Synthesis of functionalized borate building blocks for the anionic derivatization of neutral compounds

Article abstract of DOI:10.1055/s-2007-1000862

Mono- and bifunctional fluorinated borate building blocks were prepared in three to five steps with good to excellent overall yields. Compounds with both nucleophilic and electrophilic functionalities are presented, which can be used for the anionic deriv

Full text of DOI:10.1055/s-2007-1000862

PAPER
245
Synthesis of Functionalized Borate Building Blocks for the Anionic
Derivatization of Neutral Compounds
Functionalized
Bo
x
rate Building
e
Blocks l Franzke,1 Andreas Pfaltz*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
Fax +41(61)2671103; E-mail: andreas.pfaltz@unibas.ch
Received 28 August 2007; revised 22 October 2007
If the borane B(Ar)3 is easily accessible, the desired borate
can be obtained according to pathway A, in one step, by
addition of the appropriate metal organyl compound. Al-
ternatively, a neutral (RBX2) or anionic intermediate
Abstract: Mono- and bifunctional fluorinated borate building
blocks were prepared in three to five steps with good to excellent
overall yields. Compounds with both nucleophilic and electrophilic
functionalities are presented, which can be used for the anionic de-
rivatization of neutral molecules.
–
(RBX3 ) can be generated first and subsequently trans-
formed into the borate by reaction with the corresponding
aryl reagent according to pathway B.
Key words: boron, fluorine, organometallic reagents, anions
In the course of our studies concerning the effect of the
counter-ion in iridium-catalyzed asymmetric hydrogena-
tion reactions,4,11 functionalized borate building blocks
were required to prepare anionic derivatives of neutral
ligands. In this report, we describe the synthesis of a series
of functionalized borates bearing nucleophilic or electro-
philic functional groups, which can be used for the anionic
functionalization of neutral compounds.
Fluorinated tetraarylborates constitute an easily accessi-
ble class of weakly coordinating anions (WCA),2 which
have found widespread use in the synthesis of highly effi-
cient single-site olefin polymerization catalysts.3 In addi-
tion, they were successfully applied in catalytic
asymmetric transformations such as the iridium-catalyzed
hydrogenation of unfunctionalized alkenes4 and Diels–
Alder reactions.5 In most cases, the well-established rep-
resentatives tetrakis(pentafluorophenyl)borate6 or tetra-
F
F
M
kis[3,5-bis(trifluoromethyl)phenyl]borate
(BArF
or
X
M
X
TFPB)7 were employed. Furthermore, related anions with
four identical aryl moieties at the boron center have re-
cently been developed.8 However, there are only a few ex-
amples of borates having the composition [B(C6F5)3(R)]–
with different boron substituents, and only simple alkyl
derivatives [R = Me, Et, i-Pr, n-Bu, CH2C(CH3)3]9 and the
phenyl analogue (R = Ph)10 have been described in detail.
(C6F5)3B
(C6F5)3B
F
F
2
1
Figure 1 Functionalized borates with two different linkers
Initial attempts to prepare compounds of type 1 with a
benzyl linker derived from the commercially available
tris(pentafluorophenyl)borane (Figure 1) via pathway A,
proved unsuccessful. The intermediate borates suffered
from partial decomposition, presumably by proton-in-
duced deborylation of the benzyl moiety, even under only
slightly acidic conditions, for example during silica gel
chromatography. Therefore, we turned our attention to the
corresponding derivatives 2 with perfluorinated linkers.
We assumed that the reduced p-basicity of these com-
pounds would slow down the acid-driven cleavage of the
carbon–boron bond and thus enable chromatographic pu-
rification.
In principle, the synthesis of [B(Ar)3(R)]– species can be
accomplished via two synthetic pathways (Scheme 1),
differing in the order of steps through which the two sub-
stituents at the boron center are introduced.9,10
Ar–M
B(Ar)3
R–M
BX3
pathway A
R–B(Ar)3
M
Ar–M
Starting from the known benzylic alcohol 3,12 prepared in
two steps from pentafluorobenzaldehyde (see experimen-
tal section), the silyl ethers 4 and 5 were synthesized
(Scheme 2). While the tert-butyldimethylsilyl ether 4 was
obtained in virtually quantitative yield, protection with
tert-butyldiphenylsilyl chloride at room temperature fur-
nished the desired aryl bromide 5 in 72% yield after 26
hours along with 27% of re-isolated starting material 3.
pathway B
R–M
BX3
R–BX2 or R–BX3 M
Scheme 1 Two pathways for the preparation of mixed tetraarylbo-
rates
SYNTHESIS 2008, No. 2, pp 0245–0252
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x
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x
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0
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Advanced online publication: 18.12.2007
DOI: 10.1055/s-2007-1000862; Art ID: T14007SS
© Georg Thieme Verlag Stuttgart · New York
Both silyl ethers 4 and 5 were subsequently metalated at
low temperature and the resulting aryllithium compounds

246
A. Franzke, A. Pfaltz
PAPER
F
F
F
F
F
F
F
OH
OR
TBDMSCl, imidazole, CH2Cl2, 0 °C → r.t., 21 h
or TBDPSCl, Et3N, DMF, 0 °C → r.t., 26 h
Br
Br
F
3
4
5
R = TBDMS (98%)
R = TBDPS (72%)
1) n-BuLi, Et2O, –78 °C, 60 min
2) B(C6F5)3, –50 °C, 15 min → r.t., 17 h
F
F
F
F
F
F
F
F
F
F
F
NBu4
NBu4
(C6F5)3B
Li
OH
OR
Br
1) PBr3, CH2Cl2, 0 °C, 2 h
TBAF, THF
(C6F5)3B
(C6F5)3B
0 °C → r.t.,
17–22 h
F
2) NBu4Br, 0 °C, 3.5 h
→ r.t., 14 h, 97%
8 (87% from 4 and 81%
from 5 respectively)
9
6
7
R = TBDMS
R = TBDPS
Scheme 2 Synthesis of building blocks derived from tris(pentafluorophenyl)borane
were quenched with tris(pentafluorophenyl)borane. The the literature.15 For this reason, we decided to develop a
isolated borates 6 and 7 were contaminated by varying general reaction sequence according to pathway B in
–
amounts of lithium [tetrakis(pentafluorophenyl)borate], Scheme 1. As an intermediate of type RBX3 , the air- and
which was easily removed after the next step. This side- moisture-stable aryltrifluoroborate 10 was chosen
product probably resulted from aryl exchange between the (Scheme 3).16
product and the borane reagent. The amount of the impu-
rity was dependent on the protecting group and decreased
with increasing reaction temperature. However, due to the
Initial attempts to synthesize the desired aryltrifluorobo-
rate 10 from aryl bromide 4, yielded mainly the corre-
sponding diaryldifluoroborate 11. Similarly, desired bis-
arylations of trialkyl borates have been described in the
explosive nature of similar ortho-fluorinated aryllithium
compounds,13 the transformations were not performed at
literature for a few cases.17 However, after careful optimi-
temperatures above –50 °C. Under optimized conditions,
zation of the reaction conditions, we were able to obtain
using tert-butyldimethylsilyl ether 4 and slowly adding
both compounds 10 and 11 selectively (Scheme 3).
the electrophile at –50 °C, the ratio of product 6 to lithium
The aryltrifluoroborate 10, prepared according to
[tetrakis(pentafluorophenyl)borate] was found to be 13:1,
Scheme 3, contained about 10% of 11 as a side-product.
However, the corresponding mixed tetraarylborates 12
and 13 (Scheme 4) could be easily separated by column
chromatography after the next step. Interestingly, the tert-
butyldimethylsilyl ether proved to be stable under the
fluorination conditions,18 but only if all the tetrahydrofu-
ran present in the reaction mixture was removed before
addition of potassium hydrogen difluoride.
according to 19F NMR spectroscopy.
