Nickel catalyzed cross-coupling of aryl C-O based electrophiles with aryl neopentylglycolboronates

Article abstract of DOI:10.1021/jo2022982

The efficiency of mesylates, sulfamates, esters, carbonates, carbamates, and methyl ethers as C-O-based electrophiles attached to the 1- or 2-position of naphthalene and to activated and nonactivated phenyl substrates was compared for the first time in Ni-catalyzed cross-coupling with phenyl neopentylglycolboronates containing electron-rich and electron-deficient substituents in their para-position. These experiments were performed in the presence of four different Ni(II)- and Ni(0)-based catalysts. Ni(II)-based catalysts mediate the cross-coupling of most 2-naphthyl C-O electrophiles with both arylboronic acids and with neopentylglycolboronates when K ₃PO₄ is used as base. The same catalysts are not efficient when CsF is used as base. However, Ni(0)-based catalysts exhibit selective efficiency, and when reactive, their efficiency is higher than that of Ni(II)-based catalysts in the presence of both K₃PO₄ and CsF. These results provide both reaction conditions for the cross-coupling, and for the elaboration of orthogonal cross-coupling methodologies of various C-O based electrophiles with aryl neopentylglycolboronates. With the exception of mesylates and sulfamates the efficiency of all other 2-naphthyl C-O electrophiles was lower in cross-coupling with aryl neopentylglycolboronates than with arylboronic acids

Full text of DOI:10.1021/jo2022982

Article
pubs.acs.org/joc
Nickel Catalyzed Cross-Coupling of Aryl C−O Based Electrophiles
with Aryl Neopentylglycolboronates
Pawaret Leowanawat, Na Zhang, and Virgil Percec*
Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323,
United States
S
* Supporting Information
ABSTRACT: The efficiency of mesylates, sulfamates, esters, carbonates, carbamates, and methyl ethers as C−O-based
electrophiles attached to the 1- or 2-position of naphthalene and to activated and nonactivated phenyl substrates was compared
for the first time in Ni-catalyzed cross-coupling with phenyl neopentylglycolboronates containing electron-rich and electron-
deficient substituents in their para-position. These experiments were performed in the presence of four different Ni(II)- and
Ni(0)-based catalysts. Ni(II)-based catalysts mediate the cross-coupling of most 2-naphthyl C−O electrophiles with both
arylboronic acids and with neopentylglycolboronates when K3PO4 is used as base. The same catalysts are not efficient when CsF
is used as base. However, Ni(0)-based catalysts exhibit selective efficiency, and when reactive, their efficiency is higher than that
of Ni(II)-based catalysts in the presence of both K3PO4 and CsF. These results provide both reaction conditions for the cross-
coupling, and for the elaboration of orthogonal cross-coupling methodologies of various C−O based electrophiles with aryl
neopentylglycolboronates. With the exception of mesylates and sulfamates the efficiency of all other 2-naphthyl C−O
electrophiles was lower in cross-coupling with aryl neopentylglycolboronates than with arylboronic acids
than boronic acids in some reactions such as protodebor-
ylation.10
INTRODUCTION
■
Suzuki−Miyaura cross-coupling is one of the most important
reactions used in the construction of C−C bonds. Pd is widely
used as the catalyst in this reaction.1 Recently, Ni, which is less
expensive and more reactive toward C−O-based electrophiles
than Pd, has been employed as an alternative catalyst in
Suzuki−Miyaura cross-coupling.2 Aryl sulfonates,3 ethers,4
esters,3i,5 carbonates,6 carbamates,6−8 sulfamates,6,7b,8 and
phosphates9 have been elaborated as C−O-based electrophiles
in cross-coupling with arylboronic acids, and for the case of aryl
methyl ethers4 also with aryl neopentylglycolboronates, all
under various Ni-catalyzed conditions.
Since the first report on the Ni-catalyzed Suzuki−Miyaura
cross-coupling of aryl sulfonates with arylboronic acids,3a
interest in utilizing accessible and inexpensive C−O based
electrophiles in cross-coupling continues to develop. Suzuki−
Miyaura cross-coupling of aryl mesylates and tosylates with
arylboronic acids using inexpensive Ni(II)-based catalysts is
well established.3d However, in some situations, boronic esters
are more favored than boronic acids in Suzuki−Miyaura cross-
coupling reactions. For example, boronic esters can be used in
stoichiometric amounts due to their monomeric species present
under anhydrous conditions when boronic acids form
anhydrides.10 At the same time, boronic esters are less sensitive
Boronic esters are prepared by the esterification of boronic
acids11 and by transition-metal-catalyzed borylation of aryl
halides.2,11 Transition-metal-catalyzed borylation provides an
one-step synthesis of boronic esters.2 Our group is interested in
the development of inexpensive Ni-catalyzed borylation
reactions of aryl halides3g,12 and sulfonates13 and of their
subsequent cross-coupling. Aryl neopentylglycolboronates are
less expensive and more atom economic than the currently
more commonly used pinacolboronates.12a The versatile
Ni(II)-catalyzed borylation of aryl halides and sulfonates with
neopentylglycolborane generated in situ from neopentylglycol
and BH3·S(CH3)2 tolerates a variety of ortho-, meta- and para-
electrophilic functional groups and provides excellent
yields.14,15 Some preliminary data suggested that Ni-catalysts
can cross-couple aryl halides with aryl neopentylglycolboro-
nates in moderate to good yields.12a Encouraged by these
results, preliminary cross-coupling reactions between aryl
neopentylglycolboronates and aryl sulfonates were also
reported.3g A more comprehensive study on the cross-coupling
Received: November 7, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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Table 1. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methoxyphenyl Neopentylglycolboronate
Catalyzed by NiCl2(PCy3)2/ K3PO4 or CsF in Toluene at 110 °C
a
entry
OR
OMs
base
time (h)
yield (%)
1
2
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
CsF
12
12
12
24
24
24
12
12
12
24
24
24
25
54
6
OSO2NMe2
OPiv
3
4
OBoc
0
5
OCONEt2
OMe
34
13
7
6
7
OMs
8
OSO2NMe2
OPiv
CsF
1
9
CsF
0
10
11
12
OBoc
CsF
0
OCONEt2
OMe
CsF
0
CsF
0
a
Isolated yield.
of aryl sulfonates and sulfamates was recently reported.16 So far,
only aryl sulfonates,3g sulfamates,16 and methyl ethers4 have
been shown to be active in the cross-coupling reaction with aryl
neopentylglycolboronates. However, other electrophiles such as
aryl pivalates, carbonates, and carbamates have not been
investigated in cross-coupling with aryl neopentylglycolboro-
nates.
