Allenic Esters from Cyclopropenones by Lewis Base Catalysis: Substrate Scope, the Asymmetric Variant from the Dynamic Kinetic Asymmetric Transformation, and Mechanistic Studies

Article abstract of DOI:10.1002/cctc.201500466

The Lewis base catalyzed reactions of cyclopropenones with a variety of nucleophiles (alcohols, phenols, or water) were systematically investigated. We demonstrated that this kind of reaction could be used to synthesize allenic esters in moderate to excellent yields. Furthermore, more synthetically interesting axially chiral allenes in moderate to good yields and ee values were obtained in the corresponding asymmetric reaction in the presence of multifunctional chiral phosphine catalyst. The reaction mechanism was disclosed by using NMR tracing experiments, MS, and DFT calculations. Notably, the asymmetric reaction was proved to be a dynamic kinetic asymmetric transformation based on the control experiments, and the detailed mechanism of this transformation revealed by theoretical investigations.

Full text of DOI:10.1002/cctc.201500466

DOI: 10.1002/cctc.201500466
Full Papers
Allenic Esters from Cyclopropenones by Lewis Base
Catalysis: Substrate Scope, the Asymmetric Variant from
the Dynamic Kinetic Asymmetric Transformation, and
Mechanistic Studies
Yin Wei, Wen-Tao Zhao, Yuan-Liang Yang, Zhen Zhang, and Min Shi*[a]
The Lewis base catalyzed reactions of cyclopropenones with
a variety of nucleophiles (alcohols, phenols, or water) were sys-
tematically investigated. We demonstrated that this kind of re-
action could be used to synthesize allenic esters in moderate
to excellent yields. Furthermore, more synthetically interesting
axially chiral allenes in moderate to good yields and ee values
were obtained in the corresponding asymmetric reaction in
the presence of multifunctional chiral phosphine catalyst. The
reaction mechanism was disclosed by using NMR tracing ex-
periments, MS, and DFT calculations. Notably, the asymmetric
reaction was proved to be a dynamic kinetic asymmetric trans-
formation based on the control experiments, and the detailed
mechanism of this transformation revealed by theoretical in-
vestigations.
Introduction
Allenes have drawn tremendous interests in the area of syn-
thetic organic chemistry due to their various reactivity patterns
as a result of their unique chemical properties. Indeed, the
past few decades have seen an exponential growth in allene
chemistry.[1] The synthetic potential of allenes in regio- and ste-
reoselective CÀC and CÀX bond-forming transformations and
unique axis-to-center chirality transfers, and the occurrence of
allenic structures in a variety of natural products and pharma-
cologically active compounds, have generated interest from or-
ganic and medicinal chemists. Thus, the synthesis of allenes
has recently become a hot research topic.[2] The classical reac-
tion types (addition, elimination, substitution, and rearrange-
ment) have been applied to obtain allenes, and, in particular,
transition metal-catalyzed reactions have been well developed.
