A room-temperature catalytic asymmetric synthesis of allenes with ECNU-Phos

Article abstract of DOI:10.1021/ja406135t

Three-carbon axial chirality has been asymmetrically established from racemic one-carbon central chirality efficiently at room temperature: we report here the discovery of the first catalytic asymmetric carbonylation of readily available racemic propargylic carbonates to access optically active 2,3-allenoates with fairly high ee. The combination of [(π-allyl)PdCl] ₂ with [(R)-ECNU-Phos], a new chiral bisphosphine ligand based on a biphenyl skeleton, demonstrates high enantioselectivity. Both enantiomers of allenoates can be obtained at room temperature by applying either (R)- or (S)-ECNU-Phos.

Full text of DOI:10.1021/ja406135t

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Communication
A Room Temperature Catalytic Asymmetric
Synthesis of Allenes with ECNU-Phos
Yuli Wang, Wanli Zhang, and Shengming Ma
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja406135t • Publication Date (Web): 20 Jul 2013
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A Room Temperature Catalytic Asymmetric Synthesis of Al-
lenes with ECNU-Phos
Yuli Wang,† Wanli Zhang† and Shengming Ma*,†, ‡
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†Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal Univer-
sity, 3663 North Zhongshan Road, Shanghai 200062, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Lu, Shanghai 200032, China
Supporting Information Placeholder
pargyl/allenyl metallic intermediates favoring one for a high enan-
tioselectivity (cf. Scheme 4).
ABSTRACT: Three-carbon axial chirality has been
asymmetrically established from racemic one-carbon central
chirality efficiently at room temperature: we report here the
discovery of the first catalytic asymmetric carbonylation of
readily available racemic propargylic carbonates to access
optically active 2,3-allenoates in fairly high ee. The combination
of [(π-allyl)PdCl]2 with a new chiral bisphosphine ligand based on
biphenyl skeleton ((R)-ECNU-Phos) demonstrates high
enantioselectivity. Both enantiomers of allenoates can be obtained
at room temperature by applying either (R)- or (S)-ECNU-Phos.
Scheme 1 Central-to-Axial Chirality: optically active ap-
proach vs racemic approach
As a first try for this strategy (Scheme 1), we started to explore
the carbonylation of readily available racemic propargylic deriva-
tives affording 2,3-allenoates efficiently,14 provided that a suitable
chiral ligand for such a transformation may be identified (Scheme
2). After tedious work, such an axial chirality has been established
from central chirality efficiently: we here report the discovery of
the first catalytic asymmetric carbonylation of readily available
racemic propargylic carbonates bearing a central chirality to
access optically active 2,3-allenoates.15 The key point is the newly
identified chiral bisphosphine ligand based on biphenyl skeleton,
i.e., (R)- or (S)-ECNU-Phos, which is working at energy-effective
room temperature preventing possible racemization16 together
with [(π-allyl)PdCl]2 for high enantioselectivity and efficiency.
Both enantiomers of allenoates can be obtained at room tempera-
ture and 1 atm of CO by applying either (R)- or (S)-ECNU-Phos.
