A chiral cagelike copper(I) catalyst for the highly enantioselective synthesis of 1,1-cyclopropane diesters

Article abstract of DOI:10.1002/anie.201206376

Triangulation method: The catalytic enantioselective cyclopropanation of multisubstituted olefins with phenyliodonium ylide malonate has been achieved in the presence of a chiral bisoxazoline copper(I) complex (see scheme). A wide range of substrates undergo the reaction to provide optically active 1,1-cyclopropane diesters in high yield with up to >99 %-ee. A rationale for the enantioselective induction has been proposed. Copyright

Full text of DOI:10.1002/anie.201206376

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Angewandte
Communications
DOI: 10.1002/anie.201206376
Asymmetric Catalysis
A Chiral Cagelike Copper(I) Catalyst for the Highly Enantioselective
Synthesis of 1,1-Cyclopropane Diesters**
Chao Deng, Li-Jia Wang, Jun Zhu, and Yong Tang*
Optically active 1,1-cyclopropane dicarboxylates are widely
applied in the total synthesis of natural products, as well as
important chiral building blocks in organic synthesis.[1,2]
Asymmetric cyclopropanation of olefins with metallocar-
benes of malonate provides an easy and direct access to these
compounds.[3] Although the asymmetric cyclopropanation of
olefins with unsymmetric disubstituted metal carbenes,[4] such
as those derived from aryl diazoacetates,[4a,b,e] a-nitrodiazoa-
cetates,[4c,g] and a-cyanodiazoacetates,[4j,k] has proven efficient
for the highly enantioselective synthesis of 1,1-disubstituted
cyclopropanes, only a very few examples of the cyclopropa-
nation of malonate-derived metallocarbenes[5] have been
achieved with high enantioselectivity and diastereoselectivi-
ty.[5b,e] The main reason might be that the carbon atom of the
malonate metallocarbenes is not pro-stereogenic, which
causes a negative effect on the enantiocontrol.[4i,5a] Thus, the
design of chiral ligands that discriminate the two prochiral
faces during asymmetric cyclopropanation is regarded as
quite a challenging problem.[4i] The research groups of
Hayashi and Mꢀller designed C1-symmetric chiral diene-
rhodium(I)[5e] and -rhodium(II) carboxylates[5b] respectively,
which proved to be elegant catalysts for the enantioselective
cyclopropanation of terminal olefins with metallocarbenes of
malonate (16–96% yield, 29–90% ee[5e] and 56–75% yield,
65–98% ee[5b]). To date, the cyclopropanation of multisubsti-
tuted olefins with metallocarbenes of malonate has been
rarely explored and has proved to be less enantioselective
(25% ee).[5c] Very recently, we designed a cagelike bisoxazo-
line-derived CuI catalyst and found it can promote the
asymmetric cyclopropanation reaction of malonate-derived
metallocarbenes with both terminal and multisubstituted
olefins with high selectivity (Scheme 1). Herein, we wish to
report the preliminary results.
Scheme 1. Asymmetric cyclopropanation between olefins and metallo-
carbenes of malonate.
Table 1: Optimization of the reaction conditions.[a]
Entry
CuI
Ligand
R
Yield [%][b]
ee [%][c]
1
2
3
4
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
L1
L2
L3
L4
L5
L4
Me
Me
Me
Me
Me
Me
97
98
72
95
80
99
66
64
86
92
88
95
5
6[d]
[a] 1a (36.6 mg, 0.20 mmol), 2 (0.60 mmol), CuI (0.02 mmol), ligand
(0.03 mmol), and 3 ꢀ MS (200 mg), ꢀ208C, c=0.1 molLꢀ1, reaction
time: 36–43 h. [b] Yield of isolated product. [c] Determined by chiral
HPLC analysis (Chiralcel AD-H). [d] At ꢀ408C, 76 h.
We employed phenyliodonium ylide 2[6] as the carbene
transfer reagent for the study it had previously been shown to
be more active than diazomalonate for the formation of
cyclopropanation of p-bromostyrene with phenyliodonium
ylide malonate to afford 3a in 97% yield with 66% ee
(entry 1). Ligand L2, bearing a pendant benzyl group, was
able to promote the cyclopropanation in 98% yield with
64% ee (entry 2). Installing two pendant benzyl groups at the
bridged carbon atom of the phenyl-bisoxazoline resulted in
the enantioselectivity dramatically increasing (72% yield,
86% ee; entry 3). Further study showed that steric hindrance
of the pendant group also played an important role in
promoting both the enantioselectivity and reactivity. For
example, the highly sterically demanding ligand L4, which
possessed two large bulky side arms, turned out to be the
optimal one (95% yield, 92% ee; entry 4). Increasing the
steric hindrance of the pendant group further destroyed the
enantioselectivity (80% yield, 88% ee; entry 5 versus
metallocarbenes.[7] As shown in Table 1, the in situ prepared
[8,9]
bisoxazoline [Cu(L1)(CH3CN)4]PF6
could catalyze the
[*] C. Deng, Dr. L.-J. Wang, J. Zhu, Prof. Dr. Y. Tang
The State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (China)
E-mail: tangy@mail.sioc.ac.cn
[**] We are grateful for the financial support from the Natural Sciences
Foundation of China (nos. 21121062, 20932008, and 21072207), the
Major State Basic Research Development Program (grant no.