Important allenic compounds are allenic esters, as efficient syn-
thons widely used in organic synthesis,[3] thus, much effort has
been made to acquire allenic esters.[4] However, reports on the
synthesis of allenic esters by Lewis base catalyzed reactions are
rare. The relevant studies are on base-promoted alkyne isomer-
ization[5] and Lewis base catalyzed intramolecular 1,4-addition
of conjugated enynes.[6] More recently, our group has reported
the Lewis base catalyzed reactions of cyclopropenones to al-
lenic esters.[7]
simple and efficient methods to obtain them.[8] The strategies
for the synthesis of axially chiral allenes have mainly focused
on the following three aspects: 1) Central-to-axial chirality
transfer is the prevailing method by metal catalysis in the pres-
ence of Cu,[9] Zn,[10] Pd,[11] Rh,[12] Zr,[13] Li,[14] Au,[15a] or Ag,[15b] and
by the ortho-Claisen rearrangement[16] or the Mitsunobu reac-
tion.[17] 2) Central-to-axial chirality transfer can also be achieved
by the kinetic resolution of racemic compounds[18] or enzymat-
ic desymmetrization of prochiral allenic diols.[19] 3) Direct asym-
metric synthesis can proceed by the introduction of stoichio-
metric chiral reagents,[20] transition metal catalysis with Pd[21a–i]
or Cu,[21j] or organic compound-directed catalysis,[5,22] in which
the dynamic kinetic asymmetric transformation (DYKAT) proto-
col has been applied.[21d] Of these strategies for the asymmetric
synthesis of allenes, the DYKAT strategy[23] attracted our atten-
tion because the DYKAT and related dynamic kinetic resolu-
tion[24] strategies have proven to be a powerful in asymmetric
synthesis as they can efficiently transform racemic starting ma-
terials into a single stereoisomeric product.[25,26]
Based on our aforementioned observations of Lewis base
catalyzed reactions of cyclopropenones towards racemic allenic
esters,[7] we tried to further develop their asymmetric variant
to produce axially chiral allenic acetates by using multifunc-
tional chiral phosphines.[27] As a part of our continued interest
in Lewis base mediated reactions, we systematically investigat-
ed reactions of cyclopropenone derivatives 1, which were pre-
pared from propargyl esters with a variety of nucleophiles in
the presence of commonly used Lewis bases (see Table S0 in
the Supporting Information) and explored their asymmetric
variant. For comparison, the reactions of a series of cyclopro-
penones with various nucleophiles in the presence of com-
monly used Lewis bases to generate acrylate and acrylic acid
derivatives were also investigated (for details, see Schemes S1
Axially chiral allenes have been widely applied in modern or-
ganic chemistry, which has motivated many chemists to seek
[a] Dr. Y. Wei, W.-T. Zhao, Dr. Y.-L. Yang, Dr. Z. Zhang, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P.R. China)
E-mail: mshi@mail.sioc.ac.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500466.
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and S2). Herein, we report a novel method for the synthesis of
allenic esters through Lewis base catalysis and its asymmetric
variant by a DYKAT process, with detailed mechanistic studies.
sole regioisomer (Scheme 2). The structure of 2o has been un-
ambiguously determined by using X-ray diffraction. The ORTEP
drawing and its CIF data are presented in the Supporting Infor-
mation.[28b] A plausible mechanism for the formation of 2o is
proposed in Scheme S3.
Results and Discussion
Reaction scope of Lewis base catalyzed reactions of cyclo-
propenones
We have examined the scope and limitations of these 1,4-dia-
zabicyclo-[2.2.2]-octane (DABCO)- and triphenylphosphine-cata-
lyzed reactions with various cyclopropenones and methanol,
the results of which are summarized in Scheme 1 (for further
Scheme 2. PPh3-catalyzed reaction of (3-oxo-2-(thiophen-2-yl)cycloprop-1-
enyl)methyl acetate 1o with MeOH.
Next, we continued to investigate the reactions of cyclopro-
penone derivatives 1 with a variety of substituted phenols and
alcohols. The results are presented in Table 1. The reaction of
substrate 1e with substituted phenols proceeded smoothly
with a catalytic amount of DABCO in THF at room tempera-
ture, affording the corresponding products 4a and 4b in 91
and 84% yields, respectively (Table 1, entries 1 and 2). With
R2 =2-BrC6H4, the reaction also worked well with a catalytic
amount of triphenylphosphine and the corresponding product
4c was produced in 92% yield (entry 3). A series of alcohols
such as aromatic, heteroaromatic, or aliphatic alcohols were
also tested and the corresponding products 4d–i obtained in
moderate to high yields (63–97%; entries 4–9). Notably, the re-
action of substrate 1w with benzyl alcohol had to be catalyzed
by highly nucleophilic tributylphosphine (20 mol%), otherwise
no reaction could take place, presumably due to the inert reac-
tivity of 1w necessitating a highly nucleophilic catalyst to
prompt the reaction (entry 9).