Asymmetric synthesis has always been a very popular and hot
topic in science of synthesis due to the fact that there are so many
optically active naturally occurring compounds with biological
importance.1 Historically, so much attention has been focused on
establishing one-carbon central chirality with many well-
established protocols, showing even industrial applications.2 Axial
chirality is also a very important part of asymmetric synthesis: the
construction of axial chirality in biaryl compounds, such as chiral
BINAP, can be easily established through optical resolution from
the racemic mixture3 or direct introduction of phosphinyl groups
into an optically active binaphthyl framework via nickel-assisted
coupling reaction;4 however, the establishment of axial chirality
of allenes, which spreads over three carbon atoms, is still a con-
undrum.5,6
At the meantime, allenes have become more and more impor-
tant due to the fact that many naturally occurring products with
bio-potentials contain an allene unit;7 they also serve as very im-
portant building blocks for organic synthesis;8-10 (S)-2,4-bis(2-
(bis(3,5-bis-(trifluoromethyl)phenyl)phosphino)phenyl)-5,5-di-
methylhexa-2,3-diene had even been demonstrated as a chiral
ligand in Rh-catalyzed enantioselective addition of arylboronic
acids to α-keto esters.11 Therefore, efficient approaches to chiral
allenes are highly desirable. The most common and efficient ap-
proaches are the chirality transfer from optically active propargyl-
ic derivatives with a proper leaving group.12 However, in all these
known cases, at least a stoichiometric amount of optically active
starting compounds is required and racemization is in most cases
a serious problem. We envisioned a catalytic system which may
lead to highly optically active allenes from the readily available
racemic propargylic derivatives.13 The challenge would be the
interconversion between the pair of involved diastereomeric pro-
Scheme 2 Design of a Catalytic Approach to Synthesize
Chiral 2,3-Allenoates
Based on our previous results with optically active propargylic
mesylates using Pd(dba)2 and (S)-Segphos as the catalyst with 1.1
equiv of (NH4)2HPO4 as the base,14 our initial experiments began
with the carbonylation of racemic benzyl non-4-yn-3-yl carbonate
(1a) under the catalysis of Pd(dba)2 and (R)-Segphos. To our de-
light, when [(π-allyl)PdCl]2 was used, this reaction could occur at
o
55 C to afford (Ra)-2a with 70% yield and 48% ee (Table 1, En-
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try 1) (for screening of different Pd catalysts, see Table S1 in
duction of a 5-OMe group ((R)-L8, abbreviated as ECNU-Phos)
Supplementary Materials). Based on this result, we identified that
bisphosphine ligands are the best skeletons among so many well-
established chiral ligands (for some typical results with other li-
gands, see Table S2 in Supplementary Materials).3 Then, some of
the commercially available chiral diphosphine ligands were ex-
amined.17 (R)-BINAP gave a better result (66% yield and 65% ee),
but the ligand based on the bipyridyl skeleton, i.e., (R)-P-Phos,
only led to 21% ee (Table 1, Entries 2 and 3); when (R)-C3-
Tunaphos was used, (Ra)-2a could only be prepared with 44%
yield and 63% ee (Table 1, Entry 4); interestingly, higher enanti-
oselectivity was observed with the rather simple (R)-MeO-
BIPHEP (Table 1, Entry 5).
gave a much better result: 2,3-allenoate could be obtained with
o
51% yield/84% ee within 2 hours at 55 C and 69% yield/83% ee
after 7 hours of stirring at 45 oC (Table 2, Entries 8 and 9).19 It is
amazing to notice the difference between methyl and methoxy
group (compare Entry 9 with Entry 3).
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Table 2 Tuning of the Aryl Group in BIPHEP-Type Li-
gand.
Table 1 Effect of Ligand Skeletons with the Basic Setting
of PPh2.
Entry Ar ((R)-L)
t
Yield
of
1a
2a
(h)
(%)
(%)a
b
1
2
2-furyl (L1)c
4-MeC6H4 (L2)c
3,5-Me2C6H3 (L3)c
3,5-(t-Bu)2C6H3 (L4)c,d
3,5-(t-Bu)2-4-MeOC6H2 (L5)c
4-MeOC6H4 (L6)e
3-MeOC6H4 (L7)e
3,5-(MeO)2C6H3 (L8)f
5
5
0
97
2
75 (69)
58 (56)
3
4
5
6
7
8
5
5
5
9
2
2
5
47 (-60) 33
Entry
Ligand
Time
(h)
Yield (%) of 1a (%)b
(Ra)-2a a
14 (66)
48 (63)
57 (79)
51 (84)
60
3
1
2
3
4
5
(R)-Segphos
9
5
5
5
4
70 (48)
66 (65)
49 (21)
44 (63)
61 (71)
3
14
23
(R)-BINAP
16
28
43
18
(R)-P-Phos
9
3,5-(MeO)2C6H3 (L8)g
7
69 (83)
4
(R)-C3-TunaPhos
(R)-MeO-BIPHEP
a
b
Isolated yield, the ee value of 2a was shown in parenthesis. Determined by
1H NMR using 1,3,5-trimethylbenzene as the internal standard. The chiral
ligands are bought from Stream Chemicals. (S)-L4 was used since only S
isomer is commercially available. Prepared according to ref. 19. Newly
prepared for the first time according to ref. 19. This reaction was carried out
under 45 oC.
