latter category is
olefins mostly lead to oxidative Heck-type reactions. There-
I
dominated by Rh catalysis by means of a CÀH oxidative addi-
fore, it is necessary to design olefin hydroacylation systems to
expand the synthetic applicability of rhodium(III)-catalyzed CÀ
H activation.
[
4]
tion pathway.
Recently rhodium(III)-catalyzed CÀH activation has been ex-
[
5]
III
tensively explored. Rh Cp* (Cp*=pentamethylcyclopenta-
dienyl) complexes have stood out as active catalysts for CÀH
activation, leading to CÀC, CÀN, and CÀO coupling with broad
substrate scope and high activity. A large number of new cou-
pling patterns have been developed that can complement
We reasoned that N-sulfonyl 2-aminobenzaldehyde may un-
dergo cyclometalation, and subsequent olefin insertion into
III
the RhÀacyl bond could give a Rh –alkyl species (Scheme 1).
However, when compared with the reported salicylaldehyde
substrate (Scheme 1), the resulting carbonyl-chelated rhodacy-
cle should be better stabilized and is conformationally more
rigid owing to the higher donating ability of the amidate
group. Therefore, this rhodacycle is less prone to b-hydride
elimination and consequently protonolysis leading to a hydro-
acylation product is favored. In addition, the presence of
a pendant sulfonyl group often allows further useful transfor-
I
0
II
those using Rh , Pd , and Pd catalysts. In contrast to the vari-
I
ous reports on Rh-catalyzed hydroacylation reactions, no such
III
reaction has been realized using Rh catalysis. This is because
III
the CÀH substrates in Rh catalysis are mostly limited to
III
arenes and alkenes. While a few examples of Rh -catalyzed ac-
tivation of formyl CÀH bonds for CÀC coupling have been re-
[
6]
[8]
ported (Scheme 1), these systems are limited to oxidative
mations. We now report rhodium(III)-catalyzed coupling of N-
sulfonyl 2-aminobenzaldehyde with olefins.
N-p-Toluenesulfonyl (Ts) 2-aminobenzaldehyde (1a) was
readily prepared starting from 2-aminobenzylalcohol in two
steps. The reaction conditions for the coupling of 1a with
ethyl acrylate were then screened. Using [{RhCp*Cl } ] as a cata-
[
a] T. Zhang, Z. Qi, X. Zhang, Prof. Dr. X. Li
Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023 (P. R. China)
E-mail: xwli@dicp.ac.cn
2
2
[
b] T. Zhang, Prof. L. Wu
lyst, coupling occurred in the presence of Ag CO in dichloro-
2
3
College of Chemistry and Materials Science
South-Central University for Nationalities
Wuhan 430074 (P. R. China)
ethane (DCE, Table 1). Indeed, NMR analyses of the major prod-
uct pointed to a hydroacylation product 3aa, even though
Ag CO is a typical oxidant in oxidative CÀH activation reac-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400022.
2
3
[5,9]
tions.
Thus the Ag CO only functions as a base, so a stoi-
2 3
Chem. Eur. J. 2014, 20, 3283 – 3288
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[
a]
Table 1. Optimization studies.
[
b]
Entry Catalyst (mol%)
Additive (equiv) Solvent Yield [%]
1
2
3
4
5
6
7
8
9
1
[{RhCp*Cl
[{RhCp*Cl
[RhCp*(MeCN)
2
2
}
}
2
2
] (2.5)
] (2.5)
Ag
Ag
(5) Ag
Ag
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
(2)
DCE
DCE
DCE
DCM
acetone 66
tAmOH n.d.
68
87
n.d.
