Asymmetric Catalytic Insertion of α-Diazo Carbonyl Compounds into O-H Bonds of Carboxylic Acids

Article abstract of DOI:10.1021/acscatal.6b02184

An efficient enantioselective insertion of α-diazoesters and α-diazoketones into O-H bonds of carboxylic acids was realized by the use of Rh₂(OAc)₄ and a chiral guanidine. Optically active α-acyloxy carbonyl compounds were generated under mild reaction conditions in high yields (up to 99%) and good enantioselectivities (up to 97.5:2.5 er).

Full text of DOI:10.1021/acscatal.6b02184

Letter
pubs.acs.org/acscatalysis
Asymmetric Catalytic Insertion of α‑Diazo Carbonyl Compounds into
O−H Bonds of Carboxylic Acids
Fei Tan, Xiaohua Liu,* Xiaoyu Hao, Yu Tang, Lili Lin, and Xiaoming Feng
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, China
S
* Supporting Information
ABSTRACT: An efficient enantioselective insertion of α-
diazoesters and α-diazoketones into O−H bonds of carboxylic
acids was realized by the use of Rh2(OAc)4 and a chiral
guanidine. Optically active α-acyloxy carbonyl compounds
were generated under mild reaction conditions in high yields
(up to 99%) and good enantioselectivities (up to 97.5:2.5 er).
KEYWORDS: asymmetric catalysis, rhodium, chiral guanidine, insertion, carboxylic acids
ransition-metal-catalyzed asymmetric insertion of α-diazo
compounds into O−H bond donors, such as alcohols,
as the chiral proton shutters. Taking inspiration from our
recently developed asymmetric catalysis using chiral guanidines
as organocatalysts and ligands,7 we sought to extend the utility
of chiral guanidine to the O−H insertion of carboxylic acids.
We hypothesized that carboxylic acids could take precedence to
react with basic guanidine over transition metal salt, forming
chiral guanidinium salt, which can participate in an insertion
reaction of Rh carbenoids, acting as a chiral proton-transport
catalyst for the [1,2]-H shift. Herein, the first example of the
catalytic enantioselective O−H insertion of α-diazoesters and
α-diazoketones to carboxylic acids was reported. A catalytic
system containing a simple chiral guanidine-amide and
dirhodium(II) carboxylate was proven to be efficient, and
various α-acyloxy carbonyl compounds were obtained with
good enantioselectivities and yields under mild reaction
conditions.
In the initial study, we conducted the reaction of ethyl α-
diazo-α-phenylacetate (2a) with benzoic acid (1a) in chloro-
form at 0 °C, and a selection of chiral guanidine G1 and Rh(II)
salts were evaluated as the catalysts (Table 1, entries 1−4). In
the presence of 2.5 mol % of Rh2(OAc)4, the O−H insertion
product 3a was formed in 51% yield and 87:13 er (entry 1).
When Rh2(oct)4 and Rh2(TPA)4 were used, the reaction results
were comparable (entries 3 and 4). Rh2(TFA)4, which has
electron-deficient carboxylate ligands, slowed the reaction and
lowered the enantioselectivity (entry 2). In an attempt to
improve the enantioselectivity, the structures of chiral
guanidines were varied (entries 5−12). Guanidine-amides
G2−G4, which carried different amino acid backbones, were
unfavorable in terms of er value (entries 5−7). The use of
guanidine G1 derived from L-proline and ether as the reaction
solvent led to a slight improvement in the enantioselectivity
(entry 8). The 2,6-diisopropylphenyl-substituted G8 was nearly
T
phenols, H2O, and carboxylic acids is highly selective and
synthetically useful for the synthesis of α-alkoxy, α-aryloxy, α-
hydroxyl, or α-acyloxy carbonyl derivatives, respectively.1 Chiral
α-acyloxy carbonyl compounds were usually synthesized from
chiral α-hydroxy carbonyl compounds, which, for example, can
be obtained from copper- or iron-catalyzed O−H insertion of
H2O with α-diazocarbonyl compounds in excellent enantiose-
lectivities.1i,j Additionally, asymmetric Passerini reaction2 and
carboxylation reaction3 could provide access to α-acyloxy
carbonyl compounds. The O−H insertion of carboxylic acids
with α-diazocarbonyl compounds provides a direct and
convenient alternative for the α-acyloxy carbonyl compounds
synthesis. The racemic synthesis of α-acyloxy carbonyl
compounds via O−H insertion of carboxylic acids with α-
diazo compounds have been reported by several groups after
the pioneering work of Wolfrom and co-workers in 1945.4a
Metal salts, including cupric chloride,4b Cu(acac)2,4c Cu-
(OAc)2,4g Rh2(OAc)4,4e Rh2(esp)2,4h and Pd(OAc)2,4f have
been used. However, the asymmetric catalytic version of O−H
insertion of carboxylic acids has not been addressed, which is
quite different from the other O−H bond donors. The only
asymmetric example of Cu(acac)2-mediated carbenoid insertion
into acetic acid and pivalic acid was exploited with the aid of a
chiral auxiliary by Wang.4d The catalytic enantioselective O−H
insertion of carboxylic acids is difficult, and one explanation
could be that the acidity of carboxylic acids may compromise
the stability of chiral metal complex and lead to dissociation,
resulting in poor enantiocontrol.
A computational study from the Yu group suggests that a
free-ylide pathway is favored in the neutral dirhodium(II)
complex-catalyzed system in O−H insertion reaction, and water
can act as an efficient proton-transport catalyst for the [1,2]-H
shift.5 These useful inferences have been demonstrated by
cooperative catalysis in asymmetric N−H, C−H, and S−H
insertions,6 using chiral phosphoric acids or cinchona alkaloids
Received: August 1, 2016
Revised: September 9, 2016
© XXXX American Chemical Society
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a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Scope of Carboxylic Acids
b
c
entry
Rh(II)
G*
solvent
yield (%)
er
1
Rh2(OAc)4
Rh2(TFA)4
Rh2(oct)4
G1
G1
G1
G1
G2
G3
G4
G1
G5
G6
G7
G8
G8
G8
G8
G8
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
Et2O
51
51
56
51
53
35
50
54
48
61
55
54
66
79
72
89
87:13
d
2
3
4
5
6
7
8
9
75.5:24.5
85:15
Rh2(TPA)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
86.5:13.5
71:29
62.5:37.5
72.5:27.5
91:9
Et2O
73:27
10
11
12
13
14
15
16
Et2O
80:20
Et2O
78:22
Et2O
91.5:8.5
92:8
e
e
e
e
Et2O
,
,
,
f
f
f
Et2O
92.5:7.5
94:6
,
g
Et2O
,h
Et2O
94:6
a
Unless otherwise noted, all reactions were performed with Rh2X4 (2.5
mol %), G* (5 mol %), 1a (0.1 mmol), and 2a (1.0 equiv) in solvent
b
c
(0.6 mL) at 0 °C for 3 h. Isolated yield. Determined by chiral HPLC
a
d
e
Unless otherwise noted, all reactions were performed with
Rh2(OAc)4 (2.5 mol %), G8 (10 mol %), and 1 (0.1 mmol) stirred
in THF (0.2 mL) at 30 °C for 0.5 h, then evaporated in vacuo and
analysis. The reaction time was 8 h. Rh2(OAc)4 (2.5 mol %), G8 (5
mol %) and 1a (0.1 mmol) were stirred in THF (0.2 mL) at 30 °C for
0.5 h, then evaporated in vacuo and reacted with 2a (1.0 equiv) in
b
f
g
h
reacted with 2c (1.4 equiv) in Et2O (0.6 mL) at −10 °C. Isolated
Et2O (0.6 mL) at −10 °C for 2 h. G8 (10 mol %). 2c was used. 2c
c
d
yield. Determined by chiral HPLC analysis. At 0 °C.
(1.4 equiv) was used.
as effective as the chiral guanidine G1 (entries 8 and 12).
Changing the preparation procedure of the catalyst and
lowering the reaction temperature, an improvement in yield
and enantioselectivity was obtained (entry 13). Moreover, the
best results for the product 3a (79% yield, and 92.5:7.5 er)
could be achieved by the use of 2.5 mol % of Rh2(OAc)4 and 10
mol % of G8 (entry 14). The ester group of α-diazoesters
influenced the reaction minimally (SI, Table S8), and the use of
2c gave the product 3ac in 89% yield and 94:6 er when its
amount increased to 1.4 equiv (entries 15 and 16). Addition-
ally, G8−Rh2(esp)2 exhibited high reactivity but with negative
results for the desired product 3a (49% yield, 85.5:14.5 er), and
a certain amount of byproduct α-ketoester (ethyl 2-oxo-2-
phenylacetate) was generated (see SI for details). In the
absence of chiral guanidines, chiral dirhodium catalyst (Rh2(5S-
MEPY)4) alone showed low activity with the generation of
trace amount of the racemic product, and α-ketoester was
detected as the main product (see SI for details).
position and a substituent at meta-position had a minor impact
on the enantioselectivity (entries 6 and 8). 1-Naphthoic acid
(1j) was a suitable substrate for the reaction and afforded the
corresponding product 3jc in 96% yield and 93.5:6.5 er (entry
10). Thiophene-2-carboxylic acid and its derivatives reacted
smoothly to generate the related insertion products 3kc−3nc
with satisfactory enantioselectivities and good yields (75−92%
yields and 95.5:4.5−97:3 er; entries 11−14). The absolute
configuration of 3nc was determined to be R by X-ray
crystallographic analysis,8 and the others showed similar
stereoarrangement from the CD spectra analysis (see SI for
details). Additionally, cinnamic acid (1o) and aliphatic
carboxylic acid (1p) can also undergo the O−H insertion
reaction to form the products with moderate er values and
good yields (entries 15 and 16).
Then, we continued to explore the substrate scope of α-
diazoesters by using thiophene-2-carboxylic acid 1k as the O−
H bond donor (Table 3). Either electron-donating or electron-
withdrawing substituents at varied position of α-phenyl rings
affected the enantioselection, and the corresponding α-acyloxy
esters were formed in 92:8−96.5:3.5 er and 71−94% yields
(entries 1−13). The α-diazoester 2w bearing 1-naphthyl
substituent underwent the reaction efficiently, resulting in
77% yield and 95:5 er (entry 14). (E)-2-diazo-4-phenylbut-3-
enoate was successfully employed in the reaction, albeit a
Next, we investigated the scope of carboxylic acids (Table 2).
Methyl-substituted benzoic acid at meta-position reduced the
yield remarkably in comparison with those at ortho- or para-
positions (entries 2, 6, and 7). 2-Halo-substituted benzoic acids
undergo the reaction smoothly, and the desired products were
given in moderate yields with slightly improved enantioselec-
tivities (entries 3−5). A sterically hindered group at para-
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a
Table 3. Substrate Scope of α-Diazoesters
Scheme 1. Substrate Scope of Asymmetric O−H Insertion of
a
α-Diazoketones with Carboxylic Acids
a
Unless otherwise noted, all reactions were performed with
Rh2(OAc)4 (2.5 mol %), G8 (10 mol %) and 1 (0.1 mmol) in THF
(0.2 mL) at 30 °C for 0.5 h, then evaporated in vacuo and reacted with
b
4 (1.4 equiv) in Et2O (0.6 mL) at −10 °C for 3 h. At −30 °C for 23
c
d
e
h. At −30 °C for 3 h. At −50 °C for 27 h. Rh2(OAc)4 (0.5 mol %)
and G8 (2 mol %) at −10 °C for 9 h.
0.5 mol % of Rh2(OAc)4 and 2.0 mol % of guanidine G8,
affording 5f in 97% yield and 95.5:4.5 er.
In order to gain insight into the mechanism, some additional
experiments were carried out. First, we investigated the effect of
guanidine on the reactions between α-diazoester 2m and acid
1k (Scheme 2). With 2.5 mol % of Rh2(OAc)4, a 99% yield of
a
Unless otherwise noted, all reactions were performed with
Rh2(OAc)4 (2.5 mol %), G8 (10 mol %) and 1k (0.1 mmol) in
THF (0.2 mL) at 30 °C for 0.5 h, then evaporated in vacuo and
reacted with 2 (1.4 equiv) in Et2O (0.6 mL) at indicate temperature.
Scheme 2. Control Experiments for Mechanism Study
b
c
d
Isolated yield. Determined by chiral HPLC analysis. Rh2(OAc)4
(1.25 mol %), G8 (5 mol %), 1k (4.0 mmol) and 2m (5.6 mmol).
higher reaction temperature was required to achieve acceptable
yield and enantioselectivity (entry 16). Meanwhile, α-
diazoesters with 3-thienyl or isobutyl substituent performed
smoothly with moderate enantioselectivities (entries 15 and
17). The reaction between carboxylic acid 1k and α-diazoester
2m was run on a gram scale in the presence of 1.25 mol % of
Rh2(OAc)4 and 5 mol % of G8, and it performed well with 99%
yield and 95.5:4.5 er (entry 4).
The chiral catalyst for the asymmetric O−H insertion of α-
diazoesters can be applied without modification to the reaction
of α-diazoketones. Compared with α-diazoesters, the use of α-
diazoketones as carbene precursors for the X−H (X = O, B, N,
etc.) insertion reaction is far less studied.6d,9 As summarized in
Scheme 1, O−H insertion of thiophene-2-carboxylic acid with
α-diazoketone containing an acetyl substituent showed higher
enantioselectivity than benzoyl-substituted one (5a vs 5b).
Fluoro-substituted 1-diazo-1-phenylpropan-2-ones afforded the
desired products 5g−5i in slightly higher yields and similar
enantioselectivities. The O−H insertion of benzoic acid gave
better results than adamantane-1-carboxylic acid (5c vs 5d).
Moreover, n-butyl and cyclopropyl-substituted α-diazoketones
could perform the products 5e and 5f in good yields and
excellent enantioselectivities, and up to 95:5 er (5e) and
97.5:2.5 er (5f) could be given when the reaction temperature
dropped. Notably, the O−H insertion of cyclopropyl-
substituted α-diazoketone could proceed efficiently even at
the racemic product 3km was found within 1 h. Using amino
amide L1 as the chiral source instead of guanidine G8 resulted
in a 91% yield but with dramatically decreased enantioselec-
tivity after a longer reaction time. It indicates the chiral
guanidine benefits the proton transfer process in an
enantioselective manner. Using cinchonine as the chiral
proton-transfer shuttle instead resulted in a dramatically
reduced yield (18%) with moderate er (79:21 er) even at
improved reaction temperature. Moreover, by using 0.5 mol %
of Rh2(OAc)4 and 2.0 mol % of guanidine G8, a complete
conversion to the O−H insertion product was given, whereas
the yields dropped gradually, and the enantioselectivity was
maintained if the amount of guanidine G8 increased. It is
consistent with our hypothesis that carboxylic acid will tightly
interact with guanidine to generate guanidinium salt, and it
could not easily disassociate to participate in the O−H
insertion reaction. The interaction among guanidine, carboxylic
acid, and Rh2(OAc)4 was further detected by CD and NMR
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studies (see SI for details). The phenomena indicate negligible
interaction between Rh2(OAc)4 and guanidine but strong
interaction between guanidine and carboxylic acid.
for Support of Top-notch Young Professionals for financial
support.
On the basis of the aforementioned results and previous
reports,5,6,10 a plausible cooperative catalysis mechanism of the
asymmetric O−H insertion of carboxylic acid is proposed
(Scheme 3). Dinitrogen is extruded from α-diazo carbonyl
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carboxylic acids with α-diazo carbonyl compounds was achieved
under mild conditions. Dirhodium(II) complex with chiral
guanidine-amide was proved to be efficient cooperative
catalysts. Various α-acyloxy esters and ketones were obtained
in good enantioselectivities and yields. Additional studies
directed at expanding the application of chiral guanidine and
mechanism are underway.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b02184.
■
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Experimental details, analytic data (NMR, HPLC, CD,
and ESI-HRMS) (PDF)
X-ray data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: liuxh@scu.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21222206, 21332003, and 21321061), the Fok Ying
Tung Education Foundation (141115), and National Program
(8) CCDC 1403604 (3nc) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
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Letter
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