After a survey of the literature on C−O-based electrophiles
in cross-coupling reactions, we found that the reaction
conditions employed Ni(II)- or Ni(0)-based catalysts, ligands,
boron sources, bases, solvents, and temperatures differed from
one group of C−O electrophiles to another. The only
comparative study on the efficiency of different C−O
electrophiles was recently reported for cross-coupling with
potassium aryl and heteroaryl trifluoroborates.3i
In this paper, we report the first comparative analysis of the
efficiency of six different aryl-containing C−O-based electro-
philes in Ni-catalyzed Suzuki−Miyaura cross-coupling with aryl
neopentylglycolboronates. These experiments were performed
with four different catalytic systems that were developed
specifically for the cross-coupling of aryl methyl ethers4 with
aryl neopentylglycolboronates, aryl mesylates16 with aryl
neopentylglycolboronates, and respectively of aryl esters,
carbamates, and carbonates with arylboronic acids5a,6 and
arylboroxines.5b−d
dioxane at 130 °C3c and NiCl2(dppe) or NiCl2(dppp) both in
toluene and in dioxane at temperatures between 80 and 100
°C.3d,k Ni(COD)2/PCy3/K3PO4/THF was shown to cross-
couple aryl sulfonates with arylboronic acids at room
temperature.3e Preliminary results on the cross-coupling of
aryl neopentylglycolboronates demonstrated that they can
cross-couple with aryl chlorides, bromides, and iodides with
NiCl2(dppe)/dppe/K3PO4 or NaOH in dioxane at 110 °C.12a
However, the same catalyst is completely inactive in the cross-
coupling of aryl neopentylglycolboronates with aryl sulfonate-
s.12a Nevertheless, preliminary results demonstrated that
Ni(COD)2/PCy3/K3PO4 in THF at room temperature is an
excellent catalyst for the cross-coupling of aryl mesylates and
tosylates with aryl neopentylglycolboronates.3g The only other
C−O-based electrophile that was cross-coupled with aryl
neopentylglycolboronates is the aryl methyl ether.4 The catalyst
of choice for this cross-coupling is Ni(COD)2/PCy3/CsF in
toluene at 120 °C. All other C−O-based electrophiles,
including esters, carbonates, carbamates, and sulfamates, were
cross-coupled only with arylboronic acids by using
NiCl2(PCy3)2/K3PO4 in toluene, dioxane, or xylene at
temperatures ranging from 80 to 150 °C.5−8 This work was
recently reviewed.2 This manuscript reports the first compar-
ison of the efficiency of six aryl C−O-based electrophiles in
cross-coupling with aryl neopentylglycolboronates. Reaction
conditions specific for individual classes of C−O electrophiles
namely for aryl mesylates (Ni(COD)2/PCy3/K3PO4/THF/25
°C), aryl methyl ethers (Ni(COD)2/PCy3/CsF/toluene/120
°C), aryl esters, carbonates, carbamates, and sulfamates
(NiCl2(PCy3)2/K3PO4/toluene/110 °C), and a modified
catalyst specific for aryl methyl ethers were investigated.
RESULTS AND DISCUSSION
■
Selection of the Ni-Based Catalysts. Nickel catalysis was
used for the cross-coupling of aryl-containing C−O-based
electrophiles with arylboronic acids, anhydrides, and boronic
esters.2 Different reaction conditions and catalysts are required
for the cross-coupling of aryl-containing C−O-based electro-
philes with arylboronic acids, anhydrides, and boronic esters.
From the entire group of C−O-based electrophiles only aryl
mesylates and tosylates were investigated in cross-coupling with
both boronic acids and boronic esters.2 Cross-coupling of aryl
sulfonates with boronic acids proceeds in the presence of
Ni(II)-based catalysts at high temperature and in the presence
of Ni(0)-based catalysts at low temperature. The Ni(II)-based
catalysts of choice for this reaction are NiCl2(PCy3)2/K3PO4/
Efficiency of Aryl C−O-Based Electrophiles in Cross-
Coupling with Aryl Neopentylglycolboronates Cata-
lyzed by NiCl2(PCy3)2/K3PO4 in Toluene at 110 °C. The
reaction conditions employed by Garg’s laboratory5a,6,8 for the
cross-coupling of a diversity of aryl C−O-based electrophiles
including aryl-OPiv, -OCO2NEt2, -OBoc, and -OSO2NMe2
with arylboronic acids, were used to investigate the cross-
coupling of all aryl C−O electrophiles with aryl neo-
pentylglycolboronates. These conditions employ NiCl2(PCy3)2
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Table 2. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methoxyphenyl Neopentylglycolboronate
Catalyzed by Ni(COD)2/PCy3/K3PO4 in Toluene at 120 °C
a
entry
OR
OMs
time (h)
yield (%)
1
2
3
4
5
6
18
18
36
36
36
36
59
44
33
17
23
17
OSO2NMe2
OPiv
OBoc
OCONEt2
OMe
a
Isolated yield.
Table 3. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methyl Carboxylate Phenyl
Neopentylglycolboronate Catalyzed by Ni(COD)2/PCy3/K3PO4 in Toluene at 120 °C
a
entry
OR
OMs
time (h)
yield (%)
1
2
3
4
5
6
18
18
36
36
12
36
52
49
45
24
40
22
OSO2NMe2
OPiv
OBoc
OCONEt2
OMe
a
Isolated yield.
as catalyst and flame-dried K3PO4 as base in toluene at 110 °C.
The results from Table 1 showed that under these conditions
aryl-OMs, -OSO2NMe2, and -OCO2NEt2 gave cross-coupling
products in moderate yields (25%, 54% and 34%, Table 1,
entries 1, 2, and 5). However, no reaction was observed for
aryl-OBoc, and only low yields were obtained for -OPiv and
-OMe (6% and 13%, respectively, Table 1, entries 3 and 6).
Table 1 reports also the same cross-coupling experiments in
which the K3PO4 base was replaced with CsF. With the
exception of the experiments from entries 7 and 8, which
showed very low efficiency, all other experiments demonstrated
that CsF is not an active base for cross-couplings performed
under the reaction conditions reported in Table 1.
The yields of the biaryl products using aryl neopentylbor-
onates were generally lower than those obtained for the cross-
coupling of the same C−O electrophiles with arylboronic
acids.6,17 It was reported that a certain amount of water
facilitated the transmetalation step of the cross-coupling
reaction with arylboronic acids.7a,8 However, in contrast to
the reaction with arylboronic acid, no water is generated during
the cross-coupling of aryl neopentylglycolboronates. Therefore,
the Ni-catalysis conditions previously employed for arylboronic
acids might not be the most suitable for aryl boronates.
