Efficient Microwave Method for the Oxidative Coupling of Phenols

Article abstract of DOI:10.1080/00397911.2014.956370

The tert-butyl peroxide-initiated, oxidative coupling of phenols can frequently promote reactions that otherwise perform poorly under alternative conditions. Despite this utility, peroxide coupling reactions employing conventional heat often require high-

Full text of DOI:10.1080/00397911.2014.956370

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Efficient Microwave Method for the
Oxidative Coupling of Phenols
Sharday Grant-Overtona, Joshua A. Bussa, Eva H. Smitha, Elisa G.
Gutierreza, Eric J. Moorheada, Vivian S. Lina & Anna G. Wenzela
a Keck Science Department, Claremont McKenna, Pitzer, and Scripps
Colleges, Claremont, California, USA
Accepted author version posted online: 21 Oct 2014.Published
online: 13 Dec 2014.
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To cite this article: Sharday Grant-Overton, Joshua A. Buss, Eva H. Smith, Elisa G. Gutierrez, Eric
J. Moorhead, Vivian S. Lin & Anna G. Wenzel (2015) Efficient Microwave Method for the Oxidative
Coupling of Phenols, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 45:3, 331-337, DOI: 10.1080/00397911.2014.956370
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Synthetic Communications1, 45: 331–337, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2014.956370
EFFICIENT MICROWAVE METHOD FOR THE
OXIDATIVE COUPLING OF PHENOLS
Sharday Grant-Overton, Joshua A. Buss, Eva H. Smith,
Elisa G. Gutierrez, Eric J. Moorhead, Vivian S. Lin, and
Anna G. Wenzel
Keck Science Department, Claremont McKenna, Pitzer, and Scripps
Colleges, Claremont, California, USA
GRAPHICAL ABSTRACT
Abstract: The tert-butyl peroxide–initiated, oxidative coupling of phenols can frequently
promote reactions that otherwise perform poorly under alternative conditions. Despite
this utility, peroxide coupling reactions employing conventional heat often require
high-dilution conditions, multiple additions of oxidant, and long reflux times, while
regularly affording only moderate product yields. We recently developed an optimized
protocol for peroxide-promoted, oxidative coupling that employs microwave heat.
Product yields up to 98% have been attained. The performance and scope of this
method relative to other oxidative coupling methodologies will be discussed.
Keywords Biaryl diol; binaphthol; microwave chemistry; phenoxyl radical
INTRODUCTION
1,1′-Bi-2-aryl diols are highly sought after, as these compounds—both racemic
and enantiopure—are ubiquitous to many areas of catalysis,[1] materials chemistry,[2]
and medicinal chemistry,[3] among others. The establishment of convenient and
effective methods for the preparation of racemic biaryl diols is advantageous to pro-
vide general accessibility to this compound class, as enantiomerically pure derivatives
can be readily resolved from racemic materials.[1a] Of the many methods reported to
Received July 22, 2014.
Address correspondence to Anna G. Wenzel, Keck Science Department, the Claremont Colleges,
925 N. Mills Ave., Claremont, CA 91711, USA. E-mail: awenzel@kecksci.claremont.edu
331

