Lewis acid-assisted dirhodium(II)-catalyzed ketone hydroacylation

Article abstract of DOI:10.1246/cl.160862

The combination of a Rh(II) salt and a Lewisacid, Sc(OTf)₃, as a highly active and robust cooperative catalyst system for ketone hydroacylation was developed. The catalyst system showed high turnover numbers (up to ca. 400) even under air, whereas the corresponding Rh(I)/Sc(OTf)₃ system did not work well. The application of the Rh(II)/Sc system was also extended to hydroacylation of olefins and an enantioselective reaction.

Full text of DOI:10.1246/cl.160862

Advance Publication Cover Page
Lewis Acid-Assisted Dirhodium(II) Catalyzed Ketone Hydroacylation
Tomohiro Yasukawa and Shū Kobayashi*
Advance Publication on the web November 1, 2016
doi:10.1246/cl.160862
©
2016 The Chemical Society of Japan
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1
Lewis Acid-Assisted Dirhodium(II) Catalyzed Ketone Hydroacylation
Tomohiro Yasukawa and Shū Kobayashi*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
(
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp)
The combination of a Rh(II) salt and a Lewis acid,
Sc(OTf) , as a highly active and robust cooperative catalyst
system for ketone hydroacylation was developed. The catalyst
system showed high turnover numbers (up to ca. 400) even
under air, whereas the corresponding Rh(I)/Sc(OTf) system
did not work well. Application of the Rh(II)/Sc system was
also extended to hydroacylation of olefins and an
enantioselective reaction.
corresponding lactone 2a in excellent yield (entry 1), although
the reaction barely proceeded in the absence of Sc(OTf)
entry 2). The reaction could work even in the presence of
3
3
(
water under air, demonstrating robustness of this catalyst
system. A Rh(I) catalyst, which is a conventional catalyst for
3
9
ketone hydroacylation, did not work for this reaction under
the same conditions, and the starting material was fully
recovered (entry 3) Even a cationic Rh(I) complex prepared
from Rh chloride salt and AgOTf in situ, which was the one
9
Lewis acids are an important class of catalysts that can
coordinate to Lewis basic functionality such as carbonyl and
imine groups to activate electrophiles. They can catalyze
many fundamental reactions, such as aldol reactions, Mannich
of the most reactive complex for this reaction, did not afford
any products under our conditions (entry 4). For the Rh(I)
catalyst system, a high catalyst loading of Sc(OTf)3 was
required to give the product (entry 5). No catalytic activity of
3
1
reactions, Michael additions, and Diels–Alder reactions.
Sc(OTf) itself was confirmed (entry 6). It is noted that the
Compared with single Lewis acid catalyst systems, the use of
a combination of Lewis acid and another catalyst such as a
transition-metal catalyst or organocatalyst as a cooperative
catalyst can be a more powerful tool to develop new reactions
and/or to enhance catalytic turnover. Our group also recently
combination of Rh(II) and Sc(OTf) is essential to achieve a
high catalyst turnover number.
3
2
3
Table 1. Optimization of reaction conditions of ketone
hydroacylation
found
a synergistic effect between metal nanoparticle
catalysts and Lewis acid catalysts for hydrogen autotransfer
4
5
processes and asymmetric 1,4-addition reactions.
Hydroacylation, which involves aldehyde C–H bond
activation and subsequent insertion into a multiple bond, is an
atom economical and attractive transformation to produce
carbonyl compounds. Since the first report of intramolecular
Yield
Entry
Rh salt
Ligand Additive (mol%)
a
(
%)
6
1
2
3
4
5
6
7
8
9
Rh (OAc)
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppp
dppe
dppb
dppf
Sc(OTf) (1)
>95
4
0
0
89
0
>95
26
>95
71
32
3
2
4
4
3
hydroacylation by Sakai and co-workers in 1972, significant
Rh
2
(OAc)
-
progress in this field has been achieved to extend applicable
7
[Rh(C
[Rh(C
[Rh(C
2
2
2
H
H
H
4
4
4
)
)
)
2
2
2
Cl]
Cl]
Cl]
2
2
2
Sc(OTf)
3
(1)
substrates.
In
particular,
intramolecular
ketone
Ag(OTf) (0.25)
hydroacylation of 2-formylarylketones is an effective
synthetic method to access phthalide derivatives that are
frequently found in natural products exhibiting biological
activities and that can act as versatile building blocks in
Sc(OTf)
Sc(OTf)
3
3
(10)
(10)
–
Rh
Rh
Rh
Rh
Rh
Rh
2
2
2
2
2
2
(OAc)
(OAc)
(OAc)
(OAc)
(OAc)
(OAc)
4
4
4
4
4
4
Yb(OTf)
3
(1)
8
Ag(OTf) (1)
HOTf (1)
organic synthesis. In 2009, Dong and co-workers developed
chiral Rh(I) complex catalyzed asymmetric ketone
hydroacylation that can produce a broad range of chiral
1
0
1
1
1
9
11
phthalides with excellent enantioselectivity. However, their
1
2
systems required a relatively high loading of an expensive Rh
catalyst (5–10 mol%) and a relatively long reaction time and
thus more active catalyst systems are desired. Herein, we
report a novel cooperative catalyst system consisting of Rh
and a Lewis acid as a highly active catalyst system for
intramolecular hydroacylation reactions.
At the outset, ketone hydroacylation was examined in
the reaction with 2-benzoylbenzaldehyde (2a) by using a very
small amount (0.25 mol%) of a Rh catalyst and a diphosphine
ligand (Table 1). We hypothesized that use of a Lewis acid
that can coordinate to a substrate may accelerate the reaction.
It was initially discovered that, in addition to a Rh(II) catalyst,
a
1
Determined by H NMR analysis.
Other types of acid additives were also tested. Similar
with Sc(OTf) another water-compatible Lewis acid,
Yb(OTf) could efficiently promote the reaction (entry 7)
while AgOTf gave low yield (entry 8) indicating that these
additives might not be a just triflate anion supplier.
Surprisingly, it was found that strong Brønsted acid additive,
HOTf, could accelerate the reaction well too (entry 9). To
clarify whether a Lewis acid-assisted pathway really exists or
3
,
3
not, the initial reaction rate was compared. Indeed, Sc(OTf)
3
10
showed the faster reaction rate than that in the case of HOTf
and judging from relatively large pK (K = hydrolysis
constant) value of Sc , we concluded that the Lewis acid-
h
h
the use of water-compatible Lewis acid, Sc(OTf)
3
, as a co-
3
+ 11
catalyst dramatically accelerated the reaction to give the

