Structurally diverse α-substituted benzopyran synthesis through a practical metal-free C(sp3)-H functionalization

Article abstract of DOI:10.1021/ol503004a

A trityl ion-mediated practical C-H functionalization of a variety of benzopyrans with a wide range of nucleophiles (organoboranes and C-H molecules) at ambient temperature has been disclosed. The metal-free reaction has an excellent functional group tole

Full text of DOI:10.1021/ol503004a

Letter
pubs.acs.org/OrgLett
Structurally Diverse α‑Substituted Benzopyran Synthesis through a
Practical Metal-Free C(sp3)−H Functionalization
Wenfang Chen,‡ Zhiyu Xie,‡ Hongbo Zheng,‡ Hongxiang Lou,* and Lei Liu*
Key Laboratory of Chemical Biology of Ministry of Education, School of Pharmaceutical Sciences, Shandong University, Jinan
250012, PR China
S
* Supporting Information
ABSTRACT: A trityl ion-mediated practical C−H functionalization of
a variety of benzopyrans with a wide range of nucleophiles
(organoboranes and C−H molecules) at ambient temperature has
been disclosed. The metal-free reaction has an excellent functional
group tolerance and high chemoselectivity and displays a broad scope
with respect to both benzopyran and nucleophile partners, efficiently
affording a collection of benzopyrans bearing diverse skeletons and α-
functionalities in one step.
enzopyrans are one of the most common structural motifs
Bspread across biologically active natural products and
synthetic pharmaceuticals.1 For example, benzopyrans compris-
ing (iso)chromans and (iso)chromenes bearing diverse α-
functionalities (aryl, vinyl, and alkyl) have been extensively
used as antioxidants (e.g., vitamin E) and pharmaceuticals
possessing antipsychotic, antibacterial, antifungal, antiviral, and
anticancer activities (Figure 1).1 Besides that, α-functionalized
benzopyrans have also found a wide range of applications in
other high-tech applications like photochromic lenses and
sensitizers in solar cells.2
without prior incorporation of activating groups.4 Since Li’s
pioneering study in 2006, the synthesis of α-substituted
benzopyran derivatives through direct sp3 C−H functionaliza-
tion has attracted considerable attention.5,6 In the presence of
suitable metals, oxidants including 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), organic peroxides, N-hydroxyphthali-
mide (NHPI), 2,2,6,6-tetramethylpiperidine-1-oxoammonium
tetrafluoroborate (T+BF4 ), and Na2S2O8 were reported to
−
promote the cross-dehydrogenative coupling (CDC) of isochro-
mans with several types of C−H nucleophiles including reactive
carbonyl compounds, arylacetylene, and electron-rich aryl rings
including indole and anisole. Despite great innovation, each
method only focused on a single class of the nucleophile, and the
scope with respect to benzopyran is largely restricted to xanthene
and isochroman moieties. Moreover, known methods rely
heavily on the employment of a metal additive, usually requiring
high temperature, neat conditions, and long reaction time, all of
which are generally disadvantageous for developing asymmetric
variants. With respect to the synthetic utility, the specific
structures of employed C−H nucleophiles restrict the integrated
pattern of functionalities in the α-position of benzopyrans.
Therefore, the majority of structures exhibited in Figure 1 could
not be concisely achieved through these methods. Recently,
Muramatsu and Li disclosed a one-pot oxidative coupling of
isochroman with Grignard reagents, affording diverse α-
substituted derivatives in high efficiency.7 However, the
employment of Grignard reagents might bring about unfavorable
regioselectivity and functional group intolerance during the
manipulation of highly functionalized substrates. Highly
functionalized organometallic reagents might also not be readily
accessible. Therefore, the development of a practical and mild
metal-free method for direct C−H functionalization of a variety
Figure 1. Representative α-substituted benzopyrans in human
technology.
Inspired by the significance of these compounds in modern
pharmacology, extensive efforts have been devoted to their
synthesis. While the traditional strategies relying on reactive
functional group transformations have found wide utility in
chemical synthesis, the installation of the functionalities usually
requires multiple and unproductive steps.3 On the other hand,
precise and direct C−H functionalization of ethers with carbon
components presents an atom- and step-economic strategy
Received: October 13, 2014
Published: November 11, 2014
© 2014 American Chemical Society
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Letter
a
of benzopyrans with a wide range of carbon nucleophiles is highly
desired.
