Preparation of triethylammonium tetra-arylborates (TEATABs): Coupling partners for the Suzuki reaction

Article abstract of DOI:10.1080/00397911003718086

(Chemical Equation Presented) Six triethylammonium tetra-arylborates (TEATABs) were synthesized via a convenient reproducible procedure and characterized by spectroscopic methods (¹H, ¹³C, ¹¹B NMR and electrospray ionization-high-resolution mass spectrometry). The compounds could be stored at ambient temperature and were useful as reactants in the Suzuki reaction in aqueous conditions when using commercially available catalysts. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911003718086

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Preparation of Triethylammonium Tetra-
Arylborates (TEATABs): Coupling Partners
for the Suzuki Reaction
Noora M. Kuuloja a , Tuula M. Kylmälä a , Jan E. Tois a , Rainer E.
Sjöholm b & Robert G. Franzén a
a Department of Chemistry and Bioengineering, Tampere University
of Technology, Tampere, Finland
b Laboratory of Organic Chemistry, Åbo Akademi, Turku, Finland
Published online: 03 Mar 2011.
To cite this article: Noora M. Kuuloja , Tuula M. Kylml , Jan E. Tois , Rainer E. Sjholm & Robert G.
Franzn (2011) Preparation of Triethylammonium Tetra-Arylborates (TEATABs): Coupling Partners for
the Suzuki Reaction, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 41:7, 1052-1063, DOI: 10.1080/00397911003718086
To link to this article: http://dx.doi.org/10.1080/00397911003718086
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Synthetic Communications1, 41: 1052–1063, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003718086
PREPARATION OF TRIETHYLAMMONIUM
TETRA-ARYLBORATES (TEATABs): COUPLING
PARTNERS FOR THE SUZUKI REACTION
1
¨ ¨
Noora M. Kuuloja, Tuula M. Kylmala, Jan E. Tois,
1
1
2
1
´
¨
Rainer E. Sjoholm, and Robert G. Franzen
1Department of Chemistry and Bioengineering, Tampere University of
Technology, Tampere, Finland
2
˚
Laboratory of Organic Chemistry, Abo Akademi, Turku, Finland
GRAPHICAL ABSTRACT
Abstract Six triethylammonium tetra-arylborates (TEATABs) were synthesized via a
convenient reproducible procedure and characterized by spectroscopic methods (1H, 13C,
11B NMR and electrospray ionization–high-resolution mass spectrometry). The compounds
could be stored at ambient temperature and were useful as reactants in the Suzuki reaction
in aqueous conditions when using commercially available catalysts.
Keywords Carboxybiphenyls; organoboron reagents; Suzuki reaction; tetra-arylborates
INTRODUCTION
Traditionally tetra-arylborates have been utilized for precipitation of different
cations from water.[1–7] In addition, these compounds have been tested as chemical
sensors,[8] as counter anions in ionic liquids,[9] in rhodium-catalyzed addition
reactions,[10,11] and also in palladium-catalyzed reactions.[12–17] However, in most
cases only the commercially available Ph4BNa has been reported, and other sub-
stituted tetra-arylborates have not been studied.
The preparation of tetra-arylborates involves halogen–metal exchange and
subsequent reaction with different boron reagents.[1–4,7,9,18–21] Although methods
for the preparation of several tetra-arylborates have been reported, there is still a
Received January 23, 2010.
Address correspondence to Robert G. Franzen, Department of Chemistry and Bioengineering,
´
Laboratory of Chemistry, Tampere University of Technology, Korkeakoulunkatu 8, FIN-33720,
Tampere, Finland. E-mail: robert.franzen@tut.fi
1052

TRIETHYLAMMONIUM TETRA-ARYLBORATES
1053
need for a more convenient procedure for differently substituted tetra-arylborates. In
this work, we describe a reliable method for the preparation of these types of
compounds. Six different tetra-arylborates were synthesized by utilizing triethylam-
monium as a counter ion. When tetra-arylborates were isolated and purified as
Et3NHþ salts, the procedures were easy and convenient. In addition, the triethylam-
monium tetra-arylborates (TEATABs) were found to be suitable coupling partners
for the Suzuki reaction.