K3PO4, dried under vacuum at 40 °C overnight, while
maintaining toluene as solvent at 120 °C. All aryl C−O-based
electrophiles from Table 2 were cross-coupled to a certain
extent under these conditions in low to moderate yield.
However, the aryl -OBoc, which was previously inert under
the NiCl2(PCy3)2 catalysis (Table 1, entry 4), reacted under
these conditions (Table 2, entry 4).
In order to explore the electronic effect of the aryl
neopentylglycolboronates on the efficiency of cross-coupling,
both electron-rich (Table 2) and electron-deficient (Table 3)
aryl neopentylglycolboronates were studied. Under identical
conditions, the electron-deficient arylboronic ester gave higher
yields than the electron-rich arylboronic ester with all types of
C−O-based electrophiles. Significant improvement was ob-
served for −OCONEt2 (40%) compared to 23% in the reaction
with electron-rich aryl neopentylglycolboronates. The trend of
the efficiency of aryl C−O-based electrophiles was found to be:
-OMs (52%) > -OSO2NMe2 (49%) > -OPiv (45%) > -OBoc
(24%) > -OMe (22%) (Table 3). A similar trend was also
found for Ni-catalyzed cross-coupling of C−O based electro-
philes with potassium aryl and heteroaryl trifluoroborates.3i
The Efficiency of Aryl C−O-Based Electrophiles in
Cross-Coupling with Aryl Neopentylglycolboronates
Catalyzed by Ni(COD)2/PCy3/CsF in Toluene at 120 °C.
By using CsF as base at 120 °C, the C−O bond of aryl methyl
ethers was successfully cross-coupled with aryl neopentylgly-
colboronates in moderate to good yields with Ni(COD)2/PCy3
in toluene.4 Inspired by this work, we applied these unmodified
reaction conditions to all C−O-based electrophiles. The
optimized conditions for the methoxy leaving group were
The Efficiency of Aryl C−O-Based Electrophiles in
Cross-Coupling with Aryl Neopentylglycolboronates
Catalyzed by Ni(COD)2/K3PO4 in Toluene at 120 °C.
The catalytic system employed by the Chatani laboratory for
the cross-coupling of aryl methyl ethers with aryl neo-
pentylglycolboronates4 involving Ni(COD)2/PCy3/CsF/tol-
uene/120 °C was modified by changing its base from CsF to
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found to be very specific and not applicable to other leaving
groups. Aryl mesylates and sulfamates were cross-coupled with
moderate yields (36% to 61%) (Table 4, 3b and 3d; OR =
The Efficiency of Aryl C−O-Based Electrophiles in
Cross-Coupling with Aryl Neopentylglycolboronates
Catalyzed by Ni(COD)2/PCy3/K3PO4 in THF at 25 °C.
First introduced by Hu laboratory3e for cross-coupling of aryl
tosylates with arylboronic acids, the catalytic system Ni-
(COD)2/PCy3/K3PO4 in THF has been subsequently
employed for the cross-coupling of aryl mesylates and
sulfamates with aryl neopentylglycolboronates.3g,16 This cata-
lytic system is very efficient for the cross-coupling of aryl
sulfonates and sulfamates at room temperature regardless of the
electronic properties and steric hindrances of both substrates.16
Therefore, we applied these reaction conditions to all six aryl
C−O-based electrophiles. Among the C−O-based eletrophiles
investigated, mesylates and sulfamates were cross-coupled with
excellent yields (92−99%) (Table 5, 3a−f; OR = OMs,
Table 4. Cross-Coupling of Aryl Containing C−O
Electrophiles with Para-Substituted Aryl
Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/
CsF in Toluene at 120 °C
Table 5. Cross-Coupling of Aryl-Containing C−O
Electrophiles with Para-Substituted Aryl
Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/
K3PO4 in THF at 25 °C
OMs, OSO2NMe2). These results are in agreement with the
results obtained when NiCl2(PCy3)2/K3PO4 was used in
toluene at 110 °C (Table 1, entries 1 and 2) and also
Ni(COD)2/PCy3/K3PO4 in toluene at 120 °C (Table 2, entries
1 and 2). In addition, cross-coupling of aryl carbamates gave
poor yields (13−14%, Table 4, 3b and 3d; OR = OCONEt2)
while -OPiv was almost inert (<3%) (Table 4, 3b and 3d; OR =
OPiv). This low efficiency of the aryl ester leaving group
(-OPiv) was also reported by Chatani’s laboratory in cross-
coupling using the same catalytic system employed for −OAc
leaving group.4 Furthermore, the efficiency of cross-coupling of
-OBoc was found to be highly dependent on the electronic
character of the aryl neopentylglycolboronates. With electron-
deficient aryl neopentylglycolboronates, a moderate yield
(51%) was isolated (Table 4, 3b; OR = OBoc). However,
with electron-rich aryl neopentylglycolboronates, almost no
product was separated (3%) (Table 4, 3d; OR = OBoc). Cross-
coupling of ortho-substituted aryl carbonate substrate gave a
diminished yield (21%) (Table 4, 3f; OR = OBoc). Aryl methyl
ether substrates cross-coupled with aryl neopentylglycolboro-
nate with the highest yields compared to other C−O
electrophiles (50% and 71%) (Table 4, 3b and 3d; OR =
OMe). These results contrast with reactions carried out with
the same Ni-catalyst but K3PO4 as base (Table 2, entry 6 and
Table 3, entry 6). Thus, by changing the base from K3PO4 to
CsF, the efficiency of electrophiles was completely different.
This observation may have significant implications for synthetic
applications such as the orthogonal cross-coupling of aryl
derivatives containing the methoxy group as an electrophile and
the less reactive electrophiles -OPiv, -OBoc, and −OCONEt2 as
inert functional groups. Phenol derivatives generally were less
reactive than naphthol derivatives due to the higher activation
energy of C−O bond in the oxidative addition step.4 Only
substituted phenyl mesylates and sulfamates were coupled with
poor yield (Table 4, 3i and 3j; OR = OMs, OSO2NMe2).
OSO2NMe2). The efficiency of naphthyl pivalates is
determined by the electronic properties of the aryl neo-
pentylglycolboronate used in the reaction. The -OPiv was more
reactive toward eletron-rich aryl neopentylglycolboronates
(70% and 72%) (Table 5, 3a and 3d; OR = OPiv) than
electron-deficient derivatives (11%) (Table 5, 3b; OR = OPiv).
No reaction was observed for substituted phenyl pivalates
(Table 5, 3g and 3h; OR = OPiv). At room temperature, aryl
methyl ethers remained unreactive while carbonates gave
diminished yields (<10%) (Table 5, 3a, 3c; OR = OBoc).
Comparison of the Efficiency of Different Electro-
philes in Cross-Coupling Reactions with 4-Methoxy
Phenyl Neopentylglycolboronates. As observed from
previous series of experiments (Tables 1−5) and by their
summary from Figure 1 (conditions 0−5), aryl mesylates and
sulfamates are more efficient than the other C−O-based
electrophiles regardless of the catalyst used. Ni(COD)2/PCy3/
K3PO4 in THF at room temperature provides the best
conditions for the cross-coupling of aryl mesylates, sulfamates,
and pivalates with aryl neopentylglycolboronates. This catalyst
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that of the same electrophiles cross-coupled with arylboronic
acids when catalyzed with NiCl2(PCy3)2/K3PO4/toluene at 130
°C (Figure 1). This result is remarkable. The electronic
properties of aryl neopentylglycolboronates also influence on
the efficiency of C−O electrophiles. Electron-deficient aryl
boronates gave higher yields than electron-rich aryl boronates
under the same conditions.
CONCLUSIONS
■
The efficiency of aryl mesylates, sulfamates, pivalates,
carbonates, carbamates, and methyl ethers was investigated
for the first time in Ni-catalyzed cross-coupling with aryl
neopentylglycolboronates. Five different catalytic systems and
reaction conditions that are specific for aryl mesylates with aryl
neopentylglycolboronates, aryl methyl ethers with aryl neo-
pentylglycolboronates, aryl sulfamates, pivalates, carbonates,
and carbamates with arylboronic acids were applied to all six
aryl C−O-based electrophiles. A catalytic system based on
modified conditions for aryl methyl ethers was also
investigated. It was shown that the optimum catalyst for aryl
mesylates, Ni(COD)2/PCy3/K3PO4/THF/25 °C, is the most
efficient not only for mesylates but also for aryl sulfamates and
pivalates and the least efficient for carbonates, carbamates and
methyl ethers. Therefore, this catalytic system is very selective
and may be able to provide for the first time orthogonal
reaction conditions for the cross-coupling of aryl mesylates,
sulfamates, and pivalates in the presence of aryl carbonates,
carbamates and methyl ethers in cross-coupling with aryl
neopentylglycolboronates. Previously, orthogonal conditions
were reported only for the cross-coupling of arylboronic acids
with aryl sulfamates in the presence of aryl carbamates and aryl
methyl ethers.8 The reaction conditions Ni(COD)2/PCy3/
CsF/toluene/120 °C are the most efficient for the cross-
coupling of aryl methyl ethers and provide moderate yields for
aryl mesylates and sulfamates while displaying little efficiency
for aryl pivalates, carbonates, and carbamates. Therefore, these
are orthogonal reaction conditions for the cross-coupling of aryl
methyl ethers, mesylates and sulfamates with aryl neo-
pentylglycolboronates in the presence of aryl pivalates,
carbonates and carbamates. The catalytic system Ni(COD)2/
PCy3/K3PO4/toluene/120 °C and the same catalyst in the
presence of CsF are the least selective, generating comparable
yields with all C−O electrophiles. NiCl2(PCy3)2/K3PO4/
toluene/110 °C is most efficient for sulfamates, carbamates,
and mesylates in all cases with moderate yields and is inefficient
for pivalates, carbonates, and methyl ethers. Therefore, this
catalyst provides also chemical orthogonality although with
much lower efficiency. The efficiency of 2-naphthyl-containing
C−O electrophiles was also compared with literature data for
cross-coupling with NiCl2(PCy3)2/K3PO4/toluene/130 °C.
This comparison showed that with the exception of aryl
mesylates and sulfamates cross-coupled with aryl neopentylgly-
colboronates in the presence of Ni(COD)2/PCy3/K3PO4/
THF/25 °C, aryl containing C−O electrophiles are more
efficient in cross-coupling with arylboronic acids than with the
corresponding aryl neopentylglycolboronates. At the same time,
catalytic systems based on Ni(II) are nonselective toward C−O
electrophiles regardless of whether they are cross-coupled with
arylboronic acids or with aryl neopentylglycolboronates, while
all catalytic systems based on Ni(0) exhibit high selectivity for
the cross-coupling of aryl C−O-based electrophiles with aryl
neopentylglycolboronates. The selectivity of Ni(0)-based
catalysts for aryl neopentylglycolboronates toward C−O
Figure 1. Comparison of the efficiency of 2-naphthyl C−O
electrophiles in the cross-coupling reaction with 4-methoxyphenylbor-
onic acid (0) and 4-methoxyphenyl neopentylglycolboronate (0−5):
(0) 5%(pivalate), 10% (for the rest) NiCl2(PCy3)2/K3PO4/toluene/
130 °C5a,6 except for OMs where 1% NiCl2(dppp)/K3PO4/dioxane/
100 °C was used;3k (1) 10% NiCl2(PCy3)2/K3PO4/toluene/110 °C;
(2) 10% Ni(COD)2/40% PCy3/K3PO4/toluene/120 °C; (3) 6%
Ni(COD)2/12% PCy3/K3PO4/ THF/25 °C; (4) 10% Ni(COD)2/
40% PCy3/CsF/toluene/120 °C; (5) 10% NiCl2(PCy3)2/CsF/
toluene/110 °C.
is not efficient for carbonates and methyl ethers and exhibits
poor efficiency for carbamates.
By contrast, Ni(COD)2/PCy3/CsF in toluene at 120 °C is
the best catalyst for the cross-coupling of aryl methyl ethers,
although this catalyst is also active for aryl mesylates and
sulfamates (Figure 1). This catalytic system is not efficient
toward aryl pivalates and carbonates but shows low efficiency
toward aryl carbamates. Ni(COD)2/PCy3/K3PO4/toluene at
120 °C as a catalytic system was found to be active but less
efficient and at the same time less selective for all substrates.