The crude silyl ethers 6 and 7 were deprotected and the re-
sulting tetrabutylammonium borate 8 was purified by
chromatography on silica gel without any problems. As
expected, fluorine substitution of the benzyl linker re-
duced the acid sensitivity and prevented decomposition
during chromatography as observed with compounds 1.
Using the tert-butyldimethylsilyl protecting group, the
hydroxyl-functionalized borate 8 was readily synthesized
in 85% yield over three steps, starting from aryl bromide
3.
Addition of the appropriate arylmagnesium reagent, pre-
pared by halogen-magnesium exchange,19 to the respec-
tive fluoroborates 10 and 11 yielded the mixed
tetraarylborates 12 and 13 (Scheme 4). These were subse-
quently transformed into the bromides 16 and 17 via the
corresponding benzylic alcohols 14 and 15, using the pro-
cedures described in Scheme 2. In this way, the hydroxyl-
and bromo-functionalized tetraarylborates 14–17 were
Benzylic bromide 9 was prepared in high yield by treating
alcohol 8 with phosphorus tribromide and tetrabutylam-
monium bromide (TBAB); the reaction without addition
of TBAB was less efficient.
Interestingly, borates 6–9 showed more than the expected obtained in 81–71% yield over three steps.
five signals in their 19F NMR spectra. This can be ascribed
to hindered rotation of the aryl substituents around the bo-
ron–carbon bonds with concomitant collapse of the dy-
namic C3-symmetry.14
In summary, a series of mono- and bifunctional fluorinat-
ed borate building blocks have been prepared, containing
nucleophilic hydroxyl or electrophilic benzylic bromide
functionalities. The compounds are readily accessible in
A different route was chosen for the preparation of the three to five steps with 85–38% overall yield, starting
corresponding tris[3,5-bis(trifluoromethyl)phenyl]bo- from the literature-known alcohol 3. The inert nature and
rates 14 and 16. So far, no practical synthesis of tris[3,5- lipophilicity, which ensures high solubility in apolar me-
bis(trifluoromethyl)phenyl]borane has been described in dia, are attractive features of the borate unit. Reagents of
Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York

PAPER
Functionalized Borate Building Blocks
247
F
F
F
F
K
OTBDMS
F3B
10
1) i-PrMgCl, Et2O–THF (5:1), r.t., 2.5 h
2) B(Oi-Pr)3 (2.0 equiv), Et2O–THF (10:1), –78 °C → r.t., 22 h
3) KHF2, H2O–Et2O, r.t., 60 min, 68%
F
F
F
OTBDMS
Br
F
4
1) n-BuLi, Et2O, –78 °C, 90 min
2) B(Oi-Pr)3 (0.75 equiv), –78 °C, 90 min → r.t., 2 h
3) KHF2, H2O, r.t., 80 min, 54%
F
F
F
K
OTBDMS
F2B
F
11
2
Scheme 3 Preparation of arylfluoroborates 10 and 11
F
F
F
F
F
F
F
NBu4
K
1) ArFMgX, THF–Et2O, r.t., 135 h
OTBDMS
OTBDMS
F(4–n)
B
(ArF)(4–n)
B
2) Na2CO3, H2O, r.t., 30 min
3) NBu4Br, CH2Cl2, r.t., 30 min
F
n
n
10 n = 1
11 n = 2
12 n = 1 (90%)
13 n = 2 (90%)
ArF = 3,5-(CF3)2C6H3
TBAF, THF,
0 °C → r.t., 14–18 h
F
F
F
F
F
F
NBu4
(ArF)(4–n)
NBu4
Br
OH
1) PBr3, CH2Cl2, 0 °C, 3 h
B
(ArF)(4–n)
B
2) NBu4Br, 0 °C → r.t., 13–15 h
F
F
n
n
16 n = 1 (92%)
17 n = 2 (92%)
14 n = 1 (90%)
15 n = 2 (86%)
Scheme 4 Synthesis of mono- and bifunctional borates
dried employing standard procedures.20 All other commercial re-
agents were used without further purification. Chromatography was
performed on Merck silica gel 60 (Darmstadt, 40–63 nm). For TLC
analyses, pre-coated Macherey–Nagel Polygram SIL G/UV254
plates were used and the compounds were visualized with the help
of UV light. All NMR experiments were performed on Bruker
Avance 400 and 500 MHz spectrometers. 1H and 13C NMR spectra
are referenced relative to TMS using the residual solvent peaks and
the solvent signals, respectively, as internal standards.21 The 19F and
11B spectra were calibrated using CFCl3 and BF3·OEt2 as external
standards. Mass spectra were measured on VG70-250, Finnigan
MAT 95Q (EI) or Finnigan MAT LCQ apparatus (ESI). Elemental
this type can be used for the anionic derivatization of any
neutral compound possessing at least one nucleophilic or
electrophilic functional group. Thus they may find use in
ESI-MS studies or mechanistic investigations of anion ef-
fects. Application of these borate reagents in the synthesis
of anionic ligands for metal-catalyzed asymmetric trans-
formations will be reported in due course.
All reactions were performed in flame-dried glassware under argon
using Schlenk techniques. Solvents, Et3N, LiBr and TBAB were
Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York

248
A. Franzke, A. Pfaltz
PAPER
analyses were performed by the Micro Analysis Laboratory of the
University of Basel. IR spectra were measured on a Perkin–Elmer
1600 FTIR spectrometer. Melting points were determined on a Bü-
chi 535 apparatus and are uncorrected. Benzylic alcohol 3 was pre-
pared following literature procedures.12 The abbreviation ArF refers
to any fluorinated aryl moiety, whilst the abbreviations mc and dmc
refer to centered multiplet and doublet of centered multiplet, respec-
tively.
MS (EI, 70 eV): m/z (%) = 258 (100) [M+], 239 (74) [M – F]+, 210
(33), 179 (14) [M – Br]+, 162 (30), 150 (44), 130 (23), 99 (17), 81
(13).
Anal. Calcd for C7H3BrF4O: C, 32.46; H, 1.17. Found: C, 32.49; H,
1.23.
(4-Bromo-2,3,5,6-tetrafluorobenzyloxy)-tert-butyldimethylsi-
lane (4)
To a solution of benzylic alcohol 3 (1.83 g, 7.05 mmol) and imida-
zole (959 mg, 14.1 mmol) in anhyd CH2Cl2 (20 mL), TBDMSCl
(1.59 g, 10.6 mmol) in CH2Cl2 (10 mL) was slowly added at 0 °C.
After the resulting colorless suspension had been stirred for 21 h
(overnight) at r.t., it was poured into aqueous HCl (1 M, 50 mL).
The phases were separated and the aqueous phase was extracted
with CH2Cl2 (3 × 40 mL). The combined extracts were washed with
sat. NaHCO3 (20 mL), dried over MgSO4, filtered and evaporated
under reduced pressure. Purification of the remaining yellow oil by
column chromatography (silica gel, 4 × 24 cm, hexanes–EtOAc,
15:1) yielded silyl ether 4 as a colorless, viscous oil, which solidi-
fied at –22 °C to a waxy solid (2.57 g, 98%).
4-Bromo-2,3,5,6-tetrafluorobenzaldehyde12
A solution of pentafluorobenzaldehyde (11.2 mL, 17.7 g, 90.3
mmol) and dried LiBr (8.90 g, 102 mmol) in anhyd NMP (50 mL)
was stirred at 160 °C for 3 h. After the brown mixture had been
cooled to r.t., it was filtered through Celite. The filtrate was poured
into H2O (200 mL) and the resulting brownish solid was collected
by filtration, washed with H2O (3 × 20 mL) and dried over P2O5 in
a desiccator. The filtrate was extracted with Et2O (5 × 100 mL) and
the combined organic phases were dried over MgSO4, filtrated and
evaporated under reduced pressure. The remaining sticky solid was
washed with H2O (3 × 10 mL) and dried as described above. The
combined crude products were washed with Et2O (60 mL) and
dried. Concentration of the washing solutions to about half volume
and isolation of the precipitate formed yielded a second crop of
product. Aldehyde 18 was isolated as a pale-yellow solid in total
yield of 13.7 g (59%).
