Site-Selective α-Alkoxyl Alkynation of Alkyl Esters Mediated by Boryl Radicals

Article abstract of DOI:10.1021/acs.orglett.9b00985

A novel method for site-selective C-H functionalization of ethyl acetate mediated by pyridine-boryl radicals is presented, delivering a variety of 4-phenylbut-3-yn-2-yl acetate derivatives under mild conditions. A distinguishing feature of this reaction i

Full text of DOI:10.1021/acs.orglett.9b00985

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Site-Selective α‑Alkoxyl Alkynation of Alkyl Esters Mediated by
Boryl Radicals
Ao Guo,† Jia-Bin Han,† Lei Zhu,‡ Yin Wei,§ 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
‡College of Chemistry and Materials Science, Hubei Engineering University, Hubei 432000, People’s Republic of China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A novel method for site-selective C−H functionalization
of ethyl acetate mediated by pyridine−boryl radicals is presented,
delivering a variety of 4-phenylbut-3-yn-2-yl acetate derivatives under
mild conditions. A distinguishing feature of this reaction is that the
pyridine-ligated boryl radicals can abstract the inactive α-hydrogen of the
alkoxyl group instead of the α-hydrogen of carbonyl groups described in
a previous report using amine-ligated boryl radicals. Significantly,
substrates with halogen atoms are compatible under the reaction conditions.
thyl acetate (EA) is widely used as an organic solvent and
additive in food chemistry due to its good solubility,
example, it was reported that the electrochemical C−H
functionalization of EA would occur at all three types of
hydrogens (Scheme 1, c).4a Patel et al. also reported an
interesting functionalization at the α position of the ethoxyl
group to afford unsymmetrical gem-diacylates. However, only
that carbon−oxygen bond was constructed.4b Given the
significance of radical sp3 C−H functionalization,6 the
development of a site-selective carbon−carbon bond con-
struction at the α-alkoxyl group is of great importance and
should have broad applications in organic synthesis, not only
because of the synthetic challenge but also because acetyl-
group-protected alcohols could be synthesized in one step.
In the past decade, the development of ligated-boryl radicals
has received great attention from the synthetic community.7,8
In particular, NHC−boryl radicals, pioneered by Fensterbank
et al., have shown versatile reactivities in a range of useful
transformations, including in the reduction of a variety of C−X
(X = xanthates, halides, CN) bonds to C−H bonds9 (Scheme
2, a) and other related reactions.10 Moreover, the challenging
addition of NHC−boryl radicals to unsaturated C−C triple
bonds was achieved by Curran et al.,11 providing a facile
synthesis of boron-containing alkenes. Very recently, an
exciting inverse hydroboration of imines using NHC−boryl
radicals was reported by Xie et al.12 Besides NHC−boryl
radicals, the groups of Li and Zhu and Aggarwal independently
reported that the homolysis of the B−B bond in pyridine-13 or
amide14-coordinated diboranes leads to a variety of reductions
and borylations.
E
pleasant aroma, and low toxicity.1 On the other hand, it is also
a cheap and basic chemical feedstock in organic synthesis. In
general, the α-hydrogen of the ester has an increased reactivity
because of the electron-withdrawing property of the carbonyl
group. As a result, two classical synthetic methods, the Claisen
ester condensations2 and enolate alkylations,3 are well
established, affording fundamental processes for carbon−
carbon bond formation in the presence of a stoichiometric
base (Scheme 1, a). Although these groundbreaking efforts
Scheme 1. Synthetic Application of Ethyl Acetates
have made significant progress, investigations to access the less
active α-sites of alkoxyl groups are highly limited (Scheme 1,
b). Indeed, the intrinsic inert properties of these hydrogens
make them notably resistant to participating in an ionic
reaction. A radical approach may be a good choice.4 However,
the site-selective hydrogen abstraction can be problematic
because the bond dissociation energies (BDEs) of these two α-
C−H bonds (Scheme 1, b, Ha and Hb) are too close.5 For
Interestingly, Roberts discovered that amine−boryl radicals
could be used in polarity-reversal catalysis. In the presence of
Received: March 20, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00985
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Scheme 2. Previous Studies on Boryl Radicals and Our New
Method
Table 1. Optimization of the Reaction Conditions
b
entry
x/y/z
oxidant
B
temp. (°C) time (min) yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2/1/3
2/1/3
2/1/3
2/1/3
2/1/3
2/1/3
2/1/3
2/1/3
2/1/3
3/1/3
1/1/3
2/1/2
2/1/1
2/1/0
2/1/2
2/1/2
2/1/2
2/1/2
2/1/2
0/0/2
I
II
III
IV
V
B1
B1
B1
B1
B1
B1
B2
B3
B4
B2
B2
B2
B2
B2
B2
B2
B2
B2
B2
B2
20
20
20
20
20
20
20
20
20
20
20
20
20
20
30
40
50
30
30
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
30
10
10
9
3
48
-
0
32
59
-
-
58
11
60
22
-
66
62
VI
III
III
III
III
III
III
III
-
III
III
III
III
III
III
an alkoxyl radical (tBuO·), amine−boryl radicals can
selectively abstract the α-H of ethyl acetate, followed by a
radical addition of olefin to yield methyl 3-(3,3-dimethyloxir-
an-2-yl)propanoates (Scheme 2, b).15 Likewise, pyridine−
boryl radicals can also abstract a hydrogen to generate alkyl
radicals with the assistance of an oxidant. Previously, we
reported the first pyridine−boryl-radical-mediated α-C−H
functionalizations of ethers, DMF (N,N-dimethylformamide),
and amines with 4-cyanopyridine 1-oxide as the oxidant.16 To
continue our research, it was of interest to explore the unique
reactivities of such boryl radicals, and we envisioned that α-
hydrogen abstraction could occur in ethyl acetate. In fact, the
reaction took place at the α-position of the alkoxyl group. In
this study, we report an unprecedented boryl radical-initiated
C−H functionalization of ethyl acetate with ((arylethynyl)-
sulfonyl)benzenes, affording diverse 4-arylbut-3-yn-2-yl acetate
derivatives under mild conditions (Scheme 2, c). Notably, the
transformation can be extended to other aliphatic acetates, and
halogens are compatible with the reaction conditions.
We set out on our research by stirring alkyne sulfones with
ethyl acetate in the presence of boryl radical (BPin·) formed in
situ. First, pyridine 1-oxide (I), 4-nitro-pyridine 1-oxide (II), 4-
cyanopyridine 1-oxide (III), pyridine 1-oxide (IV), 2,6-
dimethylpyridine 1-oxide (V), and 2,6-dichloropyridine 1-
oxide (VI) were employed as the oxidants, and 4-
cyanopyridine 1-oxide was the best to furnish the desired
product 2a in 48% yield in 1 h (Table 1, entries 1−6). Next,
several borates were tested, and it was found that B2
[bis(neopentyl glycolato)diboron, B2(nep)2] exhibited benefi-
cial effects, and the yield of 2a increased to 59% (Table 1,
entries 7−9). When changing the amount of B2(nep)2 to 3
equiv or 1 equiv, the yields of 2a decreased in both cases
(Table 1, entries 10 and 11). If the amount of 4-cyanopyridine
1-oxide decreased to 2 equiv, interestingly, the yield of 2a
increased slightly to 60% (Table 1, entry 12). However, further
reducing the amount of oxidant to 1 equiv only gave the
desired product in 11% yield (Table 1, entry 13). The
temperature effect was also evaluated, and it was found that 30
°C was more suitable for this transformation (Table 1, entries
15−17). Notably, reaction time was also very important, as
performing the reaction in 30 and 10 min, the starting material
1a was completely consumed, producing the product 2a in
63
68
74 (71)
-
c
30
a
b
Reaction conditions: 0.3 mmol scale. Yields were determined by gas
c
chromatography using n-dodecane as an internal standard. The
isolated yield is shown in parentheses. I: pyridine 1-oxide; II: 4-nitro-
pyridine 1-oxide; III: 4-cyanopyridine 1-oxide; IV: 4-methylpyridine
1-oxide; V: 2,6-dimethylpyridine 1-oxide; VI: 2,6-dichloropyridine 1-
oxide. B1: Bis(pinacolato)diboron; B2: Bis(neopentyl glycolato)-
diboron; B3: 2,2′-Bis-1,3,2-benzodioxaborole; B4: Bis(hexylene
glycolato)diboron.
68% and 74% yields, respectively (Table 1, entries 18 and 19).
In the absence of 4-cyanopyridine 1-oxide or without the
radical generation system (cyanopyridine and borate), the
reaction did not take place (Table 1, entries 14 and 20).
With the best reaction conditions in hand, we next evaluated
the substrate scope of the reaction by varying the structure of
alkyl sulfones 1 (Scheme 3). For substrates with para-
substituents, all the reactions proceeded smoothly to furnish
the desired products 2b−2j in moderate yields, and the
electronic properties of the substituents did not have a
significant effect on the reaction outcomes. Meta- or ortho-
substituted substrates were also suitable for this reaction, and
the corresponding products 2k−2o were obtained in 60−71%
yields. Treatment of a substrate bearing a naphthyl substituent
furnished product 2p in 73% yield. Notably, heteroaromatic
rings substituted with sulfones were also tolerated in this
reaction and provided corresponding products 2q and 2r in
50% and 57% yields, respectively. Furthermore, alkynyls and
amides were also compatible with the reaction conditions, and
the corresponding products 2s and 2t were obtained in 45%
and 74% yields, respectively. Notably, the reaction of 1a could
be scaled up to 1 mmol, and 2a was obtained in 68% yield.
Encouraged by the above results, we next attempted to
expand this method to other esters instead of ethyl acetate.
Upon conducting the reactions of 1 with 2 equiv of B2(nep)2, 1
equiv of 4-cyanopyridine, and 3 equiv of 4-cyanopyridine 1-
oxide in 30 min in several commercially available ester
B
DOI: 10.1021/acs.orglett.9b00985
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
reaction probably proceeds through a radical pathway (Scheme
5, eq 1). Furthermore, upon the replacement of the radical
Scheme 3. Reactions of Various Alkyl Sulfones 1 with EA
Scheme 5. Control Experiments
initiation system by AIBN [2,2′-azobis(2-methylpropioni-
trile)], DTBP (di-tert-butyl peroxide), and TBHP (tert-butyl
hydroperoxide), no desired product was observed in any of the
cases (Scheme 5, eq 2) (see Supporting Information for
detailed information).
To further understand the polar effects in this reaction, a
calculation was conducted to determine the electron density
distribution (Figure 1). The spin density on the B atom is only
a
All reactions were carried out on a 0.3 mmol scale in 3 mL of EA.
b
c
Isolated yields. 1 mmol scale.
solvents, the desired products 3a−3n were obtained in
moderate yields with excellent site selectivities (Scheme 4).
a b
,
Scheme 4. Reactions of 1 with Other Esters
Figure 1. Computational calculation at the MP2/6-31G(d) level.
0.027, indicating that the single electron of the free radical is
not localized on the boron atom but is delocalized on the
pyridine ring. According to polar effects reported by Ryu6a and
Macmillan,6h the pyridine−boryl radicals should act as
electrophilic species to facilitate the most hydridic hydrogen
abstraction.
Based on the results of the above experiments and our
previous study,16 a plausible mechanism using 1a as a model
was proposed as shown in Scheme 6. Initially, the pyridine−
boryl radical is generated by homolysis of the B−B bond, and
the resonance A is more stable according to computational
calculation. Then, the empty orbital of the boron atom may
a
b
All reactions were carried out on a 0.3 mmol scale. Isolated yields.
It is worth noting that for halogen-containing solvents, such as
ethyl chloroacetate, ethyl bromoacetate, and ethyl fluoroace-
tate, the reactions also took place exclusively at the α-C−H
bond of the ethoxy groups. No dehalogenation9c was observed
in any of the three cases. Further, vinylation was also successful
under the standard reaction conditions, and corresponding
trans-allyl ester derivatives 3o−3q were formed in moderate
yields.
Scheme 6. Proposed Mechanism
To probe the reaction mechanism, several control experi-
ments were performed. With 1 equiv of TEMPO, the reaction
was notably sluggish. Only a small amount of 2a was obtained
after 12 h, while in the presence of 5 equiv of TEMPO, the
reaction was totally inhibited. These results indicate that the
C
DOI: 10.1021/acs.orglett.9b00985
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
coordinate with EA to generate complex B, which exists in
equilibrium with intermediate C. The following intramolecular
1,5-HAT by the 4-cyanopyridine−boryl radical at the C2
position gives intermediate D,17 which is quickly oxidized to
intermediate E together with the formation of HOBnep
byproduct and ligand. Subsequent addition of intermediate E
to the alkynyl sulfone 1a furnishes the final products 2a.
To demonstrate the synthetic utility of this method further,
derivatizations of 3m were performed. Upon treatment of 3m
with NaN3, the substitution reaction was completed in 2 h and
furnished the corresponding product 4 in 92% yield (Scheme
7, eq 1), In addition, in the presence of a base, Na2CO3, 3m
was easily hydrolyzed to give alcohol 5 in 86% yield after 40
min (Scheme 7, eq 2).
ACKNOWLEDGMENTS
■
We are grateful for the financial support provided by the
National Natural Science Foundation of China (21871100),
the Fundamental Research Funds for the Central Universities
(2017KFYXJJ166), Huazhong University of Science and
Technology (HUST), and the Opening Fund of Hubei Key
Laboratory of Bioinorganic Chemistry and Materia Medica
(No. BCMM201805). We also thank the Analytical and
Testing Center of HUST, Analytical and Testing Center of the
School of Chemistry and Chemical Engineering (HUST), for
access to their facilities.
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Scheme 7. Synthetic Applications of Product 3m
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In summary, we have developed a site-selective C−H
functionalization of esters through boryl-radical-enabled hydro-
gen abstraction under mild conditions. The substrate scope is
broad, and a variety of functional groups are compatible with
the reaction. A distinguishing feature of this reaction is that the
boryl radical specifically abstracts the hydrogen of the alkoxyl
C−H bond, instead of the α-hydrogen adjacent to the carbonyl
group. This discovery will be a good addition to electrophilic
boron radical chemistry because there was only one case of the
electrophilic ligated boryl radical (NHC−BF2·) reported by
18
́
̂
Curran, Lalevee, and Lacote. Moreover, halogens are
tolerated under the reaction conditions. Further investigations
into the mechanism and applications of this method in organic
synthesis are currently underway in our laboratory and will be
published in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00985.
Experimental procedures, characterization data for all
new compounds, and selected NMR spectra and IR
spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xytang@hust.edu.cn.
ORCID
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Lei Zhu: 0000-0003-4399-1855
Xiang-Ying Tang: 0000-0003-3959-3715
Notes
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Korol, R.; Liu, L.; Grimme, S.; Stephan, D. W. J. Am. Chem. Soc. 2017,
The authors declare no competing financial interest.
D
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DOI: 10.1021/acs.orglett.9b00985
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