RESULTS AND DISCUSSION
Tetra-arylborates have recently been prepared using a wide variety of
methods.[1–4,8,18,20] So far, the isolation and purification of substituted tetra-
arylborates have been problematic, and many operations are required. An advantage
of our method is that formed Grignard reagent reacts in situ with NaBF4, and transfer
of the sensitive Grignard reagent solution from flask to flask can be omitted. The solu-
bility of TEATABs in alcohol and water is less than corresponding sodium salts. To
take advantage of this property, cation exchange was performed, and TEATABs were
isolated in pure form by filtration. For most purposes, this crude product is pure
enough for further use. This method is suitable for all prepared TEATABs regardless
of substituent, and the preparation can be performed at gram scale (Table 1).
The solubility was tested in different solvent systems, which have been com-
monly utilized in the Suzuki reaction [dimethylformamide (DMF), DMF=H2O
(2:1), acetone, acetone=H2O (3:2), toluene, toluene=EtOH (1:1), EtOH, EtOH=
H2O (1:1), and H2O]. The Suzuki reactions with Ph4BNa are normally performed
at 0.1–0.4 M concentrations,[12–14,22] and therefore solubility was tested at concentra-
tions between 0.1 and 1 M. Ph4BNa was found highly soluble (0.6–1 M) in all tested
solvents except toluene. Solubility of TEATAB 1 was low (<0.1 M) in these solvents.
Table 1. Preparation of TEATABs
Compound
Substituent R
Yield (%)a
1
2
3
4
5
H
4-Cl
75b
81
86
44
53
3-Cl
3,5-di-Cl
4-OMe
6
50
aIsolated yield after recrystallization. No attempts was made to find the best recrystal-
lization solvents.
bPreparation started from commercially available Ph4BNa, and only cation exchange
was used.

1054
N. M. KUULOJA ET AL.
In aqueous ethanol, however, the solubility difference between Ph4BNa and
TEATAB 1 was remarkable. Also, the solubility of substituted TEATAB 2 was
examined in toluene, toluene=EtOH (1:1), EtOH, and EtOH=water (1:1). The
compound was soluble in toluene=EtOH (1:1) (0.6 M) and EtOH (0.9 M), unlike
TEATAB 1, but it was practically insoluble in EtOH=water (1:1) (<0.1 M). TEATAB
1, was not soluble in the examined solvents at concentrations where the Suzuki
reaction is normally performed, except for acetone=H2O (0.3 M) and DMF=water
(0.2 M). Solubility of TEATAB 2 was greater than solubility of TEATAB 1, but both
were practically insoluble in water.
These compounds offer a good alternative to known and routinely used orga-
noboron compounds for transition-metal-catalyzed reactions. Also, boronic acids,
boronate esters, and organotrifluoroborates have been prepared using halogen–
metal exchange and subsequent reaction with boron reagents.[23] In general, boronic
acids and organotrifluoroborates react fast and in good yields under coupling con-
ditions. However, the determination of the precise stoichiometry of boronic acids
in the reaction is difficult because of their ability to form dimers and trimers during
storage. Potassium organotrifluoroborates have the same benefits as TEATABs.
