Highly enantioselective Pd-catalyzed allylic alkylations of acyclic ketones

Article abstract of DOI:10.1002/anie.200502020

(Chemical Equation Presented) Pocket the difference: Highly enantioselective allylic alkylation of simple acyclic ketones is carried out by using a Pd catalyst with a "chiral pocket" ligand 1 (see scheme). The addition of a Lewis acid such as AgBr and the

Full text of DOI:10.1002/anie.200502020

Communications
Asymmetric Synthesis
chiral P,N ligands and employed them successfully in Pd-
catalyzed AAAs.[5,6] High regio- and enantioselectivities were
provided by using monosubstituted allyl derivatives as sub-
strates and by using cyclic ketones as nucleophiles. For Pd-
catalyzed AAAs with acyclic ketones to be enantioselective,
we have modified the ligands. Herein, we report our
preliminary results in the synthesis of a novel chiral ligand
and its application to a Pd-catalyzed asymmetric allylic
reaction that uses acyclic ketones as nucleophiles.
DOI: 10.1002/anie.200502020
Highly Enantioselective Pd-Catalyzed Allylic
Alkylations of Acyclic Ketones**
Xiao-Xia Yan, Chun-Gen Liang, Yan Zhang, Wei Hong,
Bo-Xun Cao, Li-Xin Dai, and Xue-Long Hou*
Initially, we examined the reaction of 1-phenyl-propan-1-
one (1a) with allyl acetate (2) in the presence of [{Pd-
(C3H5)Cl}2] and a variety of ligands,[6a] but we obtained no
encouraging results (Scheme 1). All reactions gave almost no
Since the first example of Pd-catalyzed asymmetric allylic
alkylation (AAA) was reported by Trost and Strege,[1] great
achievements in this area of chemistry have been made.
Today, AAA is not only a well-studied transition-metal-
catalyzed asymmetric reaction but also one of the most
important asymmetric carbon–carbon bond-forming reac-
tions.[2] As a diversity of bond types can be formed and many
chiral elements can be installed at the nucleophile, the
electrophile, or both, transition-metal-catalyzed AAAs have
become a powerful tool in organic synthesis.[2c] Although a
variety of substrates and reagents are suitable in AAA
reactions, the use of carbon nucleophiles is mainly limited to
the stabilized “soft” carbanions such as b-keto ester and
malonate derivatives.[2] Simple ketone enolates are an impor-
tant class of nucleophiles, but AAA had been ineffective with
these nucleophiles because they are nonstabilized and hard.
The breakthrough came in 1999, when Trost and Schroeder
obtained high enantioselectivity in Pd-catalyzed AAAs with
tetralone and cyclohexanone derivatives by using a “chiral
pocket” ligand.[3] Since then, several examples were reported
and excellent enantioselectivities were obtained.[4,5] Cyclic
ketones have been used as substrates in these reactions.
Recently, Braun et al. reported the Pd-catalyzed allylic
alkylation of 1,3-diphenylallyl acetate with high diastereose-
lectivity by using mesityl ethyl ketone.[4c] However, the
reaction was not asymmetric. Thus, an asymmetric version
of Pd-catalyzed AAAs with acyclic ketones remains a
challenge. Recently, we designed some ferrocene-derived
Scheme 1. a) [{Pd(C3H5)Cl}2] (2.5 mol%), ligand (5 mol%), lithium
1,1,3,3-hexamethyldisilazane (LiHMDS), THF, 08C.
desired product 3a under these conditions. According to the
reports by Trost et al.[3,4a] as well as our own previous work,[5]
chiral pocket ligands have emerged as ligands of choice for
the highly enantioselective allylic alkylation of simple cyclic
ketone enolates. Thus the modified chiral pocket imine and
amine deriviatives L1–L4[8] were synthesized from ferrocene
4[7] (Scheme 2).
In the presence of L3, the reaction of 1a with 2 gave the
allyl product 3a in 69% yield and with 31% ee, whereas in the
presence of L4, the reaction gave 3a in 23% yield and with
2.5% ee. The lower yield and enantioselectivity is attributed
to the mismatched chiralities in L4.[9] A further increase in the
enantioselectivity (from 31% to 48% ee) was observed when
L1 was used in place of L3. The choice of base is important:
the use of sodium hydride or potassium tert-butoxide in
conjunction with ligand L1 led to very low yields. The use of a
[*] X.-X. Yan, Dr. C.-G. Liang, Y. Zhang, B.-X. Cao, Prof. L.-X. Dai,
Prof. Dr. X.-L. Hou
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (P.R. China)
Fax: (+86)21-5492-5100
E-mail: xlhou@mail.sioc.ac.cn
W. Hong, Prof. Dr. X.-L. Hou
Shanghai–Hong Kong Joint Laboratory in Chemical Synthesis
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (P.R. China)
[**] Financially supported by the Major Basic Research Development
Program (Grant G 200077506), National Nature Science Founda-
tion of China, Chinese Academy of Sciences, Shanghai Committee
of Science and Technology, and the Croucher Foundation of Hong
Kong.
