A Study of the Aryl-Aryl Coupling Reactions of (4-X-C6H 4)Ph2P=O

Article abstract of DOI:10.1081/SCC-120027718

The Suzuki coupling reactions of (4-X-Ph)Ph₂P=O, where X = bromide or triflate, with a series of boronic acids were studied using tetrakis(triphenylphosphino) palladium or palladium acetate as the catalyst. The boronic acids utilized were pheny

Full text of DOI:10.1081/SCC-120027718

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A Study of the Aryl–Aryl Coupling Reactions of
(4‐X‐C6H4)Ph2P˭O
Michaela Czupik a , Nathaniel Bankey a & Eric Fossum a
a Department of Chemistry , Wright State University , 3640 Colonel Glenn Hwy., Dayton,
Ohio, 45435, USA
Published online: 16 Aug 2006.
To cite this article: Michaela Czupik , Nathaniel Bankey & Eric Fossum (2004) A Study of the Aryl–Aryl Coupling Reactions
of (4‐X‐C6H4)Ph2P˭O, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 34:4, 705-714, DOI: 10.1081/SCC-120027718
To link to this article: http://dx.doi.org/10.1081/SCC-120027718
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 4, pp. 705–714, 2004
A Study of the Aryl–Aryl Coupling
Reactions of (4-X-C6H4)Ph2P55O
*
Michaela Czupik, Nathaniel Bankey, and Eric Fossum
Department of Chemistry, Wright State University, Dayton, Ohio, USA
ABSTRACT
The Suzuki coupling reactions of (4-X-Ph)Ph2P55O, where X ¼ bromide
or triflate, with a series of boronic acids were studied using tetrakis-
(triphenylphosphino) palladium or palladium acetate as the catalyst. The
boronic acids utilized were phenyl, p-tolyl, 3-methoxy, 4-methoxy, and
4-acetyl. Yields of the corresponding biphenyl analogues ranged from 50
to 95%. Palladium acetate provided products free of triphenylphosphine
contamination, and required significantly shorter reaction times for
complete reaction (2 to 4 hours vs. 12 to 24 hours, respectively) than when
tetrakis-(triphenylphosphino) palladium was used. The methodology was
applied to bis(4-F-Ph)(4-OTf-Ph)P55O to afford, in excellent yield (99%),
a biphenyl-based AB2 monomer precursor for dendritic and hyperbranched
poly(arylene ether phosphine oxide)s.
Key Words: Triarylphosphine oxides; Coupling; Biphenyl.
*
Correspondence: Eric Fossum, Department of Chemistry, Wright State University,
3640 Colonel Glenn Hwy.Dayton, OH 45435, USA; E-mail: eric.fossum@wright.edu.
705
DOI: 10.1081/SCC-120027718
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Czupik, Bankey, and Fossum
INTRODUCTION
During the course of a project involving the synthesis of hyperbranched
poly(arylene ether phosphine oxide)s from unsymmetrically substituted
triarylphosphine oxides 1a, 1b, and 1c (Fig. 1), the need arose to prepare the
corresponding biphenyl analogues. Polymerization reactions of 1a, 1b, and 1c
using nucleophilic aromatic substitution conditions (K2CO3, NMP, reflux)
afforded hyperbranched poly(polyarylene ether phosphine oxide)s, HB
PAEPOs, with limited molecular weights (,15,000 g/mol) and broad
molecular weight distributions (2.5–3.5).[1] One potential reason for the
observed molecular weights may have been the presence of intramolecular
cyclization reactions. Two approaches to eliminate or decrease the possibility
of intramolecular cyclization are polymerization reactions in the presence of
core molecules[2–5] and the use of more rigid monomers. Polymerization
reactions of 1a, 1b, and 1c in the presence of a variety of core molecules has
led to HB PAEPOs with controlled molecular weights and molecular weight
distributions as low as 1.25.[6]
An alternative method for eliminating or minimizing intramolecular
cyclization during the polymerization reactions is the use of more rigid,
biphenyl-substituted monomers. Monomers 1a, 1b, and 1c are prepared using
standard Grignard chemistry and chlorophosphines, however the starting
materials that would be needed to prepare the desired biphenyl derivatives are
not readily available and thus, the biphenyl derivatives need to be prepared via
an alternate route. Coupling reactions of aryl halides with diphenylphosphine
oxide have been utilized to prepare triarylphosphine oxide derivatives in good
yield.[7–10] Unfortunately, bis-(4-fluorophenyl)phenylphosphine oxide is not
commercially available and the use of 4-bromo-40methoxybiphenyl is cost
prohibitive. Therefore, a more cost efficient route to the desired biphenyl
monomers using derivatives of 1a, 1b, and 1c as the starting materials has
been explored. A number of synthetic methodologies exist for aryl–aryl
coupling, but we have chosen to explore the most widely utilized method, the
Suzuki coupling reactions.[11,12]
Figure 1. AB2 monomers, 1a, 1b, and 1c, utilized to prepare hyperbranched
poly(arylene ether phosphine oxide)s via nucleophilic aromatic substitution reactions.