NiCl2(PCy3)2/K3PO4/toluene at 110 °C is the least efficient of
all catalysts but exhibits selectivity that is complementary to
Ni(COD)2-based catalytic systems. The replacement of K3PO4
base from this catalyst with CsF decreases its efficiency even
more. Column (0) of Figure 1 compares also literature data for
the cross-coupling of the same C−O electrophiles in cross-
coupling with 4-methoxyphenylboronic acid with
NiCl2(PCy3)2/K3PO4/toluene at 130 °C.5a,6 This is the catalyst
of choice employed in Garg laboratory5,6,8 for the cross-
coupling of aryl C−O electrophiles with arylboronic acids. Only
the cross-coupling of 2-naphthyl mesylate with 4-methoxyphe-
nylboronic acid was selected from experiments catalyzed with
NiCl2(dppp)/K3PO4/dioxane/100 °C.3k These results are self-
explanatory. Arylboronic acids are more efficient than aryl
neopentylglycolboronates when NiCl2(PCy3)2/K3PO4 in tol-
uene at high temperature is used (compare columns 0 and 1 in
Figure 1). When the K3PO4 base was replaced with CsF the
catalyst became inefficient for the cross-coupling of all C−O
electrophiles with aryl neopentylglycolbononates. Nevertheless,
the efficiency of the cross-coupling of 2-naphthyl mesylates and
sulfamates with aryl neopentylglycolboronate catalyzed with
Ni(COD)2/ PCy3/K3PO4/THF at 25 °C is comparable with
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was added slowly during stirring at 0 °C. The resulting clear solution
was stirred during the rapid addition of iodomethane (37.0 mmol, 5.25
g). The reaction mixture was allowed to stir at room temperature for 3
h. The reaction was quenched with water and extracted with EtOAc.
The combined organic phase was washed with 10% NaOH, water, and
brine, then dried over MgSO4 and concentrated. The crude product
was purified by column chromatography with CH2Cl2 as an eluent to
give a white solid (4.0 g, 84%): mp =73−75 °C (lit.19 mp 73−74 °C);
1H NMR (500 MHz, CDCl3) δ 7.79−7.70 (m, 3H), 7.43 (t, J = 7.5,
1H), 7.33 (dd, J = 11.0, 3.9, 1H), 7.19−7.11 (m, 2H), 3.92 (s, 3H);
13C NMR (126 MHz, CDCl3) δ 157.8, 134.7, 129.5, 129.1, 127.8,
126.9, 126.5, 123.7, 118.9, 105.9, 55.4.
based electrophiles contrasts the lack of selectivity observed for
the same electrophiles in cross-coupling with potassium
aryltrifluoroborates.3i
EXPERIMENTAL SECTION
■
General Experimental Methods. 1-Naphthol, 2-naphthol, p-
cresol, methyl 4-hydroxybenzoate, N,N-dimethylsulfamoyl chloride,
methanesulfonyl chloride, pivaloyl chloride, di-tert-butyl dicarbonate,
N,N-diethylcarbamoyl chloride, iodomethane, DMAP, NaH, Ni-
(COD)2 (98+%), PCy3, and CsF were used as received from
commercial sources. Toluene, triethylamine, pyridine, DMF, DME,
and dichloromethane were distilled over CaH2 and stored under
nitrogen prior to use. THF from commercial source was distilled over
sodium and benzophenone and stored under nitrogen prior to use.
K3PO4 from a commercial source was dried at 40 °C under vacuum
overnight and kept in a desiccator prior to use or flame dried as
specified in Results and Discussion section. NiCl2(PCy3)2, naphthalen-
1-yl pivalate, (1i), p-tolyl pivalate (1v), and naphthalen-2-yl pivalate
(1c) were synthesized according to literature procedure.5a 2-
Methanesulfonyloxynaphthalene (1a) 4-methoxyphenyl methanesul-
fonate (1r), p-tolyl methanesulfonate (1u), and methyl 4-
((methylsulfonyl)oxy)benzoate (1l) were prepared according to
literature method.13 Naphthalen-1-yl dimethylsulfamate (1h), tert-
butyl naphthalen-1-yl carbonate (1j), tert-butyl naphthalen-2-yl
carbonate (1d), naphthalen-1-yl diethylcarbamate (1k), and naph-
thalen-2-yl diethylcarbamate (1e) were synthesized according to the
literature procedure.6 Methyl 4-methoxybenzoate (1q)18 and 4-
methoxyphenyl dimethylsulfamate (1m)16 were prepared by following
the literature procedures. 2-(4-Methoxyphenyl)-5,5-dimethyl-1,3,2-
dioxaborinane (2a), methyl 4-(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-
benzoate (2b), and 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane (2c)
were prepared according to the literature procedures.12b,14,15 1H NMR
(500 MHz) and 13C NMR (125 MHz) spectra were recorded using
TMS as internal standard. High-resolution mass spectra of new
compounds were obtained on a high-resolution double focusing
chemical ionization mass spectrometer. A GC coupled with an FID
detector and column HP 19091J-413 (5%-phenyl)-methylpolysiloxane
30 m length 0.32 mm internal diameter was used to follow the reaction
conversions and to assess purity of final compounds complementary to
the NMR technique. The crude reaction mixtures were diluted with
THF and analyzed by GC as reported in previous related publications
from our laboratory.13−16
Typical Procedure for the Synthesis of Aryl Mesylates. The
aryl mesylates were prepared according to a literature procedure.13
1-Methanesulfonyloxynaphthalene (1g).3h To an oven-dried
round-bottom flask equipped with a stirring bar was added under
nitrogen atmosphere 1-naphthol (20 mmol, 2.88 g) and freshly
distilled dichloromethane (20 mL) followed by anhydrous pyridine
(8.0 mL). The reaction mixture was cooled to 0 °C before
methanesulfonyl chloride (24.0 mmol, 2.80 g) was added dropwise.
The reaction was allowed to stir at 0 °C for 4 h at room temperature
until TLC demonstrated the consumption of the starting material. The
reaction was quenched by addition of water. The aqueous phase was
extracted with CH2Cl2 three times, and all of the combined organic
layers were washed with 15% HCl and brine and dried over MgSO4.
After filtration, the solvent was evaporated under reduced pressure,
and the crude product was purified by column chromatography to give
1
a pale yellow oil (3.98 g, 89%): H NMR (500 MHz, CDCl3) δ 8.14
(d, J = 8.3, 1H), 7.90 (d, J = 7.5, 1H), 7.82 (d, J = 8.1, 1H), 7.64−7.52
(m, 3H), 7.48 (t, J = 7.9, 1H), 3.22 (s, 3H); 13C NMR (126 MHz,
CDCl3) δ 144.5, 134.1, 127.2, 126.5, 126.4, 126.14, 126.13, 124.5,
120.6, 117.54, 37.1.