c
d
e
f
g
Moreover, base and solvent effects were also examined: LiF and
toluene have been identified to be the best (for these results, see
Tables S3 and S4 in Supplementary Materials). Thus, the scope of
such an efficient strategy was explored by conducing the reaction
of racemic propargylic carbonates, [(-allyl)PdCl]2 (1~2 mol%),
(R)-ECNU-Phos (4~8 mol%), and LiF (1.1 equiv) with a CO
b
a Isolated yield, the ee value of 2a was shown in parenthesis. Determined by
1H NMR using 1,3,5-trimethylbenzene as the internal standard.
With the results of (R)-MeO-BIPHEP in hand, we started to
identify the best phenyl-substituent of the coordinating phospho-
rus center with the purpose of tuning its electronic and steric na-
ture for a practical enantioselectivity:18 no reaction occurred with
the 2-furyl ligand ((R)-L1) (Table 2, Entry 1); When the 4-
position of phenyl was substituted by methyl ((R)-L2), the yield
was improved but ee dropped slightly (Table 2, Entry 2); 3,5-
dimethyl phenyl ligand ((R)-L3) made both yield and ee drop
sharply (Table 2, Entry 3). All these facts indicated that tuning of
the electronic nature may not work very well, which made us turn
our attention to the steric effect: increasing the steric hindrance of
3,5-positions by replacing Me with t-Bu ((S)-L4) led to a higher
ee (Table 2, Entry 4); further introducing a 4-OMe to the aryl
group of (S)-L4 makes (R)-L5, which improved the enantioselec-
tivity further, albeit slightly (Table 2, Entry 5). Interestingly, re-
moving the 3,5-di-t-butyl groups from (R)-L5, i.e., (R)-L6, pro-
vided very comparable results: 48% yield and 63% ee of (Ra)-2a
(Table 2, Entry 6); at the point of nowhere, it was observed that
when relocating the methoxy group from 4-position to 3-position
of the phenyl ring ((R)-L7), the rate of the reaction was increased
together with a remarkable enantioselectivity: 57% of 2,3-
allenoate with 79% ee (Table 2, Entry 7). Excitingly, extra intro-
o
balloon in toluene at 25 C and the results are shown in Table 3.
For substrates with both R1 and R2 are alkyl groups, 2 mol% [(π-
allyl)PdCl]2 and 8 mol% of (R)-ECNU-Phos were required to
afford 2,3-allenoates (Ra)-2a-(Ra)-2e with 60-64% yield and 88-
92% ee after 24 h; interestingly, with R1 being an aryl group, the
reaction of methyl 1-phenyl-2-heptynyl carbonate (1f) could also
proceed to afford the corresponding product (Ra)-2f at room tem-
perature with an even higher enantioselectivity (93% ee) using
only 1 mol% [(π-allyl)PdCl]2 and 4 mol% of (R)-ECNU-Phos. As
expected, (Sa)-2f could also be synthesized with the similar results
by applying the enantiomer (S)-ECNU-Phos (78% yield and 94%
ee) (eq. 1); in fact, this is quite general with R1 being differently
alkyl substituted aryl group: now R2 may be alkyl ((Ra)-2g-(Ra)-
2i), phenethyl ((Ra)-2j) and cycloalkyl groups ((Ra)-2k and (Ra)-
2l); furthermore, when either electron-donating group (OMe) or
electron-withdrawing group such as Cl and Br was introduced to
the phenyl ring of R1, 1.5 mol% [(π-allyl)PdCl]2 and 6 mol% (R)-
ECNU-Phos were required, affording very decent enatioselectivi-
ties; finally, it is interesting to note that an allyl group (1r) may
also be accommodated. These versatile substituents such as OMe,
Cl, Br, allyl will surely provide opportunities for further synthetic
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elaboration. It should be noted that in some cases some of the
starting carbonate was recovered: in the case of (Ra)-2b, the start-
ing carbonate was recovered in 78% ee, indicating some level of
kinetic resolution.
