Boron-Catalyzed O-H Bond Insertion of α-Aryl α-Diazoesters in Water

Article abstract of DOI:10.1021/acs.orglett.8b01988

A catalytic, metal-free O-H bond insertion of α-diazoesters in water in the presence of B(C₆F₅)₃·nH₂O (2 mol %) was developed, affording a series of α-hydroxyesters in good to excellent yields. The reaction features easy operation and wide substrate scope, and importantly, no metal is needed as compared with the conventional methods. Significantly, this approach further expands the applications of B(C₆F₅)₃ under water-tolerant conditions.

Full text of DOI:10.1021/acs.orglett.8b01988

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Boron-Catalyzed O−H Bond Insertion of α‑Aryl α‑Diazoesters in
Water
Htet Htet San,† Shi-Jun Wang,† Min Jiang,‡ and Xiang-Ying Tang*,†
†School of Chemistry and Chemical Engineering and Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica,
Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China
‡Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry (SIOC), Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A catalytic, metal-free O−H bond insertion of α-diazoesters in water
in the presence of B(C6F5)3·nH2O (2 mol %) was developed, affording a series of α-
hydroxyesters in good to excellent yields. The reaction features easy operation and
wide substrate scope, and importantly, no metal is needed as compared with the
conventional methods. Significantly, this approach further expands the applications
of B(C6F5)3 under water-tolerant conditions.
he strong Lewis acid, tris(pentafluorophenyl)-borane,
B(C6F5)3, and other related electron-deficient boron
particular, the O−H bond insertion of α-diazoesters with water
is of great importance not only because water is one of the
most abundant and green resources for C−O bond
construction but also due to its high simplicity for the direct
synthesis of free α-hydroxyesters in an atom-economical way.17
Indeed, numerous efforts have been devoted to developing
efficient metal catalysts for this important reaction, including
those based on Cu(I),17a,d Au(I),17b Fe(II),17c and Rh(II)17e
(Scheme 1, a). In light of the importance of developing green
T
catalysts have recently drawn much attention for their broad
applications in many useful transformations,1 including
reduction of unsaturated substances,2 dehydrogenations,3
hydrosilylations,4 hydroaminations,5 hydroborations,6 dehy-
drogenative cross couplings,7 polymerizations,8 and other
reactions.9 B(C6F5)3 has also emerged as one of the most
important components in Frustrated Lewis Pair (FLP)
chemistry10 after the seminal work reported by Stephan and
Erker.11 To date, the aim of developing more practical and
widely applicable synthetic methods using the B(C6F5)3
catalyst becomes increasingly important. One of the major
concerns is that B(C6F5)3 always faces the dilemma of water
hydrolysis via B−C bond protonolysis due to its strong Lewis
acidity. Such shortcomings have seriously hampered the
development of B(C6F5)3 or FLP-involving chemistry.12
Thus, many efforts have been devoted to applying B(C6F5)3-
catalyzed reactions under water-tolerant conditions.13,14 For
example, Shibuya reported a boron-catalyzed intramolecular
double hydrofunctionalization reaction of alkynes using water
as a proton source.14a Moran disclosed that B(C6F5)3·H2O
could be used as a Brønsted acid in the azidation of tertiary
aliphatic alcohols14e and the Friedel−Crafts reactions of
tertiary aliphatic fluorides.14b The groups of Stephan,14l
Scheme 1. Transition Metal-Catalyzed or Metal-Free OH
Insertions with α-Diazoesters
14f
Ashley,14k and Soos
also independently discovered that
́
B(C6F5)3/ethers were efficient FLP catalysts in the challenging
hydrogenation of ketones/aldehydes to alcohols. In addition,
the groups of Fu14h and Ingleson14c also developed reductive
aminations of carboxylic acids and aldehydes catalyzed by
B(C6F5)3, respectively. Despite those advances, the perform-
ance of B(C6F5)3 in reactions using water as the solvent is still
unknown to the best of our knowledge.
Transition metal-catalyzed insertion of α-diazoesters into
C−H15 or X−H (X = heteroatoms)16 bonds is a powerful
method for easy construction of C−C and C−X bonds. In
Received: June 25, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01988
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
processes and also continuing our research interests in
insertion reactions,18 it is highly desirable to perform this
important reaction in water (Scheme 1, b). However, this
could be very challenging owing to the low solubility of α-
hydroxyesters in water.
To solve the problem described above, we envisaged that a
bifunctional catalyst that could activate α-hydroxyesters and
also pull a water molecule closer via hydrogen bonding would
facilitate the desired water insertion (Scheme 1, c). Recently,
an elegant ortho-selective substitution reaction of phenols with
α-aryl α-diazoesters was reported by Zhang and co-workers19
in which B(C6F5)3 acted as a bifunctional hydrogen-bonding
catalyst. Inspired by this work, we tested our hypothesis by
conducting the desired reaction in water in the presence of
B(C6F5)3·nH2O.