47
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
(0.2)
[
d]
3
][SbF
6
]
2
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
[{RhCp*Cl
2
2
2
2
2
2
2
2
}
}
}
}
}
}
}
}
2
2
2
2
2
2
2
2
] (4)
] (2.5)
] (2.5)
] (2.5)
] (2.5)
] (2.5)
] (2.5)
] (2.5)
Ag
Ag
Ag
Ag
CH
3
CN
n.d.
dioxane 84
Ag
DCE
DCE
DCE
DCE
67
32
88
82
0
K
3
PO
CsOAc (1)
PO (0.05)
4
(1)
1
1
[
c]
12
[{Rh(cod)Cl}
2
]/PPh
3
(2.5/5)
K
3
4
[a] Reactions were carried out by using a catalyst, base additive, N-Ts 2-
aminobenzaldehyde (0.2 mmol), and ethyl acrylate (4 equiv) in a solvent
(
[
2 mL) at 1008C for 18 h. [b] Isolated yield after column chromatography.
c] Performed at 708C; cod=cyclooctadiene. For limitations of these con-
ditions, see Supporting Information. [d] n.d. =not detected. [e] tAmOH=
-methyl-2-butyl alcohol.
2
chiometric amount of Ag CO seems unnecessary. Indeed, low-
2
3
ering the stoichiometry of Ag CO to 0.5 equivalents did give
2
3
rise to an improved efficiency (entries 1,2) and 3aa was isolat-
ed in 87% yield (conditions A, entry 2). Further lowering the
amount of Ag CO gave inferior results (entry 9). DCE turned
2
3
out to be the optimal solvent. Both a base additive and the
{RhCp*Cl } ] catalyst are necessary and screening of other
[
2
2
bases revealed CsOAc as the most efficient one (conditions B,
I
entry 11). In comparison, although a typical Rh catalyst sys-
[
4b]
tem also exhibited high activity in this reaction (entry 12),
I
these Rh conditions gave poor results for unactivated olefins
[
a,b]
[
10]
I
Scheme 2. Hydroacylation of different olefins. [a] Reaction conditions: N-
(Table 2). It is not our objective to exhaust Rh conditions to
sulfonyl 2-aminobenzaldehyde (0.2 mmol), olefin (4 equiv), CsOAc
] (0.005 mmol), DCE (2 mL), 1008C, 18 h, sealed tube
2 2
under nitrogen. [b] Isolated yield after column chromatography. [c] Reaction
obtained optimal results and given our continued interest in
(
0.2 mmol), [RhCp*Cl
III
III
Rh -catalyzed CÀH activation, the Rh conditions B were re-
tained for further studies.
was performed in a 6.6 mmol scale. [d] Ag
of CsOAc.
2 3
CO (0.2 mmol) was used instead
Table 2. Comparisons of yields [%] of three representative coupling reac-
tions under different conditions.
The scope and limitations of this coupling reaction were ex-
plored under conditions B (Scheme 2). Aldehyde 1a coupled
with different acrylates and an enone to give 3aa–3af in 84–
92% yields. Moreover, this reaction is scalable to multigram-
scale synthesis as in the isolation of 2.24 g of 3aa (91% yield).
In addition, moderate-to-high yields were isolated when the
olefin substrate was extended to acrylamides (3ag–3ai). Differ-
ent ring-substituted (methyl, nitro, and halogens) benzalde-
hydes are also applicable, although somewhat lower yield
tends to be isolated for a Cl- or Br-substituted benzaldehyde
I
R
Rh
Conditions
A
Conditions
B
Conditions
C
[
a,e]
[b,e]
[c,e]
[d,e]
conditions
CO
Ph
nBu
2
Et
82
71
37
87
<5
0
88
83
70
87
81
0
I
[
(
a] Rh conditions: [{Rh(cod)Cl}
5 mol%), DCE, 708C, 18 h. [b] Conditions A: [{RhCp*Cl
CO (50 mol%), DCE, 1008C, 18 h. [c] Conditions B: [{RhCp*Cl
2.5 mol%), CsOAc (1 equiv), DCE, 1008C, 18 h. [d] Conditions C:
{RhCp*Cl ], (2.5 mol%), Ag CO (2 equiv), KPF (2 equiv), DCE, 1108C,
8 h. [e] Isolated yield after column chromatography.
2
]
(2.5 mol%), PPh
3
(5 mol%),
], (2.5 mol%),
],
3 4
K PO
(
3da–3ee). Variation of the sulfonyl group to methyl and aryl
2 2
}
substituents is tolerated as shown by the isolation of (3ga–
3ja) in good-to-high yields. Furthermore, the benzaldehyde
ring can be extended to a 2-thiophenaldehyde (3ka), albeit
with a lower yield. In contrast, only the CÀH oxidative olefina-
Ag
(
[
2
3
2 2
}
2
}
2
2
3
6
1
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Communication
tion product (3ka’) was isolated in high yield when this reac-
tion was conducted under conditions A with one equivalent of
Ag CO , indicating that these two conditions might give com-
2
3
plementary results.