2
assisted pathway was dominant and more effective than
Brønsted acid-assisted pathway. The effect of diphosphine
ligand structures was then briefly surveyed (entries 10–12)
Scheme 1. Hydroacylation of olefin
1
2
Finally, we examined an enantioselective version of the
reaction using a chiral diphosphine ligand. The combination
of a Rh(II) salt and Duanphos, which was the best chiral
ligand for the reported Rh(I) catalyzed asymmetric ketone
and
the
results
showed
that
1,3-
bis(diphenylphosphino)propane (dppp) is the best choice. In
the case of 1,4-bis(diphenylphosphino)butane (dppb) and 1,1ʹ-
bis(diphenylphosphino)ferrocene (dppf), a relatively large
amount of benzophenone side product (18% and 67%,
respectively) was observed, which may form through
9
hydroacylation, could induce enantioselectivity; further
solvent screening established conditions under which product
a was formed with good enantioselectivity (Scheme 2).
1
1
2
Interestingly, no product was generated with a Rh(I) salt and
Duanphos under the conditions, even in the presence of
decarbonylation.
In
contrast,
1,2-
1
1
bis(diphenylphosphino)ethane (dppe) and dppp showed good
selectivity with respect to the production of 2a.
3
Sc(OTf) (10 mol%). This result suggests that Rh(II)/Sc is a
much more robust and highly active co-catalyst system than
the conventional chiral Rh(I) complex.
Substrate scope was surveyed under the optimized
conditions (Table 2). Substrates with electron-donating groups
substituted on the benzoyl part showed less reactivity than 1a,
and a slight increase in the catalyst loading (Rh and ligand)
afforded the desired product in high yields (2b and 2c). The
presence of a nitro group on the benzoyl part further
decreased the reactivity and a higher catalyst loading was
required to achieve a full conversion. In this case, the product
O
Rh2(OAc)4 (Rh: 0.25 mol%)
Duanphos (0.25 mol%)
Sc(OTf)3 (1 mol%)
O
P
tBu
H
H
O
tBu
O
dioxane/toluene/H2O (5/5/2)
0.33 M, air, 100 °C, 16 h
P
Ph
Ph
1a
2a
50% yield, 80% ee
Duanphos
2
d was obtained in almost quantitative yield. The co-catalyst
Scheme 2. Asymmetric ketone hydroacylation
system was also applicable for the reaction with 2-
acetylbenzaldehyde to afford the product 2e in excellent yield.
Substitution on the benzene ring of 2-acetylbenzaldehyde was
examined, and substrates bearing either a methoxy or chloro
group were smoothly converted into the desired product 2f or
2
g in high yield.
Hydroacylation of an olefin was also demonstrated by
1
3
using the Rh(II)/Sc system (Scheme 1). The desired seven-
membered ring product 4 was obtained in good yield under
the optimized conditions, whereas the eight-membered ring 5
formed as a side product.
Table 2. Substrate scope
Rh
2
(OAc)
dppp (0.25 mol%)
Sc(OTf) (1 mol%)
4
(Rh: 0.25 mol%)
O
O
3
R1
R1
Scheme 3. Plausible reaction mechanism
O
O
dioxane/H
2
O (5/1), 0.33 M, air
00 °C, 16 h
1
R2
Although there are several possible mechanisms for the
R2
14
1
2
formation of the lactone from 2-formylarylketones, such as a
1
4c
O
O
O
O
O
Tishchenko-type pathway, we assumed that it was more
likely that the reaction proceeded through i) C–H oxidation of
an aldehyde by Rh, ii) Rh–H insertion to the carbonyl group,
O
O
O
1
5
and iii) reductive elimination (Scheme 3), based on the
observation of the formation of a decarbonylated side product,
the induction of enantioselectivity by a chiral phosphine
ligand, and the finding that no catalytic activity was achieved
by using the Lewis acid catalyst alone. The Lewis acid co-
catalyst might coordinate to an aldehyde and/or a ketone
group to facilitate the oxidative addition step and/or the Rh–H
Me
OMe
NO
2
2a: 92% yield
2b: 89% yielda 2c: 80% yielda
2d: 99% yieldb
O
O
O
Cl
O
O
O
3
insertion step, respectively. The true role of Sc(OTf) is
unclear at present, but the latter function might be less likely
because no significant change of enantioselectivity was
MeO
Me
e: 94% yield
Me
Me
2
2f: 99% yield
2g: 78% yield
a
Isolated yields are shown. bRh
and dppp (0.5 mol%) were used. Rh
and dppp (1.0 mol%) were used.
2
(OAc)
(OAc)
4
(Rh: 0.5 mol%)
(Rh: 1.0 mol%)
observed by changing Sc(OTf) to either other metal triflates
3
10
2
4
or a chiral Sc(OTf)3 complex. A catalytically active Rh
species might form after coordination of a phosphine ligand to
1
6
Rh
2
(OAc)
dppp (0.25 mol%)
Sc(OTf) (1 mol%)
4
(Rh: 0.25 mol%)
the axial position of a Rh(II) dimer complex. Several
possible active species can be considered, for example, the
O
O
O
3
17
axially ligated Rh(II) dimer itself and a Rh(I) acetate
+
dioxane/H O (5/1), 0.33 M
2
complex formed in situ from disproportionation of the Rh(II)
O
air, 100 °C, 16 h
O
O
5
1
8
dimer. To elucidate the latter possibility, a control study
using [Rh(C H ) Cl] and Ag(OAc) to generate Rh(I) acetate
3
4
7
0% yield
17% yield
2
4 2
2

