Enantioselective rhodium-catalyzed allylic Ci-H activation for the addition to conjugated dienes

Article abstract of DOI:10.1002/anie.201005215

Easy and efficient: By applying the title transformation, two adjacent sp³ stereogenic centers, one of which is a quaternary carbon center, can be easily formed. This asymmetric reaction provides easy and efficient access to multifunctionalized tetrahydropyrrole, tetrahydrofuran, and cyclopentane compounds (see scheme; coe=cyclooctene, DME=1,2-dimethoxyethane, Tf=trifluoromethanesulfonyl).

Full text of DOI:10.1002/anie.201005215

Communications
DOI: 10.1002/anie.201005215
À
C H Activation
À
Enantioselective Rhodium-Catalyzed Allylic C H Activation for the
Addition to Conjugated Dienes**
Qian Li and Zhi-Xiang Yu*
À
À
Transition-metal-catalyzed C H activation has the potential
to streamline organic synthesis because it can provide novel
disconnections in the retrosynthetic analysis of a target
For enantioselective allylic C H oxidations, White and co-
workers[16a,b] achieved a 63% ee for the palladium(II)-cata-
lyzed allylic oxygenation of terminal olefins.[16c–g] Even though
it is challenging to achieve asymmetric sp C H activation
with high enantioselectivity, considering its synthetic impor-
tance, continuous endeavors to meet such a challenge are
required.
molecule.[1,2] A synthetically useful C H activation method
should be diastereoselective and more importantly, enantio-
selective, if one or more stereocenter is generated in this
process. Although significant progress has been made in
3
À
À
À
transition-metal-catalyzed C H activation reactions, the
Recently, our research group has developed a conjugated-
À
À
progress of enantioselective C H activation and subsequent
À
diene-assisted, rhodium-catalyzed addition of allylic C H
[3]
C C bond formations through metal insertion has lagged
bonds to conjugated dienes to furnish multifunctional tetra-
hydropyrrole, tetrahydrofuran, and cyclopentane compounds.
The two new stereogenic centers in the final products had
good to excellent diastereoselectivity (see the reaction shown
in Table 1).[17] We were eager to develop an asymmetric
behind.[4] Several impressive examples in this area have
emerged. For example, the research groups of Mikami,[5]
Murai,[6] and Bergman and Ellman[7] have all reported
pioneering work on the enantioselective coupling reactions
of vinylic and aromatic C H bonds with alkenes. Enantio-
selective hydroacylation reactions of alkenes and ketones
[8]
À
À
À
version of this allylic C H activation/C C bond formation
reaction, which would provide efficient and easy access to the
multifunctional chiral tetrahydropyrrole, tetrahydrofuran,
and cyclopentane compounds. The two challenges for this
À
through the cleavage of an acyl C H bond have also been
reported.[9] Yu and co-workers have developed an elegant
palladium(II)-catalyzed enantioselective coupling reaction of
À
enantioselective reaction are the asymmetric allylic C H
À
À
aromatic C H bonds with boronic acids and styrenes using a
activation/C C bond formation, and the asymmetric synthesis
desymmetrization strategy.[10a,b] By using a similar approach,
Albicker and Cramer achieved enantioselective palladium-
catalyzed direct arylations.[10c] Despite these notable advances
of a quaternary carbon center, which has been a longstanding
challenge in organic synthesis.[18] Herein, we report the first
À
example of a highly enantioselective allylic C H activation/
C C bond formation reaction through metal insertion. We
in the enantioselective sp2 C H activation reactions, the
[3]
À
À
enantioselective sp3 C H activation/C C bond formation
through the intermediate formation of carbon–metal bonds
remains elusive.[11–15] The lack of progress may be attributed
show that the present reaction provides an easy route to the
asymmetric synthesis of two adjacent sp3 carbon centers, one
of which is a quaternary carbon center.
À
À
to the limited methods for sp3 C H activation, the harsh
Our study began with the identification of an effective
chiral ligand for the target reaction (Table 1). We found that
chelating diphosphines such as binap inhibited the reaction,
therefore we focused our effort on screening the monodentate
ligands. Chiral phosphoramidites,[19] which are a class of easily
accessible and highly modulable ligands, were tested. Fortu-
nately, the application of phosphoramidite ligand Aunder our
previous reaction conditions gave a high yield and a promising
ee value (Table 1, entry 1). This encouraging result led us to
further optimize the reaction conditions. We observed that
changing the silver source from AbSbF6 to AgOTf improved
the enantioselectivity (entry 2). Better enantioselectivity was
obtained when DME or benzene were used as the solvent
(entries 3 and 4, respectively) compared with DCE (entry 2),
although the reaction in benzene was slower. The use of
[{Rh(coe)2Cl}2] as a catalyst precursor provided a faster
reaction rate (entry 5), presumably as a result of the faster
dissociation of the coe ligand from the catalyst precursor.
Next, we tested a variety of phosphoramidite ligands. A slight
increase of the steric bulk on the nitrogen center improved
the ee value, for example, phosphoramidites bearing diethyl
amine (B), diisopropyl amine (C), piperidine (D), and
morpholine (E) gave 90%, 89%, 87%, and 90% ee, respec-
À
3
À
reaction conditions required for the cleavage of an sp C H
bond, and the paucity of ligands available for enantioselective
C H activations. One example of an enantioselective sp C
H activation/C C bond formation came from Yu and co-
3
À
À
À
workers, who reported a palladium(II)-catalyzed pyridine-
directed reaction that occurred with a promising 37% ee.[10a]
[*] Q. Li, Prof. Dr. Z.-X. Yu
Beijing National Laboratory of Molecular Sciences (BNLMS)
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
College of Chemistry, Peking University, Beijing, 100871 (China)
Fax: (+86)10-6275-1708
E-mail: yuzx@pku.edu.cn
[**] We thank the National Natural Science Foundation of China
(20825205–National Science Fund for Distinguished Young Schol-
ars, 20772007, 20672005) and the Ministry of Science and
Technology (2011CB808603 and 2010CB833203–National Basic
Research Programs of China-973 Programs) for financial support.
We also thank Prof. Norio Miyaura (Hokkaido University), Prof. Qi-
Lin Zhou (Nankai University), and Prof. Shu-Li You (SIOC) for
sending us the chiral ligands.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005215.
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Angew. Chem. Int. Ed. 2011, 50, 2144 –2147

Table 1: Screening of the reaction conditions.
tively (entries 6–9). However, any further increase of the
steric bulk on the ligands (F and G) resulted in negligible
yields; this result is probably due to the inhibition of their
complexation with the catalyst (entries 10 and 11). Phosphor-
amidite ligand H with an octahydrodinaphthyl group and
ligand I with a substituent on the binaphthyl scaffold gave
high yields but slightly lower enantioselectivities (entries 12
and 13, respectively). Zhouꢀs spiro phosphoramidite ligand J
was less effective for the enantioinduction in our reaction
(entry 14).[20] When using Miyauraꢀs ligand K (entry 15) a
similar enantioselectivity was achieved as with ligand A, but a
longer reaction time was required and the yield was lower.[21]
We also examined the catalytic reaction of 1a using lower
rhodium loadings (entries 16 and 17). Pleasingly, when the
loading of [{Rh(coe)2Cl}2] was lowered to 1.5 mol%, an
excellent yield and ee value were obtained after a prolonged
reaction time (27 h). The same enantioselectivity was
achieved when the catalyst loading was lowered to
1.0 mol%, although the reaction time was extended to
36 hours. To achieve a practical reaction time, we found that
5 mol% of [{Rh(coe)2Cl}2], 12 mol% of AgOTf, 25 mol% of
ligand B, and DME were the optimal conditions.
Entry [Rh]
L* [Ag]
Solvent
t
Yield ee
[h] [%][a] [%][b]
1
2
3
4
5
6
7
8
[{Rh(cod)Cl}2]
A
A
A
A
A
B
C
D
E
F
AgSbF6 DCE
AgOTf DCE
AgOTf DME
20 95 58 (2S,3R)
12 85 70 (2S,3R)
11 87 80 (2S,3R)
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
[{Rh(coe)2Cl}2]
AgOTf benzene 40 86 80 (2S,3R)
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
AgOTf DME
8
87 80 (2S,3R)
11 90 90 (2R,3S)
11 90 89 (2R, 3S)
11 90 87 (2R,3S)
11 90 90 (2R,3S)
24 <5 ND
9
10
11
12
13
14
15
G
H
I
24 <5 ND
11 91 87 (2R, 3S)
11 90 70 (2R,3S)
36 83 14 (2R,3S)
48 57 77 (2S,3R)
27 90 89 (2R,3S)
36 53 90 (2R,3S)
J
K
B
B
16[c] [{Rh(coe)2Cl}2]
17[d] [{Rh(coe)2Cl}2]
We next investigated the generality of the enantioselec-
À
tive allylic C H activation reaction of ene-2-dienes
(Scheme 1). A phenyl substituent at the internal position of
the conjugated diene decreased the ee value, whereas a butyl
group at the same position gave a slightly improved ee value
of 94% (2b and 2c, respectively; Scheme 1). However,
increasing the length of the alkyl substituent at the internal
position did not give a higher enantioselectivity (2d and 2e).
The catalytic system was also applicable to internal allylic
[a] Yield of isolated product. [b] The ee values were determined by HPLC
on a chiral stationary phase. Sense of induction is indicated in
parentheses. [c] 1.5 mol% of [{Rh(coe)2Cl}2], 7.5 mol% of L*, and
3.6 mol% of [Ag]. [d] 1 mol% of [{Rh(coe)2Cl}2], 5 mol% of L*, and
2.4 mol% of [Ag]. cod=cyclooctadiene, coe=cyclooctene, DCE=1,2-
dichloroethane, DME=1,2-dimethoxyethane, ND=not determined,
Tf =trifluoromethanesulfonyl.
À
C H bonds. For example, the reactions of substrates that have
aryl groups at the terminal position proceeded with lower
yields but without any erosion of enantioselectivity (2 f–2h).
Significantly, the enantioselective reactions were not limited
À
to allylic C H bonds which are adjacent to a nitrogen atom, as