Both of them are stable and easily prepared and purified. However, highly corrosive
and poisonous reagents such as KF, KHF2, or HF are needed in the preparation of
organotrifluoroborates.[24]
After the successful preparation of TEATABs, the utility of these novel salts in
the Suzuki reaction was explored. Both Naþ and Et3NHþ salts were examined in
parallel. The reactivity of TEATAB 1 and commercially available sodium tetraphe-
nylborate (Ph4BNa) were compared utilizing the same reaction as Bumagin and
Bykov.[14] The reactivity of TEATAB 1 was similar or slightly better than that of
the corresponding sodium salt (Table 2, entries 1–8). When water was used as a
Table 2. Suzuki reaction using TEATAB 1 and Ph4BYa
Literature
yield[14]
Entry Halide X=R
Solvent
Borate Y Temp. (ꢀC) Time (h) Yield (%)
1
2
I=-NO2
I=-NO2
I=-NO2
I=-NO2
I=-NO2
I=-NO2
I=-NO2
I=-NO2
Br=-OMe
Br=-OMe
H2O
H2O
H2O
H2O
Naþ
100
100
25
2
2
71
79
75
14
98
98
76
86
72b
72b
96
Et3NHþ
Naþ
3
24
24
10
10
10
10
2
50
4
Et3NHþ
Acetone=H2O (3:2) Naþ
25
25
5
98
81
6
Acetone=H2O (3:2) Et3NHþ
25
25
7
DMF=H2O (2:1)
DMF=H2O (2:1)
Naþ
Et3NHþ
8
25
25
9
10
Acetone=H2O (3:2) Naþ
Acetone=H2O (3:2) Et3NHþ
25
2
aReaction conditions: tetra-arylborate (0.27 mmol), 4-iodonitrobenzene (1 mmol), Pd(OAc)2 (1 mol%),
Na2CO3 (1 mmol), and solvent (5.3 ml).
b10 mol% of catalyst was used.

TRIETHYLAMMONIUM TETRA-ARYLBORATES
1055
solvent and the reaction was performed at room temperature (entries 3 and 4), the
sodium salt produced a greater yield than TEATAB 1. This is due to the higher
solubility of Ph4BNa in water. On the other hand, when the reaction was performed
at 100 ꢀC (entries 1 and 2), the yields were almost equal. According to these experi-
ments, we concluded that acetone=H2O was the best solvent system (entries 5 and 6)
for the reaction. To further explore the compatibility of TEATABs in the Suzuki
reaction, these substrates were subjected to the reaction with an electron-rich halide
(entries 9 and 10). The reaction was rapid and completed at room temperature
regardless of the cation. In addition, the yields were comparable to those reactions
where trifluoroborates were utilized as coupling partners.[23]
Previous reports have focused on unsubstituted tetra-arylborates (Ph4BX) as
reactants in the Suzuki coupling.[12–15] Only one example where substituted tetra-
arylborates have been used as reactants can be found in the literature.[17] When
the chlorinated TEATAB 2 was used as the reactant, a dramatic difference in reac-
tivity was noticed. Surprisingly, no reaction could be observed between TEATAB 2
and 4-iodonitrobenzene in acetone=water at room temperature, unlike TEATAB 1
(Table 2, entry 6). When the reaction was performed at 85 ꢀC for 18 h, 69% of the
desired product was obtained. The reaction was repeated by using 4-bromoanisole
as the halide. Also in this case, external heating was needed, and 4-methoxy-40-
chlorobiphenyl was obtained in 62% yield.
Because of this diminished reactivity of substituted TEATABs, a revision of
reaction conditions was necessary to get greater conversion and shorter reaction
times. The reaction between TEATAB 2 and 4-bromobenzoic acid was chosen as
a model reaction (Table 3). In the context of an ongoing synthetic project in our
group, we were interested in the preparation of carboxy-functionalized biaryls.[25]
In addition, 4-bromobenzoic acid has been widely utilized in the Suzuki reaction
previously.[12–15] At first, Pd(OAc)2 was used as the catalyst, and different solvents
Table 3. Effect of the base and the catalyst on the Suzuki reaction in different solventsa
Entry
Catalyst
Solvent
Base
Temp. (ꢀC)
Time (h)
Yield (%)
1
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd=C
Acetone=H2O (3:2)
Acetone=H2O (3:2)
DMF=H2O (2:1)
EtOH=H2O (1:1)
Acetone=H2O (3:2)
EtOH=H2O (1:1)
Acetone=H2O (3:2)
EtOH=H2O (1:1)
EtOH=H2O (1:1)
EtOH=H2O (1:1)
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaOH
rt
2
18
48
18
18
18
4
50
60
85
85
85
85
85
85
85
85
85
3
74
55
4
5
26
6
Pd=C
40
7
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
86b
97b
61b
44b
8
2
9
10
18
18
Et3N
aReaction conditions: TEATAB 1 (0.068 mmol), 4-bromobenzoic acid (0.25 mmol), base (0.5 mmol),
and solvent (1 ml).
bSame results were also obtained when 5 mol% catalyst was used.