Scheme 3 Application of 2,3-Allenoate (Ra)-2f.
Table 3 Substrate Scope of Pd-Catalyzed Asymmetric
Carbonylation of Racemic Propargylic Carbonate.
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A working model to predict the absolute configuration of the
allene moiety for the highly enantioselective formation of (Ra)-2
from racemic propargylic carbonates (±)-1 is shown in Scheme 4:
after oxidative addition of (R)-ECNU-Phos coordinated palla-
dium catalyst with the starting material 1a, both (R)-and (S)-
allenyl palladium species with ECNU-Phos would be generated.
There should be an isomerization between these two diastereo-
mers through the -- rearrangement via the intermediacy of 6.23
Based on the structural analysis, (S,R)-5 is disfavored since there
is obvious steric interactions between the R1 group and the biaryl
skeleton and an Ar group of (R)-ECNU-Phos, which does not
exist in intermediate (R,R)-5. Thus, allenoate (R)-2 is formed as
the product highly enantioselectively via the intermediacy of
(R,R)-5.
Scheme 4 Prediction of the absolute configuration of the
products.
a
The reaction was carried out in 37 hours.
In summary, we have realized the efficient formation of such an
axial chirality spreading over three carbon atoms from the readily
available racemic propargylic carbonates bearing
a central
chirality with high enantioselectivity for the first time. This
reaction proceeds under 1 atm of CO at room temprature with (R)-
or (S)-ECNU-Phos, in which the 3,5-dimethoxy group may
provide the required steric and electronic environment as well as
the mild reaction temperature, which is critical for the temperature
sensitive nature of optically active allenes.16 This study will surely
stimulate the interest of forming the three-carbon axial chirality
from all types of readily available different racemic propargylic
derivatives providing the most convenient approach for the
synthesis of chiral allenes with different functionalities for the
further development of allene chemistry. Futher studies in this
area are being pursued in this laboratry
As discussed at the beginning, these 2,3-allenoates are quite
useful in asymmetric synthesis (Scheme 3): when (Ra)-2f was
treated with 2 equiv of I2 at -15 oC, the corresponding lactone (R)-
3 was obtained in 84% yield with 92% ee. The absolute configu-
ration of (R)-3 was established by its X-ray diffraction study.20
Based on this results and our previous studies,14 we assigned the
configuration of the 2,3-allenoate from (R)-ECNU-Phos as R. So
far, there is no easy way for the synthesis of optically active pri-
mary 2,4-disubstituted 2,3-allenols,21 which are also a type of
versatile allenes for compounds with central or axial chirality.22
Here, treating (Ra)-2f with DIBAL-H afforded the allenol (Ra)-4
in 64% yield and 93% ee.
ASSOCIATED CONTENT
Supporting Information
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propargylic amines, see: Lo, Y. K.-Y.; Wong, M.-K.; Che, C.-M.
Experimental procedures and analytical data and NMR spectra of
the products. This material is available free of charge via the
Internet at http://pubs.acs.org.
Org. Lett. 2008, 10, 517; (i) for ZnX2-mediated synthesis of axially
chiral allenes via optically propargylic amine, see: Ye, J.; Li, S.;
Chen, B.; Fan, W.; Kuang, J.; Liu, J.; Liu, Y.; Miao, B.; Wan, B.;
Wang, Y.; Xie, X.; Yu, Q.; Yuan, W.; Ma, S. Org. Lett. 2012, 14,
1346.
AUTHOR INFORMATION
Corresponding Author
(13) For the coversion of methyl 5-(cyclohex-1-en-1-yl)-4-
[(diethoxyphosphoryl)oxy]pent-2-ynoate to methyl (R)-(-)-5-
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masm@sioc.ac.cn
Notes
The authors declare no competing financial interests.