3
species by anion exchange was conducted in the presence of
18 J. P. Collman, R. Boulatov, J. Am. Chem. Soc. 2000, 122, 11812.
31
1
0
1
9
2 4
Preliminary P NMR analysis of the mixture of Rh (OAc) and
Duanphos and Sc(OTf)
3
.
However, no reaction occurred,
dppp failed to identify the structure of active species because of
very low concentration of active species. Probably, there were
several kinds of Rh-P species and intensity of each species was
too low to detect. See the Supporting Information for details.
which indicates that an active Rh species in our Rh(II)/Sc
1
9
system may not be a simple monomeric Rh(I) species.
In conclusion, a new cooperative catalyst system
consisting of a Rh(II) salt and a Lewis acid for ketone
hydroacylation was developed, and high turnover number (up
to ca. 400) compared with that of reported reactions was
3
achieved. The corresponding Rh(I)/Sc(OTf) system did not
work well under such low catalyst loading and thus the use of
a Rh(II) salt was essential. The Rh(II)/Sc system was also
applicable for hydroacylation of olefins, and an
enantioselective reaction was possible. The origin of the high
activity of the Rh(II) catalyst and the role of Sc(OTf)
under investigation in our laboratory.
3
are
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS), The University of Tokyo, MEXT, Japan, and
the Japan Science and Technology Agency (JST). We also
thank Mr. Elias Ken Selmi Higashi (a short-term visiting
student from University Grenoble Alpes, France) for his
technical support.
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