Rf = 0.57 (hexanes–EtOAc, 15:1).
IR (NaCl): 2955, 2935, 2891, 2860, 1637, 1487, 1261, 1100, 1051,
917, 837, 780, 717 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.12 [s, 6 H, Si(CH3)2], 0.90 [s,
9 H, C(CH3)3], 4.77 (t, J = 1.7 Hz, 2 H, CH2).
13C{1H} NMR (100 MHz, CDCl3): d = –5.4 [Si(CH3)2], 18.5
[C(CH3)3], 25.9 [C(CH3)3], 53.4 (CH2), 99.7 (mc, Ar-p-C), 118.7 (t,
J = 18 Hz, Ar-i-C), 144.9 (dmc, J = 243 Hz, Ar-C), 145.4 (dmc,
J = 250 Hz, Ar-C).
19F{1H} NMR (375 MHz, CDCl3): d = –142.9 (mc, 2 F, Ar-o-F),
–133.9 (mc, 2 F, Ar-m-F).
Mp 107–108.5 °C (Lit.12 105–108 °C); Rf = 0.47 (hexanes–EtOAc,
3:1).
IR (KBr): 2915, 1702, 1636, 1587, 1485, 1415, 1389, 1306, 1273,
1070, 1035, 970, 831, 777, 625, 584 cm–1.
1H NMR (400 MHz, CDCl3): d = 10.30 (s).
13C{1H} NMR (100 MHz, CDCl3): d = 107.5 (tt, J = 22, 2 Hz, Ar-
p-C), 114.6 (t, J = 10 Hz, Ar-i-C), 145.3 (dmc, J = 251 Hz, Ar-C),
146.8 (dmc, J = 264 Hz, Ar-C), 182.1 (mc, CHO).
19F{1H} NMR (375 MHz, CDCl3): d = –144.2 (mc, 2 F, Ar-o-F),
–131.5 (mc, 2 F, Ar-m-F).
MS (EI, 70 eV): m/z (%) = 317 (100) [M – t-Bu]+, 241 (15) [4-
+
BrC6F4CH2 ], 236 (10), 221 (13), 77 (45).
Anal. Calcd for C13H17BrF4OSi: C, 41.83; H, 4.59. Found: C, 41.76;
H, 4.54.
MS (EI, 70 eV): m/z (%) = 257 (100) [M – H]+, 229 (16) [M –
CHO]+, 149 (41) [C6F4H+], 99 (28), 79 (10).
(4-Bromo-2,3,5,6-tetrafluorobenzyloxy)-tert-butyldiphenylsi-
lane (5)
Anal. Calcd for C7HBrF4O: C, 32.72; H, 0.39. Found: C, 32.67; H,
0.46.
To a solution of benzylic alcohol 3 (5.18 g, 20.0 mmol) and anhyd
Et3N (4.18 mL, 30.0 mmol) in anhyd DMF (40 mL), TBDPSCl
(6.14 mL, 24.0 mmol) was slowly added at 0 °C. After the mixture
had been stirred for 26 h at r.t., it was diluted with Et2O (150 mL)
and poured into aq HCl (0.4 M, 80 mL). The phases were separated
and the aqueous phase was extracted with Et2O (3 × 70 mL). The
combined extracts were washed with sat. NaHCO3 (40 mL), dried
over MgSO4, filtered and evaporated under reduced pressure. After
purifying the remaining oil by column chromatography (silica gel,
7 × 16 cm, hexanes–EtOAc, 10:1) silyl ether 5 was obtained as col-
orless, viscous oil (7.14 g, 72%). In addition, some of the starting
alcohol 3 was re-isolated (1.39 g, 27%).
(4-Bromo-2,3,5,6-tetrafluorophenyl)methanol (3)12
A solution of 4-bromo-2,3,5,6-tetrafluorobenzaldehyde (18.4 g,
71.6 mmol) in anhyd MeOH (150 mL) was cooled to 0 °C and
NaBH4 (2.71 g, 71.6 mmol) was added in portions over a period of
40 min. The mixture was stirred for a further 60 min at 0 °C and then
for 3 h at r.t. The solution was poured into cold aqueous HCl (3%,
600 mL) and the resulting suspension was cooled in an ice bath. The
yellow solid was collected by filtration, washed with H2O (4 × 20
mL) and dried over P2O5 in a desiccator. The filtrate was concen-
trated under reduced pressure to a volume of about 100 mL. The re-
sulting precipitate was isolated by filtration, washed with H2O
(3 × 10 mL) and dried as above. The benzylic alcohol 3 was ob-
tained in two crops as a colorless powder (14.3 g, 77%).
Rf = 0.51 (hexanes–EtOAc, 10:1).
IR (NaCl): 3071, 2936, 2893, 2860, 1638, 1487, 1428, 1387, 1270,
1107, 1051, 916, 818, 703 cm–1.
Mp 65–66 °C (Lit.12 60–62 °C); Rf = 0.24 (hexanes–EtOAc, 3:1).
1H NMR (400 MHz, CDCl3): d = 1.05 (s, 9 H, CH3), 4.78 (s, 2 H,
CH2), 7.41 (mc, 4 H, Ph-m-H), 7.46 (mc, 2 H, Ph-p-H), 7.69 (mc,
4 H, Ph-o-H).
13C{1H} NMR (100 MHz, CDCl3): d = 19.4 [C(CH3)3], 26.8 (CH3),
54.2 (CH2), 99.8 (mc, Ar-p-C), 118.4 (t, J = 18 Hz, Ar-i-C), 128.0
(Ph-m-CH), 130.1 (Ph-p-CH), 132.8 (Ph-i-C), 135.7 (Ph-o-CH),
144.8 (dmc, J = 248 Hz, Ar-C), 145.4 (dmc, J = 252 Hz, Ar-C).
IR (KBr): 3347, 2980, 2954, 2894, 1637, 1476, 1406, 1370, 1270,
1223, 1044, 1029, 1003, 901, 830, 751, 662, 577, 437 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.96 (t, J = 6.6 Hz, 1 H, OH), 4.82
(d, J = 6.4 Hz, 2 H, CH2).
13C{1H} NMR (100 MHz, CDCl3): d = 53.0 (CH2), 100.2 (t, J = 23
Hz, Ar-p-C), 118.2 (t, J = 18 Hz, Ar-i-C), 144.9 (dmc, J = 250 Hz,
Ar-C), 145.3 (dmc, J = 251 Hz, Ar-C).
19F{1H} NMR (375 MHz, CDCl3): d = –143.4 (mc, 2 F, Ar-o-F),
–133.3 (mc, 2 F, Ar-m-F).
19F{1H} NMR (375 MHz, CDCl3): d = –142.3 (mc, 2 F, Ar-o-F),
–134.0 (mc, 2 F, Ar-m-F).
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Functionalized Borate Building Blocks
249
MS (EI, 70 eV): m/z (%) = 441 (100) [M – t-Bu]+, 254 (10), 201
9:1) yielded benzylic alcohol 8 as a colorless, foamy solid (2.19 g,
(18).
87% over two steps).
Anal. Calcd for C23H21BrF4OSi: C, 55.54; H, 4.26. Found: C, 55.70;
H, 4.15.
In an analogous manner, 8 was obtained in 81% yield over two steps
using TBDPS-ether 7. In this case TBAF·3H2O was added at r.t. and
the reaction time was 22 h.
Lithium [4-(tert-Butyldimethylsilanyloxymethyl)-2,3,5,6-tetra-
fluorophenyl]tris(pentafluorophenyl)borate (6)
Mp 53–56 °C; Rf = 0.15 (CH2Cl2).