Scheme 1. Lewis base catalyzed reactions of 1 with MeOH. Ms=mesyl.
details, see the Supporting Information). With DABCO as cata-
lyst, a variety of cyclopropenone derivatives (1a–1m) with
either electron-donating or withdrawing groups as substitu-
ents on the 2-, 3-, or 4-position of the benzene ring underwent
the reactions smoothly, affording the corresponding products
2a–2m in moderate to high yields (54–96%) [Scheme 1,
Eq. (1); for further details, see the Supporting Information].
Compared with nitrogen-centered Lewis bases such as DABCO,
phosphorus-centered Lewis bases such as triphenylphosphine
are more nucleophilic, leading to higher catalytic activities for
this type of reaction. With triphenylphosphine as catalyst, a va-
riety of cyclopropenone derivatives (1p–1z) with either elec-
tron-donating or withdrawing groups as substituents on the 2-
, 3-, and/or 4-positions of the benzene ring underwent the re-
actions smoothly, affording the corresponding products 3a–k
in moderate to high yields (57–95%) [Scheme 1, Eq. (2); for fur-
ther details, see the Supporting Information]. The structure of
2 has been unambiguously determined by performing X-ray
diffraction of 2j. The ORTEP drawing and its CIF data are pre-
sented in the Supporting Information.[28a] With compound 1o
(R1 =2-thienyl group) as substrate, the reaction did not occur
under the optimal reaction conditions. Surprisingly, with tri-
phenylphosphine as catalyst, the reaction could take place,
albeit affording an abnormal product 2o in 56% yield as the
Investigations on the asymmetric variant
In view of our results on the Lewis base catalyzed reactions of
cyclopropenone derivatives with methanol, the next logical
step was to investigate the asymmetric reaction by using chiral
bases. Initial examination was performed by using cycloprope-
none 1q (R2 =4-ClC6H4, 1.0 equiv) and nucleophile methanol
(5.0 equiv) in the presence of a catalytic amount of various
chiral mono- or biphosphines and chiral multifunctional phos-
phines. The results are summarized in Table 2. Use of chiral
phosphine CP1 or CP4 as the catalyst (mono- or biphosphine)
resulted in low yields and ee values or even no desired prod-
uct. If the catalyst was CP2 or CP3 (with phosphine and hy-
droxy substituents), the reaction proceeded smoothly, afford-
ing the corresponding product 5a in 62 and 66% yields, re-
spectively, but with poor enantioselectivities. We also screened
phosphine- and amide-substituent catalysts CP6–CP9 (bi- and
multi-functional phosphines) and found that only CP9 fur-
nished the product, in 71% yield and 40% ee. Moreover, we
examined phosphine- and thiourea/urea-substituent catalysts
such as CP5 and CP10–CP14 (multifunctional phosphines) and
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tained in moderate to good
yields with good ee values (en-
tries 1–7). The highest ee ob-
tained was 83% (entry 6). 2,3-Al-
lenoate ((R)-5h) could be con-
verted to the corresponding
iodobutenolide (R)-6 by iodolac-
Table 1. Lewis base catalyzed reactions of cyclopropenone derivatives 1 with different nucleophiles (phenols
or alcohols).
Entry
1
R1/R2/R3
Catalyst
DABCO
NuH
x
y
t [h]
Yield[a] [%]
tonization
with
iodine
4-ClC6H4/H/H (1e)
20
5
0.5
4a (91)
(Scheme 3).[11f,29] The absolute
configuration of (R)-6 was deter-
mined by an X-ray diffraction
study (Figure 1). Based on this
result, we assigned the absolute
configuration R to product 5.[11a,f]
The CIF data of (R)-6 are present-
ed in the Supporting Informa-
tion.[28c]
2
4-ClC6H4/H/H (1e)
DABCO
20
2
1.0
4b (84)
3
C6H5/2-BrC6H4/H (1t)
PPh3
10
2
3.0
4c (92)
4
5
4-ClC6H4/H/H (1e)
3-BrC6H4/H/H (1h)
DABCO
DABCO
BnOH
20
20
5
5
1.0
4d (90)
4e (68)
12.0
6
7
3-BrC6H4/H/H (1h)
3-MeC6H4/H/H (1i)
DABCO
DABCO
20
20
2
2
5.0
4.0
4 f (97)
4g (87)
Mechanistic studies
8
9
C6H5/4-ClC6H4/H (1q)
C6H5/(CH2)5/H (1w)
PPh3
PBu3
10
20
5
5
1.0
5.0
4h (74)
4i (63)
A plausible mechanism for this
BnOH
reaction
is
suggested
in
Scheme 4. The transformation is
believed to proceed through the
[a] Isolated yield.
found that CP14 gave the best results, providing the desired
product in 80% yield and 57% ee.