demonstrated by the successful reactions of homoallylic
substrates to give products 2i–2m. However, the enantiose-
lectivity decreased when the unsubstituted homoallylic sub-
strate was used. We synthesized the spiro substrate to increase
the conformational rigidity in the hope that this would
improve the enantioselectivity of the reaction. Unfortunately,
the ee value achieved was 68%, although the reaction yield
was increased to 88% (2j). Interestingly, in contrast to aryl-
substituted allylic substrates that gave 2 f–2h as products in
low yields, aryl substituents on the homoallylic substrates did
not affect the reaction yields and maintained the high
enantioselectivity (2k–2m). In addition to the nitrogen-
tethered substrates, the carbon- and oxygen-tethered sub-
strates also reacted efficiently and gave products 2n and 2o,
respectively, with good to high enantiocontrol. Importantly,
for all the substrates tested the reactions proceeded diaste-
reoselectively resulting in the cis products. Thus, this process
provides a facile method to synthesize substituted tetra-
hydropyrrole, tetrahydrofuran, and cyclopentane compounds
with excellent diastereoselectivity and good to high enantio-
selectivity. The absolute configuration of product 2 f was
determined by X-ray crystallographic analysis (Figure 1).[22]
Additionally, we performed a nonlinear effect study of the
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Communications
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Scheme 1. Scope of the asymmetric allylic C H activation/addition to conjugated dienes. The reaction time of each substrate is indicated in the
Supporting Information. The yields reported are for the isolated products. The major enantiomer is shown. The diastereoselectivity was
1
determined by H NMR spectroscopy. The ee values were determined by HPLC on a chiral stationary phase. The absolute configuration of 2 f was
determined by X-ray crystallography and the absolute configuration of the other products was determined by analogy to 2 f. Bn=Benzyl,
brsm=based on the recovered starting material, Ts=p-toluenesulfonyl.
reaction of 1a using ligand B. It turned out that there is no
stereodetermining step (see the Supporting Information for
details).
À
nonlinear effect in our asymmetric allylic C H activation
reaction, thus suggesting that there is one chiral phosphor-
amidite group coordinated to the rhodium center in the
In summary, we have developed a highly enantioselective
À
rhodium-catalyzed allylic C H activation/addition to conju-
gated dienes. The asymmetric synthesis of tetrahydropyrrole,
tetrahydrofuran, and cyclopentane compounds that contain
two adjacent sp3 carbon centers, one of which is a quaternary
carbon center, was achieved. Investigations into the mecha-
nism and additional applications of this novel method in
organic synthesis are currently underway.
Received: August 20, 2010
Revised: December 14, 2010
Published online: January 20, 2011
À
Keywords: asymmetric synthesis · C H activation ·
conjugated diene · enantioselectivity · rhodium
Figure 1. Ortep drawing of 2 f. Hydrogen atoms are omitted for clarity
and ellipsoids are drawn at 50% probability.
.
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