Table 2. Coupling of Benzopyrans with Organoboranes
We focused our attention on trityl ions as the oxidant for
benzopyran oxidation for the following reasons: trityl ions are
stable, nontoxic, easily handled, and readily prepared from trityl
chloride and the corresponding acid (e.g., HClO4 or HBF4) in a
single step.8a Moreover, trityl ions were reported to mediate
benzyl ether cleavage and the C−H functionalization of 1,3-
dioxolane.8b−d Very recently, we disclosed that the combination
of Ph3CCl and 1 equiv of GaCl3 can promote C−H
functionalization of a variety of ethers with potassium phenyl-
ethynyltrifluoroborate.9 Although isochroman was a suitable
substrate for the specific nucleophile, the oxidation system
proved to be ineffective for alkylethynylboranes and other types
of organoboranes, such as aryl-, vinyl-, benzyl-, and allylboranes,
as well as typical C−H nucleophiles like reactive carbonyls and
aryl rings.10 While the origin of the limited scope was not clear,
the different counterion effect of the trityl ion on the coupling
efficiency observed in the previous study encouraged us to
explore the suitable oxidation system for the C−H functionaliza-
tion of benzopyrans with a wide range of nucleophiles.8d
Given the paramount importance of α-arylated benzopyrans,
the arylation of isochroman 1a with potassium (4-
methoxyphenyl)trifluoroborate 2a was initially selected as a
model reaction for the optimization (Table 1). A variety of trityl
a
Table 1. Screen for the Oxidant
a
The reaction was carried out with 1a or 4 (0.2 mmol), 2 (0.4 mmol),
and Ph3CClO4 (0.2 mmol) in CH2Cl2 (2.0 mL) at 0 °C or rt in 2 h.
b
b
c
entry
trityl ion
time (h)
yield (%)
Boronic acid as nucleophile. Diethyl (p-methoxyphenyl)boronate as
d
nucleophile. Reaction at 40 °C.
1
2
3
4
5
6
7
Ph3CCl/GaCl3
Ph3CSbCl6
Ph3COTf
Ph3CPF6
2
2
10
33
49
46
82
80
0
Lewis acids.11 As shown in Table 2, the metal-free reaction is
efficient not only for the π-rich aryl (3a,b) and heteroaryl borates
(3e) but also for π-neutral (3c) and π-deficient (3d) arylboranes,
though electron-poor 2-pyridinyl borate (3f) failed to give the
desired product. The C−H functionalization strategy does not
require any additive and thus alleviates potential chemo-
selectivity problems that usually arise from the conventional
Lewis acid initiated methods for electrophile formation through
acetal collapse. Consequently, the functional group compatibility
is excellent, with halogens (3d, 3j, and 3o), benzyl ether (3l),
olefins (3g, 3h, and 3p), and alkynes (3i−l) well-tolerated for
further manipulations.
Next, we examined the scope of benzopyrans (Table 2).
Structurally and electronically varied isochromans 4a−f reacted
smoothly with potassium aryltrifluoroborate 2a. Electron-rich
isochromans, which were expected to be much less reactive than
isochroman 1a, also afforded products in good yields (5c−e).5b,7
Isothiochroman (4g) reacted well under the standard conditions.
1H-Isochromene (4h) was subjected to decomposition under
the oxidation conditions, which could be ascribed to an
incompatibility of the enol ether moiety toward the Lewis
acidity of the trityl ion.12 2H-Chromenes also proved to be
competent substrates, with electronically varied substrates 5i−m
well tolerated. The reaction is highly regioselective, delivering
predominantly C2-addition products, and no double functional-
ization was observed. High efficiency was also achieved when π-
neutral (2c) and π-deficient (2d) arylboranes were applied (5n
2
2
Ph3CClO4
Ph3CBF4
2
2
C7H7BF4
24
24
c
8
Oxidant
<5
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), and oxidant (0.2
b
c
mmol) in CH2Cl2 (2.0 mL) at rt. Isolated yield. Oxidants including
DDQ, PhI(OAc)2, K2S2O8, CAN, MnO2 and TBHP/CuBr2. Tf =
trifluoromethanesulfonyl.
salts bearing different counterions were examined, and reaction
optimization experiments identified Ph3CClO4 in CH2Cl2 at
room temperature in 2 h as ideal conditions (entries 1−6, Table
1). Other common oxidants including tropylium cation
(C7H7BF4), DDQ, PhI(OAc)2, Na2S2O8, CAN (cerium
ammonium nitrate), MnO2, and TBHP (tert-butyl hydro-
peroxide)/CuBr2 failed to effect the coupling (entries 7 and 8
and Table S1 in the Supporting Information).
The reaction proved general for a broad range of organo-
boranes, allowing for efficient coupling with potassium aryl-,
vinyl-, alkynyl-, benzyl-, and allyltrifluoroborates (Table 2).
Boronic acids (3ab, 3ga, and 3ia) and boronate ester (3ac) are
also competent components, providing results comparable to
those of the potassium trifluoroborate (3a, 3g and 3i). The
employment of organoboranes as nucleophiles is classically
confined to π-rich aryl moieties or relies on transition metals or
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Organic Letters
Letter
and 5o). In consideration of a variety of known protocols
utilizing the unsaturation in 2H-chromene as a reactive handle
for further functionalization, the direct access to α-substituted
chromenes provides excellent opportunities to develop a
structurally and stereochemically diverse library of chroman-
like compounds through diversity-oriented synthesis.13 Xan-
thene (5p) and benzoxathiole (5q) also worked well under the
standard conditions.
adapted to α,β-unsaturated aldehydes, yielding predominantly
γ-alkylated products in good efficiency (7o−q). Electronically
varied isochromans and 2H-chromenes were competent
substrates for CDC reactions with representative C−H
nucleophiles like anisole, acetophenone, and propanal (8a−h).
The trityl ion mediated C−H oxidation is typically believed to
undergo direct hydride abstraction or formal hydride transfer
involving an initial single electron transfer (SET) (Figure 2).15
The CDC of isochromans with C−H nucleophiles typically
required the employment of metal additives under harsh
conditions for reaction completion,5 and each condition is only
suitable for a single class of the nucleophile. The mild metal-free
oxidation system encouraged us to explore the scope of CDC
reactions by using the trityl ion as the sole agent (Table 3). The
a
Table 3. Scope of Trityl Ion-Mediated CDC Reactions
Figure 2. Mechanistic analysis for Ph3CClO4-mediated benzopyran
oxidation.
The reaction efficiency was not influenced when the C−H
oxidation was conducted prior to the addition of 2a, suggesting
that the organoborane might not participate in the oxidation
process. Substrates displayed a kinetic isotope effect, indicating
the C−H cleavage involved in the rate-determining step. Radical
trapping experiments were performed, in which the coupling of
1a with 2a was not affected by 1 equiv of 2,2,6,6-
tetramethylpiperidin-1-yloxy (TEMPO) or 2,6-di-tert-butyl-4-
methylphenol (BHT). The observation suggested that a radical
intermediate might not be involved in the reaction. While we
cannot conclude which of the two possibilities might be viable at
this time, we would like to reemphasize that DDQ-mediated C−
H oxidation, typically involving a formal hydride transfer initiated
by a SET,16 could be dramatically suppressed in the similar
radical trapping experiments.5b,17
In conclusion, a metal-free C−H functionalization of
benzopyrans using a readily available trityl ion as the sole
oxidizing agent has been developed. The reaction proceeds
smoothly under ambient temperature with excellent chemo-
selectivity, and it exhibits a broad scope with respect to both
benzopyran ((iso)chroman and chromene) and nucleophile
(organoborane and C−H component) partners with high
functional group tolerance. The integrated pattern of function-
alities in the α-position of benzopyrans is broad, including aryl,
vinyl, alkynyl, benzyl, allyl, and alkyl moieties, which have
potential for further functional handles. We envision that this
avenue will not only allow a facile and rapid access to series of
multiple benzopyrans through “structural core diversification”
strategy to discover biologically significant small molecules but
also provide a valuable platform for further efforts toward
inventing catalytic asymmetric C−H functionalization of
benzopyrans.
a
The reaction was carried out with 1a or 4 (0.2 mmol), 6 (0.4 mmol),
b
and Ph3CClO4 (0.2 mmol) in CH2Cl2 (2.0 mL) at 0 °C or rt in 2 h. 6
was added once the oxidation process was complete. Na2HPO4 (0.4
mmol) was added. 1.0 mmol of aldehyde used.