Subsequently, we switched substrate 1q to 1r with a sterical-
ly more bulky naphthyl group and screened a series of amino
acid-derived phosphine catalysts CP14–CP18 in the same reac-
tion under the standard conditions. We identified that the cat-
alyst CP14 gave the best performance. The reaction should be
performed in toluene in the presence of 20 mol% CP14 at
room temperature for 2.0 h, providing the desired product 5b
in 70% yield with 71% ee (Table 2). Notably, use of the racemic
substrate could give the product in a yield of over 50%,
beyond the limit of the theoretical yield for a traditional kinetic
resolution process, indicating that this could be a DYKAT proc-
ess.[23c] To gain a better understanding, we performed several
control experiments and theoretical investigations, as dis-
cussed later in the mechanistic studies.
Scheme 3. Iodolactonization of 2,3-allenoate.
In the presence of CP14, we continued to examine solvent
effects and the reaction temperature influences for this reac-
tion, the results of which are given in Table 3. Toluene
emerged as the solvent of choice (Table 3, entries 1–9) and re-
ducing the reaction temperature to 08C and À108C did not
improve the enantioselectivities under otherwise identical con-
ditions (entries 10 and 11). If the catalyst loading was reduced
to 10 mol%, the yield and ee of 5b decreased remarkably
(entry 12).
Figure 1. ORTEP drawing of (R)-6.
conjugate addition of Lewis base to cyclopropenone, affording
zwitterionic intermediate I1, which undergoes ring-opening to
give a ketene intermediate I2. Subsequently, the ketene inter-
mediate I2 reacts with methanol to afford the ion pair I3,
which releases the catalyst to give the final product 2. 31P NMR
tracing experiments were first performed to investigate the
mechanism (for further details, see the Supporting Informa-
tion). With triphenylphosphine as the catalyst, the 31P NMR
spectroscopic data of triphenylphosphine showed a signal at
d=À4.62 ppm (Figure 2). On addition of substrate 1r to the
NMR tube, three new signals appeared at d= +18.93, +28.89,
After identifying the optimized reaction conditions, we ex-
amined the generality of the reaction by using cyclopropenone
derivatives 1 (R=H: 1r, R=4-Br: 1aa, R=4-Me: 1bb, R=3-Me:
1cc) and several nucleophiles such as isopropanol, benzyl alco-
hol, 4-bromobenzyl alcohol, and ethanol. The results are sum-
marized in Table 4. The corresponding products 5c–i were ob-
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assigned to the in situ-generat-
ed zwitterionic intermediate I1,
ketene intermediate I2, and ion
pair I3 proposed in Scheme 4.
Lengthening the measurement
Table 2. Reactions of cyclopropenones with MeOH catalyzed by chiral phosphines.
time,
signal a
at
d= +
18.90 ppm gradually disap-
peared and the intensity of the
signals at d= +28.89 and
+29.07 ppm did not change
significantly. These 31P NMR trac-
ing experiment results indicated
that several equilibria existed in
the catalytic cycle. Subsequent-
ly, one intermediate formed by
the addition of triphenylphos-
phine to 1q was also detected
by using LC–MS ([M+H]+ =
575.0; see the Supporting Infor-
mation), which could be the in-
termediate I1 or I2 suggested
in Scheme 4.
To understand the detailed
mechanism for the formation of
product 2 catalyzed by Lewis
base, we also theoretically in-
vestigated the reaction path-
ways catalyzed by DABCO and
triphenylphosphine (for details,
see the Supporting Informa-
tion).[31] All calculations were
performed at the mPW1K/6-
31+G(d,p)//mPW1K/6-31G(d)
level of theory on the Gaussi-
an 09 program.[32] The relative
energies of all intermediates
and transition states along the
reaction pathway catalyzed by
DABCO are shown in Scheme 5.