IR (KBr): 3626, 3423, 2971, 2882, 1644, 1515, 1461, 1380, 1257,
1089, 977, 924, 885, 766, 694, 661, 626, 609, 575 cm–1.
1H NMR (400 MHz, acetone-d6): d = 0.98 (t, J = 7.3 Hz, 12 H,
CH3), 1.44 (sext, J = 7.4 Hz, 8 H, CH2CH3), 1.84 (mc, 8 H,
CH2CH2CH3), 3.46 (mc, 8 H, CH2CH2CH2CH3), 4.29 (t, J = 6.1 Hz,
1 H, OH), 4.64 (dt, J = 6.1, 1.4 Hz, 2 H, CH2OH).
13C{1H} NMR (125 MHz, acetone-d6): d = 13.8 (CH3), 20.4 (t, J = 1
Hz, CH2CH3), 24.4 (CH2CH2CH3), 52.4 (CH2OH), 59.4 (t, J = 3 Hz,
CH2CH2CH2CH3), 116.2 (t, J = 19 Hz, Ar-p-C), 125.5 (br, C6F5-i-C
and Ar-i-C), 137.0 (dmc, J = 244 Hz, C6F5-m-C), 139.0 (dmc,
J = 244 Hz, C6F5-p-C), 144.9 (dd, J = 244, 18 Hz, Ar-m-C), 148.9
(dd, J = 239, 13 Hz, Ar-o-C), 149.1 (dmc, J = 241 Hz, C6F5-o-C).
19F{1H} NMR (375 MHz, acetone-d6): d = –168.0 to –167.6 (m, 6 F,
C6F5-m-F), –164.0 (t, J = 20 Hz, 2 F, C6F5-p-F), –163.9 (t, J = 19
Hz, 1 F, C6F6-p-F), –150.0 (dd, J = 22, 14 Hz, 2 F, Ar-m-F), –133.5
(mc, 2 F, ArF-o-F), –132.3 (mc, 2 F, ArF-o-F), –131.9 (mc, 2 F, ArF-
o-F), –131.7 (mc, 2 F, ArF-o-F).
To a solution of aryl bromide 4 (1.01 g, 2.71 mmol) in anhyd Et2O
(15 mL), n-BuLi in hexanes (1.6 M, 1.69 mL, 2.71 mmol) was add-
ed dropwise at –78 °C over a period of 12 min (Caution: ortho-
Fluorinated aryllithium reagents are potentially explosive!). After
the mixture had been stirred for 60 min at –78 °C, the temperature
was raised to –50 °C and B(C6F5)3 (1.39 g, 2.71 mmol) in anhyd
Et2O (55 mL) was added slowly within 25 min. The slightly yellow
solution was stirred for a further 15 min at –50 °C and then for 17 h
(overnight) at r.t. The solution was concentrated under reduced
pressure to a volume of about 8 mL and anhyd pentane (50 mL) was
added. The mixture was vigorously stirred for 25 min at r.t. and the
upper phase was removed. The remaining oil was treated with an-
hyd pentane (3 × 10 mL) and dried in high vacuum. The colorless,
foamy solid obtained consisted of borate 6 and Li[B(C6F5)4] in a ra-
tio of 13:1 according to 19F NMR spectroscopy.
1H NMR (400 MHz, acetone-d6): d = 0.07 [s, 6 H, Si(CH3)2], 0.87
[s, 9 H, C(CH3)3], 4.77 (s, 2 H, CH2).
19F{1H} NMR (375 MHz, acetone-d6): d = –167.8 (mc, 6 F, C6F5-m-
F), –164.0 (t, J = 18 Hz, 2 F, C6F5-p-F), –163.9 (t, J = 18 Hz, 1 F,
C6F5-p-F), –149.6 (dd, J = 23, 13 Hz, 2 F, Ar-m-F), –133.3 (mc, 2 F,
ArF-o-F), –132.3 (mc, 2 F, ArF-o-F), –132.0 (mc, 2 F, ArF-o-F),
–131.7 (mc, 2 F, ArF-o-F).
11B{1H} NMR (160 MHz, acetone-d6): d = –15.8.
MS (ESI): m/z (%) = 805 (100) [M – Li]–.
11B{1H} NMR (160 MHz, acetone-d6): d = –15.8.
MS (ESI): m/z (%) = 691 (100) [M – NBu4]–.
Anal. Calcd for C41H39BF19NO: C, 52.75; H, 4.21; N, 1.50. Found:
C, 52.86; H, 4.24; N, 1.52.
Tetrabutylammonium (4-Bromomethyl-2,3,5,6-tetrafluorophe-
nyl)tris(pentafluorophenyl)borate (9)
To a solution of borate 8 (4.67 g, 5.00 mmol) in anhyd CH2Cl2 (35
mL), PBr3 (305 mL, 3.25 mmol) was added dropwise at 0 °C. After
the solution had been stirred for 2 h at 0 °C, dried TBAB (2.42 g,
7.50 mmol) was added. The mixture was stirred for a further 3.5 h
at 0 °C and then for 14 h (overnight) at r.t. After the resulting color-
less solution had been diluted with Et2O (300 mL), it was succes-
sively washed with H2O (50 mL), half-sat. NaHCO3 (50 mL) and
brine (50 mL). The combined aqueous phases were re-extracted
with Et2O (2 × 80 mL) and the combined extracts were dried over
MgSO4, filtered and evaporated under reduced pressure. Purifica-
tion of the crude product by column chromatography (silica gel,
4 × 11 cm, CH2Cl2) yielded benzylic bromide 9 as a colorless,
foamy solid (4.85 g, 97%).
Lithium [4-(tert-Butyldiphenylsilanyloxymethyl)-2,3,5,6-tetra-
fluorophenyl]tris(pentafluorophenyl)borate (7)
In analogy to the synthesis of 6, TBDPS-ether 5 (3.89 g, 7.82 mmol)
was reacted with n-BuLi in hexanes (1.6 M, 4.89 mL, 7.82 mmol)
and B(C6F5)3 (4.00 g, 7.82 mmol) in anhyd Et2O (165 mL in total)
for 15 min at –50 °C then for 16 h (overnight) at r.t. A colorless,
foamy solid was obtained, which consisted of borate 7 and
Li[B(C6F5)4] in a ratio of 9:1 according to 19F NMR spectroscopy.
1H NMR (400 MHz, acetone-d6): d = 1.00 (s, 9 H, CH3), 4.79 (s,
2 H, CH2), 7.37–7.49 (m, 6 H, Ph-m-H and Ph-p-H), 7.70 (d, J = 7.3
Hz, 4 H, Ph-o-H).
19F{1H} NMR (375 MHz, acetone-d6): d = –167.7 (mc, 6 F, C6F5-m-
F), –164.0 (t, J = 20 Hz, 2 F, C6F5-p-F), –163.9 (t, J = 20 Hz, 1 F,
C6F5-p-F), –149.0 (dd, J = 23, 13 Hz, 2 F, Ar-m-F), –133.2 (mc, 2 F,
ArF-o-F), –132.3 (mc, 2 F, ArF-o-F), –131.9 (mc, 2 F, ArF-o-F),
–131.6 (mc, 2 F, ArF-o-F).
11B{1H} NMR (160 MHz, acetone-d6): d = –15.8.
MS (ESI): m/z (%) = 929 (100) [M – Li]–.
Mp 54–59 °C; Rf ≤ 0.71 (CH2Cl2, tailing).
IR (KBr): 2971, 2881, 1645, 1515, 1483, 1380, 1268, 1219, 1159,
1090, 979, 883, 766, 694, 665, 613, 549 cm–1.
1H NMR (400 MHz, acetone-d6): d = 0.98 (t, J = 7.4 Hz, 12 H,
CH3), 1.43 (sext, J = 7.4 Hz, 8 H, CH2CH3), 1.84 (mc, 8 H,
CH2CH2CH3), 3.46 (mc, 8 H, CH2CH2CH2CH3), 4.63 (s, 2 H,
CH2Br).
13C{1H} NMR (125 MHz, acetone-d6): d = 13.8 (CH3), 19.1
(CH2Br), 20.4 (t, J = 1 Hz, CH2CH3), 24.4 (CH2CH2CH3), 59.4 (t,
J = 3 Hz, CH2CH2CH2CH3), 113.4 (t, J = 17 Hz, Ar-p-C), 125.0 (br,
C6F5-i-C and Ar-i-C), 137.1 (dmc, J = 250 Hz, C6F5-m-C), 139.0
(dmc, J = 247 Hz, C6F5-p-C), 144.5 (dd, J = 247, 19 Hz, Ar-m-C),
149.0 (dmc, J = 238 Hz, C6F5-o-C and Ar-o-C).
19F{1H} NMR (375 MHz, acetone-d6): d = –167.8 to –167.5 (m, 6 F,
C6F5-m-F), –163.7 (t, J = 20 Hz, 2 F, C6F5-p-F), –163.5 (t, J = 20
Hz, 1 F, C6F5-p-F), –148.2 (dd, J = 22, 13 Hz, 2 F, Ar-m-F), –132.6
(mc, 2 F, ArF-o-F), –132.3 (mc, 2 F, ArF-o-F), –132.0 (mc, 2 F, ArF-
o-F), –131.8 (mc, 2 F, ArF-o-F).
Tetrabutylammonium (2,3,5,6-Tetrafluoro-4-hydroxymethyl-
phenyl)tris(pentafluorophenyl)borate (8)
To a solution of crude silyl ether 6 in anhyd THF (25 mL),
TBAF·3H2O (2.99 g, 9.47 mmol) was added at 0 °C. After the mix-
ture had been stirred for 17 h (overnight) at r.t., all volatiles were re-
moved under reduced pressure. The remaining yellow oil was re-
dissolved in H2O (60 mL) and Et2O (80 mL), the phases were sepa-
rated and the aqueous phase was extracted with Et2O (3 × 40 mL).
The combined organic extracts were washed with H2O (15 mL),
dried over MgSO4, filtered and concentrated under reduced pres-
sure. Purification of the resulting sticky foam by column chroma-
tography (silica gel, 7 × 14 cm, 1.4 L CH2Cl2 then CH2Cl2–MeOH,
11B{1H} NMR (160 MHz, acetone-d6): d = –15.8.
Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York

250
A. Franzke, A. Pfaltz
PAPER
MS (ESI): m/z (%) = 753 (100) [M – NBu4]–.
IR (KBr): 2956, 2933, 2891, 2860, 1624, 1587, 1455, 1390, 1266,
1104, 1061, 968, 896, 841, 779, 741, 703, 608 cm–1.
1H NMR (500 MHz, acetone-d6): d = 0.10 [s, 12 H, Si(CH3)2], 0.88
[s, 18 H, C(CH3)3], 4.77 (t, J = 1.4 Hz, 4 H, CH2).
Anal. Calcd for C41H38BBrF19N: C, 49.42; H, 3.84; N, 1.41. Found:
C, 49.55; H, 3.85; N, 1.48.
Potassium [4-(tert-Butyldimethylsilanyloxymethyl)-2,3,5,6-tet-
rafluorophenyl]trifluoroborate (10)
13C{1H} NMR (125 MHz, acetone-d6): d = –5.3 [Si(CH3)2], 18.8
[C(CH3)3], 26.1 [C(CH3)3], 53.9 (CH2), 116.1 (t, J = 18 Hz, Ar-p-
C), 145.1 (dd, J = 246, 19 Hz, Ar-m-C), 148.7 (dt, J = 238, 13 Hz,
Ar-o-C). Despite prolonged data acquisition time, the signal for Ar-
i-C was not detected.
To a solution of aryl bromide 4 (1.43 g, 3.82 mmol) in anhyd Et2O
(9 mL), i-PrMgCl in THF (2.0 M, 1.91 mL, 3.82 mmol) was added
dropwise within 9 min at r.t. The mixture was stirred for 2.5 h and
the resulting colorless suspension was diluted with more Et2O (10
mL) and cooled to –78 °C. B(Oi-Pr)3 (1.76 mL, 7.64 mmol) was
quickly added and the mixture was stirred for 22 h (overnight) at r.t.
To the viscous, colorless suspension, MeOH (100 mL) was added in
order to destroy any residual aryllithium reagent. Then all volatiles
were removed under reduced pressure. The remaining sticky solid
was suspended in Et2O (30 mL) in a polypropylene vessel and KHF2
(2.54 g, 32.5 mmol) in H2O (15 mL) was added dropwise at r.t. The
two-phase system was vigorously stirred for 60 min and then evap-
orated under reduced pressure. The solid residue was extracted with
acetone (5 × 10 mL), the combined extracts were filtered and the
solvent was removed under reduced pressure. The crude product
was suspended in CH2Cl2 (6 mL) and the mixture was stirred for
35 min at r.t. and filtered. The solid was washed with CH2Cl2
(2 × 2 mL) and dried by suction to yield a colorless powder, which
consisted of product 10 and the corresponding difluoroborate 11 in
a ratio of 10:1 according to 19F NMR spectroscopy (1.22 g, ~68%).
This material was sufficiently pure for use in the next step. Pure
aryltrifluoroborate 10 could be obtained by washing this mixture
several times with H2O-saturated Et2O,17b however, this procedure
resulted in substantial loss of material.
19F{1H} NMR (375 MHz, acetone-d6): d = –149.1 (dd, J = 24, 13
–
Hz, 4 F, Ar-m-F), –147.8 (br mc, 2 F, BF2 ), –136.7 (mc, 4 F, Ar-o-
F).
11B{1H} NMR (160 MHz, acetone-d6): d = 5.0 (br t, J = 60 Hz).
MS (ESI): m/z (%) = 635 (100) [M – K]–.
Tetrabutylammonium [4-(tert-Butyldimethylsilanyloxymeth-
yl)-2,3,5,6-tetrafluorophenyl]tris[3,5-bis(trifluoromethyl)phe-
nyl]borate (12)
To bromo-3,5-bis(trifluoromethyl)benzene (1.82 mL, 10.5 mmol)
in anhyd THF (9 mL), i-PrMgCl in THF (2.0 M, 4.86 mL, 9.72
mmol) was added dropwise over a period of 6 min at 0 °C. The mix-
ture was stirred for 60 min at 0 °C and then for 60 min at r.t. This
Grignard solution was added to a suspension of aryltrifluoroborate
10 (649 mg, 1.62 mmol) in anhyd Et2O (28 mL) and the resulting
solution was stirred for 135 h at r.t. The mixture was poured into a
solution of Na2CO3 (4.40 g) in H2O (55 mL) and the two-phase sys-
tem was vigorously stirred for 30 min at r.t. The phases were sepa-
rated and the aqueous phase was extracted with Et2O (3 × 50 mL).
The combined organic extracts were dried over Na2SO4, filtered,
evaporated under reduced pressure and the remaining oil was re-dis-
solved in CH2Cl2 (30 mL). TBAB (627 mg, 1.94 mmol) in CH2Cl2
(10 mL) was added and the suspension was stirred for 30 min at r.t.,
filtered and concentrated under reduced pressure. Purification of the
resulting oil by column chromatography (silica gel, 4 × 11 cm, 240
mL Et2O then CH2Cl2) yielded borate 12 as a colorless solid (1.73
g, 90%).
IR (KBr): 2954, 2934, 2890, 2859, 1654, 1620, 1456, 1393, 1281,
1080, 1037, 979, 906, 842, 809, 781, 719, 678, 638, 599 cm–1.
1H NMR (500 MHz, acetone-d6): d = 0.11 [s, 6 H, Si(CH3)2], 0.88
[s, 9 H, C(CH3)3], 4.78 (t, J = 1.4 Hz, 2 H, CH2).