Scheme 2. Synthesis of ligands L1–L4. Reagents and conditions:
a) (S,S)- or (R,R)-cyclohexane-1,2-diamine, p-TsOH, 4-Š MS, toluene;
b) NaBH4, EtOH.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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bulkier base such as LiHMDS led to a slight increase in both
yield (77%) and enantioselectivity (57% ee), and the use of
lithium tetramethylpiperidine gave the product in 48% yield
with 56% ee. The metal ion of the base has a great effect on
the yield and enantioselectivity of the reaction. For example,
the reaction with LiHMDS gave the product in 77% yield and
with 57% ee, but the corresponding reactions with NaHMDS
and KHMDS provided products with 2% ee and 9% ee,
respectively, although the yields were higher. A study of the
effect of solvent on the reaction showed that among the
solvents tested (Et2O, 1,4-dioxane, 1,2-dimethoxyethane,
toluene), THF gave the best results.
of catalyst on the reaction was also studied. Reducing the
catalyst amount to 1mol% gave rise to a slight decrease in
both yield (93%) and enantioselectivity (90% ee). When the
amount of catalyst was decreased to 0.5 mol%, the reaction
still proceeded smoothly to give the product in 83% yield
with 90% ee. Under these optimized conditions, a variety of
acyclic ketones were tested (see Scheme 1) and the results are
given in Table 2.
Table 2: Pd-catalyzed AAA of acyclic ketones 1.[a]
Substrate 1
Product 3
Entry
R1
R2
Yield [%][b]
ee [%][c]
On the basis of these results, we envisioned that the
lithium enolate derived from an a-alkoxy-substituted ketone
would provide a general approach to the control of enantio-
selectivity in the AAA of acyclic ketones due to the
propensity of this ketone to form the Z-chelated enolate.
Our results demonstrate that the alkoxy group exhibits a
remarkable effect on the level of enantioselectivity. When 1-
phenyl-3-methoxypropan-1-one (1b) was used with LiHMDS
as base, the enantioselectivitiy significantly increased to 68%
ee and the yield was 72%. Takemoto and co-workers as well
as Wang reported that chiral ammonium salts have a dramatic
effect on the enantioselectivity of Pd-catalyzed AAA reac-
tions.[10] In our case, the ammonium salts, both chiral and
achiral, also showed an ability to improve the enantioselec-
tivity of the reaction (see Table 1). We inferred that this
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
g
h
i
Ph
Ph
Ph
Ph
Me
93
98
77
93
OMe
OEt88
OiPr
OMe
OMe
Ph
OMe
Me
OAc
OPh
SO2Ph
92
95
83
91
89
76
50
50
89
83
79
94
91
73
87
80
86
71
0
p-MeOC6H4
p-ClC6H4
Ph
cHex
2-furyl
Ph
10
11
12
j
k
l
Ph
Ph
[a] All reactions were performed at À208C; [{Pd(C3H5)Cl}2] (2 mol%); L1
(4 mol%); 1 (100 mol%); LiHMDS (120 mol%); 2 (130 mol%); AgBr
(10 mol%). [b] Yield of isolated product. [c] Determined by chiral HPLC.
Table 1: The effect of additives on the reaction.[a]
All acyclic nonstabilized hard ketones gave products in
good to excellent yields and enantioselectivities. However,
the stabilized soft ketone 1l gave a racemic product (entry 12,
Table 2). The nature of the alkoxy substituent has a great
effect on the enantioselectivity. Substrates 1b and 1c with less
sterically hindered MeO and EtO groups gave the corre-
sponding allylated products with ee values of 93% and 92%,
respectively (entries 2 and 3, Table 2). However, the sub-
strates with the bulky iPrO, 1d, and PhO, 1k, furnished
products with ee values of 79% and 71%, respectively,
although the yields were good to excellent (entries 4 and 11,
Table 2). It was unexpected that the reactivity of ketone 1j
with the ester substituent would be so low, although the
enantioselectivity was good (50% yield and 86% ee; entry 10,
Table 2). The electronic property of the substituent on the
phenyl ring of the substrate has little effect on the reaction.