Initially, the addition of catalyst
DABCO to compound 1a led to
the formation of zwitterionic in-
termediate I4. The cyclopro-
pene ring-opening resulted in
intermediate I5 through transi-
tion state TS1 with an energy
barrier of 11.0 kcalmolÀ1. Then,
the OAc group left via transition
state TS2 with a small energy
barrier of 0.1 kcalmolÀ1 to give
the contact ion pair I6. The
methanol was bonded to give
complex I7. Passing through
[a] Isolated yield. [b] T=À208C. [c] Catalyst loading=20 mol%. [d] Toluene as reaction solvent.
and +29.07 ppm after 5 min (Figure 2a–c). Previously, the
31P NMR signal of triphenylphosphine oxide had been mea-
sured at d= +30.6 ppm.[30] Therefore, none of these new sig-
nals corresponded to triphenylphosphine oxide but could be
transition state TS3 with an energy barrier of 2.1 kcalmolÀ1 af-
forded the corresponding intermediate I8. Subsequently, the
catalyst DABCO was eliminated through transition state TS4,
leading to the complex of product 2. An alternative reaction
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Table 3. Optimization of the reaction conditions.
Entry
Solvent
T [8C]
t [h]
Yield[a] [%]
ee[b] [%]
1
2
3
4
5
6
7
8
toluene
p-xylene
1,4-dioxane
benzene
CH2Cl2
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
2.0
3.0
12.0
5.0
2.0
5.0
7.0
2.0
5.0
70
68
32
46
53
80
67
73
60
53
30
60
71
62
53
58
56
6
59
61
55
70
67
65
MeCN
methyl tert-butyl ether
fluorobenzene
trifluoromethylbenzene
toluene
toluene
toluene
Scheme 4. Plausible reaction mechanism.
9
10
11
12[c]
12.0
24.0
4.0
pathway catalyzed by DABCO was also investigated theoreti-
cally (for details, see the Supporting Information), however, it
involved a couple of steps with high energy barriers.
À10
RT
[a] Isolated yield. [b] Determined by using chiral HPLC. [c] Catalyst load-
ing=10 mol%.
In the same manner, the relative energies of all intermedi-
ates and transition states along the reaction pathway catalyzed
by triphenylphosphine were calculated, the results of which
are shown in Scheme 6. The catalyst triphenylphosphine was
first added to compound 1p, leading to the zwitterionic inter-
mediate I9. The cyclopropene ring-opening resulted in the in-
termediate I10 via transition state TS5 with a low energy bar-
rier of 0.1 kcalmolÀ1, indicating that the cyclopropene ring-
opening step was much more facile with the phosphine cata-
lyst. Notably, the OAc group leaving step with triphenylphos-
phine catalyst was more difficult than with DABCO as catalyst,
as this step passed through transition state TS6 with an
energy barrier of 11.8 kcalmolÀ1 to give the contact ion pair
I11. The methanol was bonded to give complex I12. Passing
through transition state TS7 with an energy barrier of 5.4 kcal
molÀ1 afforded the corresponding intermediate I13. Subse-
quently, the catalyst triphenylphosphine was eliminated
through transition state TS8, leading to the complex of prod-
uct 2. The calculation results provided theoretical evidences
for the reaction pathway suggested in Scheme 4. Notably,
both tertiary amine and phos-
Table 4. Multifunctional chiral phosphine CP14-catalyzed reactions of
various cyclopropenones 1 with different nucleophiles.
Entry
R
NuH
Yield[a] [%]
ee[b] [%]
1
2
H (1r)
H (1r)
iPrOH
BnOH
76 ((R)-5c)
74 ((R)-5d)
82
82
3
H (1r)
84 ((R)-5e)
75
4
5
6
7
4-Br (1aa)
4-Me (1bb)
4-Me (1bb)
3-Me (1cc)
iPrOH
EtOH
iPrOH
iPrOH
81 ((R)-5 f)
82 ((R)-5g)
77 ((R)-5h)
64 ((R)-5i)
77
82
83
81
[a] Isolated yield. [b] Determined by using chiral HPLC.
phine can catalyze this type of
reaction, however, their catalytic
properties have differences.
