Enantioselective palladium-catalyzed insertion of α-aryl-α- diazoacetates into the O-H bonds of phenols

Article abstract of DOI:10.1002/anie.201309820

A palladium-catalyzed asymmetric O-H insertion reaction was developed. Palladium complexes with chiral spiro bisoxazoline ligands promoted the insertion of α-aryl-α-diazoacetates into the O-H bond of phenols with high yield and excellent enantioselectivity under mild reaction conditions. This palladium-catalyzed asymmetric O-H insertion reaction provided an efficient and highly enantioselective method for the preparation of synthetically useful optically active α-aryl-α-aryloxyacetates.

Full text of DOI:10.1002/anie.201309820

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Angewandte
Communications
DOI: 10.1002/anie.201309820
Asymmetric Catalysis
Hot Paper
Enantioselective Palladium-Catalyzed Insertion of a-Aryl-a-
À
diazoacetates into the O H Bonds of Phenols**
Xiu-Lan Xie, Shou-Fei Zhu,* Jun-Xia Guo, Yan Cai, and Qi-Lin Zhou*
À
Abstract: A palladium-catalyzed asymmetric O H insertion
reaction was developed. Palladium complexes with chiral spiro
bisoxazoline ligands promoted the insertion of a-aryl-a-
À
diazoacetates into the O H bond of phenols with high yield
and excellent enantioselectivity under mild reaction conditions.
À
This palladium-catalyzed asymmetric O H insertion reaction
provided an efficient and highly enantioselective method for
the preparation of synthetically useful optically active a-aryl-a-
aryloxyacetates.
P
alladium is an indispensable and versatile catalyst in
modern organic synthesis.[1] Palladium efficiently catalyzes
the cyclopropanation of olefins with diazomethane, a widely
used carbene-transfer reaction,[2] and various new palladium-
catalyzed carbene-transfer reactions based on migratory
insertion have recently been developed.[3] However, asym-
metric palladium-catalyzed carbene-transfer reactions are
rare and remain a challenge. The early examples of palla-
dium-catalyzed asymmetric carbene-transfer reactions
involved cyclopropanations and carbenylative amination,
however, the resulting enantioselectivity was unsatisfactory.[4]
Recently, Hu and co-workers[5] reported highly enantioselec-
tive palladium-catalyzed three-component reactions of pyr-
role, diazoesters, and imines. Herein, we report a palladium-
catalyzed asymmetric insertion of a-aryl-a-diazoacetates into
Scheme 1. Palladium-catalyzed asymmetric insertion of a-diazo-a--
À
phenylacetates into the O H bond of phenols, and selected important
compounds derived from chiral a-aryl-a-aryloxyacetates.
(Scheme 1). The established methodologies for the synthesis
of optically active a-aryl-a-aryloxyacetates are based on
either multistep transformations from chiral starting materi-
als or the use of chiral auxiliaries.[7,8] To our knowledge, no
highly enantioselective catalytic asymmetric method for
preparing these important compounds has been reported.[9]
The challenges lie mainly in the high acidity of the a-
hydrogen of a-aryl-a-aryloxyacetates. Because these com-
pounds readily epimerize under basic conditions or at high
temperature, any reliable method for their preparation should
involve mild, neutral conditions. The transition-metal-cata-
the O H bond of phenols (Scheme 1).[6] Palladium complexes
with chiral spiro bisoxazoline ligands efficiently catalyzed the
À
À
O H insertion reaction, which provides a new method for the
preparation of a-aryl-a-aryloxyacetates 3 with good yields
and excellent enantioselectivity under mild, neutral reaction
conditions.
The a-aryl-a-aryloxyacetate moiety is ubiquitous in
biologically active molecules[7] and it is also present in
a
chiral solvating agent for NMR spectroscopy[8]
À
lyzed O H insertion reaction meets this requirement. In 2006,
[*] X.-L. Xie, Prof. S.-F. Zhu, J.-X. Guo, Y. Cai, Prof. Q.-L. Zhou
State Key Laboratory and Institute of Elemento-organic Chemistry
Nankai University, Tianjin 300071 (China)
and
Maier and Fu[6c] attempted an O H bond insertion reaction
À
between a-diazo-a-phenylacetates and phenols by using
chiral copper bisazaferrocene complexes as catalysts. How-
À
ever, the desired O H insertion product (a-phenoxy-a-
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin)
E-mail: sfzhu@nankai.edu.cn
qlzhou@nankai.edu.cn
Homepage: http://zhou.nankai.edu.cn
phenylacetate) was obtained in 56% yield with only 11%
ee. We[6d] and Uozumi and co-workers[6g] used chiral copper
spiro bisoxazoline complexes and a chiral copper imidazoin-
dolephosphine complex, respectively, for the same reaction
and also obtained extremely low ee values (10% ee and 7%
ee, respectively).