With the optimal reaction conditions in hand (Table 1, entry
3), we next turned our interest to evaluating the substrate
scope of the reaction by varying the structure of diazoesters. As
can be seen in Scheme 2, the reactions of para-substituted
a
Scheme 2. Substrate Scope
Upon treatment of 1a in 2 mL of water in the presence of 2
mol % of B(C6F5)3·nH2O at 40 °C for 2 h, the reaction gave
the corresponding water insertion product 2a in 68% yield
(Table 1, entry 1). Considering that alcohol 2a could also take
a
Table 1. Screening of the Reaction Conditions
b
entry
x
y (mL)
time (min)
yield (%)
c
1
2
3
4
5
6
2
2
2
5
1
2
2
2
2
2
2
2
2
4
180
240
30
30
30
30
50
50
50
30
30
30
68 (55)
65
0.5
0.5
0.5
0.1
0.5
0.5
0.5
0.5
0.9
0.5
80
71
75
70
62
70
74
68
75
56
a
Reactions were carried out on a 0.2 mmol scale under air at 40 °C in
b
c
deionized water. Isolated yields. On a 1.0 mmol scale.
d
7
e
8
f
substrates 1 all went smoothly to give the corresponding
products 2b−2i in good yields, and the electronic properties of
the substituents did not have a significant effect on the reaction
outcomes. For substrates with meta-substituents, the reaction
also furnished products 2j−2l in moderate yields. When ortho-
substituted diazoesters were treated under the standard
reaction conditions, products 2m−2p were obtained in 65−
80% yields, respectively. As for disubstituted diazoesters,
products 2q and 2r were obtained in 79 and 57% yields,
respectively. It is worth noting that naphthyl and thienyl
groups were also compatible with this reaction, and products
2s and 2t were produced in 65 and 71% yields. In addition,
changing the ethyl group of the diazoesters to methyl,
isopropyl, benzyl, and isopentyl groups, corresponding
products 2o−2u and 2v−2x were delivered in moderate
yields. Notably, upon conducting the reaction of 1a on a 1.0
mmol scale, 2a could still be produced in 74% yield.
To gain some insight into the reaction mechanism, we
conducted several control experiments (Scheme 3). Treating
diazoester 1a in D2O, the reaction gave isolated 2a-d in 75%
yield with 95% D content. In the presence of 2a, the water
insertion of 1a was not inhibited, and no alcohol insertion
product was observed, indicating that the binding between 2a
and B(C6F5)3·nH2O cannot be too strong. Substrate 3 with a
benzyl group could not undergo such water insertion,
presumably due to the fact that the putative cation
intermediate could not be stabilized by the benzyl group. It
was reported by Wasa and co-workers20 that the conjugated
acids in FLPs could act as Brønsted acids; therefore, PPh3 and
9
g
10
11
12
h
i
a
Reactions were carried out on a 0.2 mmol scale under air in
b
c
deionized water. Isolated yields. Reaction temperature of 25 °C.
d
e
f
With 0.4 mL of MeNO2. With 0.3 mL of MeNO2. With 0.2 mL of
g
h
i
MeNO2. With 0.1 mL of MeNO2. With 0.1 mL of tBuOH. With
0.5 mL of tBuOH.
part in the reaction as a nucleophile, we performed the reaction
in diluted solution; however, the yield of 2a slightly decreased
to 65% after 4 h (Table 1, entry 2). When performing the
reaction in 0.5 mL of H2O, the reaction proceeded much faster
and delivered 2a in 80% yield (Table 1, entry 3). Increasing or
decreasing the loadings of B(C6F5)3·nH2O, the reactions
furnished product 2a in 71 and 75% yields, respectively (Table
1, entries 4 and 5). Higher concentration did not increase the
efficiency of this reaction (Table 1, entry 6). MeNO2 was
demonstrated by Moran and co-workers to have a cocatalytic
effect in the B(C6F5)3·H2O-catalyzed azidation of tertiary
aliphatic alcohols.14e Therefore, different amounts of MeNO2
were added to the reaction system, but the yields of 2a were
not increased (Table 1, entries 7−10). Meanwhile, the reaction
was also performed in mixed solvent (tBuOH/H2O), and 2a
was obtained in 75 and 56% yields, respectively (Table 1,
entries 11 and 12).
B
DOI: 10.1021/acs.orglett.8b01988
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Organic Letters
Letter
4). As water was consumed, all three peaks, which were not
identical with anhydrous B(C6F5)3,21 showed no significant
changes (Figure 1, 4−6), suggesting that 1a may coordinate
with B(C6F5)3 in the absence of water.