The DYKAT process
As described above, the asym-
metric reaction of racemic sub-
strate 1 with a nucleophile cata-
lyzed by
a chiral phosphine
could be a DYKAT process. To
gain a better understanding, we
performed the following control
experiments, the results of
which are summarized in
Table 5. The first examination
was performed by using racemic
Figure 2. 31P NMR tracing experiments.
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Scheme 5. DFT studies on the DABCO-catalyzed reaction pathway.
Scheme 6. DFT studies on the PPh3-catalyzed reaction pathway.
cyclopropenone 1r (1.0 equiv) as the starting material in the
presence of 20 mol% CP14 at room temperature. After 1 h,
the reaction mixture was purified by using silica gel column
chromatography to afford the desired product (R)-5b in 67%
yield and 70% ee and the recovered starting material (R)-1r in
18% yield and 27% ee (entry 1). The same procedure per-
formed by using (R)-1r and (S)-1r as starting material, respec-
tively, gave the desired product (R)-5b in 67% yield and 85%
ee and the recovered starting material (R)-1r in 24% yield and
42% ee (entry 2), and (R)-5b in 73% yield and 69% ee and the
recovered starting material (R)-1r in 21% yield and 23% ee
(entry 3). The experimental results show that the stereoselec-
tivity of final product does not depend on the configuration of
the chiral center in the starting material, and provides evi-
dence that the asymmetric reaction of racemic substrate 1
with nucleophile catalyzed by CP14 is indeed a DYKAT pro-
cess.
Based on the control experiments and the aforementioned
theoretical studies, we suggest that the reaction proceeds by
a DYKAT process, as shown in Scheme 7. The key intermediate
shown originates from racemic starting material that loses its
chiral center, thus, the stereoselectivity solely depends on the
subsequent step in the presence of chiral phosphine catalyst.
Therefore, this is a very special DYKAT process in which the
stereochemical properties of the starting material are not
transferred to the product.
To check the detailed mechanism for the asymmetric reac-
tion by a DYKAT process catalyzed by chiral phosphine catalyst,
we also theoretically investigated the reaction pathway cata-
lyzed by CP14. All calculations were performed at the mPW1K/
6-31+G(d,p)//mPW1K/6-31G(d) level of theory with the Gaussi-
an 09 program.[32] The relative energies of all intermediates and
transition states along the reaction pathway catalyzed by CP14
are shown in Scheme 8. The absolute configurations of the dia-
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firstly adds to the racemic com-
pound 1p, leading to the zwit-
terionic intermediate diastereo-
mers (R)-I14 and (S)-I14. These
pass through their correspond-
ing transition states (R)-TS9 and
(S)-TS9 to the ring-opening in-
termediates (R)-I15 and (S)-I15
with energy barriers of 0.6 and
1.1 kcalmolÀ1, respectively. Then,
the OAc group leaving step
passes through transition states
(R)-TS10 and (S)-TS10 with
energy barriers of 10.1 and
8.9 kcalmolÀ1, respectively, to
give the contact ion pair (E)-I16
Table 5. Control experiments for the investigation of the DYKAT process.
Entry
Starting
material
(R)-5b
(R)-1r
Yield[a] [%]
ee[b] [%]
Yield[a] [%]
ee[b] [%]
1
2
3
rac-1r
(R)-1r
(S)-1r
67
67
73
70
85
69
18
24
21
27
42
23
[a] Isolated yield. [b] Determined by using chiral HPLC.