2009CB825300), and the Chinese Academy of Sciences.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206376.
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entry 4). Noticeably, the enantioselectivity could be enhanced
by lowering the reaction temperature, which provided the
desired product in 99% yield with 95% ee (entry 6).
The scope of terminal alkenes with a variety of different
substituents was next examined under the optimized reaction
conditions. As shown in Table 2, high yields and high to
Table 2: Reaction of terminal alkenes with phenyliodonium ylide.[a]
Entry
R1, R2
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
p-BrC6H4, H (1a)
Ph, H (1b)
99 (3a)
85 (3b)
93 (3c)
99 (3d)
79 (3e)
99 (3 f)
99 (3g)
97 (3h)
96 (3i)
95 (3j)
21 (3k)
95
91(S)[d]
95
p-ClC6H4, H (1c)
p-CF3C6H4, H (1d)
p-ClCH2C6H4, H (1e)
p-MeC6H4, H (1 f)
o-MeC6H4, H (1g)
m-MeC6H4, H (1h)
p-MeOC6H4, H (1i)
p-PhC6H4, H (1j)
PhOCH2, H (1k)
96
94
92
92
93
87
92
80
9[e]
10
11
Scheme 2. Cyclopropanation of nonterminal alkenes. Reaction condi-
tions: 1 (0.40 mmol), 2 (400 mg, 1.20 mmol), CuI (14.9 mg,
[a] 1 (0.40 mmol), 2 (400 mg, 1.20 mmol), CuI (14.9 mg, 0.04 mmol), L4
(35.9 mg, 0.06 mmol), toluene (4.0 mL) and 3 ꢀ MS (300 mg), ꢀ408C,
c=0.1 molLꢀ1, reaction time: 50–131 h. [b] Yield of isolated product.
[c] Determined by HPLC analysis on a chiral stationary phase. [d] The
absolute configuration of 3b was determined as S by comparing the
optical rotation with the literature values.[4h,10] [e] L6 was employed with
5 mol% of catalyst; L6: iPr-bisoxazoline, R1 =R2 =p-tBuC6H4.
0.04 mmol), L4 (35.9 mg, 0.06 mmol), toluene (4.0 mL), and 3 ꢀ MS
(300 mg), at ꢀ408C, c=0.1 molLꢀ1, reaction time: 69–133 h. The yield
is of the isolated product. The ee values were determined by HPLC
analysis on a chiral stationary phase. [a] The absolute configuration of
5b was determined as S,S by X-ray crystallographic analysis.[11] [b] At
ꢀ208C. [c] Toluene (2.0 mL), c=0.2 molLꢀ1. MS=molecular sieves.
was analyzed by X-ray crystallography (Figure 1).[11] The
copper center adopts a distorted square-planar geometry,
with N1 and N2 of ligand L4 and a nitrile molecule (the sum of
the bond angles of N3-Cu-N1, N3-Cu-N2, and N1-Cu-N2 is
359.998). Both pendant phenyl groups swing towards the
copper center and shield both the upper and lower faces of the
excellent enantioselectivities (87–96% ee) were obtained
(entries 1–10) for both electron-withdrawing and -donating
groups at the para, ortho, and meta positions of the phenyl
ring of substrates 1a–1j. Aliphatic substrate 1k showed lower
reactivity and gave the cyclopropane 3k in 21% yield with
80% ee (entry 11).
Further studies showed that the current catalytic system
exhibited excellent enantiocontrol for nonterminal olefins
(Scheme 2). Various electron-poor and -rich indenes 4a–4 f
with different substitution patterns on the aromatic ring
reacted smoothly with phenyliodonium ylide malonate (2) to
give the corresponding cyclopropanes in good to high yields
with excellent enantioselectivities (70–99% yields, 97– >
99% ee). Six- and seven-membered cyclic alkenes 4g and
4h readily participated in this transformation, and almost
enantiopure products were obtained in high yields. Moreover,
trisubstituted alkene 4i gave rise to product 5i with 95% ee.
The acyclic cis-alkenes 4j and 4k were also suitable substrates
for the cyclopropanations, and led to 5j in 84% yield with
99% ee and 5k in 79% yield with 93% ee, respectively.
Cyclopentadiene (4l) and 1,3-cyclohexadiene (4m) also
worked well, affording the monocyclopropanation products
in moderate yield with up to 91% ee. In addition, aliphatic-
substituted olefin 4n afforded cyclopropane 5n in 75% yield
and 77% ee.
Figure 1. X-ray structure of [Cu(L4)(CH3CN)]+ (unless labeled, hydro-
gen atoms and PF6 are omitted for clarity).
To understand the asymmetric induction of the current
reaction, a single crystal of the [Cu(CH3CN)4]PF6/L4 complex
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bisoxazoline–copper(I) coordination plane (N1-Cu-N2-C5-
C1-C2). As the bond angle of N3-Cu-N1 is distinctively larger
than that of N3-Cu-N2 (148.38(10)8 versus 118.69(10)8) and
of the [Cu(CH3CN)4]PF6/L4 complex led to a proposed
stereochemical model. The readily accessible starting mate-
rial, cheap catalyst, high diastero- and enantioselectivities
make the present reaction potentially useful in organic
synthesis. Further application of this method in useful trans-
formations is underway.
ꢀ
ꢀ
the Cu N1 bond is shorter than that of Cu N2 (1.945(2) ꢁ
versus 2.012(2) ꢁ), a nonsymmetric chiral cage is formed.
According to previous studies by the research groups of
Doyle and Charette, malonate-derived metallocarbenes can
adopt three different conformations (out-out, in-out, and in-
in).[12] The in-out arrangement was regarded as the reactive Experimental Section
Typical procedure for the asymmetric cyclopropanation (3a as an
conformation because one of the ester groups prefers to adopt
an out-of plane conformation that could stabilize the partial
positive charge formed on the b carbon atom of the alkene.[9c]
On the basis of these results, and by combining the molecular
structure of [Cu(CH3CN)/L4]+ with the model developed by
Pfaltz and co-workers,[13] a stereocontrol model has been
developed to explain the stereochemistry of the reaction. As
shown in Figure 2, two competing transition-state structures
example): A mixture of [Cu(CH3CN)4]PF6 (14.9 mg, 0.04 mmol) and
the ligand (L4, 35.9 mg, 0.06 mmol) in toluene (4 mL) with activated
3 ꢁ MS (300 mg) was stirred at room temperature for 2 h under
nitrogen. Alkene 1a (53 mL, 0.4 mmol) was then added and the
resulting mixture cooled to ꢀ408C. After stirring the mixture for
10 min, the phenyliodonium ylide (400 mg, 1.20 mmol) was added in
one portion. After the reaction was complete (monitored by TLC),
the suspension was filtered through a glass funnel with a thin layer
(20 mm) of silica gel (100–200 mesh) and eluted with CH2Cl2
(ca. 150 mL). The filtrate was concentrated under reduced pressure
and the residue was purified by flash chromatography to afford 3a
(124.7 mg), as a colorless oil [Rf = 0.6 (1:6 (v/v), EtOAc/petroleum
ether] in 99% yield with 95% ee (Chiralcel AD-H, iPrOH/hexanes =
2:98, 0.70 mLminꢀ1, l = 254 nm: tS(major) = 12.8 min, tR(minor) =
20
14.4 min); ½aꢁD ¼ꢀ109.28 (c = 1.5, CHCl3); 1H NMR (300 MHz,
CHCl3): d = 7.40 (d, J = 8.4 Hz, 2H), 7.07 (d, J = 8.4 Hz, 2H), 3.79
(s, 3H), 3.41 (s, 3H), 3.17 (t, J = 8.55 Hz, 1H), 2.15 (dd, J = 8.1, 5.7 Hz,
1H), 1.75 ppm (dd, J = 9.3, 5.4 Hz, 1H).
Received: August 8, 2012
Published online: October 12, 2012
Keywords: asymmetric catalysis · carbenes · copper ·
.
cyclopropanation · homogeneous catalysis
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cyclopropanation of multisubstituted olefins.
A and B were proposed, in which A more favorably gives the
major S,S-configured cyclopropane than does B, because of
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the out-of plane ester and the phenyl group of the chiral
skeleton of the bisoxazoline, as well as the steric hindrance
between the olefin with the chiral ligand. Thus, the S or
S,S isomer[12] would be obtained as the major product, which
is completely consistent with the experimental results
observed.
In summary, we have developed a facile cagelike chiral
catalyst for the catalytic enantioselective cyclopropanation of
multisubstituted olefins with phenyliodonium ylide malonate.
Remarkably, this newly designed and cheap bisoxazoline–
copper(I) complex resulted in the cyclopropanation of a range
of substrates such as terminal, disubstituted, and trisubsti-
tuted olefins, giving the desired products in excellent yield (up
to 99%) with enantioselectivity (up to > 99% ee). This
protocol provides an efficient method for the synthesis of
chiral 1,1-cyclopropane diesters. The single-crystal structure
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