Thus, the methoxy-substituted aryl ketone 1e and the chloro-
substituted aryl ketone 1 f gave allylated products with 94%
and 91% ee, respectively. Importantly, the aliphatic cyclo-
hexyl methoxymethyl ketone (1h) afforded the correspond-
ing product in 76% yield with 87% ee (entry 8, Table 2). The
heteroaryl-substituted simple ketone 1i gave rise to the
corresponding product in 50% yield with 80% ee.
Entry
Additives
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
Bu4NBr
Hex4NBr
5
ZnCl2
CuClO4
AgOTf
Ag2O
AgBr
48
62
60
66
70
84
89
96
98[b]
75
75
74
83
89
87
73
89
93[b]
AgBr
[a] Reactions performed at 08C; [{Pd(C3H5)Cl}2] (2.5 mol%); L1
(5.0 mol%); additives (10 mol%); LiHMDS (120 mol%); 1b
(100 mol%); 2 (130 mol%). [b] Reaction carried out at À208C.
increase in enantioselectivity in the presence of ammonium
salts is caused to some extent by their Lewis acid character.
Thus, several Lewis acids were tested (see Table 1).[11]
Lewis acids have a greater effect on the reaction than
ammonium salts. Among the Lewis acids tested, silver
bromide gave the best result both in terms of enantioselec-
tivity and yield. When the reaction proceeded at À208C in the
presence of a catalytic amount of AgBr, the product was
obtained in 98% yield with 93% ee. The effect of the amount
Under different conditions, Z or E forms of an enolate are
favored,[12] which may influence the enantioselectivity of the
alkylation. Xie et al. reported that the Z form of an enolate
was formed exclusively by using Ph2NLi at À788C, while Z
and E forms of an enolate were provided in a ratio of 48:52
with Me3CN(TMS)Li (TMS = trimethylsilyl) as base at room
temperature.[12] In the reactions reported here, we found that
different forms of an enolate dramatically affected the
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Communications
[6] a) L.-X. Dai, T. Tu, S.-L. You, W.-P. Deng, X.-L. Hou, Acc.
Chem. Res. 2003, 36, 659 – 667; b) S.-L. You, X.-Z. Zhu, Y.-M.
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enantioselectivity of a reaction. In the presence of Ph2NLi at
À788C, the reaction of ketone 1a gave the product 3a with
88% ee. Even phenyl propyl ketone (1m) gave the product
3m with 87% ee, but only enantioselectivities of 34% and
76% ee were obtained with Me3CN(TMS)Li (Scheme 3).
[8] J.-X. Gao, X.-D. Yi, P.-P. Xu, C.-L. Tang, H.-L. Wan, T. Ikariya, J.
Organomet. Chem. 1999, 592, 290 – 295.
[9] a) K. Muꢀiz, C. Bolm, Chem. Eur. J. 2000, 6, 2309 – 2316; b) S.-L.
You, X.-L. Hou, L.-X. Dai, Y. H. Yu, W. Xia, J. Org. Chem. 2002,
67, 4684 – 4695.
[10] a) M. Nakoji, T. Kanayama, T. Okino, Y. Takemoto, J. Org.
Chem. 2002, 67, 7418 – 7423; b) Y. Wang, Ph.D. Thesis, Shanghai
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[11] Other metal salts, including Sc(OTf)3, AgNO3, AgClO4, AgBF4,
AgPF6, AgCl, AgOAc, and AgSbF6, were also tested. All of
them gave the product in 68–97% yield with enantioselectivites
of 76–87% ee.
Scheme 3. The reactions of 1a and 1m to form 3a and 3m, respectively.
[12] For example: a) L. Xie, K. Vanlandeghem, K. M. Isenberger, C.
Bernier, J. Org. Chem. 2003, 68, 641– 643; b) L. Xie, K. M.
Isenberger, G. Held, L. M. Dahl, J. Org. Chem. 1997, 62, 7516 –
7519.
These results may explain why substrates with an alkoxy
group at the a-position of the carbonyl group led to products
with high enantioselectivities. The presence of AgBr did not
influence the ratio of Z and E isomers of an enolate: the
reaction of 1a with 2 in the presence of NaHMDS and AgBr
gave 3a with 9% ee.
In conclusion, we have synthesized a novel chiral imino
ferrocene ligand and used this ligand to carry out highly
enantioselective Pd-catalyzed allylic alkylations of simple
acyclic ketones. The dramatic effect of AgBr as well as the
dependency of the enantioselectivity on the different forms of
the enolate have been demonstrated. Further investigations
on the role of AgBr and the applications of the ligand L1 in
other types of acyclic ketones are in progress.
Received: June 11, 2005
Published online: September 20, 2005
Keywords: alkylation · enantioselectivity · enolates · ketones ·
.
palladium
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