(E denotes the configuration of the C=C bond in I16). In these
two steps, the intermediates and transition states with S confi-
guration are always a few kcalmolÀ1 lower than those with
R configuration and the barrier (8.9 kcalmolÀ1) involving the
S configuration is smaller than that involving the R configura-
tion (10.1 kcalmolÀ1) in the rate-determining step. These calcu-
lation results indicate that the starting material 1p with S con-
figuration reacts faster than that with R configuration, which
accounts for the R configuration of the experimentally recov-
ered starting material. The OAc group leaving step leads to an
intermediate (E)-I16 in which the chiral center is lost, thus, the
stereoselectivity of final product does not depend on the con-
figuration of the chiral center in the starting material. This
result agrees with experimental findings. The methanol is
bonded to the intermediate (E)-I16 to give complex I17. Pass-
ing through transition state TS11 with an energy barrier of
12.5 kcalmolÀ1 affords the corresponding intermediate I18.
Subsequently, the chiral phosphine catalyst is eliminated
Scheme 7. Proposed DYKAT process.
stereoisomeric transition states and intermediates were as-
signed according to the chiral center in 1p, and those of the
diastereoisomeric product complexes according to the allene
moiety.
The calculated reaction pathway catalyzed by chiral phos-
phine catalyst CP14 is similar to that catalyzed by triphenyl-
phosphine (see Scheme 8). The chiral phosphine catalyst CP14
Scheme 8. DFT studies on the chiral phosphine-catalyzed reaction pathway.
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through transition state TS12b with an energy barrier of
12.6 kcalmolÀ1, leading to the complex of product (R)-3a. The
intermediate I18 can go through a rotation transition state
TS12a with an energy barrier of 17.3 kcalmolÀ1 to reach inter-
mediate I19. Subsequently, the chiral phosphine catalyst is
eliminated through transition state TS13 with an energy barri-
er of 18.5 kcalmolÀ1, leading to the complex of product (S)-3a.
The energy of TS12b is lower than that of TS13 by 5.9 kcal
molÀ1 owing to the hydrogen bonding interaction of the NÀH
moiety of the chiral phosphine, ester moiety, and acetic acid in
TS12b (for their structures, see Figure 3). Although there are
hydrogen bonds between the NÀH moiety of the chiral phos-
phine and ester moiety in TS13, the steric repulsion between
the phenyl moiety and naphthyl moiety in TS13 gives it
a higher energy. Based on the calculation results, the product
with R configuration should be acquired, which is in line with
experimental findings.
racemic cyclopropenones catalyzed by multifunctional chiral
phosphine, affording the axially chiral allenic esters in moder-
ate to good yields and ee values. A plausible reaction mecha-
nism was proposed, based on our investigations by NMR trac-
ing experiments, MS, and DFT calculations. The control experi-
ments provided evidence that the asymmetric reaction of race-
mic cyclopropenones with nucleophile in the presence of
chiral phosphine catalyst was indeed a DYKAT process. The
theoretical investigations revealed the detailed mechanism of
the DYKAT process for this reaction, showing it to be a special
kind of DYKAT.
Experimental Section
General
1H NMR spectra were recorded on a Bruker AM-400 spectrometer
with solutions in CDCl3 and TMS as internal standard; J values are
shown in [Hz]. Mass spectra were recorded on a HP-5989 instru-
ment (Agilent Technologies). All of the compounds reported in this
paper gave satisfactory HRMS analytic data. Melting points were
determined on a digital melting point apparatus and temperatures
were uncorrected. Optical rotations were determined at 589 nm
(Na d line) by using a PerkinElmer-341 MC digital polarimeter; [a]d
values are given in [10 degÀ1 cm2 gÀ1]. IR spectra were recorded on
a PerkinElmer PE-983 spectrometer; absorptions are given in
[cmÀ1]. Chiral HPLC was performed on a Shimadzu SPD-10 A vp
series with chiral columns (Chiralpak AD-H, IC-H columns 4.6”
250 mm, Daicel Chemical Ind., Ltd.). THF, toluene, and Et2O were
distilled from Na in an Ar atmosphere. CH3CN, 1,2-DCE, and CH2Cl2
were distilled from CaH2 in an Ar atmosphere. Commercially ob-
tained reagents were used without further purification. All reac-
tions were monitored by using TLC with Huanghai GF254 silica gel
coated plates. Flash column chromatography was performed by
using 300–400 mesh silica gel at increased pressure.