1056
N. M. KUULOJA ET AL.
were tested (Table 3, entries 1–4). DMF=H2O (entry 3) was found to be a slightly
better solvent system than acetone=H2O, but an extended reaction time was needed
to obtain a good yield. The previously reported catalyst, Pd=C,[13] was also tested,
but only poor yields were obtained (entries 5 and 6). The ligandless catalyst system
was replaced by the commonly used Pd(PPh3)4 system, and reactions were per-
formed in acetone=water and ethanol=water solvent mixtures. Complete conversion
and excellent yields were obtained after 2 h in ethanol=water and after 4 h in acet-
one=water (Table 3, entries 7 and 8). The choice of base plays an important role
in the Suzuki reaction[26,27] and therefore sodium carbonate, sodium hydroxide,
and triethylamine were tested (entries 8–10). Sodium carbonate proved to be the best
base, under these reaction conditions (entry 8). When NaOH was used as a base, the
yield of the product was reduced (entry 9). Et3N was also less effective as a base
(entry 10). In summary, entry 8 gave the best results, taking into account the reac-
tion time (2 h) and yield (97%).
There is a reason to assume that cation exchange takes place in the reaction
media.[13] To prove this hypothesis, TEATAB 1 (1 equiv, 0.24 mmol) was placed in
a test tube with EtOH=H2O (1:1) (2.4 ml) and Na2CO3 (aq.) (6.7 equiv). The mixture
was heated to 80 ꢀC and kept at that temperature for 2 h. After cooling to room tem-
perature, the mixture was filtered, and the filtrate was evaporated to dryness. An
NMR sample was prepared, and only sodium tetra-arylborate could be observed.
This experiment clearly shows that cation exchange takes place under these reaction
conditions. To examine the influence of cation exchange on reactivity, TEATAB 2
and the corresponding sodium salt were subjected to the Suzuki reaction. Triethyla-
mine and sodium carbonate were used as bases, and the yields were compared
(Table 4). With both tetra-arylborates, the best results were obtained when both
Naþ and Et3NHþ were present in the reaction media (Table 4, entries 2 and 3).
The best result was obtained when Et3NHþ was used as a counter ion and Na2CO3
as a base (Table 4, entry 3). When triethylamine was used as a base, the yield was
lower (Table 4, entry 2). This behavior can be explained by the fact that triethyla-
mine has been reported to be a far less efficient base in the Suzuki reaction.[28]
The main reason for this base effect is not only the basicity of the anion but also
the contribution of the cation, because metal cations accelerate the formation of
palladium–boron complexes.[28] Organic amine bases have been reported to have a
Table 4. Effect of use of different bases and cations (Y) of the tetra-arylboratea
Entry
Y
Base
Yield (%)
1
2
3
4
Naþ
Na2CO3
Et3N
50
68
97
44
Naþ
Et3NHþ
Et3NHþ
Na2CO3
Et3N
aReaction conditions: tetra-arylborate (0.068 mmol), 4-bromobenzoic acid
(0.25 mmol), base (0.5 mmol), and EtOH=H2O (1 ml).

TRIETHYLAMMONIUM TETRA-ARYLBORATES
1057
co-operative role in the coupling reaction.[29] These results indicate that the presence
of triethylamine increases the reactivity, but the reaction also needs the smaller cat-
ion Naþ for greater conversion. The reaction mechanism is more complicated also
because free carboxylic acid could play an active role in this reaction. It has been
reported that carboxylic acid and amine groups attach to the tetracoordinated boron
by replacement of two phenyl groups.[30] As a conclusion, both amines and car-
boxylic acids can play an active role in the Suzuki reaction. Currently, the
co-operative role of carboxy- and amino- functionalities in the Suzuki reaction is
under further investigation in our laboratory.
Finally the differentially substituted TEATABs 1–6 were subjected to the
reaction conditions according to Table 3, entry 8. The results are collected in
Table 5. Monochloro-substituted TEATABs 2 and 3 furnished the coupling products
in good to excellent yields (entries 2 and 3). However, when the TEATAB was
adorned with two chloro-substituents, the desired biphenyl was not obtained. This
effect of the extension of electron-withdrawing properties indicates that electron-
donating substituents would make the TEATABs more reactive. Accordingly,
4-methoxy-40-carboxybiphenyl was obtained in 95% yield (entry 5). Another
electron-withdrawing group containing TEATAB 6 (entry 6) was tested, but it was
totally unreactive under these reaction conditions. One explanation could be the
interaction of the oxazoline group with the Pd catalyst.
Table 5. Suzuki reaction between TEATABs 1–6 and 4-bromobenzoic acida
Entry
Ar4BNHEt3
Product
Yield (%)
1
2
1
2
90
97
3
4
3
4
73b
Tracesb,c
95
5
6
5
6
d
—
aReaction conditions: TEATAB (0.18 mmol), 4-bromobenzoic acid (0.60 mmol), Pd(PPh3)4
(0.030 mmol, 5 mol%), Na2CO3(aq) (1.19 mmol), and EtOH=H2O (1:1) (0.5 ml) at 85 ꢀC, 2 h.
bReaction time was 18 h.
cNMR analysis indicated that traces of desired compound were formed as well as many side products.
Isolation was not attempted.
dAfter 48 h, the product was not observed.

1058
N. M. KUULOJA ET AL.
The mechanism of this reaction has not been fully explored. In the proposed
mechanism, the oxidative addition is followed by the coupling of one aryl group of
Ar4BNa.[13] The generated Ar3B, Ar2B(OH), and ArB(OH)2 take part in the presence
of base in the next catalytic cycles. This means that the first catalytic cycle takes place
in the absence of base.[31] However, in the case of trifluoroborate salts, no reaction was
observed when the reaction was performed without a base,[24b] suggesting that one or
more fluorine groups must be replaced by hydroxyl groups to form a new type of tet-
racoordinated species [for example, ArBF2(OH)ꢁ]. This kind of replacement cannot
take place when TEATABs are used because otherwise quantitative yields could
not be obtained. Braga et al. have proposed that palladium and boron are connected
to each other via an oxygen bridge during the transmetallation step.[32] The structure
of this intermediate in the transmetallation step of the reactions of tetra-arylborates is
unknown. Further mechanistic studies are ongoing in our laboratory.
In conclusion, substituted TEATABs were prepared using convenient and
reproducible methods, and their properties in the Suzuki reaction were investigated.
TEATABs could be obtained as stable solids and are easier to prepare and purify than
their corresponding Naþ salts. In addition, the reactivity of TEATABs in the Suzuki
reaction is comparable and even better. Moreover, TEATABs are new and interesting
orgonoboron reagents, which offer good atom economy in cross-coupling reactions.
The large reactivity difference between TEATAB 1 and TEATAB 2 shows that sub-
stituents in the aromatic rings have considerable effect in the tetracoordinated boron.
Electron-withdrawing substituents were found to reduce the yield. Also, the counter
cation and steric hindrance appears to have an effect on the reactivity. Further mech-
anistic studies need to be performed before any further conclusions can be made.
EXPERIMENTAL
Solvents and reagents were purchased from Sigma-Aldrich or Merck. Tetrahy-
drofuran (THF) was dried over sodium and freshly distilled before use. Other sol-
vents and reagents were used as received. Thin-layer chromatography (TLC) was
performed on precoated (silica gel 60 F254) aluminum plates, and visualization was
performed by ultraviolet light. Silica gel 60, particle size 0.040–0.063 nm, was used
for column chromatography. 1H and 13C NMR spectra were measured with a Varian
Mercury 300 spectrometer at 300 and 75 MHz, respectively, using CDCl3, acetone-
d6, or dimethylsulfoxide (DMSO-d6) as solvents. Chemical shifts are reported in d
values (ppm) relative to TMS. 11B NMR spectra were measured with a Bruker
AV-600 spectrometer as c. 50 mg=ml solutions in acetone-d6 at 192.66 MHz. BF3-
etherate was used as an external standard (d ¼ 0.0 ppm). Melting points were determ-
ined with a Stuard 110 melting-point apparatus. High-resolution mass spectra
(HMRS-ESI) were recorded using a MicroTOF-Q mass spectrometer. The syntheses
of TEATABs 2–6 were carried out under an argon atmosphere. The Suzuki reactions
were performed in sealed tubes.