332
S. GRANT-OVERTON ET AL.
date,[4] the oxidative coupling of phenols[5] has proven to be the most abundant, as it
often involves the best compromise between providing synthetically useful reaction
yields while typically utilizing readily available, inexpensive starting materials. Meth-
ods generally employed are either metal-mediated (e.g., FeCl3,[6] K3Fe(CN)6,[7] Mn
(acac)3,[8] Cu salt systems,[9] etc.[10]) or involve the use of discrete radicals formed
from peroxide initiators.[5,11] Most metal-promoted oxidative coupling reactions uti-
lize either a stoichiometric or excess amount of metal salts. This often complicates
metal removal from the diol product, particularly if the reaction is conducted
in the solid state.[6c] Reactions that employ discrete radicals also frequently mandate
the addition of excess reagent (1.5–2 equiv peroxide), often in multiple portions over
the course of the reaction.[11c,d,f,g] In addition, high-dilution conditions (0.03 M) and/
or long reaction times (24–50 h) at high temperatures (140 °C) are typically required
to attain synthetically useful product yields.[11]
The ability to rapidly and consistently access biaryl diols has often proven to be
the rate-limiting step in executing projects within our research program.[12,13] Herein,
we report a facile protocol for the peroxide-promoted, microwave coupling of
phenols and naphthols. This protocol affords greater product yields (up to 98%,
following flash chromatography), while enabling shorter reaction times, decreased
solvent use, and reduced quantities of peroxide reagent relative to other methods
reported in the literature.
RESULTS AND DISCUSSION
In 2010, we became interested in the preparation of 2,20-biphenyl diol 2a to
incorporate it into Brønsted acid–type catalysts.[12] Given that the starting phenol
1a was readily accessible, an oxidative coupling reaction quickly emerged as the
logical choice for the preparation of our desired diol. Initially, we attempted
to dimerize 1a via a potassium dichromate–promoted oxidation reaction (Table 1,
entry 1).[14] While somewhat successful, yields were poor, and the harsh reagents
not only damaged the reaction glassware via etching but also led to undesired
reaction by-products, such as hydrolysis and subsequent quinone formation.
Table 1. Oxidative dimerization of phenol 1a
Entry
Conditions
Yield (%)a
1
2
3
4
5
K2Cr2O7, H2SO4, H2O/HOAc, 60 °C, 5 h
Cu/montmorillonite, PhCl, O2, 140 °C, 6.5 h
FeCl3, H2O, 50 °C, 5 h
FeCl3, H2O, microwave at 150 °C, 20 min
tBu2O2, PhCl (0.05 M), 140 °C, 2 h " ambient temp, 3 h
19
13
2
8
40
aIsolated yields.

OXIDATIVE COUPLING OF PHENOLS
333
Seeking milder reaction conditions, we next employed a protocol using copper-
exchanged montmorillonite clay;[9b] this method had previously been reported to
couple a chloro-substituted tetramethylbenzodioxepin precursor in 70% yield.[15]
Unfortunately, in the dimerization of 1a, diol 2a was isolated in only 13% yield (entry
2). We next turned to exploring FeCl3-promoted oxidative couplings,[6a,b,d,g] which
also afforded poor product yields (entries 3 and 4).[16] Ultimately, the reaction of
1a in the presence of tert-butyl peroxide (1.5 equiv) in refluxing chlorobenzene
(0.05 M) afforded the most consistent results;[11d] diol 2a was isolated in 40% yield.
After observing that the peroxide-promoted oxidative coupling reaction was
responsive to higher reaction temperatures, we explored microwave heating of the
reaction mixture to further improve product yields.[17] After thorough investigation,
the best results were obtained by heating a solution of 1a (1.0 M) in chlorobenzene in
the presence of tert-butyl peroxide to 200 °C for 10 min. Under these conditions,
reduced quantities of tert-butyl peroxide (1.05 equiv) were required to achieve
complete starting material conversion (Scheme 1). In addition, 20 times less solvent
than the conventional heating protocol was utilized. Diol 2a was isolated in 98%
yield following flash chromatography.
With these reaction conditions in hand, we next sought to investigate the scope
of our new protocol relative to three literature methods that we had previously found
to be effective in our hands: copper-exchanged montmorillonite clay,[9b] aqueous
iron(III) trichloride in the presence of microwave heat,[6b] and the tert-butyl
peroxide–promoted coupling of phenols using conventional heat[11d] (Table 2). It is
important to note that while the oxidation reaction using potassium dichromate[14]
had previously afforded promising results (19% yield; Table 1, entry 1), it was not
included in our study because of safety concerns associated with its general use.[18]
All reactions were run in triplicate to ensure consistency of results. Presented
values represent the median yields. Reaction yields in Table 2 were reported
1
as H NMR yields to accurately reflect each reaction's performance, particularly
those resulting in poor product yields. However, it is important to note that the
percentages of isolated yields for the highlighted microwave/peroxide coupling
1
protocol were found to match the H NMR yields listed. In addition, microwave/
peroxide coupling reactions for 1a and 1b have been scaled up to 5 mmol with no
observed degradation in reaction yield.
As illustrated in Table 2, the microwave/peroxide coupling protocol
consistently provided greater yields relative to the other protocols, tolerating both
electron-rich and electron-deficient phenols and naphthols in good yield. The use
of microwave heat in the peroxide-promoted reactions consistently provided yields
between 23 and 58% greater than peroxide reactions utilizing conventional heat.
Scheme 1. Optimized conditions for the peroxide-promoted microwave coupling of phenol 1a.