(cyclohex-1-en-1-yl)penta-2,3-dienoate with
a
stoichiometric
amount of (R)-pantolactone as proton source, see: (a) Mikami, K.;
Yoshida, A. Angew Chem., Int. Ed. 1997, 36, 858; (b) Mikami, K.;
Yoshida, Tetrahedron 2001, 57, 889; (c) for Cr-mediated reduction
of Pd/Sm system using chiral proton source, see: Verniere, C.; Cazes,
B. Tetrahedron Lett. 1981, 22, 103.
ACKNOWLEDGMENT
(14) Wang, Y.; Ma, S. Adv. Synth. Catal. 2013, 355, 741.
Financial support from National Natural Science
Foundation of China (21232006) and State Basic Research
Program of China (2009CB825300) is greatly appreciated.
We also thank Mr. Pengbin Li in this group for reproducing
the results of (Ra)-2c, (Ra)-2i and (Ra)-2o presented in
Table 3.
(15) For reports on the asymmetric synthesis of 2,3-allenoates, see: (a)
Wittig reaction of ketenes with stiochiometric amount of optically
active ylides affording chiral 2,3-allenoates, see: Li, C.-Y.; Wang,
X.-B.; Sun, X.-L.; Tang, Y.; Zheng, J.-C.; Xu, Z.-H.; Zhou, Y.-G.;
Dai, L.-X. J. Am. Chem. Soc. 2007, 129, 1494; (b) for kinetic
resolution of racemic 2,3-allenoates affording chiral 2,3-allenoates
and 3-methylenepyrrolidine derivatives, see: Yu, J.; Chen, W.-J.;
Gong, L.-Z. Org. Lett. 2010, 12, 4050; (c) for isomerizations of 3-
alkynoates affording chiral 2,3-allenoates, see: Liu, H.; Leow, D.;
Huang, K.-W.; Tan, C.-H. J. Am. Chem. Soc. 2009, 131, 7212; (d)
For N-heterocyclic carbene-catalyzed internal redox reaction from
alkynals to allenoates, see: Zhao, Y.-M.; Tam, Y.; Wang, Y.-J.; Li,
Z.; Sun, J. Org. Lett. 2012, 14, 1398; (e) For the synthesis of chiral
tetrasubstituted allenoates via deprotonation and 1,2-addition or
substitution by using asymmetric phase-transfer catalysts, see:
Hashimoto, T.; Sakata, K.; Tamakuni, F.; Dutton, M. J.; Maruoka, K.
Nat. Chem. 2013, 5, 240; (f) For the preparation of chiral 2,3-
allenoate via asymmetric β-hybride elimination of 3-OTf-2(E)-
enoates, see: Crouch, I. T.; Neff, R. K.; Frantz, D. E. J. Am. Soc.
Chem. 2013, 135, 4970.
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alkoxycarbonylation of allenyl palladium intermediates from
optically active propargylic mesylates affording chiral 2,3-allenoates,
see: Marshall, J. A.; Wolf, M. A. J. Org. Chem. 1996, 61, 3238; (f)
for palladium-catalyzed cross-coupling reactions of optically active
propargylic esters, i.e., optically active propargylic carbonates, see:
Dixneuf, P. H.; Guyot, T.; Ness, M. D.; Roberts, S. M. Chem.
Commun. 1997, 2083; (g) for copper-catalyzed substitution of
optically active propargylic carbonates with diboron or substituted
boronates affording chiral allenes, see: Ito, H.; Sasaki, Y.; Sawamura,
M. J. Am. Chem. Soc. 2008, 130, 15774; (h) for gold- or silver-
catalyzed synthesis of chiral allenes from optically active
(22) Axial chirality of chiral 2,3-allenols has been transferred smoothly to
axial and central chirality in our recent work: Ye, J.; Fan, W.; Ma, S.
Chem. Eur. J. 2013, 19, 716.
(23) For the intercoversion of such optically active allenyl Pd(II) species,
see: Ogoshi, S.; Nishida, T.; Shinagawa, T.; Kurosawa, H. J. Am.
Chem. Soc. 2001, 123, 7164.
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