Triethylammonium Tetraphenylborate (1)
Commercially available sodium tetraphenylborate (11 g, 32 mmol) was dis-
solved in 100 ml of MeOH=water (1:1), and 200 ml of a 2.5 wt% solution (aq.) of

TRIETHYLAMMONIUM TETRA-ARYLBORATES
1059
triethylamine (1.5 equiv.) was added. During the addition, a white precipitate was
formed. The precipitate was filtered, washed with water, and dried in a vacuum.
1
White solid (10.14 g, 75% yield); mp 197–198 ꢀC; H NMR (300 MHz, acetone-d6)
d (ppm): 7.35–7.31 (m, 8H, H2=H6), 6.96–6.94 (m, 8H, H3=H5), 6.91–6.79 (t, 4H,
J ¼ 7.9 Hz, H4), 3.22 (q, 6H, J ¼ 7.3 Hz, CH2), 1.26 (t, 9H, J ¼ 7.3 Hz, CH3); 13C
NMR (75 MHz, acetone-d6)
d
(ppm): 164.96 (JBC ¼ 49.4 Hz, C1), 136.93
(JBC ¼ 1.4 Hz, C2=C6), 125.99 (JBC ¼ 2.8 Hz, C3=C5), 122.26 (JBC < 1 Hz, C4);
48.13 (CH2), 9.39 (CH3); 11B NMR (192.55 MHz, acetone-d6) d (ppm): ꢁ6.514.
ESI-HRMS m=z calcd. for C36H52BN2: 523.4224; found 523.4226 [M þ Et3NH]þ.
Triethylammonium Tetra(4-chlorophenyl)borate (2)
Sodium tetrafluoroborate (0.982 g, 0.009 mol), Mg turnings (0.932 g,
0.038 mol), and a crystal of I2 were suspended in THF (2 ml) in a reaction flask.
One-fifth of a solution of 4-chlorobromobenzene (7.20 g, 0.038 mol) in dry THF
(20 ml) was added dropwise to the reaction mixture. The reaction mixture was stirred
with gently warming to initiate the reaction. The rest of the solution was then slowly
added to maintain reflux for 1 h. The mixture was stirred overnight at room tempera-
ture and quenched by pouring it into 100 ml of Na2CO3=NaOH=H2O (4:1:200) sol-
ution with vigorous stirring. The aqueous solution was saturated with NaCl and
extracted with ether (4 ꢂ 50 ml). The ether was removed in vacuo, and a yellow syrup
was obtained. The residue was dissolved in H2O=MeOH (1:1, 100 ml) and filtered
through a pad of celite to obtain a clear solution. The solution was treated with
2.5 wt% Et3N solution (aq.) (2.6 equiv., 100 ml). The crude product precipitated
and was filtered by suction. The crude product was dissolved in acetone (10 ml),
and the insoluble material was filtered off. Water was added dropwise to the clear
solution with vigorous stirring until a white precipitate formed. After filtration, a
1
white solid was obtained (4.08 g, 81% yield); mp 101–103 ꢀC; H NMR (300 MHz,
acetone-d6) d (ppm): 7.20–7.15 (m, 8H, H2=H6), 6.98 (d, 8H, J ¼ 8.5 Hz,
H3=H5), 3.44 (q, 6H, J ¼ 7.3 Hz, CH2), 1.30 (t, 9H, J ¼ 7.3 Hz, CH3);13C NMR
(75 MHz, acetone-d6) d (ppm): 161.78 (JBC ¼ 49.6 Hz, C1), 138.07 (JBC ¼ 1.7 Hz,
C2=C6)), 128.54 (JBC < 1 Hz, C4), 126.29 (JBC ¼ 3.1 Hz, C3=C5), 48.00 (CH2), 9.48
(CH3); 11B NMR (192.55 MHz, acetone-d6) d (ppm): ꢁ7.426. ESI-HRMS m=z calcd.
for C30H32BNNaCl4: 580.1280; found 580.1284 [M þ Na]þ.
