Recyclable and reusable Pd(OAc)2/P(1-Nap)3/[bmim][PF6]/H2O system for the addition of arylboronic acids to aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2014.11.020

A stable and efficient Pd(OAc)₂/P(1-Nap)₃[tri(1-naphthyl)phosphine] catalytic system for the addition of arylboronic acids to aldehydes has been developed. In the presence of Pd(OAc)₂ and P(1-Nap)₃, the addition reaction of arylboronic acids with aldehydes was carried out smoothly at 65 °C to give a variety of carbinol derivatives in good to excellent yields using a mixture of [bmim][PF₆] and water as the solvent. The isolation of the products was readily performed by the extraction with diethyl ether, and the Pd(OAc)₂/P(1-Nap)₃/[bmim][PF₆]/H₂O system could be easily recycled and reused six times without significant loss of catalytic activity. Our system not only avoids the use of easily volatile THF or toluene as solvent but also solves the basic problem of palladium catalyst and these phosphine ligand reuse.

Full text of DOI:10.1016/j.jorganchem.2014.11.020

Journal of Organometallic Chemistry 777 (2015) 50e56
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Recyclable and reusable Pd(OAc)2/P(1-Nap)3/[bmim][PF6]/H2O system
for the addition of arylboronic acids to aldehydes
,
, *
Hong Zhao a b, Mingzhu Cheng a, Tinli Zhang a, Mingzhong Cai a
a Key Laboratory of Functional Small Organic Molecule, Ministry of Education and Department of Chemistry, Jiangxi Normal University, Nanchang 330022,
PR China
b School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A stable and efficient Pd(OAc)2/P(1-Nap)3[tri(1-naphthyl)phosphine] catalytic system for the addition of
arylboronic acids to aldehydes has been developed. In the presence of Pd(OAc)2 and P(1-Nap)3, the
addition reaction of arylboronic acids with aldehydes was carried out smoothly at 65 ꢀC to give a variety
of carbinol derivatives in good to excellent yields using a mixture of [bmim][PF6] and water as the
solvent. The isolation of the products was readily performed by the extraction with diethyl ether, and the
Pd(OAc)2/P(1-Nap)3/[bmim][PF6]/H2O system could be easily recycled and reused six times without
significant loss of catalytic activity. Our system not only avoids the use of easily volatile THF or toluene as
solvent but also solves the basic problem of palladium catalyst and these phosphine ligand reuse.
© 2014 Elsevier B.V. All rights reserved.
Received 28 August 2014
Received in revised form
12 November 2014
Accepted 20 November 2014
Available online 29 November 2014
Keywords:
Palladium
Ionic liquid
Carbinol
Arylboronic acid
Green chemistry
Introduction
moisture, and good functional group tolerance [23e25]. Meanwhile
the palladium-catalyzed addition of arylboronic acids to aldehydes
Diarylmethanols are important intermediates for the prepara-
tion of a number of biologically active compounds [1e3]. For
example, the 1,1-diarylalkyl structural moiety is present in some
compounds with reported activity such as antimuscarinics [4],
antidepressants [5], and endothelin antagonists [6]. Although the
addition of strong organometallics (RLi, RMgX) to aldehydes pro-
vides one of the most versatile methods to prepare secondary al-
cohols, limitations as to their use arise from their high reactivity as
nucleophile or base, which often gives rise to undesired side re-
actions in the preparation of multifunctional compounds. The
addition of organozinc reagents to aldehydes has also been re-
ported [7e9], but examples of aryl transfer are often limited to the
introduction of a phenyl group [10e15]. Thus, there is still a need
for practical synthetic methodologies for the synthesis of a variety
of carbinol derivatives. In the past few years, the rhodium-
catalyzed addition of organoboron reagents to aldehydes has
received much attention [16e22] since organoboron reagents enjoy
high prestige in the transition-metal-catalyzed CeC coupling re-
action due to their advantages of low toxicity, stability to air or
has also emerged as a promising alternative for the synthesis of
carbinol derivatives [26e30]. Generally, these rhodium- or
palladium-catalyzed addition reactions of arylboronic acids with
aldehydes are performed in easily volatile and toxic organic sol-
vents such as toluene or THF using homogeneous rhodium or
palladium complexes as catalysts, however, industrial applications
of homogeneous rhodium or palladium complexes remain a chal-
lenge because they are expensive, cannot be separated from the
product mixture and reused. These disadvantages have so far pre-
cluded their practical applications.
The development of environmentally friendly synthetic pro-
cedures has become a major concern throughout the chemical in-
dustry due to continuing depletion of natural resources and
growing environmental awareness [31e34]. One of the prime
concerns of industry and academia is the search for replacements to
the environmentally damaging organic solvents used on a large
scale, especially those which are volatile and difficult to contain.
There are also significant economical and environmental reasons
for developing recyclable catalytic reactions from both academic
and industrial perspectives. To satisfy both recyclability and envi-
ronmental concerns, a more facile method is to immobilize the
catalyst in a liquid phase by dissolving it into a nonvolatile and non-
* Corresponding author. Fax: þ86 791 88120388.
E-mail address: caimzhong@163.com (M. Cai).
http://dx.doi.org/10.1016/j.jorganchem.2014.11.020
0022-328X/© 2014 Elsevier B.V. All rights reserved.

H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
51
mixing liquid, such as ionic liquids. Room temperature ionic liquids,
especially those based upon the 1,3-dialkylimidazolium cation,
have received much attention in the last decade [35e39]. They offer
an alternative and ecologically sound medium compared to con-
ventional organic solvents because they are non-volatile, recy-
clable, thermally robust and excellent solvents for a wide range of
organic and inorganic reagents. Furthermore, their high compati-
bility with transition metal catalysts and limited miscibility with
common solvents, enables easy product and catalyst separation
with the retention of the stabilized catalyst in the ionic phase [40].
These and related ionic liquids have been successfully applied to
palladium-catalyzed carbonecarbon bond formations such as Heck
reaction [41,42], Suzuki reaction [43e46], Sonogashira reaction
[47], Stille reaction [48,49], and carboneheteroatom bond forma-
tion reactions [50,51]. Recently, the use of water as solvent in
organic reactions has also attracted considerable interest due to the
low cost, non-toxicity, safety, availability, and greater chemo-
selectivity compared with organic solvents [52e55]. We herein
report the application of Pd(OAc)2/P(1-Nap)3/[bmim][PF6]/H2O
system as an extremely effective and reusable catalytic medium for
the addition reaction of arylboronic acids with aldehydes. The
developed methodology has important practical advantages
deserving special note.
(4-Chlorophenyl)phenylmethanol, 3b [19]
White solid, m.p. 61e62 ꢀC. IR (KBr): nmax/cmꢂ1 3375, 3064,
1640, 1601, 1488, 1138, 1013, 848, 760, 700. 1H NMR (400 MHz,
CDCl3)
d
(ppm): 7.35e7.26 (m, 9H), 5.81 (s, 1H), 2.26 (s, 1H). 13C
NMR (100 MHz, CDCl3)
127.4, 126.0, 75.1.
d (ppm): 143.0, 141.7, 132.8, 128.2, 128.1,
(4-Cyanophenyl)phenylmethanol, 3c [30]
White solid, m.p. 65e66 ꢀC. IR (KBr): nmax/cmꢂ1 3406, 2197,
1629, 1487, 1067, 755, 699. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.61
(d, J ¼ 8.4 Hz, 2H), 7.50 (d, J ¼ 8.4 Hz, 2H), 7.39e7.32 (m, 5H), 5.86 (s,
1H), 2.52 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 148.9, 142.8,
d
132.3, 128.9, 128.4, 127.1, 126.6, 118.8, 111.2, 75.6.
(3-Nitrophenyl)phenylmethanol, 3d [17]
Yellow solid, m.p. 43e44 ꢀC. IR (KBr): nmax/cmꢂ1 3435, 1639,
1619, 1530, 1351, 1138, 1122, 1068, 953. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.28 (s, 1H), 8.10 (d, J ¼ 8.0 Hz, 1H), 7.70 (d, J ¼ 7.6 Hz, 1H),
7.48 (t, J ¼ 8.0 Hz, 1H), 7.36e7.25 (m, 5H), 5.90 (s, 1H), 2.58 (br, 1H).
13C NMR (100 MHz, CDCl3)
(ppm): 145.8, 142.8, 134.6, 132.5, 130.8,
d
129.0, 128.4, 126.7, 122.4, 121.3, 75.4.
(4-Methoxycarbonylphenyl)phenylmethanol, 3e [17]
Colorless oil. IR (film): nmax/cmꢂ1 3451, 1716, 1595, 1281, 1180,
1067, 751. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.99 (d, J ¼ 8.4 Hz,
2H), 7.46 (d, J ¼ 8.4 Hz, 2H), 7.36e7.24 (m, 5H), 5.86 (s, 1H), 3.88 (s,
3H), 2.54 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 167.0, 148.8,
Experimental
d
General remarks
143.2, 129.7, 128.8, 128.3, 127.9, 126.7, 126.3, 75.8, 52.2.
All chemicals were reagent grade and used as purchased. The
products were purified by flash chromatography on silica gel.
Mixture of EtOAc and hexane was generally used as eluent. All
addition products were characterized by comparison of their
spectra and physical data with authentic samples. IR spectra were
determined on a PerkineElmer 683 instrument. 1H NMR spectra
were recorded on a Bruker Avance 400 (400 MHz) spectrometer
with TMS as an internal standard in CDCl3 as solvent. 13C NMR
spectra were recorded on a Bruker Avance 400 (100 MHz) spec-
trometer in CDCl3 as solvent. Melting points are uncorrected.
Phenyl(4-trifluoromethylphenyl)methanol, 3f [17]
White solid, m.p. 95e97 ꢀC. IR (KBr): nmax/cmꢂ1 3302,1318,1164,
1107, 1056, 743. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.60 (d,
J ¼ 8.4 Hz, 2H), 7.49 (d, J ¼ 8.4 Hz, 2H), 7.35e7.25 (m, 5H), 5.84 (s,
1H), 2.47 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 147.6, 143.2,
d
128.7, 128.2, 126.7, 126.6, 125.5, 125.4, 122.8, 75.8.
(Biphenyl-4-yl)phenylmethanol, 3g [30]
White solid, m.p. 83e85 ꢀC. IR (KBr): nmax/cmꢂ1 3412, 3056,
1640, 1601, 1487, 1453, 1159, 1072, 759, 697. 1H NMR (400 MHz,
CDCl3)
d
(ppm): 7.58e7.55 (m, 4H), 7.47e7.27 (m, 10H), 5.90 (s, 1H),
(ppm): 143.6,142.8,140.7,
2.25 (br, 1H). 13C NMR (100 MHz, CDCl3)
d
General procedure for the palladium-catalyzed addition of
arylboronic acids to aldehydes in [bmim][PF6]/H2O
140.5, 128.7, 128.4, 127.6, 127.3, 127.1, 127.0, 126.8, 126.5, 76.0.
A Schlenk reaction tube was charged with [bmim][PF6] (3 mL),
distilled water (2 mL), Pd(OAc)2 (0.05 mmol), P(1-Nap)3
(0.05 mmol), aldehyde (1.0 mmol), aryl-boronic acid (2.0 mmol),
and K2CO3 (3.0 mmol) under an argon atmosphere. The mixture
was stirred at room temperature for 0.5 h, and then heated at 65 ꢀC
for 6e24 h. After being cooled to room temperature, the reaction
mixture was extracted three times with diethyl ether (3 ꢁ 10 mL).
The residue of the extraction was subjected to a second run of the
addition reaction by charging with the same substrates (arylbor-
onic acid, aldehyde, and K2CO3) under the same conditions without
further addition of Pd(OAc)2 or P(1-Nap)3. The combined ether
phase was concentrated under reduced pressure and the residue
was purified by flash column chromatography on silica gel (hex-
aneeethyl acetate ¼ 10:1) to give the desired product.
(4-Methoxyphenyl)phenylmethanol, 3h [30]
White solid, m.p. 66e67 ꢀC. IR (KBr): nmax/cmꢂ1 3412,1612,1587,
1517, 1494, 1178, 1034, 810, 727. 1H NMR (400 MHz, CDCl3)
d
(ppm):
7.38e7.24 (m, 7H), 6.87 (d, J ¼ 8.8 Hz, 2H), 5.81 (s, 1H), 3.79 (s, 3H),
2.17 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 158.9, 144.1, 136.2,
d
128.4, 127.9, 127.5, 126.4, 113.9, 75.8, 55.3.
(4-Methylphenyl)phenylmethanol, 3i [17]
White solid, 54e55 ꢀC. IR (KBr): nmax/cmꢂ1 3307, 2936, 1623,
1501, 1165, 751. 1H NMR (400 MHz, CDCl3)
5H), 7.26 (d, J ¼ 8.0 Hz, 2H), 7.15 (d, J ¼ 8.0 Hz, 2H), 5.99 (s, 1H), 2.36
(s, 3H), 2.18 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 143.9,
d (ppm): 7.39e7.28 (m,
d
140.9, 137.4, 129.3, 128.4, 127.5, 126.6, 126.4, 76.2, 21.2.
(2-Chlorophenyl)phenylmethanol, 3j
(4-Nitrophenyl)phenylmethanol, 3a [30]
Colorless oil. IR (film): nmax/cmꢂ1 3433, 1638, 1066, 752, 699. 1H
Yellow solid, m.p. 68e69 ꢀC. IR (KBr): nmax/cmꢂ1 3449, 1601,
NMR (400 MHz, CDCl3)
(m, 8H), 6.15 (s, 1H), 2.32 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 141.2, 140.0, 131.5, 128.5, 127.7, 127.5, 127.0, 126.8, 126.1,
d
(ppm): 7.53 (d, J ¼ 7.6 Hz, 1H), 7.33e7.12
1514, 1340, 1048, 705. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.18 (d,
J ¼ 8.8 Hz, 2H), 7.57 (d, J ¼ 8.8 Hz, 2H), 7.38e7.29 (m, 5H), 5.91 (s,
d
1H), 2.49 (br, 1H). 13C NMR (100 MHz, CDCl3)
142.7, 129.1, 128.4, 127.1, 126.7, 123.7, 75.5.
d
(ppm): 150.7, 142.9,
125.9, 71.7. Anal. Calcd for C13H11OCl: C, 71.39; H, 5.07. Found: C,
71.14; H, 4.86.

52
H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
(2,5-Dichlorophenyl)phenylmethanol, 3k
2-Methylphenyl(4-nitrophenyl)methanol, 3s [19]
Colorless oil. IR (film): nmax/cmꢂ1 3339, 1590, 1562, 1381, 1020,
Yellow oil. IR (film): nmax/cmꢂ1 3419, 2928, 1604, 1521, 1489,
697. 1H NMR (400 MHz, CDCl3)
7.35e7.23 (m, 7H), 6.16 (s, 1H), 2.33 (br, 1H). 13C NMR (100 MHz,
CDCl3) (ppm): 141.8, 139.7, 133.9, 133.1, 129.3, 129.0, 128.6, 128.0,
d
(ppm): 7.58 (d, J ¼ 8.4 Hz, 1H),
1347, 1108, 1034, 758. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.19 (d,
J ¼ 8.8 Hz, 2H), 7.53 (d, J ¼ 8.4 Hz, 2H), 7.34e7.18 (m, 4H), 6.11 (s,
1H), 2.32 (s, 3H), 2.28 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm):
d
d
127.4, 126.9, 72.3. Anal. Calcd for C13H10OCl2: C, 61.66; H, 3.98.
Found: C, 61.43; H, 3.67.
150.2, 147.2, 140.5, 135.6, 131.1, 128.4, 127.5, 127.0, 126.6, 123.6, 72.8,
19.4.
2-Methoxyphenyl(4-nitrophenyl)methanol, 3t
4-Methylphenyl(4-nitrophenyl)methanol, 3l [29]
Yellow oil. IR (film): nmax/cmꢂ1 3421, 2940, 2839, 1601, 1521,
Yellow solid, m.p. 105e106 ꢀC. IR (KBr): nmax/cmꢂ1 3423, 2920,
1490, 1347, 1245,1028, 734. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.14
1639, 1509, 1344, 1178, 1063, 796. 1H NMR (400 MHz, CDCl3)
(d, J ¼ 8.8 Hz, 2H), 7.55 (d, J ¼ 8.4 Hz, 2H), 7.32e7.21 (m, 2H), 6.96 (t,
d
(ppm): 8.18 (d, J ¼ 8.4 Hz, 2H), 7.57 (d, J ¼ 8.4 Hz, 2H), 7.22 (d,
J ¼ 8.0 Hz, 2H), 7.17 (d, J ¼ 8.0 Hz, 2H), 5.88 (s, 1H), 2.34 (s, 3H), 2.29
(br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 150.9, 147.2, 139.9,
J ¼ 7.6 Hz, 1H), 6.90 (d, J ¼ 8.0 Hz, 1H), 6.09 (d, J ¼ 4.4 Hz, 1H), 3.80
(s, 3H), 3.23 (d, J ¼ 4.4 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d (ppm):
d
156.5, 151.0, 147.0, 130.8, 129.4, 127.7, 127.1, 123.4, 121.1, 111.0, 71.5,
55.4. Anal. Calcd for C14H13NO4: C, 64.86; H, 5.05; N, 5.40. Found: C,
64.58; H, 5.27, N, 5.58.
138.3, 129.6, 127.0, 126.7, 123.6, 75.4, 21.1.
4-Chlorophenyl(4-nitrophenyl)methanol, 3m [30]
Yellow solid, m.p. 130e131 ꢀC. IR (KBr): nmax/cmꢂ1 3421, 1638,
1512, 1346, 1067, 799. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.20 (d,
1-Naphthyl(4-nitrophenyl)methanol, 3u
Yellow solid, m.p. 129e130 ꢀC. IR (KBr): nmax/cmꢂ1 3421, 1637,
1599, 1514, 1344, 1068, 954, 853, 783. 1H NMR (400 MHz, CDCl3)
J ¼ 8.8 Hz, 2H), 7.56 (d, J ¼ 8.8 Hz, 2H), 7.34 (d, J ¼ 7.6 Hz, 2H), 7.29
(d, J ¼ 8.8 Hz, 2H), 5.91 (s, 1H), 2.37 (br 1H). 13C NMR (100 MHz,
d
(ppm): 8.18 (d, J ¼ 8.8 Hz, 2H), 8.03 (d, J ¼ 7.6 Hz, 1H), 7.91e7.86
(m, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H), 7.50e7.45 (m, 4H), 6.58 (s, 1H), 2.53
(br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 150.3, 147.2, 137.8,
CDCl3)
74.7.
d (ppm): 150.3, 147.4, 141.2, 134.3, 129.2, 128.1, 127.2, 123.8,
d
134.2, 130.5, 129.4, 129.0, 127.5, 126.6, 126.0, 125.6, 125.3, 123.7,
123.6, 73.3. Anal. Calcd for C17H13NO3: C, 73.11; H, 4.69; N, 5.01.
Found: C, 72.85; H, 4.52, N, 5.22.
3-Methylphenyl(4-nitrophenyl)methanol, 3n
Yellow solid, m.p. 60e61 ꢀC. IR (KBr): nmax/cmꢂ1 3501, 2870,
1602,1513,1345,1147,1041, 857. 1H NMR (400 MHz, CDCl3)
d (ppm):
8.18 (d, J ¼ 8.8 Hz, 2H), 7.57 (d, J ¼ 8.8 Hz, 2H), 7.27e7.13 (m, 4H),
4-Fluorophenyl(4-methoxyphenyl)methanol, 3v [17]
5.88 (s, 1H), 2.41 (br, 1H), 2.34 (s, 3H). 13C NMR (100 MHz, CDCl3)
White solid, m.p. 51e52 ꢀC. IR (KBr): nmax/cmꢂ1 3420, 2938,
d
(ppm): 149.9, 147.5, 146.4, 127.2, 126.9, 125.9, 125.8, 124.1, 123.9,
1618, 1255, 1187, 1065, 750. 1H NMR (400 MHz, CDCl3)
d (ppm):
74.9, 29.7. Anal. Calcd for C14H13NO3: C, 69.13; H, 5.39; N, 5.76.
Found: C, 68.87; H, 5.58, N, 5.47.
7.34e7.32 (m, 2H), 7.27 (d, J ¼ 8.8 Hz, 2H), 7.03e7.01 (m, 2H), 6.88
(d, J ¼ 8.8 Hz, 2H), 5.79 (s, 1H), 3.80 (s, 3H), 2.04 (br, 1H). 13C NMR
(100 MHz, CDCl3)
d (ppm): 160.9, 159.2, 136.1, 128.2, 128.1, 115.4,
4-Methoxyphenyl(4-nitrophenyl)methanol, 3o [29]
115.2, 113.9, 75.3, 55.4.
Yellow solid, m.p. 76e77 ꢀC. IR (KBr): nmax/cmꢂ1 3410, 2917,
2849, 1612, 1604, 1514, 1351, 1251, 1042, 800. 1H NMR (400 MHz,
4-Methoxyphenyl(4-trifluoromethylphenyl)methanol, 3w [19]
White solid, m.p. 87e88 ꢀC. IR (KBr): nmax/cmꢂ1 3349, 2938,
2840, 1615, 1587, 1516, 1335, 1259, 1107, 1065, 814. 1H NMR
CDCl3)
(d, J ¼ 8.0 Hz, 2H), 6.88 (d, J ¼ 8.0 Hz, 2H), 5.88 (s, 1H), 3.79 (s, 3H),
2.41 (br, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 159.7, 151.0, 147.2,
134.9, 128.2, 126.9, 123.6, 114.3, 75.1, 55.4.
d
(ppm): 8.18 (d, J ¼ 8.8 Hz, 2H), 7.56 (d, J ¼ 8.8 Hz, 2H), 7.25
d
(400 MHz, CDCl3)
2H), 7.22 (d, J ¼ 8.4 Hz, 2H), 6.85 (d, J ¼ 8.8 Hz, 2H), 5.78 (s, 1H), 3.77
(s, 3H), 2.55 (s, 1H). 13C NMR (100 MHz, CDCl3)
(ppm): 159.4,147.9,
d
(ppm): 7.56 (d, J ¼ 8.0 Hz, 2H), 7.47 (d, J ¼ 8.0 Hz,
d
4-Nitrophenyl(4-trifluoromethylphenyl)methanol, 3p [30]
135.5, 129.5, 128.1, 126.9, 126.6, 125.3, 114.1, 75.3, 55.3.
Yellow solid, m.p. 104e105 ꢀC. IR (KBr): nmax/cmꢂ1 3488, 2916,
1598, 1513, 1333, 1160, 1120, 1070, 804. 1H NMR (400 MHz, CDCl3)
4-Cyanophenyl(4-methylphenyl)methanol, 3x [29]
White solid, m.p. 69e71 ꢀC. IR (KBr): nmax/cmꢂ1 3406, 2962,
d
(ppm): 8.21 (d, J ¼ 8.8 Hz, 2H), 7.63 (d, J ¼ 8.0 Hz, 2H), 7.57 (d,
J ¼ 8.8 Hz, 2H), 7.50 (d, J ¼ 8.0 Hz, 2H), 5.99 (s, 1H), 2.51 (br, 1H). 13
C
2198, 1605, 1496, 1165, 1067, 848, 746. 1H NMR (400 MHz, CDCl3)
NMR (100 MHz, CDCl3)
d (ppm): 149.9, 147.5, 146.4, 130.5, 127.2,
d
(ppm): 7.58 (d, J ¼ 8.4 Hz, 2H), 7.49 (d, J ¼ 8.0 Hz, 2H), 7.20 (d,
J ¼ 8.0 Hz, 2H), 7.14 (d, J ¼ 8.4 Hz, 2H), 5.80 (s, 1H), 2.53 (br,1H), 2.32
(s, 3H). 13C NMR (100 MHz, CDCl3)
(ppm): 149.1,139.9,138.1,132.2,
126.9, 126.6, 125.9, 123.9, 74.9.
d
4-Acetylphenyl(4-nitrophenyl)methanol, 3q [17]
129.5, 126.9, 126.6, 118.8, 110.9, 75.4, 21.1.
Yellow solid, m.p. 121e122 ꢀC. IR (KBr): nmax/cmꢂ1 3476, 2925,
1676, 1601, 1517, 1345, 1054, 961, 800. 1H NMR (400 MHz, CDCl3)
4-Methoxyphenyl(2-methylphenyl)methanol, 3y [17]
Colorless oil. IR (film): nmax/cmꢂ1 3403, 1607, 1295, 1252, 1065,
d
(ppm): 8.15 (d, J ¼ 8.8 Hz, 2H), 7.88 (d, J ¼ 8.4 Hz, 2H), 7.55 (d,
754. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.57 (d, J ¼ 7.6 Hz, 1H), 7.23
J ¼ 8.8 Hz, 2H), 7.45 (d, J ¼ 8.4 Hz, 2H), 5.97 (d, J ¼ 2.8 Hz, 1H), 3.38
(d, J ¼ 3.2 Hz, 1H), 2.55 (s, 3H). 13C NMR (100 MHz, CDCl3)
d (ppm):
(d, J ¼ 8.8 Hz, 2H), 7.28e7.17 (m, 2H), 7.12 (d, J ¼ 7.4 Hz, 1H), 6.83 (d,
J ¼ 8.8 Hz, 2H), 5.95 (d, J ¼ 4.0 Hz, 1H), 3.88 (s, 3H), 2.22 (s, 3H), 2.05
198.1, 150.2, 147.7, 147.2, 136.5, 128.9, 127.2, 126.6, 123.8, 74.8, 26.6.
(d, J ¼ 4.0 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d (ppm): 141.7, 135.2,
3-Nitrophenyl(4-nitrophenyl)methanol, 3r [30]
135.1, 130.6, 127.5, 126.6, 126.1, 125.9, 113.7, 72.9, 55.3, 19.4.
Yellow solid, m.p. 117e118 ꢀC. IR (KBr): nmax/cmꢂ1 3457, 1603,
1517, 1345, 1082, 1047, 861, 736. 1H NMR (400 MHz, CDCl3)
d
(ppm):
2-Nitrophenyl(o-tolyl)methanol, 4a [30]
8.28e8.14 (m, 4H), 7.70 (d, J ¼ 7.6 Hz,1H), 7.61e7.52 (m, 3H), 6.02 (d,
Yellow solid, m.p. 92e93 ꢀC. IR (KBr): nmax/cmꢂ1 3308, 1610,
J ¼ 2.4 Hz, 1H), 2.73 (d, J ¼ 2.4 Hz, 1H). 13C NMR (100 MHz, CDCl3)
1522, 1493, 1343,1037, 850, 791. 1H NMR (400 MHz, CDCl3)
d (ppm):
d
(ppm): 149.3, 148.5, 147.5, 144.6, 132.4, 129.9, 127.2, 124.1, 123.1,
7.97 (d, J ¼ 8.8 Hz,1H), 7.60e7.45 (m, 3H), 7.25e7.17 (m, 4H), 6.54 (d,
121.2, 74.5.
J ¼ 4.4 Hz, 1H), 2.82 (d, J ¼ 4.4 Hz, 1H), 2.24 (s, 3H). 13C NMR

H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
53
Table 1
Addition reaction of 4-methoxyphenylboronic acid with 4-nitrobenzaldehyde in different conditions.a
Entry
Solvent
Pd source
Base
Temp. (ꢀC)
Yield (%)b
1
2
3
4
5
6
7
8
[bmim][PF6]
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
Et3N
n-Bu3N
DABCO
DMAP
DBU
K2CO3
K2CO3
K2CO3
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
80
95
50
Trace
44
83
65
37
0
93
84
79
83
72
14
85
90
9
11
8
7
10
90
89
77
[bmim][PF6]/H2O (4/1)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (2/3)
[bmim][PF6]/H2O (1/4)
H2O
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
[bmim][PF6]/H2O (3/2)
Pd(OAc)2
PdCl2(PPh3)2
PdCl2(PhCN)2
Pd(PPh3)4
Pd2(dba)3
Pd/C
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
9
10
11
12
13
14
15
16
17
18
19
20
21
22c
a
Reaction conditions: 4-nitrobenzaldehyde (1.0 mmol), 4-methoxyphenylboronic acid (2.0 mmol), base (3.0 mmol), Pd source (5 mol%), P(1-Nap)3 (5 mol%) in 5 mL of
solvent for 6 h under Ar.
b
Isolated yields.
For 24 h.
c
Table 2
Addition of various arylboronic acids to aldehydes.a
Entry
Ar
R
Product
Time (h)
Yield (%)b
1
2
3
4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-O2NC6H4
4-ClC6H4
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
6
8
8
8
8
93
91
90
91
92
93
55
77
83
79
74
92
91
96
93
64
68
62
95
96
87
86
92
90
78
4-NCC6H4
3-O2NC6H4
4-MeOCOC6H4
4-CF3C6H4
4-BrC6H4
4-MeOC6H4
4-MeC6H4
2-ClC6H4
2,5-Cl2C6H3
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-FC6H4
5
6
6
7c
8
10
15
12
20
20
6
6
6
6
20
20
20
10
10
14
10
6
9
Ph
Ph
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4-MeC6H4
4-ClC6H4
3-MeC6H4
4-MeOC6H4
4-CF3C6H4
4-MeCOC6H4
3-O2NC6H4
2-MeC6H4
2-MeOC6H4
1-Naphthyl
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
2-MeC6H4
3s
3t
3u
3v
3w
3x
3y
4-CF3C6H4
4-NCC6H4
4-MeOC6H4
6
24
a
Reaction conditions: aldehyde (1 mmol), arylboronic acid (2 mmol), K2CO3 (3 mmol), Pd(OAc)2 (5 mol%), P(1-Nap)3 (5 mol%), [bmim][PF6] (3 mL), water (2 mL) at 65 ꢀ
C
under Ar.
b
c
Isolated yields.
Biphenyl-4-yl(phenyl)methanol was obtained.

54
H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
(100 MHz, CDCl3)
129.3, 128.4, 128.1, 126.2, 126.0, 124.6, 68.6, 19.0.
d
(ppm): 148.6, 139.4, 138.1, 135.7, 133.3, 130.5,
(d, J ¼ 8.4 Hz, 1H), 7.90e7.78 (m, 3H), 7.49e7.37 (m, 7H), 7.03 (d,
J ¼ 4.4 Hz, 1H), 3.47 (d, J ¼ 4.4 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d
(ppm): 148.6, 138.2, 136.9, 134.0, 133.5, 130.9, 129.6, 129.1, 128.9,
128.7, 126.7, 125.9, 125.4, 124.9, 124.5, 123.4, 68.5.
2-Methoxyphenyl(2-nitrophenyl)methanol, 4b [19]
Yellow solid, m.p. 80e81 ꢀC. IR (KBr): nmax/cmꢂ1 3498, 1596,
1518, 1340, 1110, 1032, 863. 1H NMR (400 MHz, CDCl3)
d (ppm): 7.88
Results and discussion
(d, J ¼ 8.4 Hz, 1H), 7.62e7.26 (m, 3H), 7.24e6.86 (m, 4H), 6.65 (d,
J ¼ 4.4 Hz, 1H), 3.75 (s, 3H), 3.22 (br, 1H). 13C NMR (100 MHz, CDCl3)
Optimization of reaction conditions
d
(ppm): 156.4, 148.7, 137.8, 133.0, 130.2, 129.4, 129.2, 128.1, 126.7,
124.2, 120.6, 110.6, 66.7, 55.3.
Initially, to determine the optimum conditions, the palladium-
catalyzed addition of 4-methoxyphenylboronic acid to 4-
nitrobenzaldehyde in the presence of P(1-Nap)3 as a ligand was
chosen as a model reaction, the influences of various reaction pa-
rameters such as solvents, Pd sources, bases, and reaction tem-
peratures on the reaction were examined and the results are
summarized in Table 1. At first, the solvent effect was examined,
and a significant solvent effect was observed. It is evident that the
reaction proceeded very slowly and only trace of desired product
was obtained when the moisture stable and commercially available
1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])
was used as solvent (Table 1, entry 1). However, a mixture of
[bmim][PF6] and H2O was found to be effective (Table 1, entries
2e5). The reaction run in [bmim][PF6]/H2O (V/V ¼ 3/2) gave 4-
methoxyphenyl(4-nitrophenyl)methanol in 83% yield (Table 1, en-
try 3). It may be due to the fact that the ionic liquids and water
dissolve the catalyst/reactants and the base, respectively, to make
the reaction proceed smoothly. The reaction did not occur at all
using neat water as solvent (Table 1, entry 6). Among the palladium
sources tested [PdCl2, Pd(OAc)2, PdCl2(PPh3)2, PdCl2(PhCN)2,
Pd(PPh3)4, Pd2(dba)3, and Pd/C], Pd(OAc)2 exhibited the highest
2-Methoxyphenyl(2-trifluoromethylphenyl)methanol, 4c [30]
Colorless oil. IR (film): nmax/cmꢂ1 3475, 1603, 1492, 1314, 1126,
1042, 847. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.70 (d, J ¼ 7.6 Hz,
1H), 7.58e7.53 (m, 1H), 7.45e7.43 (m, 1H), 7.31e7.28 (m, 1H), 6.98
(d, J ¼ 7.4 Hz, 1H), 6.94e6.88 (m, 3H), 6.55 (s, 1H), 3.84 (s, 3H), 3.18
(br, 1H). 13C NMR (100 MHz, CDCl3)
d (ppm): 156.6, 140.9, 131.9,
131.2, 129.3, 128.8, 127.7, 125.8, 122.5, 121.3, 120.4, 112.5, 110.4, 67.2,
55.3.
Bis(2-methoxyphenyl)methanol, 4d [56]
Colorless oil. IR (film): nmax/cmꢂ1 3446, 1598, 1487, 1128, 1056,
845. 1H NMR (400 MHz, CDCl3)
d (ppm): 7.36e7.30 (m, 4H),
7.05e6.92 (m, 4H), 6.47 (d, J ¼ 4.8 Hz, 1H), 3.83 (s, 6H), 3.72 (d,
J ¼ 4.8 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d (ppm): 156.6, 130.9,
128.3, 127.6, 120.4, 110.2, 67.0, 55.2.
Naphthalen-1-yl(2-nitrophenyl)methanol, 4e [30]
Yellow solid, m.p. 67e69 ꢀC. IR (KBr): nmax/cmꢂ1 3304, 1604,
1513, 1489, 1342, 803, 725. 1H NMR (400 MHz, CDCl3)
d (ppm): 7.99
Table 3
Addition of hindered arylboronic acids to hindered aromatic aldehydes.a
Entry
1
Ar
Ar1
Product
Time (h)
20
Yield (%)b
83
2
3
4
20
24
24
85
78
72
5
20
82
a
Reaction conditions: aldehyde (1 mmol), arylboronic acid (2 mmol), K2CO3 (3 mmol), Pd(OAc)2 (5 mol%), P(1-Nap)3 (5 mol%), [bmim][PF6] (3 mL), water (2 mL) at 65 ꢀ
C
under Ar.
b
Isolated yields.

H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
55
catalytic activity and afforded a 93% yield (Table 1, entry 7). Our
next studies focused on the effect of base and reaction temperature
on the model reaction. When Cs2CO3, K2CO3 and Na2CO3 were used
as the base, good to excellent yields were obtained, whereas
organic bases such as Et3N, n-Bu3N, DABCO, DMAP, and DBU
afforded low yields (Table 1, entries 7, 13e19), so K2CO3 was finally
selected as the base for the reaction. For the temperatures evalu-
ated [50, 65, 80, and 95 ꢀC], 65 ꢀC gave the best result. It was found
that the reaction was accomplished when it was carried out in
[bmim][PF6]/H2O (V/V ¼ 3/2) at 65 ꢀC for 6 h. Reducing reaction
temperature to 50 ꢀC resulted in a decrease in yield and a longer
reaction time was needed (Table 1, entry 22). Therefore, the optimal
catalytic system involved the use of Pd(OAc)2 (5 mol%), P(1-Nap)3
Encouraged by the above results, the scope and the generality of
the reaction by varying the arylboronic acids were further inves-
tigated. We were pleased to find that the standard conditions were
compatible with various arylboronic acids bearing methyl, chloro,
methoxy, trifluoromethyl, acetyl and nitro groups. It was found that
both electron-rich and electron-deficient arylboronic acids under-
went the addition reaction efficiently and generated the corre-
sponding diarylmethanols in moderate to excellent yields. For
example, arylboronic acids with 4-methyl, 4-chloro, 4-methoxy
and 3-methyl on the benzene rings underwent the reaction with
4-nitrobenzaldehyde smoothly and the desired products 3leo were
isolated in 91e96% yields (Table 2, entries 12e15). Arylboronic
acids bearing an electron-withdrawing substituent, which are less
nucleophilic and, hence, transmetalate more slowly than the
electro-neutral analogs, are prone to homocoupling and proto-
deboronation side reactions [57]. The reaction of arylboronic acids
bearing 4-trifluoromethyl, 4-acetyl and 3-nitro groups also gave the
corresponding diarylmethanols 3per in 62e68% yields (Table 2,
entries 16e18). The reaction of sterically hindered arylboronic acids
such as 2-methylphenylboronic acid and 2-methoxyphenylboronic
acid could proceed efficiently and the products 3s and 3t were
isolated in 95% and 96% yield, respectively (Table 2, entries 19 and
20). The bulky 1-naphthylboronic acid also reacted with 4-
nitrobenzaldehyde smoothly to give the desired product 3u in
87% yield (Table 2, entry 21). In addition, the reactions of phe-
nylboronic acids bearing substituents with various substituted
benzaldehydes afforded the corresponding diarylmethanols 3vey
also in good to high yields (Table 2, entries 22e25). We also tried
to carry out the reaction of phenylboronic acid with aliphatic al-
dehydes such as 3-phenylpropanal or hexanal using this catalytic
system, however, the addition reactions were very slow at stan-
dard condition and only traces of desired products were detected
due to their lower electrophilicity than that of aromatic
aldehydes.