[**] We thank the National Natural Science Foundation of China, the
National Basic Research Program of China (2012CB821600), the
“111” project (B06005) of the Ministry of Education of China, and
the Program for New Century Excellent Talents in University (NCET-
10-0516) for financial support.
À
In this work, O H bond insertion reactions between
methyl a-diazo-a-phenylacetate (1a) and phenol (2a) were
performed in chloroform at 408C in the presence of various
transition-metal catalysts prepared in situ from the corre-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309820.
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sponding metal salts and a chiral spiro bisoxazoline ligand
Increasing the amount of NaBArF to 2 equiv relative to
[10]
(Sa,S,S)-4a with NaBArF
as an additive. As shown in
[Pd(PhCN)2Cl2] shortened the reaction time to 1.5 h and
markedly improved both the yield and the enantioselectivity
(entry 12). By contrast, the yield dropped dramatically to less
than 10% in the absence of NaBArF. The role of NaBArF
remains unclear; however, the bulky and noncoordinating
BArF anion of the resulting palladium catalyst may increase its
Lewis acidity and stability, which is helpful for getting higher
Table 1, chiral catalysts derived from copper, iron, nickel,
À
cobalt, gold, iridium, and ruthenium produced the desired O
H insertion product with low enantioselectivity (entries 1–7).
By contrast, when PdCl2 was used as the catalyst precursor,
promising enantioselectivity (85% ee) was observed,
although the yield was still low (entry 8). The nature of the
palladium precursor strongly affected both the yield and the
enantioselectivity of the reaction; [Pd(CH3CN)2Cl2] and
[Pd(PhCN)2Cl2], which have nitrile ligands, provided higher
yields and enantioselectivities (entries 9 and 10).
À
reactivity and enantioselectivity. Because an O H insertion
reaction with water was detected as a side reaction, molecular
sieves were introduced to absorb the water in the reaction
system and this change further increased the yield of desired
product to 83% and the enantioselectivity to 98% ee
(entry 13). The reaction could be performed at a catalyst
loading of 1 mol% without compromising enantioselectivity;
however, the reaction rate and yield decreased (entry 14).
Ligands (Sa,S,S)-4b, (Sa,S,S)-4c and (Sa)-4d gave the same
level of enantioselectivity (entries 15–17) as that of ligand
(Sa,S,S)-4a, thus indicating that the substituents at the oxazo-
line rings have a negligible influence on the chiral induction of
the catalyst. A bisoxazoline ligand with a binaphthyl back-
bone, (Sa,S,S)-5, also gave good yield and enantioselectivity
(entry 18). However, a bisoxazoline ligand with a pyridine
spacer, (S,S)-6, gave very low enantioselectivity (entry 19).
The optical purity of product 3aa remains unchanged even
after stirring under the standard reaction conditions for an
additional 20 h, a result that highlights the advantage of the
current neutral and mild reaction conditions in the synthesis
of a-aryl-a-aryloxyacetates.
Furthermore, [Pd(CH3CN)2Cl2] and [Pd(PhCN)2Cl2] were
stable under the reaction conditions, whereas palladium black
formed when PdCl2 was used. A Pd0 precursor, [Pd(dba)2],
gave a faster reaction but no chiral induction (entry 11).
À
Table 1: Palladium-catalyzed asymmetric O H insertion of methyl a-
diazo-a-phenylacetate into phenol.[a]
Under the optimal reaction conditions, various phenols
and a-aryl-a-diazoacetates were evaluated as substrates
(Table 2). Impressively, all the reactions exhibited good to
high yields (68–89%) and excellent enantioselectivity (96–
99% ee; entries 1–27). Although the highly sterically hin-
dered reactants phenol 2k and diazoester 1o reduced the
reaction rate (entries 11 and 27), the steric and electronic
properties of the phenol and diazoester substituents had
a negligible effect on the enantioselectivity, thus indicating
that the spiro palladium catalyst is highly tolerant of various
substrate structures. The broad substrate scope observed
suggests that this reaction could prove a powerful method for
the preparation of optically active a-aryl-a-aryloxyacetates.