Triethylammonium Tetra(3-chlorophenyl)borate (3)
Compound 3 was prepared in a way similar to compound 2. The crude product
was recrystallized from methanol=water (1:1). White solid (4.33 g, 86% yield); mp
1
182–185 ꢀC; H NMR (300 MHz, acetone-d6) d (ppm): 7.24–7.20 (m, 4H, H2 or
H6), 7.18–7.13 (m, 4H, H2 or H6), 7.02 (t, 4H, J ¼ 7.6 Hz, H5), 6.92–6.88 (m, 4H,
H4), 3.42 (q, 6H, J ¼ 7.3 Hz, CH2), 1.38 (t, 9H, J ¼ 7.3 Hz, CH3). 13C NMR
(75 MHz, acetone-d6) d (ppm): 165.86 (JBC ¼ 49.4 Hz, C1), 135.74 (JBC ¼ 1.7 Hz,
C6), 134.85 (JBC ¼ 1.3 Hz, C2), 132.87 (JBC ¼ 3.9 Hz, C3), 128.19 (JBC ¼ 3.1 Hz,
C5), 123.16 (JBC < 1 Hz, C4), 48.14 (CH2), 9.40 (CH3); 11B NMR (192.55 MHz, acet-
one-d6) d (ppm): ꢁ6.766. ESI-HRMS m=z calcd. for C30H32BNNaCl4: 580.1280;
found 580.1265 [M þ Na]þ.

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N. M. KUULOJA ET AL.
Triethylammonium Tetra(3,5-dichlorophenyl)borate (4)
Compound 4 was prepared in a way similar to compound 2. The crude product
was recrystallized from MeOH=water. White solid (2.68 g, 44% yield); mp 198–201 ꢀC;
1H NMR (300 MHz, acetone-d6) d (ppm): 7.10–7.05 (12H, m, ArH), 3.47 (q, 6H,
J ¼ 7.3 Hz, CH2), 1.41 (t, 9H, J ¼ 7.3 Hz, CH3). 13C NMR (75 MHz, acetone-d6) d
(ppm): 165.45 (JBC ¼ 49.4 Hz, C1), 133.94 (C2=C6, overlap with peak 133.88),
133.88 (C3=C5, overlap with peak 133.94), 123.91 (C4), 48.17 (CH2), 9.42 (CH3);
11B NMR (192.55 MHz, acetone-d6) d (ppm): ꢁ6.939. ESI-HRMS calcd. for
C30H28BNCl8K: 731.9460; found 731.9452 [M þ K]þ.
Triethylammonium Tetra(4-methoxyphenyl)borate (5)
Sodium tetrafluoroborate (0.23 g, 0.002 mol) and Mg turnings (0.40 g,
0.017 mol) were placed in a reaction flask under argon. Dry THF (1 ml) and a crystal
of I2 crystal were added. A solution of 4-bromoanisole (3.12 g, 0.017 mol) in dry
THF (10 ml) was added dropwise. Isolation and purification was performed by a
procedure similar to that described previously. White solid (0.57 g, 53% yield); mp
1
106–109 ꢀC; H NMR (300 MHz, acetone-d6) d (ppm): 7.22–7.18 (m, 8H, H2=H6),
6.55 (d, 8H, J ¼ 8.5 Hz, C3=C5), 3.32 (q, 6H, J ¼ 7.3 Hz, CH2), 1.33 (t, 9H,
J ¼ 7.3 Hz, CH3); 13C NMR (75 MHz, acetone-d6) d (ppm): 156.72 (JBC ¼ 50.1 Hz,
C1), 156.23 (JBC < 1 Hz, C4), 137.41 (JBC ¼ 1.7 Hz, C2=C6), 111.72 (JBC ¼ 3.0 Hz,
C3=C5), 54.87 (OCH3), 48.02 (CH2), 9.40 (CH3); 11B NMR (192.55 MHz, acetone-
d6) d (ppm): ꢁ7.667. ESI-HRMS calcd. for C40H60BN2O4: 643.4646; found
643.4650 [M þ Et3NH]þ.
