Aryne Trifunctionalization Enabled by 3-Silylaryne as a 1,2-Benzdiyne Equivalent

Article abstract of DOI:10.1021/acs.orglett.8b00469

An unprecedented aryne 1,2,3-trifunctionalization protocol from 2,6-bis(silyl)aryl triflates was developed under transition-metal-free conditions. The reaction of generated 3-silylaryne with both pyridine N-oxide and N-hydroxylamide afforded o-silyl triflate/tosylate in a one-pot transformation, allowing the formation of 2,3-aryne precursors with various vicinal pyridinyl/amido substituents. These pyridinyl-substituted 2,3-aryne intermediates exhibit a broad scope of reactivity with diverse arynophiles in good yields and high selectivity.

Full text of DOI:10.1021/acs.orglett.8b00469

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Aryne Trifunctionalization Enabled by 3‑Silylaryne as a
1,2‑Benzdiyne Equivalent
Chunjie Lv,† Caiwen Wan,† Song Liu, Yu Lan,* and Yang Li*
School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Chongqing, 400030, P. R. China
S
* Supporting Information
ABSTRACT: An unprecedented aryne 1,2,3-trifunctionalization protocol
from 2,6-bis(silyl)aryl triflates was developed under transition-metal-free
conditions. The reaction of generated 3-silylaryne with both pyridine N-
oxide and N-hydroxylamide afforded o-silyl triflate/tosylate in a one-pot
transformation, allowing the formation of 2,3-aryne precursors with
various vicinal pyridinyl/amido substituents. These pyridinyl-substituted
2,3-aryne intermediates exhibit a broad scope of reactivity with diverse
arynophiles in good yields and high selectivity.
highly desirable in terms of diversifying the constitution of 1,2-
s one of the most active organic intermediates, arynes have
Abeen employed in numerous chemical transformations, benzdiyne equivalents as well as developing more arene
multifunctionalization means.2
which led to the syntheses of diverse vicinal difunctionalized
Along with our study on aryne multifunctionalization,5c−f,h
we were curious about the possibility of “reprogramming” the
structural constitution of 1,2-benzdiyne synthons in order to
accommodate broader combinations of functional groups. It is
known that all conventional 1,2-benzdiyne building blocks
contain two leaving groups (LGs) and one accepting group
(AG) on three consecutive positions, the AG of which is either
a halogen or silyl group and the LG can be halogen, OTf, or
OTs (see examples in Scheme 1a).2 Alternatively, is that
plausible to utilize a set of two AGs and one LG to assemble a
1,2-benzdiyne equivalent (Scheme 1b)? After carefully
searching for suitable candidates, 3-silylarynes were chosen,
the silyl groups of which have shown both steric repulsion
character7 and inductive electron-donating ability8 (Scheme
1c). Consequently, we postulated that a trimethylsilyl (TMS)
group on intermediate iii (Scheme 1d) would play dual roles:
manipulating the regioselectivity in 1,2-aryne reaction step and
acting as a new AG in 2,3-aryne precursor.9 Meanwhile, we
intended to use substrates with O-nucleophile in the 1,2-aryne
step so that the new C−O bond could be converted to C-OTf/
OTs on the ortho-position of the TMS group, constituting a
new AG-LG set. Herein, we would like to present an
unprecedented approach toward arene trifunctionalization
enabled by 2,6-bis(trimethylsilyl)phenyl triflates, which in-
cludes an umpolung 1,2-aryne reaction step with either pyridine
N-oxide or N-hydroxylamide (Scheme 1d).