334
S. GRANT-OVERTON ET AL.
Table 2. Relative scope of the peroxide-promoted microwave coupling protocola,b
tBu2O2
tBu2O2 FeCl3 ꢀ 6H2O
Cu=clay
O2, 4
e
g
Entry
1
Substrate
Product
microwavec,d
4
Microwavef
98%
89%
81%
40%
8%
13%
16%
83%
2
3
24%
62%
10%
34%
4
5
>98%
93%
76%
20%
62%
32%
91%
91%
6
7
>98%
83%
38%
23%
47%
52%
>98%
92%
8
93%
67%
22%
8%
aReactions were run on a 1.5-mmol scale.
bResults reported as 1H NMR yields (%).
cPercent isolated yields matched 1H NMR yields.
ꢁ
dPhenol (1.0 M), tBu2O2 (1.05 equiv), chlorobenzene, microwave 200 C, 10 min.
ePhenol (0.05 M), tBu2O2 (1.5 equiv), chlorobenzene, 140 ꢁC, 2 h →(ambient temp), 3 h.[11d]
fPhenol (0.33 M), FeCl3 ꢀH2O (1.05 equiv), water, microwave 150 ꢁC, 10 min.[6b]
gPhenol (0.10 M), 0.5 g Cu-exchanged montmorillonite, chlorobenzene (15 mL), 140 ꢁC, 6 h, air.[9b]

OXIDATIVE COUPLING OF PHENOLS
335
Regardless of the heating method, the use of peroxide clearly outperformed reactions
employing iron trichloride.
Although the copper-exchanged montmorillonite clay protocol frequently led
to good yields, this method performed quite poorly in the presence of the chlorinated
(entry 1, 13%) and fluorinated (entry 2, 16%) substrates studied. An advantage of
the peroxide-promoted microwave protocol is the commercial availability of the
peroxide reagent;[19] in contrast, copper-exchanged montmorillonite clay must be
separately prepared via a 24 intercalation protocol, followed by centrifugal rinsing
of the clay and extensive drying in a vacuum oven.[20] In addition, the montmorillonite
method employs 10 times more chlorobenzene reaction solvent relative to the micro-
wave/peroxide coupling protocol, as well as longer reactions times (6 h vs. 10 min).
In summary, we have presented an efficient protocol for the oxidative coupling
of phenols and naphthols. The method is facile, rapid, and high yielding, directly
employing commercially available reagents without any purification required prior
to use. In addition, the use of minimal reaction solvent minimizes waste, while
simultaneously facilitating product purification by enabling the direct subjection of
the reaction mixture to flash chromatography.
EXPERIMENTAL
Nuclear magnetic resonance (NMR) spectra were obtained using either a Bruker
AvanceIII-500 or Bruker 300 NMR spectrometer. All compounds were purchased
from Aldrich, Alfa Aesar, TCI America, Strem, and/or Fisher. NMR solvents were
purchased from Cambridge Isotope Laboratories, Inc. Chlorobenzene (Acros) was
used as received. The flash chromatography of all compounds was performed utilizing
silica gel 60 (32–63 micron) purchased from Bodman Industries. Mass-spectrometric
data were obtained at the Caltech Mass Spectrometry Facility. A Biotage Initiator
microwave reactor was used for all microwave experiments. Microwave reactions were
conducted in septum-sealed, glass vials equipped with magnetic stirbars; an external
surface sensor was used to monitor reaction conditions.
2-tert-Butyl-4-chloro-5-methylphenol[12] (1.5 mmol, 1.0 equiv) was dissolved
in chlorobenzene (1.5 mL) in a 0.5–2-mL microwave reaction vial equipped with a
stirbar. Di-tert-butyl peroxide (287 µL, 1.58 mmol, 1.05 equiv) was then added in one
portion. The vial was capped and placed in the microwave reactor. After mixing for
5 min to ensure that the peroxide was thoroughly distributed throughout the reaction
mixture, the reaction was heated to 200 °C for 10 min. Upon cooling, the reaction was
directly purified by flash chromatography on silica gel to afford the desired product.
ACKNOWLEDGMENTS
Mona Shahgholi and Naseem Torian of the Caltech Mass Spectrometry
Facility are gratefully acknowledged for performing mass spectral analyses.
FUNDING
This work was supported by the National Science Foundation [Grant Number
1151933], partial support of this research. E. G. G. and S. G.-O. were supported by

336
S. GRANT-OVERTON ET AL.
summer research grants from the Rose Hills Foundation; E. S. was funded through a
Keck Foundation Summer Research Grant.
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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