Triethylammonium Tetra[4-(4,4-dimethyl)-4,5-dihydro-
oxazoles)phenyl]borate (6)
Compound 6 was prepared in a way similar to compound 2. The crude product
was dissolved in THF=water (1:2) and filtered through celite. A solution of triethy-
lamine (2.5 wt%, 2.6 equiv.) was added to the clear solution with vigorous stirring.
The formed white precipitate was filtered, washed with toluene, and recrystallized
from THF=H2O. White solid (3.64 g, 50% yield); mp 247–248 ꢀC; 1H NMR
(300 MHz, CDCl3) d (ppm): 7.60–7.57 (d, 8H, J ¼ 8.3 Hz, H2=H6), 7.41–6.44 (m,
8H, H3=H5), 4.06 (s, 8H, CH2O,), 1.32 [s, 24H, (CH3)2C], 2.47 (q, 6H, J ¼ 7.3 Hz,
CH2), 0.92 (t, 9H, J ¼ 7.3 Hz, CH3). 13C NMR (75 MHz, CDCl3) d (ppm): 165.11
(OC¼N), 135.88 (C1), 125.93 (C2=C6 and C3=C5), 121.0 (C4), 79.05 (OCH2),
66.57 [(CH3)2C], 46.41 (CH2), 28.26 [(CH3)2C], 8.83 (CH3); 11B NMR
(192.55 MHz, acetone-d6) d (ppm): ꢁ6.401. ESI-HRMS calcd. for C44H50BN4O4:
709.3925; found 709.3947 [M-Et3NH þ 2H]þ.
Sodium Tetra(4-chlorophenyl)borate[18]
The preparation was carried out using a published method.[18] 1H NMR
(300 MHz, acetone-d6) d (ppm): 7.21–7.15 (m, 8H, H2=H6), 6.99 (d, 8H, J ¼ 6.42 Hz,
H3=H5). 13C NMR (75 MHz, acetone-d6) d (ppm): 162.53 (JBC ¼ 49.6 Hz, C1),

TRIETHYLAMMONIUM TETRA-ARYLBORATES
1061
138.81 (JBC ¼ 1.7 Hz, C2=C6), 129.27 (JBC < 1 Hz, C4), 127.02 (JBC ¼ 3.0 Hz, C3=
C5). 11B NMR (192.55 MHz, acetone-d6) d (ppm): ꢀ6.689.
General Procedure for the Suzuki Coupling: Preparation of
4’-Chlorobiphenyl-4-carboxylic Acid from TEATAB 2
TEATAB 2 (101 mg, 0.18 mmol), 4-bromobenzoic acid (115 mg, 0.60 mmol),
Pd(PPh3)4 (35 mg, 0.030 mmol, 5 mol%), 2 M Na2CO3 (aq.) (595 ml, 1.19 mmol)
and EtOH=H2O (1:1) (0.5 ml) were placed in a sealed tube. The reaction mixture
was stirred at 85 ꢁC for 18 h. The reaction was quenched by adding 3 M HCl
(0.5 ml) and diluted with EtOAc (30 ml). The reaction mixture was filtered through
celite and was washed with water (2 ꢂ 15 ml) and brine (1 ꢂ 15 ml). After drying
(Na2SO4) and filtration, the solvents were removed by a rotary evaporator. The
crude product was purified using column chromatography [eluent CHCl3–MeOH
(90:10)] or recrystallization (EtOAc=MeOH), and the desired compound was
obtained as a white solid (135 mg, yield 97%).
ACKNOWLEDGMENTS
This work was supported by the National Graduate School of Organic Chem-
istry and Chemical Biology. We thank also the Academy of Finland for financial
˚
support (No. 110043) and Markku Reunanen (Abo Akademi, Finland) for mass
spectral measurements.
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