The conditions B are equally viable for the coupling with
styrenes. In line with the hydroacylation of acrylates, benzalde-
hyde 1a coupled with different styrenes with consistently high
efficiency (3aj–3ao). Consistent with the scope of benzalde-
hydes in the coupling with activated olefins, variation of the
sulfonyl group is well-tolerated (3gk–3jk). Variation of the sub-
stituent in the benzaldehyde ring (3bk–3 fk), including intro-
ducing nitro and halogens groups, has no significant effect on
the isolated yield. These functional groups should readily pro-
vide handles for further functionalization of these ketone prod-
ucts. The high yield obtained here for the halogen-substituted
benzaldehydes stands in contrast to those in the coupling with
acrylates. The same observation also applies to the coupling of
a 2-thiophenaldehyde substrate (3kk).
Gratifyingly, the current conditions are also applicable to the
coupling for aliphatic olefins. Of note, these olefins are usually
much less reactive in CÀH activation reactions. Indeed, only
a few systems of rhodium(III)-catalyzed olefination reactions
Scheme 4. Mechanistic studies.
[
11]
for aliphatic olefins have been reported. We found that this
transformation is compatible with aliphatic alkenes such as 1-
hexene, vinylcyclohexane, 4-methyl-1-pentene, and allylben-
zene, and the coupled product was isolated in 56–73% yield.
In contrast, no desired reaction occurred when conditions A
were applied.
Several experiments have been performed to explore the
mechanism of this hydroacylation reaction (Scheme 4). To dem-
onstrate the relevancy of CÀH activation, two rhodacyclic com-
plexes 7a and 7b have been readily prepared from the reac-
tion of 1a and [RhCp*(OAc) ] in different solvents (Scheme 4a),
2
in which a concerted metalation–deprotonation mechanism is
[1c,14]
To demonstrate the synthetic applicability of the hydroacyla-
tion product, derivatization of 3aa has been performed
likely operational with the assistance of an acetate ligand,
and which differs from the CÀH oxidative addition mechanism
I
(
Scheme 3). Desulfonylation by using H SO afforded aniline 4
in Rh-catalysis. Both complexes were fully characterized by
2
4
NMR and IR spectroscopy, and HRMS. In particular, the RhÀC(O)
resonates characteristically at d=249.8 ppm (d, J
=27.3 Hz)
RhÀC
13
[15]
for 7b in the C NMR spectrum. Both 7a and 7b (6 mol%)
proved active for the coupling of 1a and 2a, as shown by the
isolation of 3aa in 82 and 68% yield, respectively (Scheme 4b).
The lower catalytic efficiency observed for 7b is likely caused
by stronger pyridine inhibition. To further probe this CÀH acti-
vation process, the coupling between [D]-1a and 2a was
1
stopped at about 45% conversion. H NMR analysis of the re-
covered [D]-1a revealed no H/D exchange at the acyl position,
indicating that this CÀH activation process is irreversible
(
Scheme 4c). H/D scrambling (30% D) was observed at the a-
position of the product, which agrees with proposed protonol-
ysis of the Rh–alkyl bond in the catalytic cycle. The kinetic iso-
tope effect (KIE) has been estimated for the competitive cou-
pling of an equimolar mixture of 1a and [D]-1a with ethyl ac-
Scheme 3. Derivatization of a hydroacylation product.