Conclusions
We have investigated novel and interesting reactions of cyclo-
propenones with alcoholic nucleophiles catalyzed by Lewis
bases, affording the corresponding allenic esters in moderate
to high yields under mild conditions. We also found an inter-
esting dynamic kinetic asymmetric transformation (DYKAT) of
Preparation of 2: General procedure: To
a mixture of 1a
(0.20 mmol, 46 mg), MeOH (10.0 mmol), DABCO (0.04 mmol,
5.0 mg), and 4 Š molecular sieves (50 mg) was added THF (2.0 mL)
at RT (258C) under Ar. The reaction solution was monitored by
using TLC. After the reaction was complete, the solution was con-
centrated under reduced pressure and the residue purified by
using silica gel column chromatography [EtOAc/PE, 1:16] to give
the target product 2a as a yellow oil (33 mg, 95% yield).[33] 1H NMR
(400 MHz, CDCl3, TMS): d=7.50 (2H, d, J=7.2 Hz), 7.37–7.33 (2H,
m), 7.29 (1H, d, J=7.2 Hz), 5.42 (2H, s), 3.83 ppm (3H, s); 13C NMR
(CDCl3, 100 MHz): d=215.6, 166.4, 131.9, 128.4, 128.3, 127.7, 102.8,
80.1, 52.4 ppm.
Preparation of 3: General procedure: To
a mixture of 1p
(0.20 mmol, 56 mg), MeOH (10.0 mmol), PPh3 (0.02 mmol, 5.2 mg),
and 4 Š molecular sieve (50 mg) was added THF (2.0 mL) at RT
(258C) under Ar. The reaction solution was monitored by using
TLC. After the reaction was complete, the solution was concentrat-
ed under reduced pressure and the residue purified by using silica
gel column chromatography [EtOAc/PE, 1:16] to give the target
1
product 3a as a yellow oil (46 mg, 92% yield). H NMR (400 MHz,
CDCl3, TMS): d=7.57 (2H, d, J=7.2 Hz), 7.39–7.24 (8H, m), 6.82
(1H, s), 3.83 ppm (3H, s); 13C NMR (CDCl3, 100 MHz): d=214.6,
166.0, 133.8, 133.6, 132.0, 131.5, 129.0, 128.4, 128.3, 128.1, 128.0,
127.5, 106.4, 99.5, 52.5 ppm; IR (CH2Cl2): n˜ =2961, 2926, 1720, 1598,
1493, 1466, 1434, 1260, 1216, 1092, 1018, 998, 798 cmÀ1; MS: m/z
Figure 3. Optimized structures of TS12b (top) and TS13 (bottom).
ChemCatChem 2015, 7, 3340 – 3349
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Full Papers
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(%): 250 (51), 235 (13), 207 (14), 191 (100), 179 (10), 165 (13), 105
(23); HRMS (EI): m/z: calcd for C17H14O2: 250.0994, found: 250.0995.
Preparation of 5: General procedure: To
a mixture of 1r
(0.10 mmol, 33 mg), MeOH (0.20 mmol), CP14 (0.02 mmol,
11.9 mg), and 4 Š molecular sieve (50 mg) was added toluene
(1.0 mL) at RT (258C) under Ar. After the reaction was complete,
the solution was concentrated under reduced pressure and the res-
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0.4 mLminÀ1, l=230 nm, tmajor =37.78 min, tminor =40.49 min). m.p.
101–1038C; [a]d = +42.2 (c=0.7, CH2Cl2).
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Acknowledgements
We are grateful for financial support from the National Basic Re-
search Program of China ((973)-2015CB856603), Shanghai Munic-
ipal Committee of Science and Technology (11JC1402600), and
the National Natural Science Foundation of China (20472096,
21372241, 21361140350, 20672127, 21421091, 21372250,
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Keywords: allenes
·
asymmetric catalysis
·
chirality
·
cyclopropenone · Lewis bases
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Received: April 28, 2015
Published online on September 3, 2015
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www.chemcatchem.org
3349
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim