Rhodium(II)/Chiral Phosphoric Acid-Cocatalyzed Enantioselective O–H Bond Insertion of α-Diazo Esters

Article abstract of DOI:10.1002/adsc.201700572

A rhodium(II)/chiral phosphoric acid system has been developed for the asymmetric catalytic insertion of α-diazo esters into the O–H bond of carboxylic acids to generate an array of synthetically useful α-hydroxy ester derivatives in good ee (up to 95% ee). Furthermore, the substrate scope could be successfully extended to a range of phenols and alcohols with high yield (up to 92%) and excellent enantioselectivity (up to 97%) under mild reaction conditions. Additionally, a density functional theory (DFT) study was performed to elucidate the reaction mechanism. (Figure presented.).

Full text of DOI:10.1002/adsc.201700572

Title: Rhodium(II)/Chiral Phosphoric Acid Co-Catalyzed
Enantioselective O–H Bond Insertion of α-Diazoesters
Authors: Yiliang Zhang, Yuan Yao, Li He, Yang Liu, and Lei Shi
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700572
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rhodium(II)/Chiral Phosphoric Acid Co-Catalyzed Enantio-
selective O–H Bond Insertion of -Diazoesters
Yiliang Zhang,+a,b Yuan Yao,+a Li He,a,b Yang Liu,a and Lei Shi*a,b,c
a
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry
and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail: lshi@hit.edu.cn, Web:
http://homepage.hit.edu.cn/pages/shilei
Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
b
c
Hubei Key Laboratory of Drug Synthesis and Optimization, Jingchu University of Technology, Jingmen 448000,
China
+
These authors contributed equally to this work and should be considered co-first authors
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
ss of carbene insertion into N–H,[3] S–H,[4] B–H[5]
Abstract. A rhodium(II)/chiral phosphoric acid system has
been developed for the asymmetric catalytic insertion of - and Si–H bonds[6] etc. Over the past few decades,
diazoesters into the O–H bond of carboxylic acids to
generate an array of synthetically useful -hydroxy ester
derivatives in good ee (up to 95% ee). Furthermore, the
substrate scope could be successfully extended to a range
of phenols and alcohols with high yield (up to 92%) and
excellent enantioselectivity (up to 97%) under mild
reaction conditions. Additionally, a density functional
theory (DFT) study was performed to elucidate the reaction
mechanism.
many works have specifically concentrated on
catalytic asymmetric O–H insertion reactions. The
corresponding product, chiral -hydroxy ester is a
key substructure of many naturally occurring
compounds and active pharmaceutical ingredients[7]
(Figure.1). It was demonstrated that through the
combination of metal catalysts and chiral ligands
(cinchona alkaloid,[8] bisoxazoline,[9] bisazaferro-
cene[10] and imidazoindolephosphine[11] etc.), highly
enantioselective insertion of α-diazo compounds into
the O–H bonds of alcohols, phenols, and water have
been realized.
Keywords: -diazoester; chiral phosphoric acid; rhodium;
asymmetric catalysis; reaction mechanism
Although these examples show a tremendous
advance toward the synthesis of optically active α-
hydroxy carbonyl containing compounds, additional
studies on various metal complexes to achieve highly
enantioselective carbenoid insertion into O–H bond
are still needed. On the other hand, the catalytic
enantioselective version of O–H insertion of
carboxylic acids has not been fully addressed,
probably due to the superior acidity of carboxylic
acids which compromise the stability of chiral metal
complex.
Construction of carbon-heteroatom bonds in a
controlled and selective manner is an important task
in synthetic organic chemistry.[1] In particular, one of
the most direct and effective methods for the
enantioselective formation of C–X bonds is the
transition metal catalyzed insertion of carbenes into
heteroatom–hydrogen bonds. Remarkable progress[2]
has been made in this area. For example, high
enantioselectivities have been achieved in the proce-
So far, there are only a few asymmetric examples.
In 2001, Wang and co-workers[12] investigated
diastereoselective insertion into carboxylic O–H
bonds using camphorsultam as a chiral auxiliary, in
which moderate to good selectivity was achieved
(24%-90%).
Feng[13]
reported
an
efficient
enantioselective (85%-95%) insertion of α-diazo
carbonyl compounds into carboxylic acid using
Rh2(OAc)4 and chiral guandine catalysts, in which
most of the successful examples were obtained with
aromatic carboxylic acids. Very recently, Ryu[14]
successfully developed a method of the highly
enantioselective protonation/ nucleophilic addition of
substituted benzyl diazoesters with aromatic carboxy-
Figure 1. Selected important -hydroxy ester-containing
compounds.
1
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
Table 1. Optimization of the reaction conditions.a)
17
(R)-6b
Reaction condition: 1a (0.1 mmol), 2a (1.3 equiv),
Rh2(TPA)4 (2 mol %), catalyst (10 mol %), MgSO4 (100
DMF
40
10
17
a)
b)
mg) in 1 ml solvent.
Yield of isolated product after
c)
column chromatography. Determined by HPLC analysis.
d) Using Rh2(OAc)4.
lic acids using a chiral oxazaborolidinium ion as an
activator.
Inspired by previous works, and also in
conjunction with our ongoing interest in asymmetric
catalysis,[15] we present herein a rhodium/chiral
phosphoric acid[16] co-catalyzed enantioselective O–H
insertion of -diazoesters into O–H bonds of a
diverse set of phenols, alcohols and carboxylic acids,
which provides direct access to highly useful
carboxylates 7, 2-aryloxy-2-aryl esters 8 and 2-
alkoxy-2-aryl esters 9 with good yields and excellent
enantioselectivities under mild and neutral reaction
conditions.
We began our research with asymmetric insertion
reaction of benzoic acid 2a with methyl 2-diazo-2-
phenylacetate 1b. The reaction was performed in
CHCl3 at 40°C with catalytic amount of Rh2(TPA)4 (2
mol %), SPA (S)-5c (10 mol %) and MgSO4 (100 mg).
It was completed in 5 min. Unfortunately, the desired
product 2-methoxy-2-oxo-1-phenylethyl benzoate 4n
was observed in only 52% yield and 6% ee. However,
Entry catalyst
Solvent
T
Yield ee(%)c)
(%)b)
(°C)
1
2
3
4
5
6
7
8
9
(R)-6a
(R)-6b
(R)-6c
(R)-6d
(R)-6e
(S)-5a
(S)-5b
(S)-5c
(S)-5d
(S)-5e
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
c-hexane
DCE
40
40
40
40
40
40
40
40
40
40
40
0
62
85
81
78
73
60
62
70
39
53
73
78
65
80
46
N.R
35
93
82
23
5
21
65
60
44
48
81
87
88
90
93
-
10
11d) (R)-6b
12
13
14
15
16
(R)-6b
(R)-6b
(R)-6b
(R)-6b
(R)-6b
40
40
40
40
toluene
NMP
Table 2. Asymmetric O–H insertion of -diazoesters into carboxylic acids. a)
a) Unless otherwise noted, the reaction was carried out on a 0.1 mmol scale under nitrogen in CHCl3 at 40 °C for 30 min,
the ratio of 1:2 was 1:1.3.Yields referred to isolated yields, ees were determined by HPLC; b) Using Rh2(TFA)4.
2
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
when we use benzyl 2-diazo-2-phenylacetate 1a as
the substrate, the expected 2-(benzyloxy)-2-oxo-1-
phenylethyl benzoate 7a could be provided in 70%
yield and 60% ee (Table 1, entry 8). Therefore, we
decided to use 1a as model substrate in the further
reaction optimization. Then, we evaluated CPAs with
various substituents (entries 1–10). 9-Anthracenyl
substituted BINOL-derived phosphoric acid catalyst
benzyl 2-phenoxy-2-phenylacetate 8a or benzyl 2-
butoxy-2-phenylacetate 9a was formed in low or
moderate enantioselectivity (12% or 81% ee). Aiming
to get better reaction results, we restarted the
optimization of the reaction conditions and then
found that SPINOL-derived acid (S)-5b and BINOL-
derived acid (R)-6c could be applied to these two
transformations with both good yields (82% and
(R)-6b performed exceptionally in the model reaction, 75%) and enantioselectivities (94% and 90%),
providing the highest enantioselectivity (93% ee,
entry 2). Variation of dirhodium(II) carboxylates to
Rh2(OAc)4 slightly decreased enantioselectivity
(entry 11). Thus Rh2(TPA)4 still led to the best result.
Having identified Rh2(TPA)4/BPA (R)-6b as our
optimal co-catalyst system, an extensive screen of
solvent was undertaken (Table 1, entries 13–17).
Notably, it was observed that enantioselectivity of
reaction sometimes depended on the solvent polarity,
and high enantioselectivity was also achieved in
toluene with an obvious lower yield (46%, entry 15).
When the reaction was cooled to 0°C, the
corresponding yield and enantioselectivity slightly
decreased to 78% and 87% (entry 12), respectively.
With the optimized reaction conditions established
(Table 1, entry 2), we first investigated the substrate
scope by carrying out reactions of various carboxylic
acids with benzyl 2-diazo-2-phenylacetate 1a (Table
2). Both aromatic and aliphatic carboxylic acids
could undergo the O−H insertion reaction with good
yields and high ee values. Substituents on the
aromatic ring of benzoic acid seem to have a
negligible effect on enantioselectivity (7b–7g).
Cinnamic type acid 1n and thiophene-2-carboxylic
acid 1i were proved to be suitable substrates for this
reaction. Interestingly enough here, when using
salicylic acid as reactant, the desired product 7h was
obtained in 74% isolated yield. It could be an indirect
proof that the O–H bond of acid occured much faster
than the O–H bond of phenol under our reaction
condition. A preliminary study showed that the
reaction exhibited a significant first order kinetic
isotope effect (see in SI). Notably, this reaction
proceeded with aliphatic carboxylic acid smoothly to
generate the related insertion products 7j–7m with
satisfactory enantioselectivities and moderate to good
yields. Then we continued to explore the substrate
scope of -diazoesters. To our delight, a good result
was obtained when using benzyl 2-diazo-4-
phenylbut-3-enoate 1o (76% yield, 89% ee) as
reactant. Nevertheless, when diazo phosphonates or
diazo (phenylsulfonyl)acetate was used in the
reaction, the corresponding product 7r or 7s was
obtained as a racemic compound, probably due to the
larger steric hindrance of the substrates. These
substrates could possibly coordinate differently from
the successful examples, and that would lower the
enantioselectivity significantly.
separately. Moreover, when Mg(OTf)2 (2 mol %) was
added to the phenol type O–H insertion reaction with
Rh2(TPA)4 (2 mol %) and S-(5b) (2 mol %), the
enantioselectivity slightly improved to 96%.
Under the optimal reaction condition, various
phenols and -diazoesters were evaluated as
substrates (Table 3, 8a–8p). Impressively, all
reactions exhibited good to high yields (75–96%) and
excellent
electronic properties of substituents on phenyl ring of
the -diazoesters affected yields and
enantioselectivities
(83–96%).
The
enantioselectivities slightly, whereas the ortho-methyl
substituent phenol 2d remarkably increased the
enantioselectivity. Then we studied the substituent
effect of -diazoesters. Better enantioselectivity was
obtained when changing the methyl ester to either
ethyl or benzyl ester, suggesting that a larger ester
group may lead to a higher ee. The substrates with
fused rings such as 2-naphthyl, thienyl, and indolyl
were proved to be tolerated in the reaction with a
little loss of ee value.
In addition to phenols, we also evaluated alcohols
4 with a new condition (Table 3, 9a–9f). The
reactions of alcohols and -diazoesters proceeded
smoothly to afford the corresponding O–H insertion
products with moderate to good yields (62–76%) and
high enantioselectivities (81–90%).
We next turned our attention to asymmetric
intramolecular O–H insertion, and the results were
presented in Scheme 1. Unfortunately, the
corresponding products benzyl 2,3-dihydrobenzofur-
an-2-carboxylate 8q and benzyl chromane-2-
carboxylate
8r
were
obtained
in
low
enantioselectivities.[9e]
Scheme 1. Asymmetric intramolecular O–H insertion.
On the basis of aforementioned results and
previous reports,[18, 4a] a possible mechanism of the
asymmetric O–H insertion process is proposed
(Scheme 2). First step is rhodium-catalyzed nitrogen
extrusion from diazoacetate to form rhodium
carbenoid. Subsequently, nucleophilic attacked by
free O–H either from alcohol, phenol or carboxylic
acid gives metal-associated ylide intermediates.
Considering the similar chemical process of O–H
insertion using ROHs or carboxylic acids,
asymmetric O–H insertion of -diazoesters 1 into
phenols or alcohols was subsequently explored.
Under the standard condition, the desired product
3
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
Table 3. Asymmetric O–H insertion of -diazoesters into ROHs.
a)Unless otherwise noted, the reaction was carried out on a 0.1 mmol scale under nitrogen in CHCl3 at 40 °C for 30 min,
the ratio of 1:3 was 1.2:1, and the ratio of 1:4 was 1:1.5. Yields referred to isolated yields, ees were determined by HPLC.
Scheme 2. Proposed reaction mechanism.
4
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
Figure 2. The computed energy surfaces for the phosphoric acid-catalyzed proton shift.
Previous DFT calculation by Yu et al.[18] and Zhou et
al.[4a] have confirmed that free ylide was generated in
X–H insertion reaction when the neutral
dirhodium(II) complex is used as the catalyst. For
alcohols and phenols in the present system, we
hypothesize that once the free ylide is generated, the
plausible pathway could be either [1,2]-proton shift
or tandem [1,4]-proton shift followed by [1,3]-proton
shift processes. Lastly, the pathway that involves
chiral phosphate acid assisted proton transfer to enol
under the optional condition continued for 2 days,
and there were no ees in the re-isolated products.
In order to check whether the asymmetric O–H
insertion of carboxylic acid shares the mechanism
mentioned above, we performed the density
functional theory (DFT) calculations at B3LYP/6-
311++G(2d,2p)//B3LYP/6-31G(d) level in chloro-
form solution using SMD model to reveal the detailed
mechanism. Calculations first showed that O–H
insertion of carboxylic acid catalysted by Rh catalyst
generated a stable enol intermediate, not the
corresponding ylide form (see in SI).
intermediates
or
oxonium
ylides
affords
enantiomerically enriched products.
Then we ruled out enantioselective enol keto
tautomerism in the products under the influence of
the phosphoric acid. Reactions of racemic product 7a
and 8a with chiral phosphoric acid (R)-6b or (S)-5c
Once the enol intermediate is formed, it could
generate the final insertion products catalyzed by
BINOL-derived phosphoric acid catalyst (R)-6b
through a [1,3]-proton transfer pathway as shown in
5
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
References
Figure 2. One can see that the enol intermediate first
forms two complexes (R)-CP-Pre and (S)-CP-Pre
with the catalyst. Then they can undergo the
concerted BPA-assisted [1,3]-proton shift to afford
(R)- and (S)-products, respectively. It should be
pointed out that (S)-CP-Pre is 8.9 kcal/mol lower in
energy than (R)-CP-Pre, indicating that it is much
more favorable to form (S)-CP-Pre. In addition, both
(S)-TS and (S)-CP-Post locate below (R)-TS and (R)-
CP-Post, showing that the reaction pathway affording
the product with S configuration is more favorable in
both dynamics and thermodynamics. This is in
consistent with our experimental result discussed
above. Obviously, the reaction mechanism for O–H
insertion of carboxylic acid is significantly different
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reaction. The mild and neutral reaction conditions,
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Experimental Section
General procedure for asymmetric O–H insertion of
acids:
Under N2 atmosphere, the Rh2(TPA)4 (1 mol %),
MgSO4 (100 mg), acid (0.10 mmol) and catalyst R-(6b)
(10 mol %) were added into an oven-dried Schlenk tube.
After CHCl3 (0.5 mL) being injected into the Schlenk tube,
the solution was stirred at 40 °C for 1 min. The diazo
compound (0.15 mmol) was dissolved in 0.5 mL CHCl3
and introduced into the reaction mixture. The resulting
mixture was stirred at 40 °C for 30 min and purified by
flash chromatography.
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Institute of Technology (HIT), the “Fundamental Research Funds
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(21202027), the NCET (NCET-12-0145), the Open Project
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analysis of 7u.
6
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10.1002/adsc.201700572
Advanced Synthesis & Catalysis
COMMUNICATION
Rhodium(II)/Chiral
Phosphoric
Acid
Co-
Catalyzed Highly Enantioselective O–H Bond
Insertion of -Diazoesters
Adv. Synth. Catal. Year, Volume, Page – Page
Yiliang Zhang,+a,b Yuan Yao,+a Li He,a,b Yang Liu,a
and Lei Shi*a,b,c
8
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