Typical Procedure for the Synthesis of Aryl Pivalates. The
aryl pivalates were prepared according to a literature procedure.5a
Methyl 4-(Pivaloyloxy)benzoate (1n). Dry triethylamine (36.1
mmol, 5 mL) and DMAP (3.0 mmol, 0.39 g) were added at room
temperature into a solution of methyl 4-hydroxybenzoate (30.0 mmol,
4.56 g) and dry CH2Cl2 (25 mL). Pivaloyl chloride (36.2 mmol, 4.5
mL) was added slowly during stirring, and the reaction mixture was
allowed to stir at room temperature for 12 h. The organic phase was
washed with saturated ammonium chloride solution and brine. The
aqueous phase was extracted with EtOAc. The organic phase was
combined, dried over MgSO4, and filtered. The solvent was evaporated
under reduced pressure and the crude product was purified by column
chromatography (SiO2, CH2Cl2) to give a white solid (6.95 g, 98%),
Typical Procedure for the Synthesis of Aryl Sulfamates. The
aryl sulfamates were prepared according to a literature procedure.6
Naphthalen-2-yl Dimethylsulfamate(1b). To an oven-dried
round-bottom flask equipped with a stirring bar was added under
nitrogen atmosphere NaH (25.0 mmol, 0.58 g). The flask was cooled
to 0 °C, and 2-naphthol (20.8 mmol, 3.0 g) in dried DME (25 mL)
was added dropwise at 0 °C. The reaction mixture was allowed to stir
at room temperature for 10 min and then was cooled to 0 °C.
Dimethyl sulfamoyl chloride (25.0 mmol, 3.74 g) in DME (4 mL) was
added dropwise and the reaction was allowed to stir at room
temperature for 12 h. The reaction was quenched by addition of water
followed by the evaporation of the solvent. The solid was dissolved by
Et2O and washed with 1 M KOH and water. The combined aqueous
layers were extracted with Et2O, washed with brine, and dried over
MgSO4. The solvent was evaporated and the crude product was
purified by column chromatography with dichloromethane/hexane
(3/7) as eluent to give a white solid (4.49 g, 86%): mp =74−75 °C;
1H NMR (500 MHz, CDCl3) δ 7.92−7.82 (m, 3H), 7.77 (d, J = 2.3,
1
mp = 58.5−60 °C; H NMR (500 MHz, CDCl3) δ 8.07 (d, J = 8.8,
2H), 7.14 (d, J = 8.8, 2H), 3.91 (s, 3H), 1.37 (s, 9H); 13C NMR (126
MHz, CDCl3) δ 176.7, 166.5, 155.0, 131.3, 121.7, 90.6, 52.3, 39.4,
27.2; HRMS (CI+) calcd for C13H17O4 (M+ + H) 237.1127, found
237.1118.
Typical Procedure for the Synthesis of Aryl Carbonates. The
aryl carbonates were prepared according to a literature procedure.6
Methyl 4-((tert-Butoxycarbonyl)oxy)benzoate20 (1o). To an oven-
dried round-bottom flask equipped with stirring bar under nitrogen
were added methyl 4-hydroxybenzoate (8.0 mmol, 1.22 g),
dimethylaminopyridine (0.80 mmol, 0.98 g), and freshly distilled
CH2Cl2 (20 mL). Triethylamine (8.8 mmol, 0.89 g) and di-tert-butyl
dicarbonate (8.8 mmol, 1.87 g) were added at room temperature. The
reaction mixture was allowed to stir until the bubbling subsided. The
solution was washed with 0.5 M NaHSO4, and the aqueous layer was
extracted with CH2Cl2. The organic phase was combined, dried over
MgSO4, and filtered. The solution was evaporated under reduced
pressure, and the crude product was purified by column chromatog-
raphy (SiO2, CH2Cl2) to give the product as a white solid (2.00 g,
1H), 7.52 (m, 2H), 7.43 (dd, J = 8.9, 2.4, 1H), 3.03 (s, 6H); 13C NMR
(126 MHz, CDCl3) δ 147.9, 133.8, 131.9, 130.1, 127.93, 127.91, 127.0,
126.3, 120.9, 119.1, 38.9; HRMS (CI+) calcd for C12H13NNaO3S (M+
+ Na) 274.0514, found 274.0526.
Typical Procedure for the Synthesis of Aryl Ethers. 2-
Methoxynaphthalene(1f). To an oven-dried round-bottom flask
equipped with a stirring bar was added under nitrogen atmosphere
NaH (30.0 mmol, 0.72 g). The flask was cooled to 0 °C, and
anhydrous DMF (25 mL) was added. 2-Naphthol (30.0 mmol, 4.32 g)
1
99%): mp = 83−84 °C; H NMR (500 MHz, CDCl3) δ 8.06 (d, J =
8.6, 2H), 7.25 (d, J = 8.7, 2H), 3.91 (s, 3H), 1.56 (s, 9H); 13C NMR
(126 MHz, CDCl3) δ 166.4, 154.8, 151.2, 131.2, 127.7, 121.3, 84.2,
52.3, 27.8; HRMS (CI+) calcd for C13H17O5 (M++H) 253.1076, found
253.1067.
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tert-Butyl (4-Methoxyphenyl)carbonate (1s). Following the typical
solvent was evaporated, and the product was purified by column
chromatography with dichloromethane/hexane.
procedure for the synthesis of aryl carbonates: white solid (90%); mp
= 65−66.5 °C (lit.21 mp 66−67 °C); H NMR (500 MHz, CDCl3) δ
1
Method B (in Tables 2 and 3). To an oven-dried test tube (15 × 85
mm) were added the naphthyl C−O electrophile (0.3 mmol),
neopentylglycol boronic ester (0.45 mmol, 1.5 equiv), and K3PO4,
dried under vacuum at 40 °C overnight (1.35 mmol, 4.5 equiv). The
tube was taken into the glovebox, and PCy3 (0.12 mmol, 0.40 equiv)
and Ni(COD)2 (0.03 mmol, 0,10 equiv) were added. Dried THF (1.0
mL) was then added, and the tube was capped with rubber septum,
which was wrapped with copper wire. The tube was taken outside the
glovebox and stirred at 120 °C for 12−36 h (see Tables 2 and 3). The
crude mixture was filtered through a short column of silica gel and
washed with THF. The solvent was evaporated, and the product was
purified by column chromatography with dichloromethane/hexane.
Method C (in Table 4).4 To an oven-dried test tube (15 × 85 mm)
were added the aryl C−O electrophile (0.3 mmol) and the aryl
neopentylglycol boronic ester (0.45 mmol, 1.5 equiv). The tube was
taken into the glovebox, and CsF (1.35 mmol, 4.5 equiv), PCy3 (0.12
mmol, 0.40 equiv), and Ni(COD)2 (0.03 mmol, 0,10 equiv) were
added. Dried toluene (1.0 mL) was then added, and the tube was
capped with rubber septum, which was wrapped with copper wire. The
tube was taken outside the glovebox and stirred at 120 °C for 12 or 24
h (see Table 4). The crude mixture was filtered through a short
column of silica gel and washed with THF. The solvent was
evaporated, and the product was purified by column chromatography
with dichloromethane/hexane or hexane as eluent.