(5 mol%), K2CO3 (3 equiv) in [bmim][PF6]/H2O (V/V ¼ 3/2) at 65 ꢀ
C
under Ar for 6 h (Table 1, entry 7).
Scope of Pd(OAc)2-catalyzed addition of various arylboronic acids to
aldehydes in [bmim][PF6]/H2O
With this promising results in hand, we started to investigate
the scope of this reaction under the optimized conditions. The
scope of both aldehydes and arylboronic acids was explored, and
the results are summarized in Table 2. At the beginning of the
determination of the aldehydes substrate scope, phenylboronic
acid was used as one of the model substrate. As shown in Table 2,
the addition reaction of phenylboronic acid with various aldehydes
proceeded smoothly under the optimized reaction conditions to
afford the corresponding diarylmethanols in good to excellent
yields and a wide range of functional groups including nitro, cyano,
ester, methoxy, methyl, trifluoromethyl, and chloro groups on the
benzene rings were well tolerated (Table 2, entries 1e11).
Furthermore, electron-deficient aromatic aldehydes reacted with
phenylboronic acid easily and furnished the desired products in
excellent yields (Table 2, entries 1e6). Interestingly, the reaction of
4-chlorobenzaldehyde with phenylboronic acid afforded selectively
(4-chlorophenyl)phenylmethanol 3b in 91% yield and keep the
chloro group untouched (Table 2, entry 2). However, the reaction of
4-bromobenzaldehyde with phenylboronic acid gave biphenyl-4-
yl(phenyl)methanol 3g in 55% yield due to the presence of Suzuki
coupling of the aryl bromide (Table 2, entry 7). On the other hand,
electron-rich aromatic aldehydes such as 4-methoxybenzaldehyde
or 4-methylbenzaldehyde as less active substrates also gave the
corresponding diarylmethanols 3h and 3i in good yields (Table 2,
entries 8 and 9). Finally, it should be worth noting that the addition
of phenylboronic acid to the sterically hindered aromatic aldehydes
such as 2-chlorobenzaldehyde and 2,5-dichlorobenzaldehyde
could also proceed effectively under the standard conditions to
afford desired products 3j and 3k in 79% and 74% yields, respec-
tively (Table 2, entries 10 and 11).
In order to further show the usability of this palladium-
catalyzed addition reaction of arylboronic acids with aromatic al-
dehydes in [bmim][PF6]/H2O as a practical method for the synthesis
of hindered diarylmethanols, we next examined the addition re-
action of hindered arylboronic acids with hindered aromatic alde-
hydes under the optimized conditions, and the results are
summarized in Table 3. As shown in Table 3, a monosubstitution in
the ortho position of both arylboronic acids and aromatic aldehydes
could reduce their reactivity slightly. For instance, the reaction of 2-
methylphenylboronic acid with 2-nitrobenzaldehyde gave 2-
nitrophenyl(o-tolyl)methanol 4a in 83% yield (Table 3, entry 1). 2-
Methoxyphenylboronic
acid
could
react
with
2-
nitrobenzaldehyde, 2-trifluoromethylbenzaldehyde, and 2-
methoxybenzaldehyde to afford the corresponding desired prod-
ucts 4b, 4c, and 4d in 85%, 78%, and 72% yield, respectively (Table 3,
Table 4
Ionic liquid and catalyst recycling in the addition reaction of 4-methoxyphenylboronic acid with 4-nitrobenzaldehyde.a
Cycle
Yield (%)b
Cycle
Yield (%)b
1
2
3
93
92
91
4
5
6
92
91
90
a
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), 4-methoxyphenylboronic acid (2 mmol), K2CO3 (3 mmol), Pd(OAc)2 (5 mol%), P(1-Nap)3 (5 mol%), [bmim][PF6]
(3 mL), water (2 mL) at 65 ꢀC under Ar for 6 h.
b
Isolated yields.

56
H. Zhao et al. / Journal of Organometallic Chemistry 777 (2015) 50e56
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the reaction, 4-methoxyphenylboronic acid, 4-nitrobenzaldehyde
and K2CO3 were allowed to react in this catalytic system at 65 ꢀC
for 6 h and then the product was extracted with diethyl ether for
three times affording the cleaned catalytic solution. After removal
of diethyl ether under the reduced pressure, a second amount of
reactants were added to the solution and the process was repeated
up to 3 times. After third cycle, the ionic liquid was washed with
distilled water (2 ꢁ 5 mL) and mixed with distilled water (2 mL),
and then recycled for another three times without addition of
Pd(OAc)2 and P(1-Nap)3. The yield of the reaction by using of the
recycling catalytic system was 92% for the first time, 91% for the
second time, 92% for the third time, 91% for the fourth time, and
90% for the fifth time, respectively. It seems that there is no effect
on the rate and yield of the reaction during each cycle (Table 4), the
result is important from a practical point of view. Additionally, this
ionic liquid/water catalytic system could be stored for several
weeks with no special precautions to exclude air and still afford
comparable results to the fresh ionic liquid/water catalytic system.
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Acknowledgments
We thank the National Natural Science Foundation of China (No.
21272044) and Key Laboratory of Functional Small Organic Mole-
cule, Ministry of Education (No. KLFS-KF-201213) for financial
support.
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