Palladium-Catalyzed Directed para C?H Functionalization of Phenols

Article abstract of DOI:10.1002/anie.201601999

Various practical methods for the selective C?H functionalization of the ortho and recently also of the meta position of an arene have already been developed. Following our recent development of the directing-group-assisted para C?H functionalization of toluene derivatives, we herein report the first remote para C?H functionalization of phenol derivatives by using a recyclable silicon-containing biphenyl-based template. The effectiveness of this strategy was illustrated with different synthetic elaborations and by the synthesis of various phenol-based natural products.

Full text of DOI:10.1002/anie.201601999

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201601999
German Edition: DOI: 10.1002/ange.201601999
À
C H Activation
À
Palladium-Catalyzed Directed para C H Functionalization of Phenols
Tuhin Patra, Sukdev Bag, Rajesh Kancherla, Anirban Mondal, Aniruddha Dey,
Sandeep Pimparkar, Soumitra Agasti, Atanu Modak, and Debabrata Maiti*
À
Abstract: Various practical methods for the selective C H
functionalization of the ortho and recently also of the meta
position of an arene have already been developed. Following
our recent development of the directing-group-assisted para
À
C H functionalization of toluene derivatives, we herein report
À
the first remote para C H functionalization of phenol
derivatives by using a recyclable silicon-containing biphenyl-
based template. The effectiveness of this strategy was illustrated
with different synthetic elaborations and by the synthesis of
various phenol-based natural products.
À
R
ecent years have witnessed the emergence of C H
activation as an effective method for the synthesis of complex
molecules.[1] As evident from several examples in nature,
chemical transformations that involve the functionalization of
À
C H bonds are coined to be efficient when they proceed with
a commendable degree of selectivity.[2–5] One strategy for
ensuring a highly selective functionalization entails the use of
a directing group.[6] This approach has been significantly
exploited in the last two decades to selectively activate the
À
Scheme 1. Directed para C H functionalization of phenols.
ortho[7] and more recently the meta C H bonds
of arenes
Following our previous report[10] and in order to optimize
the template design, a biphenyl moiety was employed as part
of the template chain along with a silicon center with two
bulky isopropyl groups as the ligation center (Scheme 1b,
S1).[14] Switching the connecting carbon and oxygen atoms in
our previous template led to a modified system for the para
[8,9]
À
À
containing multiple C H bonds with very similar intrinsic
reactivity. However, the effective employment of such
À
a strategy for the selective activation of a para C H bond
has remained extremely difficult. This was attributed to the
problems associated with forming a large cyclophane-like
metallacycle, which is required for the functionalization of
C H functionalization of phenol.[15] This modification ensures
À
À
a C H bond situated far away from the coordinating func-
that the donor nitrile group will be placed at a position
suitable for the formation of the desired metallacycle in the
transition state. The end-on coordination by the directing
moiety leads to a suitable placement of the weakly held
electrophile through the cyclophane 17-membered transition
state.
With template S1 in hand, we attempted the palladium-
catalyzed olefination of phenol in the presence of AgOAc as
the oxidant. Consistent with our expectations, the para C H
olefination product was formed in 84% yield (isolated in
82%) with 10:1 selectivity in the presence of N-acetylglycine
as the ligand. The crucial role of the template morphology was
corroborated by the complete loss of selectivity for the
olefination of anisole, tert-butyldimethyl(phenoxy)silane, and
simple phenol.[16] Importantly, the requirement for a suitably
tional group. The need for an optimum chain length poses
a further impediment towards devising a template that would
enable the facilitated delivery of the electrophile through
a weakly coordinating donor group. Overcoming these
challenges, our group recently reported the first template-
[10]
À
directed para C H functionalization of toluene derivatives.
À
To date, C H activation reactions with phenol derivatives
À
have predominantly occurred at the ortho and meta position
in the presence of a suitable template,[11,12] or selectivity was
based exclusively on steric and electronic bias.[5,13] The
directing-group-assisted para-selective functionalization of
such a molecule would effectively overrule its vulnerability
towards electrophilic attack at both the 2- and 4-position
(Scheme 1a).
À
located donor to activate the para C H bond was further
confirmed by the poor yield achieved with template S2.
[*] T. Patra, S. Bag, Dr. R. Kancherla, A. Mondal, A. Dey, S. Pimparkar,
S. Agasti, A. Modak, Prof. Dr. D. Maiti
Department of Chemistry
À
Proceeding further with our investigation, the para C H
functionalization of various substituted phenol derivatives
with both electron-donating and electron-withdrawing groups
was explored (2a–2x, Table 1). Mono-ortho-substituted
derivatives were found to be suitable substrates (2b–2e). To
confirm the role of the directing template in overruling the
probable electronic bias, electron-withdrawing groups were
Indian Institute of Technology Bombay
Powai, Mumbai 400076 (India)
E-mail: dmaiti@chem.iitb.ac.in
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601999.
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[a]
À
À
Table 1: Arene scope of the template-assisted para C H olefination of
Table 2: Olefin scope of the para C H olefination of phenols.
phenols.[a]
[a] Phenol (0.2 mmol), olefin (0.4 mmol), Pd(OAc)2 (10 mol%), Ac-Gly-
OH (20 mol%), AgOAc (0.4 mmol), DCE/TFE (1.2/0.4 mL), 608C, 32 h.
[b] Yields in parentheses correspond to reactions on 2.5 mmol scale.
TFE=2,2,2-trifluoroethanol.
[a] Phenol (0.2 mmol), olefin (0.4 mmol), Pd(OAc)2 (10 mol%), Ac-Gly-
OH (20 mol%), AgOAc (0.4 mmol), DCE/TFE (1.2/0.4 mL), 608C, 32 h.
[b] Yields in parentheses correspond to reactions on 2.5 mmol scale.
Following this, we investigated the olefination of direct-
ing-template-modified phenol with a wide range of olefins
(Table 2). A diverse set of functional groups, including esters,
amides, aldehydes, ketones, sulfonyls, and phosphonates, were
found to be compatible with the reaction conditions (3a–3g,
3q). Olefins with long-chain alkyl substituents as well as
internal and cyclic olefins were also tolerated (3h–3n).
After para functionalization, the silanol template 4 was
recovered using catalytic amounts of para-toluenesulfonic
acid. Silanol 4 could be then reinstalled in good yield (71%,
Scheme 2). Alternatively, the template could be removed
in situ with TBAF after para functionalization to provide 5 in
excellent yield (Scheme 2).
installed at the meta position with respect to the hydroxy
group (2 f, 2g, 2l, and 2t). The successful olefination of these
substrates established the pivotal role of the directing
template. Sterically demanding substituents were also toler-
ated, as demonstrated in particular by 2m–2x where the para
selectivity remained unaffected. The efficacy of the directing
group was further confirmed by the fact that no effect on the
selectivity was observed in the presence of molecules with
À
multiple vulnerable C H bonds (2e, 2n, and 2u). The
presence of oxygen- and sulfur-based heterocycles did not
affect the reactivity and selectivity (2q–2s).
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Scheme 2. Template removal and reinstallation.[16] DMAP=4-dimethyl-
aminopyridine, p-TSA=para-toluenesulfonic acid, TBAF=tetrabutyl-
ammonium fluoride.
This para-functionalization strategy was then applied to
several phenol-based natural products (Scheme 3 and
Scheme 4). For example, phenol can thus be employed as
the starting material of the synthesis of para-coumaric acid (6;
4 steps, 47%), which is an inhibitor of stomach cancer aside
from being a potent antioxidant.[17] Terpene derivative 7,
found in petroleum extracts of the roots of Seseli sibiricum,
was also synthesized from phenol (3 steps, 49%).[18] The
phenolic phytochemical ferulic acid (8)[19] was also synthe-
sized using the present approach (4 steps, 47%). Intriguingly,
the potent growth inhibitor factor 9 was prepared by this
template-directed para-functionalization approach (3 steps,
48%).[20] Aside from 10 (3 steps, 46%), anti-inflammatory
artepellin C (11; 5 steps, 23%), antimicrobial plicatin B (12;
4 steps, 34%), and drupanin (13; 5 steps, 29%) were also
synthesized (Scheme 4).[21,22]
The importance of this para-olefination method was
further established by postsynthetic modifications of both
the phenol and olefin core. By making use of the phenolic
hydroxy group, 5 was converted into benzofuran 14 and
Scheme 3. Synthesis of phenol-based natural products.[16]
coumarin 23 (Scheme 5).[23] A C O coupling yielded deriva-
À
tive 15.[16] Alternatively, the phenolic hydroxy group could be
transformed into a synthetically useful triflate group in
excellent yield, enabling subsequent Suzuki, Sonogashira,
[16]
À
and C N couplings (17–19). Alternatively, reduction fol-
lowed by hydrolysis of the acrylate core was used to
synthesize 1-indenone 21 and spirocycle 22.[24]
In conclusion, we have described a highly selective
À
method for the para C H functionalization of phenol
derivatives that is based on the use of a silyl–biphenyl
template. The applicability of this approach was demon-
strated by the synthesis of various natural products. In future,
the mechanistic details of this process as well as the usefulness
of this strategy in drug development and polymer and
material science will be studied.
Acknowledgements
Scheme 4. Synthesis of artepillin C, plicatin B, and drupanin.[16]
This work was supported by the SERB (EMR/2015/000164).
Fellowships from UGC India (T.P. and S.B.), DST Fast Track
(R.K.), and CSIR India (S.A. and A.M.) are gratefully
acknowledged.
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À
Keywords: C H activation · olefination · palladium · silicon ·
synthetic methods
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7751–7755
Angew. Chem. 2016, 128, 7882–7886
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Received: February 26, 2016
Revised: March 26, 2016
Published online: May 9, 2016
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