7.08 (d, J = 9.0, 2H), 6.88 (d, J = 9.1, 2H), 3.79 (s, 3H), 1.55 (s, 9H);
13C NMR (126 MHz, CDCl3) δ 201.2, 156.4, 151.5, 143.9, 121.2,
113.5, 82.5, 54.8, 26.9.
Typical Procedure for the Synthesis of Aryl Carbamates. The
aryl carbamates were prepared according to a literature procedure.6
Methyl 4-((Diethylcarbamoyl)oxy)benzoate22 (1p). To an oven-
dried round-bottom flask equipped with a stirring bar was added under
nitrogen atmosphere NaH (15.6 mmol, 0.37 g). The flask was cooled
to 0 °C, and methyl 4-hydroxybenzoate (13.0 mmol, 1.98 g) in dried
DME (5 mL) was added dropwise at 0 °C. The reaction mixture was
allowed to stir at room temperature for 10 min and then was cooled to
0 °C. Diethylcarbamoyl chloride (15.6 mmol, 2.12 g) in DME (5 mL)
was added dropwise, and the reaction was allowed to stir at room
temperature for 12 h. The reaction was quenched by addition of water
followed by the evaporation of the solvent. The solid was dissolved by
Et2O and washed with 1 M KOH and water. The combined aqueous
layers were extracted with Et2O, washed with brine, and dried over
MgSO4. The solvent was evaporated, and the crude product was
purified by column chromatography with 0−4% EtOAC/CH2Cl2 as
1
eluent to give a colorless oil (0.89 g, 27%): H NMR (500 MHz,
CDCl3) δ 8.05 (d, J = 8.5, 2H), 7.20 (d, J = 8.6, 2H), 3.90 (s, 3H),
3.47−3.36 (m, 4H), 1.26 (t, J = 6.9, 3H), 1.21 (t, J = 6.9, 3H); 13C
NMR (126 MHz, CDCl3) δ 166.6, 155.4, 153.5, 131.1, 127.0, 121.7,
52.2, 42.5, 42.1, 14.4, 13.4.
Method D (in Table 5).16 To an oven-dried test tube (15 × 85 mm)
were added the aryl C−O electrophile (0.3 mmol), the aryl
neopentylglycol boronic ester (0.36 mmol, 1.2 equiv), and K3PO4,
dried under vacuum at 40 °C overnight (0.9 mmol, 3 equiv). The tube
was taken into the glovebox, and PCy3 (0.036 mmol, 0.12 equiv) and
Ni(COD)2 (0.018 mmol, 0.06 equiv) were added. Dried THF (1.0
mL) was then added, and the tube was capped with a rubber septum.
Inside the glovebox, the reaction was stirred at room temperature
under nitrogen for 12−48 h (see Table 5). The crude mixture was
filtered through a short column of silica gel and washed with THF.
The solvent was evaporated and the product was purified by column
chromatography with hexane, dichloromethane/hexane, or dichloro-
methane as eluent.
4-Methoxyphenyl Diethylcarbamate22 (1t). Following the typical
procedure of the synthesis of aryl carbamates. Column chromatog-
raphy (SiO2; 0 −20% EtOAc/CH2Cl2), colorless oil (73%): 1H NMR
(500 MHz, CDCl3) δ 7.03 (d, J = 9.0, 2H), 6.87 (d, J = 9.0, 2H), 3.79
(s, 3H), 3.48−3.31 (m, 4H), 1.26−1.17 (m, J = 23.5, 6H); 13C NMR
(126 MHz, CDCl3) δ 155.9, 155.8, 153.8, 144.3, 121.7, 113.4, 54.8,
41.4, 41.0, 13.4, 12.5.
Preparation of Neopentylglycolborane. A procedure elabo-
rated previously in our laboratory was used.13−15 To a cooled solution
(0 °C) of neopentylglycol (6.0 mmol, 2.0 equiv) in toluene (3 mL)
was slowly added (CH3)2S·BH3 (6.0 mmol, 2.0 equiv) under nitrogen.
The reaction was allow to stir at 0 °C for 30 min and then at room
temperature for 90 min. The neopentylglycolborane was used directly
without further purification.
General Procedure for Neopentylglycolborylation. The
arylboronic esters were prepared according to literature proce-
dures.13−15 To an oven-dried 25 mL Schlenk tube were added Zn
powder (6.0 mmol, NiCl2(dppp) (1.5 mmol), and PPh3 (3.0 mmol)
along with the appropriate aryl halides (if it is solid) (3.0 mmol). The
aryl halide, catalyst, and PPh3 were degassed by pumping and
backfilling with nitrogen three times. Dry toluene (3 mL) was added
to the reaction mixture along with the appropriate aryl halide (if it is
liquid) and Et3N (9.0 mmol). Neopentylglycolborane was added
dropwise to the reaction mixture. The reaction was placed into an oil
bath at 100 °C with stirring under nitrogen. After completion of the
starting material, the reaction was quenched by the addition of a
saturated NH4Cl solution and extracted with EtOAc three times. The
combined organic fractions were dried over MgSO4, followed by
filtration and evaporation of the solvent. The crude product was
purified by column chromatography.
General Procedure for Cross-Coupling. Method A (in Table
1).5a,6,8 To an oven-dried test tube (15 × 85 mm) were added the
naphthyl C−O electrophile (0.3 mmol), neopentylglycol boronic ester
(1.2 mmol, 4 equiv), and NiCl2(PCy3)2 (0.03 mmol, 0.10 equiv). The
tube was taken into the glovebox, and the anhydrous base (CsF, used
as received and kept in the glovebox after first opened, and K3PO4,
dried by flame drying and kept in the glovebox) (2.16 mmol, 7.2
equiv) was added. Dried toluene (1.0 mL) was then added, and the
tube was capped with a rubber septum, which was wrapped with
copper wire. The tube was taken outside the glovebox and stirred at
110 °C for 12 or 24 h (see Table 1). The crude mixture was filtered
through a short column of silica gel and washed with THF. The
2-(4-Methoxyphenyl)naphthalene (3a): white solid; mp 130−131
°C (lit.23 mp 131−133 °C); 1H NMR (500 MHz, CDCl3) δ 7.75 (d, J
= 8.2, 1H), 7.72 (d, J = 8.5, 2H), 7.47−7.40 (m, 1H), 7.37−7.29 (m,
1H), 7.18−7.09 (m, 2H), 3.90 (s, 3H); 13C NMR (126 MHz, CDCl3)
δ 158.4, 137.3, 132.9, 132.8, 131.5, 127.6, 127.5, 127.2, 126.8, 125.4,
124.8, 124.6, 124.2, 113.5, 54.5.