Scheme 3. Control Experiments
A considerable amount of 2a was formed in the above
titration experiments because the addition of 1a into the NMR
tube would introduce moisture.22 For further excluding the
influence of alcohol product, 20.0 equiv of 1f was added in one
portion to the solution of B(C6F5)3·nH2O in an NMR tube to
avoid moisture interruption. As can be seen in Figure 2, the set
Figure 2. 19F-NMR experiments of 1a with B(C6F5)3·nH2O. (1)
B(C6F5)3·nH2O (5.0 mg, 1.0 equiv) in 0.6 mL of toluene-d8; (2) 1f
(41.6 mg, 0.2 mmol, 20.0 equiv) was added; and (3) tested by 19F
NMR 1 h later.
TMP were added to the mixture of 1a and B(C6F5)3 in
toluene, respectively. In the presence of PPh3, the reaction was
very slow, and 2a was formed in 45% yield, whereas the
reaction with TMP only gave trace amount of product after 24
h. In addition, if conducting the reaction in mixed solvent
(THF/toluene), the reaction was also inhibited. These results
indicated that B(C6F5)3·nH2O is more active than correspond-
ing conjugated acids A−C, probably because these conjugated
acids cannot serve as bifunctional hydrogen bonding catalysts.
To probe the real catalyst of this water insertion further, we
performed several NMR experiments (Figure 1). No obvious
chemical shifts changed when the first two equiv of 1a were
added (Figure 1, 1−3); however, all three peaks shifted
downfield when another 3.0 equiv of 1a was added (Figure 1,
of three peaks also shifted downfield (Figure 2, 2) and were
almost identical with the final 19F NMR spectrum of 1a with
B(C6F5)3 obtained in the above titration experiments (Figure
1, 4−6). We also tested the NMR again after 1 h. No obvious
chemical shift was observed, and a large amount of 1f remained
unreacted (Figure 2, 3). These results further confirmed that 1f
could coordinate with B(C6F5)3 in the absence of water.
On the basis of the discovery presented above, we believed
that the reaction of phenol with 1a could also occur under our
reaction conditions. Indeed, upon treatment of phenols with α-
aryl α-diazoester 1a in dry DCE in the presence of a catalytic
amount of B(C6F5)3·nH2O, the reaction also proceeded
smoothly to give ortho-substituted product 3a in 65% yield
together with 2a as a minor product. We assumed that water
insertion would take place first. After water was consumed, the
Friedel−Crafts reaction then occurred. This result also
provides good support for bifunctional hydrogen-bonding
catalyst of B(C6F5)3 proposed by Zhang.19 Furthermore,
classic Brønsted acids were used as catalysts in the reaction
of 1a with water. It was found that both HCl and HOTf were
less effective than B(C6F5)3·nH2O, indicating hydrogen
bonding may have a significant influence on the reaction
outcome (Scheme 4).
On the basis of the results presented above and reported
literature,14a,19 two mechanisms are proposed in Scheme 5.
First, diazoester 1a can be protonated by B(C6F5)3·nH2O to
form intermediate D followed by nucleophilic attack by H2O
to furnish the desired product 2a (path a). On the other hand,
we assume that B(C6F5)3·nH2O acts as a bifunctional catalyst
in the reaction. B(C6F5)3·nH2O can protonate 1a to generate
activated cationic intermediate D′. Meanwhile, water is pulled
closer to 1a via hydrogen bonding and then attacks the carbon
Figure 1. 19F-NMR titration experiments of 1a with B(C6F5)3·nH2O.
(1) B(C6F5)3·nH2O (10.0 mg, 1.0 equiv) in 0.6 mL of toluene-d8. (2)
1a (4.0 mg, 1.0 equiv) was added; (3) another 4.0 mg of 1a (1.0
equiv) was added; (4) 12.0 mg of 1a (3.0 equiv) was added; (5) 20.0
mg of 1a (5.0 equiv) was added; and (6) 28.0 mg of 1a (7.0 equiv)
was added.
C
DOI: 10.1021/acs.orglett.8b01988
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Letter
Scheme 4. Evaluation of Hydrogen Bonding
ACKNOWLEDGMENTS
■
We are grateful for financial support from Huazhong
University of Science and Technology (HUST), Opening
fund of Hubei Key Laboratory of Bioinorganic Chemistry &
Materia Medica (No. BCMM201805), and the Fundamental
Research Funds for the Central Universities
(2017KFYXJJ166). We also thank Analytical & Testing Center
of HUST for HRMS analysis.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xytang@hust.edu.cn.
ORCID
Xiang-Ying Tang: 0000-0003-3959-3715
Notes
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The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.8b01988
Org. Lett. XXXX, XXX, XXX−XXX