13C{1H} NMR (125 MHz, acetone-d6): d = –5.3 [Si(CH3)2], 18.8
[C(CH3)3], 26.1 [C(CH3)3], 53.9 (CH2), 116.2 (t, J = 18 Hz, Ar-p-
C), 145.1 (dmc, J = 243 Hz, Ar-m-C), 148.9 (dmc, J = 244 Hz, Ar-
o-C). Despite prolonged data acquisition time, the signal for Ar-i-C
was not detected.
Mp 89.5–91 °C; Rf ≤ 0.75 (CH2Cl2, tailing).
IR (KBr): 2968, 2885, 1612, 1440, 1359, 1281, 1128, 1037, 941,
888, 842, 785, 728, 681, 624, 586, 448 cm–1.
19F{1H} NMR (375 MHz, acetone-d6): d = –149.0 (dd, J = 24, 15
1H NMR (500 MHz, CDCl3): d = 0.09 [s, 6 H, Si(CH3)2], 0.87 (t,
J = 7.3 Hz, 12 H, CH2CH3), 0.89 [s, 9 H, C(CH3)3], 1.22 (sext,
J = 7.4 Hz, 8 H, CH2CH3), 1.45 (mc, 8 H, CH2CH2CH3), 2.90 (mc,
8 H, CH2CH2CH2CH3), 4.75 (s, 2 H, CH2O), 7.49 (s, 3 H, ArF-p-H),
7.77 (s, 6 H, ArF-o-H).
Hz, 2 F, Ar-m-F), –135.9 (mc, 2 F, Ar-o-F), –134.9 (br q, J = 41 Hz,
–
3 F, BF3 ).
11B{1H} NMR (160 MHz, acetone-d6): d = 2.5 (q, J = 43 Hz).
MS (ESI): m/z (%) = 361 (100) [M – K]–.
13C{1H} NMR (125 MHz, CDCl3): d = –5.4 [Si(CH3)2], 13.2
(CH2CH3), 18.6 [C(CH3)3], 19.5 (CH2CH3), 23.6 (CH2CH2CH3),
25.9 [C(CH3)3], 53.7 (CH2O), 58.9 (t, J = 3 Hz, CH2CH2CH2CH3),
114.8 (t, J = 18 Hz, Ar-p-C), 117.3 (sept, J = 4 Hz, ArF-p-CH),
124.7 (q, J = 272 Hz, CF3), 128.8 (q, J = 31 Hz, ArF-m-C), 134.1
(ArF-o-CH), 145.1 (dd, J = 246, 17 Hz, Ar-m-C), 148.2 (dt, J = 238,
12 Hz, Ar-o-C), 160.8 (br mc, ArF-i-C). Despite prolonged data ac-
quisition time, the signal for Ar-i-C was not detected.
19F{1H} NMR (375 MHz, CDCl3): d = –147.6 (dd, J = 25, 14 Hz,
2 F, Ar-m-F), –127.2 (dd, J = 24, 14 Hz, 2 F, Ar-o-F), –62.5 (s,
18 F, CF3).
11B{1H} NMR (160 MHz, CDCl3): d = –8.5.
MS (ESI): m/z (%) = 943 (100) [M – NBu4]–.
Potassium Bis[4-(tert-butyldimethylsilanyloxymethyl)-2,3,5,6-
tetrafluorophenyl]difluoroborate (11)
To a solution of aryl bromide 4 (1.34 g, 3.58 mmol) in anhyd Et2O
(15 mL), n-BuLi in hexanes (1.6 M, 2.24 mL, 3.58 mmol) was add-
ed dropwise over a period of 10 min at –78 °C. After the mixture
had been stirred for 90 min at this temperature, B(Oi-Pr)3 (620 mL,
2.69 mmol) was added within 5 min to the colorless suspension.
The mixture was stirred for a further 90 min at –78 °C and then for
2 h at r.t. before being transferred to a polypropylene vessel and
KHF2 (1.12 g, 14.3 mmol) in H2O (7 mL) was slowly added. The
two-phase system was vigorously stirred for 80 min at r.t. and then
all volatiles were removed under reduced pressure. The remaining
solid was extracted with acetone (5 × 10 mL) and the combined ex-
tracts were filtered and evaporated under reduced pressure. The
crude product was suspended in CH2Cl2 (5 mL), the mixture was
stirred for 30 min at r.t. and then filtered. The solid was washed with
CH2Cl2 (2 × 2 mL) and dried by suction to yield the diaryldifluoro-
borate 11 as a colorless powder (656 mg, 54%).
Anal. Calcd for C53H62BF22NOSi: C, 53.68; H, 5.27; N, 1.18.
Found: C, 53.83; H, 5.23; N, 1.22.
Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York

PAPER
Functionalized Borate Building Blocks
251
Tetrabutylammonium Bis[4-(tert-butyldimethylsilanyloxy-
methyl)-2,3,5,6-tetrafluorophenyl]bis[3,5-bis(trifluoro-
methyl)phenyl]borate (13)
11B{1H} NMR (160 MHz, CDCl3): d = 8.4.
MS (ESI): m/z (%) = 829 (100) [M – NBu4]–.
Anal. Calcd for C47H48BF22NO: C, 52.68; H, 4.51; N, 1.31. Found:
C, 52.73; H, 4.42; N, 1.37.
In analogy to the synthesis of 12, bromo-3,5-bis(trifluorometh-
yl)benzene (792 mL, 4.59 mmol) was reacted with i-PrMgCl in THF
(2.0 M, 2.04 mL, 4.08 mmol) and diaryldifluoroborate 11 (689 mg,
1.02 mmol) in anhyd THF (4 mL) and anhyd Et2O (12 mL) for 135
h at r.t. After aqueous workup as described above, the magnesium
borate was treated with TBAB (395 mg, 1.22 mmol) in CH2Cl2
(25 mL) for 30 min at r.t. Purification of the yellow crude product
by column chromatography (silica gel, 3 × 12 cm, 140 mL Et2O
then CH2Cl2) furnished borate 13 as a colorless, sticky foam (1.16
g, 90%).
Tetrabutylammonium Bis(2,3,5,6-tetrafluoro-4-hydroxymeth-
ylphenyl)bis[3,5-bis(trifluoromethyl)phenyl]borate (15)
In analogy to the synthesis of 8, silyl ether 13 (1.11 g, 0.878 mmol)
was treated with TBAF·3H2O (1.11 g, 3.51 mmol) in anhyd THF
(10 mL) for 18 h (overnight) at r.t. Purification of the yellowish
crude product by column chromatography (silica gel, 3 × 12 cm,
300 mL CH2Cl2 then CH2Cl2–MeOH, 10:1) yielded diol 15 as a col-
orless, foamy solid (787 mg, 86%).
Rf ≤ 0.74 (CH2Cl2, tailing).
Mp 67–70 °C; Rf = 0.33 (CH2Cl2–MeOH, 10:1).
IR (NaCl): 2962, 2885, 1612, 1443, 1359, 1277, 1128, 1038, 937,
886, 841, 778, 735, 679 cm–1.
IR (KBr): 3627, 3386, 2971, 2883, 1612, 1447, 1361, 1279, 1128,
1005, 934, 886, 839, 722, 679, 628 cm–1.
1H NMR (500 MHz, CDCl3): d = 0.88 (t, J = 7.3 Hz, 12 H, CH3),
1.25 (sext, J = 7.3 Hz, 8 H, CH2CH3), 1.48 (mc, 8 H, CH2CH2CH3),
1.92 (br s, 2 H, OH), 2.95 (mc, 8 H, CH2CH2CH2CH3), 4.68 (s, 4 H,
CH2OH), 7.48 (s, 2 H, ArF-p-H), 7.91 (s, 4 H, ArF-o-H).