Moreover, a gram-scale synthesis of the insertion product 3aa
was performed with excellent yield and enantioselectivity
(entry 28). This experiment further highlighted the potential
Entry [M]
Ligand
t [h]
Yield [%][b] ee [%][c]
1[d]
2
CuCl
FeCl2
(Sa,S,S)-4a
3
71
10
59
22
2
(Sa,S,S)-4a 24 (8)[e] 15
(Sa,S,S)-4a 24 (8)[e] 11
(Sa,S,S)-4a 24 (8)[e] 29
(Sa,S,S)-4a 24 (8)[e] 12
(Sa,S,S)-4a 24 (8)[e] 25
3
NiCl2
4
CoBr2
5
AuCl
22
3
6
[Ir(COD)Cl]2
7
[RuCl2(benzene)]2 (Sa,S,S)-4a 15
10
23
51
47
54
69
83
66
66
72
72
74
74
36
85
87
92
rac
96
98
98
97
98
95
90
19
8
PdCl2
(Sa,S,S)-4a 18
(Sa,S,S)-4a 18
(Sa,S,S)-4a 11
(Sa,S,S)-4a
(Sa,S,S)-4a
(Sa,S,S)-4a
(Sa,S,S)-4a
(Sa,S,S)-4b
(Sa,S,S)-4c
(Sa)-4d
9
[Pd(CH3CN)2Cl2]
[Pd(PhCN)2Cl2]
[Pd(dba)2]
10
11
12[f]
3.5
1.5
2
[Pd(PhCN)2Cl2]
13[f,g] [Pd(PhCN)2Cl2]
14[g,h] [Pd(PhCN)2Cl2]
15[f,g] [Pd(PhCN)2Cl2]
16[f,g] [Pd(PhCN)2Cl2]
17[f,g] [Pd(PhCN)2Cl2]
18[f,g] [Pd(PhCN)2Cl2]
19[f,g] [Pd(PhCN)2Cl2]
À
of this palladium-catalyzed asymmetric O H insertion reac-
tion.
5
1.5
1.5
5
2
2
In addition to a-aryl-a-diazoacetates, ethyl and benzyl a-
diazopropionates (1p and 1q) also reacted with excellent
enantioselectivity (96% ee and 94% ee, respectively), but the
yields were low (Scheme 2).
(Sa,S,S)-5
(S,S)-6
À
In addition to phenols, other O H donors, including n-
butanol and water, were also tested (Scheme 3). The reactions
[a] Reaction conditions: 1a/2a/[M]/ligand/
NaBArF =0.3:0.45:0.015:0.018:0.018 (mmol), in 4 mL solvent at 408C.
[b] Yield of isolated product. [c] Determined by supercritical fluid
chromatography (SFC) on a Chiralcel OD-H column. [d] The data is from
Ref. [6d]. [e] The diazoester 1a was not completely consumed after 24 h
at 408C so the reaction was heated at 608C for additional 8 h. [f] With
12 mol% NaBArF. [g] With 300 mg 5 ꢀ molecular sieves as additive.
[h] With 1 mol% catalyst and 2.4 mol% NaBArF. COD=1,5-cycloocta-
diene, dba=dibenzylideneacetone.
of n-butanol and water proceeded smoothly to afford the
corresponding O H insertion products with moderate yields
À
and enantioselectivity.
À
The O H insertion products 3 could easily be transformed
into various important chiral compounds (Scheme 4). For
instance, the hydrolysis of methyl 2-(naphthalen-2-yloxy)-2-
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À
Table 2: Palladium-catalyzed asymmetric O H insertion of a-aryl-a-
diazoacetates into phenols.[a]
À
Scheme 3. Palladium-catalyzed asymmetric O H insertion into n-buta-
nol and water.
Entry Ar1 (1)
Ar2 (2)
3
t [h] Yield ee [%]
[%]
1
2
3
4
5
6
7
8
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
4-MeC6H4 (1b)
4-PhC6H4 (1c)
4-ClC6H4 (1d)
4-BrC6H4 (1e)
C6H5 (2a)
4-MeOC6H4 (2b) 3ab
3aa
2
83
98(R)
98
98
98
99
98
98
98
97
98
97
99(R)
98
98
98
98
99
97
97
98
98
96
97
99
96(R)
96
98
1.5 87
1.5 85
1
0.7 83
0.2 80
1.5 79
4-MeC6H4 (2c)
4-tBuC6H4 (2d)
4-PhC6H4 (2e)
4-ClC6H4 (2 f)
3-MeC6H4 (2g)
3-ClC6H4 (2h)
3-CF3C6H4 (2i)
2-MeC6H4 (2j)
2,6-Me2C6H4 (2k) 3ak 12
2-naphthyl (2l)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
3ac
3ad
3ae
3af
3ag
3ah
3ai
86
2
76
9
0.2 74
0.2 78
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3aj
69
3al
0.2 84
0.2 78
0.5 82
2
1
2
1
3ba
3ca
3da
3ea
3 fa
3ga
3ha
3ia
87
79
86
77
C6H5 (2a)
3-MeOC6H4 (1 f) C6H5 (2a)
Scheme 4. Transformations of the insertion products. DIBAL-H=diiso-
butylaluminum hydride, THF=tetrahydrofuran.