c
d
arylation of 1a with anisole proceeded smoothly at 0 °C in 2 h to
deliver 7a in a yield comparable to that reported by Todd
through DDQ/CuCl2-mediated coupling at 100 °C after 36 h.5b
3-Methoxyanisole, a challenging substrate for the DDQ system,
afforded the desired 7b in 15% yield.5b Trityl ion mediated mild
conditions allowed a variety of ketones (7c−i) including volatile
acetone (7h) to participate in the coupling efficiently. The
aldehyde component was next studied. However, when propanal
6k was subjected to the standard conditions, no desired 7k was
detected, with aldehyde self-condensation identified as the major
pathway probably because of the Lewis acidity of the trityl ion.12
After extensive investigation of a variety of basic additives,
Na2HPO4 was finally found to be essential to suppress the
undesired pathway, affording 7k in 73% yield (see Table S2,
Supporting Information). Under the optimized conditions, both
linear aldehydes like acetaldehyde (7j) and pentanal (7l) and
sterically hindered branched ones like 7m and 7n joined in the
coupling efficiency.14 Additionally, the modified condition
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and spectral data for new compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
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Organic Letters
Letter
́
́
Pinter, A.; Dehn, S.; Schulze, P.; Klussmann, M. Angew. Chem., Int. Ed.
AUTHOR INFORMATION
Corresponding Authors
■
2013, 52, 13228. (k) Li, Z.; Li, H.; Guo, X.; Cao, L.; Yu, R.; Li, H.; Pan, S.
Org. Lett. 2008, 10, 803. (l) Clausen, D. J.; Floreancig, P. E. J. Org. Chem.
2012, 77, 6574. (m) Xu, Y.; Kohlman, D. T.; Liang, S. X.; Erikkson, C.
Org. Lett. 1999, 1, 1599. (n) Liu, X.; Sun, B.; Xie, Z.; Qin, X.; Liu, L.; Lou,
H. J. Org. Chem. 2013, 78, 3104. (o) Pan, X.; Hu, Q.; Chen, W.; Liu, X.;
Sun, B.; Huang, Z.; Zeng, Z.; Wang, L.; Zhao, D.; Ji, M.; Liu, L.; Lou, H.
Tetrahedron 2014, 70, 3447. (p) Meng, Z.; Sun, S.; Yuan, H.; Lou, H.;
Liu, L. Angew. Chem., Int. Ed. 2014, 53, 543. (q) Richter, H.; García
*E-mail: louhongxiang@sdu.edu.cn.
*E-mail: leiliu@sdu.edu.cn.
Author Contributions
‡W.C., Z.X., and H.Z. contributed equally.
Notes
Mancheno, O. Eur. J. Org. Chem. 2010, 4460.
̃
The authors declare no competing financial interest.
(6) For examples of C−H functionalization of saturated ethers, see:
(a) Chen, L.; Shi, E.; Liu, Z.; Chen, S.; Wei, W.; Li, H.; Xu, K.; Wan, X.
Chem.Eur. J. 2011, 17, 4085. (b) Kumar, G. S.; Pieber, B.; Reddy, K.
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H.; Wang, Z.; Guo, X.; Li, Z. Org. Lett. 2010, 12, 1932. (d) Li, Z.; Yu, R.;
Li, H. Angew. Chem., Int. Ed. 2008, 47, 7497. (e) Liu, D.; Liu, C.; Li, H.;
Lei, A. Angew. Chem., Int. Ed. 2013, 52, 4453. (f) Liu, D.; Liu, C.; Li, H.;
Lei, A. Chem. Commun. 2014, 50, 3623. (g) He, T.; Yu, L.; Zhang, L.;
Wang, L.; Wang, M. Org. Lett. 2011, 13, 5016. (h) Huang, X. F.; Zhu, Z.
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Biomol. Chem. 2012, 10, 1587. (k) Kamijo, S.; Hoshikawa, T.; Inoue, M.
Org. Lett. 2011, 13, 5928. (l) Hoshikawa, T.; Kamijo, S.; Inoue, M. Org.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Foundation of
China (21202093, 21472112), the Program for New Century
Excellent Talents in University (NCET-13-0346), Young
Scientist Foundation Grant of Shandong Province
(BS2013YY001), and the Fundamental Research Funds of
Shandong University (2014JC005) for financial support. We are
also grateful to the Program for Changjiang Scholars and
Innovative Research Team in University (IRT13028).
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