1H NMR (500 MHz, CDCl3): d = 0.07 [s, 12 H, Si(CH3)2], 0.88 [s,
18 H, C(CH3)3], 0.88 (t, J = 7.3 Hz, 12 H, CH2CH3), 1.24 (sext,
J = 7.4 Hz, 8 H, CH2CH3), 1.47 (mc, 8 H, CH2CH2CH3), 2.93 (mc,
8 H, CH2CH2CH2CH3), 4.70 (s, 4 H, CH2O), 7.47 (s, 2 H, ArF-p-H),
7.88 (s, 4 H, ArF-o-H).
13C{1H} NMR (125 MHz, CDCl3): d = 13.3 (CH3), 19.5 (CH2CH3),
23.7 (CH2CH2CH3), 53.2 (CH2OH), 58.9 (CH2CH2CH2CH3), 113.5
(t, J = 18 Hz, Ar-p-C), 117.5 (sept, J = 4 Hz, ArF-p-CH), 124.7 (q,
J = 272 Hz, CF3), 128.7 (q, J = 31 Hz, ArF-m-C), 133.2 (ArF-o-CH),
134.3 (br, Ar-i-C), 144.5 (dd, J = 246, 19 Hz, Ar-m-C), 147.6 (dt,
J = 237, 14 Hz, Ar-o-C), 159.0 (br, ArF-i-C).
19F{1H} NMR (375 MHz, CDCl3): d = –149.0 (dd, J = 24, 13 Hz,
4 F, Ar-m-F), –129.7 (dd, J = 25, 13 Hz, 4 F, Ar-o-F), –62.4 (s,
12 F, CF3).
13C{1H} NMR (125 MHz, CDCl3): d = –5.4 [Si(CH3)2], 13.3
(CH2CH3), 18.6 [C(CH3)3], 19.6 (CH2CH3), 23.7 (CH2CH2CH3),
26.0 [C(CH3)3], 53.7 (CH2O), 58.9 (CH2CH2CH2CH3), 113.9 (t,
J = 18 Hz, Ar-p-C), 117.3 (sept, J = 4 Hz, ArF-p-CH), 124.8 (q,
J = 273 Hz, CF3), 128.6 (q, J = 31 Hz, ArF-m-C), 133.3 (ArF-o-CH),
144.7 (dd, J = 245, 17 Hz, Ar-m-C), 147.7 (dt, J = 237, 12 Hz, Ar-
o-C), 159.3 (br, ArF-i-C). Despite prolonged data acquisition time
the signal for Ar-i-C was not detected.
19F{1H} NMR (375 MHz, CDCl3): d = –148.3 (dd, J = 24, 13 Hz,
4 F, Ar-m-F), –130.0 (dd, J = 24, 13 Hz, 4 F, Ar-o-F), –62.3 (s,
12 F, CF3).
11B{1H} NMR (160 MHz, CDCl3): d = –10.6.
MS (ESI): m/z (%) = 1023 (100) [M – NBu4]–.
11B{1H} NMR (160 MHz, CDCl3): d = –10.6.
MS (ESI): m/z (%) = 795 (100) [M – NBu4]–.
Anal. Calcd for C46H48BF20NO2: C, 53.25; H, 4.66; N, 1.35. Found:
C, 53.13; H, 4.62; N, 1.36.
Tetrabutylammonium (2,3,5,6-Tetrafluoro-4-hydroxymethyl-
phenyl)tris[3,5-bis(trifluoromethyl)phenyl]borate (14)
In analogy to the synthesis of 8, silyl ether 12 (1.56 g, 1.31 mmol)
was treated with TBAF·3H2O (830 mg, 2.63 mmol) in anhyd THF
(15 mL) for 14 h (overnight) at r.t. Purification of the yellowish
crude product by column chromatography (silica gel, 3 × 8 cm, 200
mL Et2O then CH2Cl2) yielded benzylic alcohol 14 as a colorless,
sticky foam, which gradually solidified to a vitreous solid (1.26 g,
90%).
Tetrabutylammonium (4-Bromomethyl-2,3,5,6-tetrafluorophe-
nyl)tris[3,5-bis(trifluoromethyl)phenyl]borate (16)
In analogy to the synthesis of 9, borate 14 (6.13 g, 5.72 mmol) was
treated with PBr3 (403 mL, 4.29 mmol) and subsequently TBAB
(2.40 g, 7.44 mmol) in anhyd CH2Cl2 (50 mL) at 0 °C for 3 h in total
and then for 13 h (overnight) at r.t. Purification of the crude product
by column chromatography (silica gel, 5 × 14 cm, CH2Cl2) fur-
nished benzylic bromide 16 as a colorless, vitreous solid (5.97 g,
92%).
Mp 120–121 °C; Rf = 0.24 (CH2Cl2).
Mp 89–90 °C; Rf ≤ 0.73 (CH2Cl2, tailing).
IR (KBr): 3636, 3421, 2973, 2883, 1780, 1612, 1444, 1359, 1281,
1128, 1004, 940, 887, 839, 799, 714, 681, 657, 620, 585, 450 cm–1.
IR (KBr): 2973, 2881, 1782, 1612, 1446, 1359, 1281, 1161, 1127,
970, 889, 839, 733, 713, 681, 656, 610 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.90 (t, J = 7.3 Hz, 12 H, CH3),
1.26 (sext, J = 7.4 Hz, 8 H, CH2CH3), 1.48 (mc, 8 H, CH2CH2CH3),
1.88 (br s, 1 H, OH), 2.93 (mc, 8 H, CH2CH2CH2CH3), 4.75 (s, 2 H,
CH2OH), 7.49 (s, 3 H, ArF-p-H), 7.79 (s, 6 H, ArF-o-H).
1H NMR (500 MHz, CDCl3): d = 0.88 (t, J = 7.3 Hz, 12 H, CH3),
1.25 (sext, J = 7.4 Hz, 8 H, CH2CH3), 1.48 (mc, 8 H, CH2CH2CH3),
2.94 (mc, 8 H, CH2CH2CH2CH3), 4.52 (s, 2 H, CH2Br), 7.49 (s, 3 H,
ArF-p-H), 7.78 (s, 6 H, ArF-o-H).
13C{1H} NMR (125 MHz, CDCl3): d = 13.2 (CH3), 19.5 (CH2CH3),
23.6 (CH2CH2CH3), 53.2 (CH2OH), 58.9 (t, J = 3 Hz,
CH2CH2CH2CH3), 114.4 (t, J = 18 Hz, Ar-p-C), 117.3 (sept, J = 4
Hz, ArF-p-CH), 124.7 (q, J = 272 Hz, CF3), 128.8 (q, J = 31 Hz,
ArF-m-C), 134.0 (ArF-o-CH), 145.0 (dd, J = 247, 18 Hz, Ar-m-C),
148.2 (dt, J = 239, 13 Hz, Ar-o-C), 160.7 (br mc, ArF-i-C). Despite
prolonged data acquisition time, the signal for Ar-i-C was not de-
tected.
13C{1H} NMR (125 MHz, CDCl3): d = 13.2 (CH3), 18.5 (CH2Br),
19.6 (CH2CH3), 23.7 (CH2CH2CH3), 58.9 (t, J = 3 Hz,
CH2CH2CH2CH3), 112.2 (t, J = 17 Hz, Ar-p-C), 117.3 (sept, J = 4
Hz, ArF-p-CH), 124.5 (q, J = 272 Hz, CF3), 128.7 (q, J = 31 Hz,
ArF-m-C), 133.9 (ArF-o-CH), 135.5 (br, Ar-i-C), 144.7 (dd, J = 249,
19 Hz, Ar-m-C), 148.2 (dt, J = 238, 12 Hz, Ar-o-C), 160.5 (br mc,
ArF-i-C).