Methyl 4-(naphthalen-2-yl)benzoate24 (3b): white solid; mp 149−
150 °C; 1H NMR (500 MHz, CDCl3) δ 8.14 (d, J = 8.5, 2H), 8.07 (d,
J = 1.3, 1H), 7.96−7.84 (m, 3H), 7.77 (d, J = 8.5, 2H), 7.74 (dd, J =
8.5, 1.8, 1H), 7.54−7.47 (m, 2H), 3.94 (s, 3H); 13C NMR (126 MHz,
CDCl3) δ 167.1, 145.6, 137.4, 133.7, 133.1, 130.3, 129.0, 128.8, 128.5,
127.8, 127.4, 126.6, 126.5, 126.5, 125.3, 52.3.
2-Phenylnaphthalene (3c): white solid; mp 98−99 °C (lit.23 mp
1
100−101 °C); H NMR (500 MHz, CDCl3) δ 8.04 (s, 1H), 7.93−
7.84 (m, 3H), 7.77−7.70 (m, 3H), 7.53−7.45 (m, 4H), 7.38 (dd, J =
10.6, 4.2, 1H); 13C NMR (126 MHz, CDCl3) δ 141.3, 138.7, 133.8,
132.8, 129.0, 128.6, 128.4, 127.8, 127.6, 127.5, 126.4, 126.07, 125.95,
125.8.
1-(4-Methoxyphenyl)naphthalene (3d): white solid; mp 110−111
°C (lit.3i mp 112−113 °C); H NMR (500 MHz, CDCl3) δ 7.99 (s,
1
1H), 7.92−7.83 (m, 3H), 7.72 (dd, J = 8.5, 1.8, 1H), 7.67 (d, J = 8.8,
2H), 7.52−7.44 (m, 2H), 7.03 (d, J = 8.8, 2H), 3.88 (s, 3H); 13C
NMR (126 MHz, CDCl3) δ 158.1, 139.1, 133.0, 132.3, 131.0, 130.3,
127.4, 126.5, 126.1, 125.2, 125.1, 124.9, 124.6, 112.9, 54.5.
1-Phenylnaphthalene23 (3e): colorless liquid; 1H NMR (500 MHz,
CDCl3) δ 7.88 (d, J = 7.6, 2H), 7.83 (d, J = 8.1, 1H), 7.52−7.43 (m,
6H), 7.43−7.37 (m, 3H); 13C NMR (126 MHz, CDCl3) δ 140.9,
140.4, 133.9, 131.7, 130.2, 128.4, 127.8, 127.4, 127.0, 126.2, 125.9,
125.5.
Methyl 4-(naphthalen-1-yl)benzoate (3f): white solid; mp 62−63
°C (lit.25 mp 65.5−66.5 °C); 1H NMR (500 MHz, CDCl3) δ 8.17 (d, J
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= 8.3, 2H), 7.91 (dd, J = 13.6, 8.2, 2H), 7.84 (d, J = 8.5, 1H), 7.58 (d, J
= 8.3, 2H), 7.56−7.49 (m, 2H), 7.47−7.41 (m, 2H), 3.97 (s, 3H); 13C
NMR (126 MHz, CDCl3) δ 167.2, 145.8, 139.3, 133.9, 131.4, 130.3,
129.7, 129.2, 128.6, 128.4, 127.1, 126.5, 126.1, 125.8, 125.5, 52.3.
4-Methoxy-4′-methyl-1,1′-biphenyl (3g): white solid, mp 103−104
°C (lit.26 mp 103−104 °C); 1H NMR (500 MHz, CDCl3) δ 7.51 (d, J
= 8.8, 2H), 7.45 (d, J = 8.1, 2H), 7.23 (d, J = 7.8, 2H), 6.97 (d, J = 8.8,
2H), 3.85 (s, 3H), 2.38 (s, 3H); 13C NMR (126 MHz, CDCl3) δ
158.8, 137.9, 136.3, 133.7, 129.3, 127.9, 126.5, 114.1, 55.3, 21.0.
Methyl 4′-methoxy(1,1′-biphenyl)-4-carboxylate (3h): white solid;
mp 172−173 °C (lit.12a mp 173−174 °C); 1H NMR (500 MHz,
CDCl3) δ 8.07 (d, J = 8.3, 2H), 7.62 (d, J = 8.3, 2H), 7.57 (d, J = 8.7,
2H), 6.99 (d, J = 8.7, 2H), 3.93 (s, 3H), 3.86 (s, 3H); 13C NMR (126
MHz, CDCl3) δ 167.0, 159.8, 145.1, 132.3, 130.0, 128.3, 128.2, 126.4,
114.3, 55.3, 52.0.
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(14) Moldoveanu, C.; Wilson, D. A.; Wilson, C. J.; Leowanawat, P.;
Resmerita, A. M.; Liu, C.; Rosen, B. M.; Percec, V. J. Org. Chem. 2010,
75, 5438−5452.
Dimethyl (1,1′-biphenyl)-4,4′-dicarboxylate (3i): white solid; mp
212−214 °C (lit.27 mp 215.5−216.5 °C); 1H NMR (500 MHz,
CDCl3) δ 8.13 (d, J = 8.2, 4H), 7.69 (d, J = 8.2, 4H), 3.95 (s, 6H); 13
C
NMR (126 MHz, CDCl3) δ 166.9, 144.5, 130.3, 129.9, 127.4, 52.3.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectra of compounds 1b, 1f, 1g, 1n−
p, 1s, 1t, and 3a−j. This information is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: percec@sas.upenn.edu.
(15) Leowanawat, P.; Resmerita, A.-M.; Moldoveanu, C.; Liu, C.;
Zhang, N.; Wilson, D. A.; Hoang, L. M.; Rosen, B. M.; Percec, V. J.
Org. Chem. 2010, 75, 7822−7828.
ACKNOWLEDGMENTS
■
Financial support by the NSF (DMR-1066116 and DMS-
0935165) and by the P. Roy Vagelos Chair at Penn is gratefully
acknowledged. P.L. is a graduate fellow of the Royal Thai
Government. Dr. Rakesh Kohli (University of Pennsylvania) is
acknowledged for obtaining HRMS data.
(16) Leowanawat, P.; Zhang, N.; Resmerita, A.-M.; Rosen, B. M.;
Percec, V. J. Org. Chem. 2011, 76, 9846−9955.
(17) The yields of biaryl product obtained with arylboronic acids are
92% for -OPiv, 85% for -OBoc, and 47% for -OCONEt2 as in ref 6.
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