1
rylate. H NMR analysis of the level of deuteration of the recov-
in 61% yield. Oxime formation followed by nucleophilic cycli-
ered aldehyde mixture at 29% conversion (HPLC) gave k /k =
H
D
zation afforded a mixture of N-Ts indazone and its tautomer
1.9 (see Scheme 4d and Supporting Information). This moder-
ate value of KIE suggests that cleavage of the CÀH bond might
be involved in the rate-limiting step. Thus in a proposed mech-
anism following cyclometalation, migratory insertion of the
acyl group into the olefin affords a rhodium(III) alkyl species
(Scheme 1). Protonolysis of the RhÀC(alkyl) bond furnishes the
final coupled product. The observation of hydroacylation as
the exclusive pathway suggests that the protonolysis is
(
(
5). Significantly, I oxidation readily gave a 3-oxoindolinylidene
2
[
12]
6), a synthetically useful exocyclized olefin, which is the aza
analogue of the product obtained in the rhodium(III)-catalyzed
oxidative coupling of salicylaldehyde with acrylates
[
6a]
(
Scheme 1). Furthermore, Chang and Cheng have independ-
ently documented additional functionalization reactions of
[
13]
closely related 2-aminoacetophenones.
Chem. Eur. J. 2014, 20, 3283 – 3288
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Communication
a lower energy pathway than the alternative b-hydride elimina-
tion process.
Experimental Section
General procedure for the synthesis of 3: N-Sulfonyl 2-aminoben-
zaldehyde (0.2 mmol), olefin (0.8 mmol), [{Cp*RhCl } ] (2.5 mol%),
To better define the scope of activated olefins, N-substituted
maleimides were examined. However, no desired hydroacyla-
tion product was generated under conditions B. We then
turned to oxidative conditions using Ag CO (1 equiv) as an ox-
2
2
and CsOAc (0.2 mmol) were charged into a pressure tube, to which
DCE (2 mL) was added under argon. The reaction mixture was
stirred at 1008C for 18 h. After cooled to room temperature, the
solvent was removed under reduced pressure and the residue was
purified by silica gel chromatography to afford the hydroacylation
product.
2
3
idant (Scheme 5). Interestingly, a series of spiroketones (8a–8 f)
was isolated in good-to-high yield, possibly by following a se-
General procedure for oxidative cyclization with maleimides—
synthesis of 8: N-Sulfonyl 2-Aminobenzaldehyde (0.2 mmol), N-
substituted maleimides (0.4 mmol), [{RhCp*Cl } ] (2.5 mol%), and
2
2
Ag CO (0.2 mmol) were charged into a pressure tube, to which
2
3
DCE (2 mL) was added under argon. The reaction mixture was
stirred at 1208C for 18 h. After cooled to room temperature, the
solvent was removed under reduced pressure and the residue was
purified by silica gel chromatography using ethyl acetate/petrole-
um ether to afford the final product.
Acknowledgements
This work was supported by the Dalian Institute of Chemical
Physics, Chinese Academy of Sciences. We thank Prof. Yong-
Gui Zhou for fruitful discussions.
Keywords: CÀH activation · cyclization · hydroacylation ·
olefins · rhodium
Scheme 5. Oxidative cyclization with maleimides. See Supporting Informa-
tion for detailed conditions.
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2
2
III
tively, a Rh tertiary alkyl complex is possible, as has been pro-
III
posed by us in related Rh -catalyzed spiroamide formation
[
16]
(
see Supporting information). An asymmetric version of this
reaction has been attempted for the synthesis of 8a using
[
RhCp*{(R)-BINOLate}] (BINOL=1,1’-bi-2-naphthol) as a cata-
[
17]
lyst.
Although good yield was isolated (78%), very poor
enantioselectivity (3% ee) was obtained, indicating that design
of chiral variants of the Cp* ring, instead of using a chiral auxil-
iary ligand (which dissociates to generate necessary vacant
sites), should be the right choice.
[
In summary, we have achieved different coupling reactions
of N-sulfonyl 2-aminobenzaldehydes with olefins. In most
cases, hydroacylation has been realized for conjugated olefins
as well as aliphatic olefins under operationally simple condi-
tions. This hydroacylation process occurs by means of a CÀH
[
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I
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2
3
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have been demonstrated. These coupling systems serve to
broaden the scope of rhodium(III)-catalyzed CÀH activation re-
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portant applications.
Chem. Eur. J. 2014, 20, 3283 – 3288
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Communication
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I
Although further optimization of the Rh conditions may give improved
III
1
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5
III
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Received: January 3, 2014
Chem. Eur. J. 2014, 20, 3283 – 3288
www.chemeurj.org
3287
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