19F{1H} NMR (375 MHz, CDCl3): d = –146.1 (dd, J = 23, 13 Hz,
2 F, Ar-m-F), –126.2 (dd, J = 23, 13 Hz, 2 F, Ar-o-F), –62.5 (s,
18 F, CF3).
19F{1H} NMR (375 MHz, CDCl3): d = –148.3 (dd, J = 24, 14 Hz,
2 F, Ar-m-F), –126.7 (dd, J = 24, 14 Hz, 2 F, Ar-o-F), –62.5 (s,
18 F, CF3).
11B{1H} NMR (160 MHz, CDCl3): d = –8.4.
Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York

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A. Franzke, A. Pfaltz
PAPER
MS (ESI): m/z (%) = 891 (100) [M – NBu4]–.
(4) (a) Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem. Int.
Ed. 1998, 37, 2897. For reviews, see: (b) Roseblade, S. J.;
Pfaltz, A. Acc. Chem. Res., in press. (c) Källström, K.;
Munslow, I.; Andersson, P. G. Chem. Eur. J. 2006, 12,
3194. (d) Cui, X.; Burgess, K. Chem. Rev. 2005, 105, 3272.
(e) Pfaltz, A.; Blankenstein, J.; Hilgraf, R.; Hörmann, E.;
McIntyre, S.; Menges, F.; Schönleber, M.; Smidt, S. P.;
Wüstenberg, B.; Zimmermann, N. Adv. Synth. Catal. 2003,
345, 33.
(5) Kündig, E. P.; Saudan, C. M.; Bernardinelli, G. Angew.
Chem. Int. Ed. 1999, 38, 1219.
(6) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2,
245.
(7) Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.;
Kobayashi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2600.
(8) (a) van den Broeke, J.; de Wolf, E.; Deelman, B.-J.; van
Koten, G. Adv. Synth. Catal. 2003, 345, 625. (b) van den
Broeke, J.; Deelman, B.-J.; van Koten, G. Tetrahedron Lett.
2001, 42, 8085. (c) van den Broeke, J.; Lutz, M.; Kooijman,
H.; Spek, A. L.; Deelman, B.-J.; van Koten, G.
Organometallics 2001, 20, 2114. (d) Jia, L.; Yang, X.;
Ishihara, A.; Marks, T. J. Organometallics 1995, 14, 3135.
(9) (a) For R = Me, see: Carpentier, J.-F.; Wu, Z.; Lee, C. W.;
Strömberg, S.; Christopher, J. N.; Jordan, R. F. J. Am. Chem.
Soc. 2000, 122, 7750. (b) For R = n-Bu, see: Döring, S.;
Erker, G.; Fröhlich, R. J. Organomet. Chem. 2002, 643-644,
61. (c) For R = Me, Et, i-Pr and CH2C(CH3)3, see: Bavarian,
N.; Baird, M. C. Organometallics 2005, 24, 2889.
(10) Hewavitharanage, P.; Danilov, E. O.; Neckers, D. C. J. Org.
Chem. 2005, 70, 10653.
Anal. Calcd for C47H47BBrF22N: C, 49.76; H, 4.18; N, 1.23. Found:
C, 49.63; H, 4.09; N, 1.34.
Tetrabutylammonium Bis(4-bromomethyl-2,3,5,6-tetrafluoro-
phenyl)bis[3,5-bis(trifluoromethyl)phenyl]borate (17)
In analogy to the synthesis of 9, diol 15 (2.21 g, 2.13 mmol) was
treated with PBr3 (260 mL, 2.77 mmol) and subsequently TBAB
(1.71 g, 5.33 mmol) in anhyd CH2Cl2 (25 mL) at 0 °C for 3.5 h in
total and then for 15 h (overnight) at r.t. Purification of the yellow-
ish crude product by column chromatography (silica gel, 4 × 10 cm,
CH2Cl2) furnished dibromide 17 as a colorless, foamy solid (2.29 g,
92%).
Mp 57–62 °C; Rf ≤ 0.72 (CH2Cl2, tailing).
IR (KBr): 2971, 2881, 1645, 1612, 1576, 1448, 1360, 1277, 1127,
971, 888, 840, 725, 680, 610, 544 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.90 (t, J = 7.3 Hz, 12 H, CH3),
1.28 (sext, J = 7.4 Hz, 8 H, CH2CH3), 1.51 (mc, 8 H, CH2CH2CH3),
2.98 (mc, 8 H, CH2CH2CH2CH3), 4.49 (s, 4 H, CH2Br), 7.50 (s, 2 H,
ArF-p-H), 7.89 (s, 4 H, ArF-o-H).
13C{1H} NMR (125 MHz, CDCl3): d = 13.3 (CH3), 18.8 (CH2Br),
19.6 (CH2CH3), 23.7 (CH2CH2CH3), 59.1 (t, J = 2 Hz,
CH2CH2CH2CH3), 111.5 (t, J = 17 Hz, Ar-p-C), 117.7 (sept, J = 4
Hz, ArF-p-CH), 124.7 (q, J = 272 Hz, CF3), 128.8 (q, J = 31 Hz,
ArF-m-C), 133.3 (ArF-o-CH), 135.4 (br, Ar-i-C), 144.2 (dd, J = 249,
19 Hz, Ar-m-C), 147.7 (dt, J = 238, 12 Hz, Ar-o-C), 158.5 (br, ArF-
i-C).
19F{1H} NMR (375 MHz, CDCl3): d = –146.7 (dd, J = 22, 11 Hz,
4 F, Ar-m-F), –129.3 (dd, J = 24, 13 Hz, 4 F, Ar-o-F), –62.4 (s,
12 F, CF3).
11B{1H} NMR (160 MHz, CDCl3): d = –10.5.
MS (ESI): m/z (%) = 921 (100) [M – NBu4]–.
(11) (a) Smidt, S. P.; Zimmermann, N.; Studer, M.; Pfaltz, A.
Chem. Eur. J. 2004, 10, 4685. (b) Blackmond, D. G.;
Lightfoot, A.; Pfaltz, A.; Rosner, T.; Schnider, P.;
Zimmermann, N. Chirality 2000, 12, 442.
(12) Robson, M. J.; Williams, J. Brit. UK Pat. Appl. GB 2171994,
1986; Chem. Abstr. 1987, 107, 2700.
(13) Wang, C.; Erker, G.; Kehr, G.; Wedeking, K.; Froehlich, R.
Organometallics 2005, 24, 4760.
Anal. Calcd for C46H46BBr2F20N: C, 47.49; H, 3.98; N, 1.20. Found:
C, 47.46; H, 4.00; N, 1.27.
(14) For the DFT-optimized [BP86/SV(P)] molecular structure of
[B(C6F6)4]–, see the supporting information of: Krossing, I.;
Raabe, I. Chem. Eur. J. 2004, 10, 5017.
Acknowledgment
(15) (a) B(ArF)3 was only isolated as a pyrolysis product of a
platinum complex, see: Konze, W. V.; Scott, B. L.; Kubas,
G. J. Chem. Commun. 1999, 1807. (b) For the pyridine
adduct B(ArF)3·py, see: Bourke, S. C.; MacLachlan, M. J.;
Lough, A. J.; Manners, I. Chem. Eur. J. 2005, 11, 1989.
(16) For reviews, see: (a) Molander, G. A.; Ellis, N. Acc. Chem.
Res. 2007, 40, 275. (b) Darses, S.; Genet, J.-P. Eur. J. Org.
Chem. 2003, 4313.
A.F. thanks the “Fonds der Chemischen Industrie” (Frankfurt) and
the German Federal Ministry for Science and Technology (BMBF)
for a Kekulé Fellowship. Financial support from the Swiss National
Science Foundation is gratefully acknowledged. We also thank Jür-
gen Rotzler for his help in the synthesis of 15.
References
(17) (a) Ito, T.; Iwai, T.; Mizuno, T.; Ishino, Y. Synlett 2003,
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(19) For a review, see: Knochel, P.; Dohle, W.; Gommermann,
N.; Kneisel, F. F.; Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V.
A. Angew. Chem. Int. Ed. 2003, 42, 4302.
(1) Current address: Department of Chemistry, