(C6F5)3B Catalyzed Chemoselective and ortho-Selective Substitution of Phenols with α-Aryl α-Diazoesters

Article abstract of DOI:10.1002/anie.201608937

The development of an efficient method for the site-selective substitution of unprotected phenols has long been considered as an attractive but challenging task. Herein, we describe a highly chemo- and ortho-selective substitution reaction of phenols with α-aryl α-diazoacetates with commercially available (C₆F₅)₃B as the catalyst. This reaction proceeds under simple and mild conditions with high efficiency, it features a wide substrate scope and can be easily scaled up.

Full text of DOI:10.1002/anie.201608937

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Communications
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International Edition: DOI: 10.1002/anie.201608937
German Edition: DOI: 10.1002/ange.201608937
À
C H Functionalization
(C6F5)3B Catalyzed Chemoselective and ortho-Selective Substitution of
Phenols with a-Aryl a-Diazoesters
Zhunzhun Yu, Yongfeng Li, Jiameng Shi, Ben Ma, Lu Liu, and Junliang Zhang*
À
Abstract: The development of an efficient method for the site-
selective substitution of unprotected phenols has long been
considered as an attractive but challenging task. Herein, we
describe a highly chemo- and ortho-selective substitution
reaction of phenols with a-aryl a-diazoacetates with commer-
cially available (C6F5)3B as the catalyst. This reaction proceeds
under simple and mild conditions with high efficiency, it
features a wide substrate scope and can be easily scaled up.
C H bond substitution of phenols, on the other hand, has long
been considered as an attractive but challenging task.
Recently, our group[7] and Shi[8] and co-workers independ-
ently developed gold-catalyzed highly chemoselective and
para-selective substitution reactions of phenols by making use
of the specific carbophilicity of gold and the strong directing
ability of hydroxy groups. In continuation of our interest in
[9]
À
C H bond substitution by carbene transfer, we wished to
develop a new catalytic system to realize an intermolecular
À
D
iazo compounds are essential and useful reactive sub-
strates that can undergo a series of transformations, including
ortho-selective C H bond substitution of phenols with
diazoesters.[10] However, this ortho-selective substitution
reaction poses more challenges than the para-selective one
owing to the small differences in the nucleophilicities of the
ortho and para positions of phenols and the greater steric
hindrance for the ortho position.[11]
alkene cyclopropanation, metal carbene migratory insertion,
À
À
C H bond functionalization, X H insertion (X = O, N, Si,
etc.), and ylide formation.[1] Among these transformations,
2
À
transition-metal-catalyzed C(sp ) H bond insertions with
carbenes represent atom- and step-economic methods for
To overcome the problems described above, we reasoned
that a bifunctional hydrogen-bonding catalyst, in which the
hydrogen-bond acceptor recognizes the hydrogen-bond
donor of the phenol and orients the ortho-C H bond,
would facilitate the desired ortho substitution of phenols.
carbon–carbon bond formation.[2] However, direct C H bond
À
À
substitution reactions of aromatic compounds with X H
[12]
À
bonds, such as phenols, which are widely found in numerous
natural products, bioactive compounds, pharmaceuticals, and
polymers and also constitute common versatile building
With this idea in mind, (C6F5)3B, a strong Lewis acid that is
blocks in organic synthesis,[3] are rather challenging as X H
used for H H and Si H bond activation and alkene
polymerization and plays a significant role as a component
of frustrated Lewis pairs, attracted our attention.[13,14] We
hypothesized that a hydrogen bond between a fluorine atom
and the hydroxy group could direct the diazo compound to
the ortho position of phenol, and the boron catalyst could
serve as a Lewis acid to activate the diazo compound.[15]
Herein, we present the first boron-catalyzed highly chemo-
À
À
À
insertion is more favorable in the presence of various metal
catalysts, such as those based on Rh, Cu, Ru, Fe, or Pd
(Scheme 1).[4] The Fu[5] and Zhou[6] groups have developed
elegant metal-catalyzed enantioselective versions for this type
of reaction. The development of methods for the site-selective
À
selective and ortho-selective C H bond substitution reaction
of phenols with a-aryl a-diazoacetates under mild conditions.
This method provides reliable and efficient access to diaryl
acetates, which are important motifs in biologically active
compounds, pharmaceuticals, and natural products
(Figure 1).[16]
Our initial experiment was performed with phenol (1a)
and a-phenyl a-diazoacetate 2a in the presence of (C6F5)3B
(10 mol%) in CH2Cl2 at room temperature. As expected, the
Scheme 1. Transformations of phenols with diazoesters.
À
desired ortho-C H bond substitution product 3aa was
[*] Z. Yu, Y. Li, B. Ma, Prof. Dr. L. Liu, Prof. Dr. J. Zhang
Shanghai Key Laboratory of Green Chemistry and Chemical Pro-
cesses, Department of Chemistry
East China Normal University
3663 North Zhongshan Road, Shanghai 200062 (China)
E-mail: jlzhang@chem.ecnu.edu.cn
J. Shi
Department of Physics, East China Normal University
3663 North Zhongshan Road, Shanghai 200062 (China)
Figure 1. Diaryl acetate subunits in natural products, pharmaceuticals,
and bioactive molecules.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608937.
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obtained as a single regioisomer in promising yield (57%),
À
along with the O H insertion product 5aa (9.5%) and the
water insertion product 6aa (18%; Table 1, entry 1). Then,
various boron catalysts were screened but much poorer
results were obtained (entries 2–4). These results indicated
that the nature of the boron catalyst is crucial for the desired
Table 1: Optimization of the reaction conditions.
Entry
R (2)
Catalyst
Yield[e] [%]
3/4/5/6
1[a]
2[a,b]
3[a,b]
4[a,b]
5[a,c]
6[a,b]
7[d]
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Et (2b)
(C6F5)3B
Ar3B
(C6F5)2BCl
BF3·Et2O
(C6F5)3B
FeCl3
(C6F5)3B
(C6F5)3B
(C6F5)3B
(C6F5)3B
57/0/9.5/18
0/0/5/69
0/0/6/92
17/0/39/8
44/0/8.8/37
5/15/23/25
84(75)/0/13/–
82(74)/0/11/–
92 (90)/0/8.5/–
0/11/0/–
Scheme 2. Variation of the diazo coupling partner. The ratios in
À
À
parentheses are the ratio of ortho-C H bond substitution to O H
À
insertion product. Yields of isolated ortho-C H bond substitution
8[d]
products are given.
9[d]
iPr (2c)
tBu (2d)
10[d]
[a] 1a (0.6 mmol), 2a (0.4 mmol), catalyst (10 mol%). [b] Run for 12 h.
[c] With 4 ꢀ M.S. [d] 1a (0.4 mmol), 2 (0.6 mmol), catalyst (5 mol%).
[e] Determined by NMR spectroscopy using CH2Br2 as an internal
standard. Yields of isolated products are given in parentheses. Ar=2,6-
F2C6H3.
isopropyl diazo esters with both electron-donating and
electron-withdrawing groups on the aryl ring, affording the
À
desired ortho-C H bond substitution products in moderate to
good yields (50% to 83%) with high chemoselectivity
(> 10:1). The amount of diazo compound had to be increased
(2.0 equiv) when sterically hindered diazo esters, such as 2k
and 2k’, were employed. a-Naphth-2-yl a-diazoacetate 2l also
worked well, providing product 3al in 57% yield with very
high chemoselectivity (28:1). Aside from isopropyl a-aryl
a-diazoacetates, methyl a-aryl a-diazoacetates 2e’–2m’ could
also be used in this transformation but reacted with lower
reaction to occur. A solvent screen showed that CH2Cl2 is best
for this transformation. To suppress the formation of the
water-insertion side product 6aa, 4 ꢀ molecular sieves (M.S.)
were added. Unfortunately, this was not beneficial to the
reaction (entry 5). We wondered whether certain metals are
also suitable catalysts for this unprecedented process. How-
ever, no satisfactory results were obtained upon testing
a series of metals (see the Supporting Information). The
À
chemoselectivities and gave the C H substitution products in
reduced yields, except for 3ak’ and 3al’. The structure of 3aj
was confirmed by single-crystal X-ray crystallography.[17] It is
noteworthy that all of the reactions are ortho-selective, and
À
ortho-C H bond substitution product 3aa was observed only
À
when using FeCl3 as the catalyst, albeit in low yield with low
site and chemoselectivity (entry 6). Having identified
(C6F5)3B as the best catalyst and CH2Cl2 as the best solvent,
we turned our attention to other reaction variables. Gratify-
ingly, the yield was improved to 84% when the amount of
diazo compound 2a was increased, and the catalyst loading
could be reduced to 5 mol% (entry 7). Further studies
demonstrated that the ester substituent of diazo ester 2 has
a significant effect on yield and selectivity. The best result was
obtained with an isopropyl group. (entry 9). Astonishingly,
that the para-C H bond functionalization products were not
observed.
The reactions between various substituted phenols and
a-phenyl a-diazoacetates were then examined. As depicted in
Scheme 3, the reactions proceeded smoothly, affording the
desired ortho-functionalized products in moderate to good
yields and chemoselectivity. The reactions also showed
a remarkable substituent effect. For meta-substituted phenols,
two regioisomers were usually obtained. However, only one
À
isomer (3sc)was formed through substitution of the C H
À
only the para-C H substitution product 4ad (11%) was
bond para to the substituent when a phenol derivative with
a bulky tert-butyl substituent in the meta position was
employed. On the other hand, ortho-substituted phenols
were found to be incompatible with this transformation,
furnishing only trace amounts or no product at all. We
surmised that this might be due to the hydrogen-bonding
interaction being prevented by steric hindrance. Interestingly,
formed when the more bulky tert-butyl diazo ester was used
(entry 10).
With optimized reaction conditions in hand, we next
investigated the scope of this highly chemoselective and
À
ortho-selective C H bond substitution reaction of phenol 1a
with various isopropyl-substituted a-aryl a-diazoacetates 2
À
(Scheme 2). Our strategy is indeed applicable to a range of
lactone 3wa was obtained by tandem C H substitution and
2
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Scheme 4. Gram-scale reaction and synthetic applications. Reaction
conditions: a) (C6F5)3B (1 mol%), 2c (1.5 equiv), CH2Cl2 (0.53m), RT;
b) 3ac, NBS (2.5 equiv), CH2Cl2/DMF (5:1), 08C; c) 3ac, pyridine (2.0
equiv), Tf2O (2.0 equiv), CH2Cl2, 08C to RT; d) 8, phenylboronic acid
(1.5 equiv), Pd(PPh3)4 (10 mol%), Cs2CO3 (1.5 equiv), THF/H2O
(10:1), 708C; e) 3ac, LiAlH4 (2.0 equiv), THF, 08C; f) 3ac, TFA
(20 mol%), toluene, 908C; g) 3ac, (2,4-tBu2C6H3O)3PAuSbF6
(5 mol%), 2c (0.67 equiv), CH2Cl2, RT. NBS=N-bromosuccinimide,
Tf =trifluoromethanesulfonyl, TFA=trifluoroacetic acid.
À
C H bond substitution products could be used as versatile
synthons. For example, bromination of 3ac afforded 7 in 90%
yield. In addition, hydroxy groups are common precursors for
coupling reactions. Coupling product 9 was obtained in 79%
yield after converting the hydroxy group into triflate 8.
Reduction of 3ac with LiAlH4 gave alcohol 10 in 99% yield,
and TFA-catalyzed lactonization could afford benzofuranone
11 in excellent yield, which is a prominent structural motif in
natural products.[18] Finally, further para-C H functionaliza-
À
tion could be achieved with a gold catalyst, furnishing 12 in
52% yield.
To gain mechanistic insight, several control experiments
were carried out (Scheme 5). First and foremost, we won-
dered whether the ortho selectivity arise from hydrogen-
bonding interactions or not. With this mind, NMR titration
experiments were carried out (see the Supporting Informa-
tion for details). 19F NMR analysis showed that the single 19
F
resonance of (C6F5)3B was shifted downfield when more
phenol was mixed with (C6F5)3B while the 11B resonance
(À0.68 ppm) of (C6F5)3B did not undergo any changes; this
finding is incompatible with the formation of a four-coordi-
nate boron intermediate.[19] These results indicated that the
predominant interaction between phenol and (C6F5)3B was
due to hydrogen bonding and not to coordination between B
and OH. Based on the above results, we hypothesized that the
Scheme 3. Variation of the phenol component. The ratios in parenthe-
À
À
ses are the ratio of the ortho-C H bond substitution to the O H
À
insertion product. Yields of isolated ortho-C H bond substitution
products are given. [a] (2,4-tBu2C6H3O)3PAuSbF6 (5 mol%), CH2Cl2
(0.08m), RT. [b] 2a was used. [c] 2c was used. Bn=benzyl, TBS=tert-
butyldimethylsilyl.
À
cyclization when sterically congested 3,4,5-trimethylphenol
was used.
key to ortho-C H substitution is the hydrogen bond between
a fluorine atom and the hydroxy group. Subsequently, the
In previous work, we had disclosed the gold-catalyzed
reaction between anisole and 2a was performed, which gave
À
À
ortho-C H bond functionalization of para-substituted phe-
the para- and ortho-C H functionalization products in only
nols with diazoacetates.[7] We thus compared the gold-
catalyzed process with the present catalyst system, and the
results revealed that the present (C6F5)3B catalyst system has
obvious advantages over the gold method (Scheme 3; 3bc,
3ec, 3ic).
13% and 9% yield, respectively (determined by NMR
spectroscopy, Scheme 5a). Furthermore, competition experi-
ments between phenol and anisole with 2a under the standard
reaction conditions showed that the anisole has a much lower
reactivity than the corresponding phenol. These results
further support the hypothesis that the ortho selectivity is
due to a hydrogen-bonding interaction between the phenol
and the reactive intermediate (Scheme 5b). Moreover, the
reaction does not exhibit a kinetic isotope effect, revealing
To our delight, the present method can be easily scaled up
(Scheme 4). A gram-scale reaction of 1a with 2c was
performed at lower catalyst loading (1 mol%), and afforded
the desired product 3ac (1.1 g, 77%). Moreover, these ortho-
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In conclusion, we have described the first catalytic ortho-
À
selective C H substitution of unprotected phenols with
(C6F5)3B as the catalyst. This transformation constitutes
a simple, efficient, and reliable approach to a variety of
useful diaryl acetates under mild conditions. This reaction is
a hydrogen-bond-directed process, which was supported by
NMR studies and control experiments. This work represents
[14]
À
a rare example of B(C6F5)3 catalyzed C C bond formation
and will broaden the application of (C6F5)3B in organic
synthesis.
Acknowledgements
We are grateful to the 973 Program (2015CB856600), the
National Natural Science Foundation of China (21372084,
21425205, 21572065), and the Changjiang Scholars and
Innovative Research Team in University (PCSIRT) for
financial support.
Keywords: boron catalysis · chemoselectivity ·
À
C H functionalization · diazo compounds · phenols
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À
that the C H cleavage is not involved in the rate-determining
step, and the reaction may proceed by electrophilic addition
(Scheme 5c). Furthermore, control experiments showed that
the hydroxy group acts as a proton source during the reaction
as a deuterium was incorporated in the final product when
[D]-1a was used. The proton at the a-position of the acetate
in the product, however, did not undergo proton–deuterium
exchange under the reaction conditions, indicating that no
enolization occurred (Scheme 5d). Stephan and co-workers
have demonstrated that a very interesting C6F5 group migra-
tion reaction occurs upon mixing (C6F5)3B with a-alkyl diazo
esters.[20] To determine whether a similar C6F5 group migra-
tion reaction takes place in our process, control experiments
were carried out, which showed that the water insertion
reaction is preferred over C6F5 migration with the a-phenyl
diazo ester as the water insertion product was obtained in
99% yield (see the Supporting Information). It is noteworthy
that the migration product was indeed formed under the same
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was subjected to the reaction with phenol, the ortho
substitution and the OH insertion product were obtained in
8 and 11% yield, respectively (determined by NMR spec-
troscopy; see the Supporting Information for details). These
results are consistent with previous observations that a-aryl
diazo esters often react very differently to a-alkyl diazo
esters.[1,21]
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Received: September 13, 2016
Published online: && &&, &&&&
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Communications
À
C H Functionalization
Z. Yu, Y. Li, J. Shi, B. Ma, L. Liu,
J. Zhang*
&&&& — &&&&
(C6F5)3B Catalyzed Chemoselective and
ortho-Selective Substitution of Phenols
with a-Aryl a-Diazoesters
Commercially available (C6F5)3B as the
catalyst enables the highly chemo- and
ortho-selective substitution of phenols
with a-aryl a-diazoacetates. This reaction
proceeds under simple and mild condi-
tions with high efficiency, features a wide
substrate scope, and can be easily scaled
up.
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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