arenes.1 A step further toward aryne multifunctionalization2
could be realized by 1,4-,3 1,3-,3e,f,4 and 1,2-benzdiyne5 and
benztriyne equivalents.6 Among benzdiynes, 1,2-benzdiyne is
unique, primarily because of the double utilization of its C2
position on both 1,2-aryne i and 2,3-aryne ii as well as arene
trifunctionalization on consecutive positions, making 1,2-
benzdiyne a “sesquibenzyne” synthon (Scheme 1a). In contrast
to the recent advances in single aryne transformations,
however, the 1,2-benzdiyne chemistry is still in its infancy. In
view of the enormous aromatic compounds bearing multiple
substituents in biologically active natural products, medicines,
and functional materials, further exploration in this field is
Scheme 1. Background and Our Proposal
Our study commenced with the reaction of 2,6-bis-
(trimethylsilyl)aryl triflate (1) with pyridine N-oxide 2. It has
been previously reported that benzyne reacts with 2 to give
either 3- or 2-(2-hydroxyphenyl)pyridines through a [3 + 2]
cycloaddition−rearrangement cascade.10 As shown in Scheme
Received: February 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00469
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
2, after screening on the reaction conditions (conditions A, see
the Supporting Information (SI) for detailed optimization
dimethyl group was used, 3m was obtained as a 1.2:1 mixture of
regioisomers (Scheme 2).
Although the mechanism between benzyne and pyridine N-
oxide has been previously studied by both the Abramovitch10a
and Larock10b groups, it remains ambiguous on the origin of
the meta vs ortho selectivity. As depicted in Scheme 3, after [3 +
Scheme 2. One-Pot Reaction with Pyridine N-Oxides
Scheme 3. Mechanistic Investigation
2] cycloaddition of iii with 2, intermediate v could be
conceived (X = H), which then undergoes rearrangement to
generate intermediate vi (path a). Hydrogen abstraction on vi
would either produce A (via deprotonation of H3) or B (via
deprotonation of H2). Alternatively, direct deprotonation of H2
on intermediate v will result in the formation of B with an aryl
group on the ortho-position of the pyridine ring (path b). Our
computational study (DFT method M11 with 6-311+G(d)
basis set) reveals that the energy of vi (−62.7 kcal/mol) is
significantly lower than that of v (−46.2 kcal/mol), suggesting
that vi would be readily formed from v irreversibly. Meanwhile,
the transition state (TS) energy (−31.6 kcal/mol for TS1) of
this process is much lower than that of TS2 (−25.9 kcal/mol)
for the formation of B (path b), indicating that path a is
preferred over path b. After the formation of vi, the TS energy
for the deprotonation of H3 (TS3, −38.0 kcal/mol) is lower
than that of H2 (TS4, −32.6 kcal/mol), which is consistent
with Larock’s hypothesis on the higher acidity of H3.10b In
addition, our deuterium labeling experiment between Kobaya-
shi aryne precursor and pyridine-3,5-d2 N-oxide gave neither
meta- nor ortho-arylated pyridine product (see Supporting
Information for details), suggesting that compound B could not
be formed from intermediate vi via path a. In order to explain
the formation of 3g, the reaction pathway was calculated when
4-cyanopyridine N-oxide was employed (with a set of pink
energies in Scheme 3, X = CN). Interestingly, in the presence
of 4-cyano group, the TS energy of TS2′ (−30.4 kcal/mol)
becomes lower than that of TS1′ (−28.5 kcal/mol), making
path b a favored means to produce compound B.
After harvesting a series of 2,3-aryne precursors 3 with either
pyridin-3-yl or pyridin-2-yl substituents on the C3 position of
aryne precursors in Scheme 2, compounds 3a and 3e were
chosen to examine their reactivity toward various arynophiles
(Table 1). Upon activation with fluoride, 3a could generate the
corresponding 2,3-aryne intermediates at room temperature;
whereas elevated temperature (100 °C in 1,4-dioxane) was
necessary in order to activate 3e. This different activation
conditions between 3a and 3e can be explained by the weaker
leaving ability of OTs group over OTf group, which has also
been observed by the Suzuki group in their 1,4-benzdiyne
study.3c When morpholine was used, compounds 4 and 6 were
a
Conditions A: (i) 1 (1.0 mmol), 2 (0.5 mmol), and CsF (2.0 mmol)
in MeCN (25 mL), rt; (ii) NaH (60%, 0.75 mmol) and PhNTf2 (0.75
mmol) in THF (30 mL), rt. Conditions B: (i) 1 (1.0 mmol), 2 (0.5
mmol), and TBAF (1.5 mmol) in MeCN (25 mL), rt; (ii) NaH (60%,
0.75 mmol) and TsCl (0.75 mmol) in DCM (30 mL), rt.