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707
Aryl–aryl coupling reactions of triarylphosphine oxide derivatives are
not well known. McGrath et al. have prepared linear poly(arylene phosphine
oxide)s via the nickel catalyzed coupling reaction of bis-(4-chlorophenyl)-
phenylphosphine oxide.[13] During the course of our investigation we became
aware of work by Xiao et al. in which the Suzuki coupling of (2-Br-
Ph)Ph2P55O with a variety of boronic acids using tetrakis(triphenylphos-
phino) palladium was studied as a means to prepare bulky, triarylphosphine
ligands.[14] Similar work has been reported by Buchwald et al. for the
preparation of binaphthyl based phosphine ligands.[15] This paper will
describe our efforts to develop and apply a synthetic strategy to prepare
biphenyl substituted triarylphosphine oxides for subsequent use as monomers
for HB PAEPOs.
RESULTS AND DISCUSSION
Initial screening reactions have been performed using (4-bromophenyl)-
diphenylphosphine oxide, 2, a variety of aryl boronic acids, 3a–e, and
tetrakis(triphenylphosphino)palladium, (Ph3P)4Pd, in toluene as shown in
Sch. 1 (Method 1). The yields are listed in Table 1. While the reactions using
(Ph3P)4Pd have provided the desired biaryl derivatives in reasonably good
yields, 50–89%, removal of the residual triphenylphosphine from the catalyst
has proven difficult. “Phosphine-free” palladium catalyzed aryl–aryl coupling
reactions using palladium acetate, Pd(OAc)2 have been reported by a number
of research groups with the key advantage being the absence of Ph3P
impurities at the conclusion of the reaction.[16–19] Application of Pd(OAc)2
Scheme 1.

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Czupik, Bankey, and Fossum
Table 1. Results from Suzuki Coupling Reaction of 2 using tetrakis(triphenylphos-
phino)palladium or palladium acetate.
Boronic acid
Solvent
Catalyst
Product
% yield (GC/MS)
3a
3b
3c
3e
3a
3b
3c
3d
Toluene
Toluene
Toluene
(Ph3P)4Pd
(Ph3P)4Pd
(Ph3P)4Pd
(Ph3P)4Pd
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
4a
4b
4c
4e
4a
4b
4c
4d
60
89
50
70
95
97
92
95
Toluene
Acetone/H2O
Acetone/H2O
Acetone/H2O
Acetone/H2O
conditions (Sch. 1, Method 2) to aryl coupling reactions of 2 with a number of
aryl boronic acids has provided the desired biphenyl compounds in excellent
yields, 92–97%, free of triphenylphosphine impurities. Reaction times using
Pd(OAc)2 have also decreased significantly. For example, a typical reaction
with (Ph3P)4Pd requires from 12–24 hours to reach completion whereas those
with Pd(OAc)2 are complete within 2–4 hours. Purification of the desired
products is afforded by recrystallization or column chromatography as
necessary. Confirmation of the desired structures has been provided by 1H and
13C NMR spectroscopy, GC/MS, and either elemental analysis or high
resolution MS as necessary.
In order to apply the Pd(OAc)2 protocol to the synthesis of biphenyl AB2
monomers, the appropriate starting material, (4-bromophenyl)-bis-(4-fluoro-
phenyl)phenylphosphine oxide, was needed. Since its synthesis proved
elusive, an alternative route was explored. Suzuki coupling reactions with aryl
triflates are well known and the synthesis of triflates from phenols, in our case
1a, 1b, and 1c, is straightforward.
Therefore, to test the feasibility of aryl coupling reactions with triaryl-
phosphine oxide-based triflate derivatives, (4-OTf-Ph)Ph2P55O, 6, was
prepared according to the route shown in Sch. 2. Treatment of (4-hydroxy-
phenyl)diphenylphosphine oxide, 5, with an excess of triflic anhydride in
dichloromethane afforded the triflate derivative, 6, in excellent yield.
Unreacted starting material, 5, was removed by dissolving the reaction
product mixture in toluene and filtering off the toluene insoluble starting
material. Subsequent removal of the toluene under reduced pressure and
drying in vacuo afforded 6 as yellow, analytically pure crystals.
As shown in Sch. 2 coupling reactions with 6 and a number of arylboronic
acids, 3a, 3d, and 3e, were performed in toluene at 908C using (Ph3P)4Pd
(Method 1b). The corresponding biphenyl derivatives, 4a, 4d, and 4e, were

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709
Scheme 2.
afforded in good yields (Table 2). Comparison of spectral data and GC/MS
data of 4a, 4d, and 4e prepared from the triflate derivate, 6, with those
prepared from the bromide derivate, 2, confirmed their identity.
The synthetic methodology developed with model compound 6 was then
applied to prepare the desired AB2 monomer precursor, 8 (Sch. 3). Reaction of
1a with an excess of triflic anhydride provided the corresponding triflate
derivative, 7, in 90% yield after workup. Coupling of 7 with 3b using the
conditions of Method 1b provided the desired biphenyl derivative, 8, in 99%
1
yield after workup (Sch. 3). Characterization by H and 13C NMR spectro-
scopy as well GC/MS and elemental analysis confirmed the structure of 8.
Deprotection of the phenol group in 8 to provide the corresponding AB2
monomer and subsequent polymerization to prepare a HB PAEPO is currently
being investigated.
A general route to biphenyl substituted triarylphosphine oxides via the
use of Suzuki coupling of either bromo or trifluorosulfonyl derivatives of
triphenylphosphine oxide and arylboronic acids has been developed.
Reactions using Pd(OAc)2 with the bromo derivative, 2, provided the desired
biphenyl analogues, 4a–d, in excellent yields (92–97%) free of triphenyl-
phosphine impurities. Reactions using (Ph3P)4Pd with 2 and 6 provided the
corresponding biphenyl derivatives, 4a–e, in good yields, 50 to 89%. The
Table 2. Results from Suzuki Coupling Reactions of the triarylphosphine oxide
triflate derivative, 6.
Boronic acid
Solvent
Catalyst
Product
% yield (GC/MS)
3a
3d
3e
Toluene
Toluene
Toluene
(Ph3P)4Pd
(Ph3P)4Pd
(Ph3P)4Pd
4a
4d
4e
77
70
65

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Czupik, Bankey, and Fossum
Scheme 3.
methodology has been applied to prepare an AB2 monomer precursor, 8, in
excellent yield (99%). The polymerization and subsequent property studies of
the HB PAEPO prepared from 8 will be described elsewhere.
EXPERIMENTAL
Materials
All reactions were performed under a nitrogen atmosphere and all
transfers were done using syringes or cannula as necessary. The boronic acids
were purchased from the Aldrich Chemical Co. and used as received.
Tetrakis(triphenylphosphino) palladium and palladium acetate were pur-
chased from the Aldrich Chemical Co. and used as received. Toluene was
dried over and distilled from sodium/benzophenone prior to use. Both
4-bromophenyldiphenylphosphine oxide[20] and bis(4-fluorophenyl)-4-hydroxy-
phenylphosphine oxide[1] were synthesized according to literature procedures.
1H, 13C, and 31P NMR spectra were obtained using a Bruker Avance DMX
300 MHz instrument operating at 300, 75.5, and 121.5 MHz, respectively.
Samples were dissolved in CDCl3. Elemental analyses were obtained from
Midwest Microlabs, Inc., Indianapolis, IN. High-resolution mass spectra were
obtained at the Campus Chemical Instrumentation Center—Mass Spec-
trometry and Proteomics Facility at Ohio State University.
(4-Tolyphenyl)diphenylphosphine oxide, 4b. Method 1. A 25 mL RB
flask was charged with 0.40 g (1.12 mmol) of 2, 0.22 g (1.624 mmol) of

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711
4-tolyboronic acid, 0.44 g of K2CO3, 5 mL of nitrogen sparged toluene, and
1 mol% Pd(PPh3P)4. The mixture was heated to 908C overnight at which point
GC/MS analysis showed the presence of unreacted 2. An additional 0.05 g
(0.37 mmol) of 4-tolyboronic acid were added and the mixture was heated for
24 hours further. GC/MS analysis showed complete conversion of 2 to 4b.
Additional toluene, 5 mL, was added and the organic layer was washed 3 times
with 10 mL of distilled water, followed by drying over MgSO4. Removal of
the solvent under reduced pressure afforded the desired compound as a white
powder. The identity of 4b was confirmed via comparison of its analytical data
with the analytical data obtained for 4b from Method 2 below.
Method 2. A method similar to that first reported by Novak et al. was
utilized for Pd(OAc)2 catalyzed reactions.[16] A 25 mL Schlenk flask was
charged with 0.40 g (1.12 mmol) of 2, 0.18 g (1.32 mmol) of 4-tolyphenyl-
boronic acid, and 3.0 mL of reagent grade acetone. In separate Schlenk flasks
were placed 0.38 g of K2CO3 in 3.0 mL of distilled water and 1 mol%
Pd(OAc)2 in acetone, respectively. The contents of the Schlenk flasks were
subjected to three freeze-pump-thaw cycles, back-filled with nitrogen, and
combined. The reaction mixture was heated to 608C for 2 hours at which point
an additional 10% of the boronic acid was added followed by heating for an
additional 4 hours. The layers were separated and the organic layer was diluted
with toluene (ꢀ10 mL), washed with brine and dried over MgSO4. Removal of
the solvent under reduced pressure followed by recrystallization from
ethanol/water afforded 0.40 g (99%) of 4b as white crystals (m.p. ¼ 159–
1618C). 1H NMR (CDCl3, d): 2.42 (s, 3 H), 7.28 (m, 2 H), 7.46–7.58 (m, 8 H),
7.66–7.80 (m, 8 H); 13C NMR (CDCl3, d): 21.5 (s), 127.3 (d), 127.5 (s), 128.9
(d) 130.1 (s), 131.2 (d), 132.5 (d), 133.0 (d), 133.4 (d), 137.4 (s), 138.6 (s)
145.0 (d). Elem. Anal. Calc’d for C25H21OP: C, 81.50%, H, 5.75%. Found: C,
81.44%, H, 5.86%.
4-Biphenyldiphenylphosphine oxide, 4a. 95% yield; preparation of 4a
1
from 0.40 g (1.12 mmol) of 2 using Method 2 gave 0.39 g (98%) of 4a. H
NMR (CDCl3, d): 7.52 (m), 7.59 (m), 7.67 (m); 13C NMR (CDCl3, d): 127.6
(d), 127.7 (s), 128.6 (s), 129.0 (d), 129.4 (s), 131.5 (d), 132.4 (d), 132.5 (d),
133.1 (d), 134.1, (d), 140.3 (s), 145.2 (d). HRMS: molecular mass þ sodium
m/z, calc. For C24H19OPNa 377.1071, found 377.1088.
1
4-(30-Methoxy)biphenyldiphenylphosphine oxide, 4c. 92% yield; H
NMR (CDCl3, d): 3.87 (s, 3 H), 6.95 (d, 1 H), 7.20 (m, 2 H), 7.40 (t, 1 H), 7.55
(m, 6 H), 7.74 (m, 8 H); 13C NMR (CDCl3, d): 55.8 (s), 113.5 (s), 113.9 (s),
120.2 (s), 127.7 (d), 129.0 (d), 130.4 (s), 131.5 (d), 132.4 (d), 132.5 (d), 132.8
(d), 141.8 (s), 145.1 (d), 160.3 (s). HRMS: molecular mass þ sodium m/z,
calc. For C25H21OPNa 407.1177, found 407.1158.
4-(40-Methoxy)biphenyldiphenylphosphine oxide, 4d. 95% yield;
1
(m.p. ¼ 146–1508C). H NMR (CDCl3, d): 3.87 (s, 3 H), 7.00 (d, 2 H),

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Czupik, Bankey, and Fossum
7.47–7.58 (m, 8 H), 7.65–7.76 (m, 8 H); 13C NMR (CDCl3, d): 55.8 (s), 114.8
(s), 127.1 (d), 128.8 (d), 129.1 (s), 130.5 (d), 132.2 (d), 132.5 (d), 132.6 (s),
132.9 (d), 133.0 (d), 144.8 (d), 160.3 (s). HRMS: molecular mass þ sodium
m/z, calc. For C25H21OPNa 407.1177, found 407.1158.
4-(40-Acetyl)biphenyldiphenylphosphine oxide, 4e. 70% yield;
1
(m.p. ¼ 201–2038C) H NMR (CDCl3, d): 2.64 (s, 3 H), 7.48–7.57 (m, 6
H), 7.69–7.82 (m, 10 H), 8.05 (d, 2 H); 13C NMR (CDCl3, d): 27.1 (s), 127.1
(d), 127.9 (s), 129.0 (d), 129.4 (s), 132.2 (d), 132.5 (d), 132.6 (d), 132.7 (d),
132.9 (d), 137.0 (s), 143.8 (d), 144.8 (s), 198.0 (s). Elem. Anal. Calc’d for
C26H21O2P: C, 78.78%, H, 5.34%. Found: C, 78.00%, H, 5.54%.
(4-Trifluoromethylsulfonylphenyl)diphenylphosphine oxide, 6. An
oven-dried RB flask was charged with 3.37 g (11.5 mmol) of 5, 45 mL of
CH2Cl2, and 2.3 mL of pyridine. The flask was immersed in an ice bath (08C)
and a solution of 1.95 mL (11.6 mmol) of triflic anhydride dissolved in 10 mL
of CH2Cl2 was added drop-wise. The resulting cloudy mixture was allowed to
warm to room temperature and subsequently stirred overnight. The mixture
was filtered and the CH2Cl2 solution was poured slowly into an excess of ice
water. The layers were separated and the aqueous layer was washed twice with
10 mL of CH2Cl2. The organic layers were combined and dried over MgSO4.
Removal of the solvent under reduced pressure afforded a mixture of 5 and 6
that was separated by dissolution of 6 in toluene leaving behind insoluble 5.
Removal of the toluene afforded 3.61 g (74%) of 6 as yellow crystals
(m.p. ¼ 120–1248C). 1H NMR (CDCl3, d): 7.39 (dd, 2 H), 7.51 (m, 4 H), 7.59
(m, 2 H), 7.67 (m, 4 H), 7.79 (dd, 2 H); 13C NMR (CDCl3, d): 122.0 (d), 129.3
(d), 131.8 (d), 132.5 (d), 132.9 (d), 133.8 (d), 134.8 (d), 152.3 (d). Elem. Anal.
Calc’d for C19H14F3O4P: C, 53.53%, H, 3.31%. Found: C, 53.57%, H, 3.71%.
bis-(4-Fluorophenyl)-(4-trifluoromethylsulfonylphenyl)phosphine
oxide, 7. The synthesis of 7 was carried out as described for 6 starting with
2.55 g (7.73 mmol) of 1a to provide 3.21 g (90%) of 7 as yellow crystals
(m.p. ¼ 96.5–998C). 1H NMR (CDCl3, d): 7.20 (dt, 4 H), 7.41 (dd, 2 H), 7.67
(m, 4 H), 7.78 (dd, 4 H); 13C NMR (CDCl3, d): 116.7 (dd), 122.2 (d), 127.2
(dd), 133.6 (d), 134.1 (dd), 134.8 (d), 152.5 (d), 165.8 (dd), 167.3 (d). HRMS:
molecular mass þ sodium m/z, calc. For C19H12F5O4PNa 485.0012, found
485.0016.
bis-(4-Fluorophenyl)-(4-methoxybiphenyl)phosphine oxide, 8. An
oven dried RB flask was charged with 0.5 g (1.10 mmol) of 7, 0.25 g
(mmol) of p-methoxy phenyl boronic acid, 0.5 g of potassium carbonate,
15 mg of Pd(0) tetrakis triphenyl phosphine and 5.0 mL of sparged toluene.
The reaction mixture was heated to reflux for 16 hours at which point the
solution was slowly poured into 150 mL distilled water. The layers were
separated and the aqueous layer was extracted three times with 35 mL of
chloroform. The chloroform layers were combined, washed three times with

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713
distilled water (3 ꢀ 40 mL), dried over MgSO4 and the solvent was removed
under reduced pressure to afford 8 (99%) as white crystals (m.p. 149–1508C).
1H NMR (CDCl3, d): 3.87 (s, 3 H), 7.01 (d, 2 H), 7.20 (m, 4 H), 7.57 (d, 2 H),
7.71 (m, 8 H); 13C NMR (CDCl3, d): 55.8 (s), 116.5 (m), 127.2 (d), 130.9 (s),
132.8 (d), 134.9 (m), 45.0 (d), 160.4 (s), 163.9 (d), 167.3 (d); 31P NMR
(CDCl3, d): 28.70 (s). Elem. Anal. Calc’d for C25H19F2O2P: C, 71.43%, H,
4.56%. Found: C, 71.61%, H, 4.61%.
ACKNOWLEDGMENTS
The authors wish to thank the Research Corporation, the donors of the
Petroleum Research Fund, administered by the American Chemical Society,
and Wright State University for financial support. The Ohio NMR Consortium
is acknowledged for providing access to high field NMR facilities.
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Received in the USA August 28, 2003

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