3-MeC6H4 (1g)
3-ClC6H4 (1h)
3-CF3C6H4 (1i)
2-MeC6H4 (1j)
2-ClC6H4 (1k)
2-naphthyl (1l)
3-thienyl (1m)
4-ClC6H4 (1d)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
3-CF3C6H4 (2i)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
3.5 81
77
2.5 77
5
3ja
3ka
3la
The detailed mechanism of the palladium-catalyzed
1
2
81
68
À
asymmetric O H insertion reaction is unclear. A preliminary
study showed that the reaction exhibits a significant first-
order kinetic isotope effect (Scheme 5), thus indicating that
3ma 0.5 79
3di
3na
3oa
3aa
2
4
8
4
85
89
87
92
À
26[b] 1n
the breaking of the phenolic O H bond or the formation of
27[c] 1o
À
the C H bond in the product was involved in the rate-
28[d] C6H5 (1a)
99(R)
determining step.
[a] The reaction conditions and analysis were the same as for Table 1,
entry 13. The diazoesters 1 were slowly added over 2 h by using a syringe
pump. For entries 1–3 and 7, the diazoesters was added in one portion.
See the Supporting Information for details. The absolute configurations
of products 3aa, 3al, and 3na were determined according to reported
procedures.[11] [b] 1n=benzyl 2-diazo-2-phenylacetate. [c] 1o=tert-butyl
2-diazo-2-phenylacetate. [d] Performed on a gram-scale: 1a/2a/[Pd-
(PhCN)2Cl2]/(Sa,S,S)-4a/NaBArF =6:9:0.3:0.36:0.36 (mmol); 21 mL
CHCl3; 1.0 g 5 ꢀ molecular sieves; 1a was dissolved in 5 mL CHCl3 and
slowly added over 2 h by using a syringe pump. 1.34 g 3aa was obtained.
Scheme 5. Kinetic isotope effect.
À
In summary, a palladium-catalyzed asymmetric O H
bond insertion reaction between a-aryl-a-diazoacetates and
phenols was developed. Palladium complexes of chiral spiro
bisoxazoline ligands were shown to be efficient and highly
enantioselective catalysts for the reaction. The reaction
provides a mild, neutral method for the preparation of
chiral a-aryl-a-aryloxyacetates and broadens the application
of palladium catalysts in carbene transformations.
À
Scheme 2. Palladium-catalyzed asymmetric O H insertion of ethyl and
benzyl a-diazopropionates into phenol.
phenylacetate (3al) in acidic media produced acid 9, an
important chiral solvating reagent for NMR spectroscopy, in
quantitative yield with retained optical purity. The reduction
of methyl a-phenyl-a-(o-tolyloxy)acetate (3aj) gave alcohol
10 (97% yield), which was easily transformed into tomox-
etine, a well-known chiral drug for the treatment of psychi-
atric disorders.[12]
Experimental Section
À
Typical procedure for palladium-catalyzed O H insertion: powdered
PdCl2(PhCN)2 (5.8 mg, 0.015 mmol, 5 mol%), (Sa,S,S)-4a (9.2 mg,
0.018 mmol, 6 mol%), NaBArF (33.8 mg, 0.036 mmol, 12 mol%) and
0.3 g 5 ꢀ molecular sieves were introduced into an oven-dried
Schlenk tube in an argon-filled glovebox. After CHCl3 (3 mL) was
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injected into the Schlenk tube, the solution was stirred at room
temperature under an argon atmosphere for 2 h. o-Cresol (49 mg,
0.45 mmol) was added to the reaction mixture at 408C. Then
a solution of methyl 2-diazo-2-phenylacetate (52.8 mg, 0.3 mmol) in
1 mL CHCl3 was introduced into the reaction mixture by a syringe
pump over 2 h. The resulting mixture was continually stirred at 408C
until the full consumption of 1a was fully consumed. After filtering
and removing the solvent under vacuum, the product was isolated by
flash chromatography (petroleum ether/ethyl acetate = 10:1, v/v) as
a colorless oil and the ee value was determined by SFC with a chiral
column.
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Received: November 12, 2013
Published online: February 5, 2014
Keywords: a-aryl-a-aryloxyacetates · asymmetric catalysis ·
.
bisoxazoline ligands · carbenes · palladium
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