b
conditions), a one-pot process was accomplished from 1a and
pyridine N-oxide to produce 3-arylpyridine 3a with concom-
itant incorporation of a Tf group on OH in 75% yield. When
substrates possess electron-donating groups on the 4-position
of pyridine N-oxide, comparable yields of 3b and 3c were
obtained as well. The reaction between 1a and 2-methylpyr-
idine N-oxide afforded product 3d in 63% yield, presumably
attributed to the steric repulsion of 2-methyl group in the [3 +
2] cycloaddition step. In contrast, the reaction with quinoline
N-oxide changed the reaction outcome, affording 2-arylquino-
line 3e in 69% yield after one-pot incorporation of a Ts group
on OH. Unexpectedly, there were no suitable conditions to
introduce a Tf group on the phenoxy intermediate in this step;
instead, the incorporation of a Ts group turned out to be highly
efficient (conditions B, see the SI for detailed optimization
conditions).11 When 3,5-dimethylpyridine N-oxide was utilized,
the reaction afforded 3f in 67% yield using conditions B.
Interestingly, 4-cyanopyridine N-oxide altered the reaction
orientation and afforded 2-arylpyridine 3g in 58% yield under
conditions B. The employment of 2,6-dimethylpyridine N-
oxide could also produce 3h in 70% yield under conditions B.
Replacing one of the TMS groups on 1a with tert-
butyldimethylsilyl (TBS) group as the substrate gave 3i in
75% yield. Furthermore, substituents, such as chloro, methyl,
and methoxy groups, could be introduced onto aryne precursor
framework, and their reactions with 2 afforded 3j−l in good to
high yields. When unsymmetrical aryne precursor with 4,5-
B
DOI: 10.1021/acs.orglett.8b00469
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presumably because of the incompatible aryne generation
conditions of 3e with this arynophile.16 All of the above
examples in Table 1 indicate that both 3a and 3e are efficient
aryne precursors with excellent regiocontrol, the products of
which are normally not readily accessible through traditional
methods. Although aryls, such as a phenyl group, have been
studied as the factors to tune the regioselectivity in aryne
reactions,17 this is the first time aryne chemistry has been
explored in the presence of proximal pyridinyl substituents
using Kobayashi aryne generation conditions, the excellent
regioselectivity of which is primarily attributed to the inductive
electron-withdrawing effect of pyridinyl groups.
Table 1. Reactions of 3a and 3e with Arynophiles
With our desire to expand the synthetic application of this
protocol, we also studied the reaction behavior of 3-
(trimethylsilyl)benzyne (iii) with N-hydroxylamides. The
Wang group has reported that an aryne can insert into their
N−O bond.18 As shown in Scheme 4, satisfyingly, the reaction
a
Scheme 4. One-Pot Reaction with N-Hydroxylamides
a
conditions C: (i) 1 (0.8 mmol), 24 (0.4 mmol), and CsF (1.6 mmol)
in MeCN (20 mL), 50 °C; (ii) NaH (60%, 0.6 mmol) and Tf2O (0.6
mmol) in DCM (20 mL), 0 °C.
of 1a with N-hydroxylamide 24 is highly regioselective and
afforded 25a in 53% overall yield after one-pot protection of
the resulting OH group with Tf (conditions C, see the SI for
detailed optimization conditions). Altering the N-hydroxyla-
mide structure harvested 25b in 62% overall yield. Moreover, a
comparable yield could be achieved on 25c when chloro-
substituted 1 was employed. The examples in Scheme 4
indicate that this 1,2-benzdiyne approach could also lead to the
ready preparation of amide-containing aryne precursors.
Among aryne generation methods, an iodo group has been
used as an effective AG in place of silyl group. In view of the
low electron negativity and large atom radius of an iodo group,
we wanted to learn if we could replace the silyl group on aryne
iii with an iodo group. Consequently, compound 26 was
prepared with iodo as one of the AGs (Scheme 5). When 26
was treated with pyridine N-oxide in the presence of CsF, 27a
a
3a (0.3 mmol), arynophile (0.6 mmol), and CsF (0.6 mmol) in
b
MeCN (10 mL), rt. 3e (0.3 mmol), arynophile (0.6 mmol), CsF (0.6
mmol), and 18-c-6 (0.3 mmol) in 1,4-dioxane (10 mL), 100 °C.
obtained in 85% and 67% yields, respectively; whereas N-
methylpiperazine gave 5 and 7 in 68% and 62% yields,
respectively (entry 1, Table 1). Benzenethiol could also act as a
good nucleophile, generating 8 and 9 in good yields (entry 2).
Diels−Alder reactions of both 3a and 3e with furans could
produce the corresponding products 10−13 in good to high
yields (entry 3). A [3 + 2] cycloaddition of 3a with nitrone
afforded cycloadduct 14 in 62% yield as a 5:1 mixture of
regioisomers, and the same reaction with 3e gave 15 in 60%
yield as a single product (entry 4).12 When benzyl azide was
employed, its reactions with 3a and 3e produced 16 and 17 in
65% and 57% yields, respectively, both of which are single
isomer (entry 5).13 Next, [2 + 2] cycloaddition with 1,1-
dimethoxyethene was tested, and both 18 and 19 were obtained
in moderate yields with high regioselectivity (entry 6).14
Moreover, σ-bond insertion of both aryne precursors with 1,3-
dimethyl-2-imidazolidinone (DMI) performed smoothly to
afford 20 and 21 as single isomer and in good yield (entry
7).15 At last, the reaction of 3a with in situ generated N-
chloromorpholine from morpholine/NCS produced 22 in 56%
yield, whereas no desired product 23 was observed with 3e,
Scheme 5. Reactivity of Aryne Precursor 26
C
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and 27b were obtained in 45% overall yield with a ratio of 1.2:1,
significantly different from the high selectivity of 26 with both
N-methylaniline and benzyl azide reported by the Garg and
Houk groups (Scheme 5, path a).9 The absent selectivity with
compound 26 in this reaction can be rationalized by the
comparable opposite strength of the iodo group on
intermediate vii between its weak inductive electron-with-
drawing effect and steric repulsion with pyridine N-oxide in a [3
+ 2] cycloaddition step, hence resulting in a mixture of
regioisomers. Interestingly, by altering the aryne generation
method, compound 26 could also serve as a precursor of aryne
iii. As shown in path b of Scheme 5, in the presence of
benzylmagnesium bromide, compound 26 could be activated
and generate intermediate iii, which was then captured by
pyridine N-oxide. After one-pot protection with Tf group, aryne
precursor 3a was obtained in 47% yield. These experiments
show that two orthogonal activating methods could be applied
on 26 in terms of generating different aryne intermediates,
either iii or vii.
In summary, 2,6-bis(trimethylsilyl)aryl triflate was utilized as
an unconventional formal 1,2-benzdiyne equivalent for arene
trisubstitution. Its reactions with both pyridine N-oxides and N-
hydroxylamides result in the regioselective incorporation of an
OH group on its C2 position in 1,2-aryne reaction step,
allowing a one-pot preparation of 2,3-aryne precursor. Further
investigation on pyridinyl-substituted aryne precursors ex-
hibited broad reaction scope with excellent regioselectivity.
This approach not only provides opportunities for the
application of pyridinyl/amido-substituted aryne transforma-
tions under transition-metal-free conditions but also sets up a
new method for further convenient construction of various
1,2,3-trisubstituted arenes using this 1,2-benzdiyne approach.
Ongoing work includes the study of this protocol toward other
arene multifunctionalizations.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00469.
Experimental details for all chemical reactions and
measurements (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lanyu@cqu.edu.cn.
*E-mail: y.li@cqu.edu.cn.
ORCID
Yu Lan: 0000-0002-2328-0020
Yang Li: 0000-0002-0090-2894
Author Contributions
†C.L. and C.W. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge research support of this
work by the NSFC (21372268, 21772017) and Fundamental
Research Funds for the Central Universities
(106112016CDJZR228806).
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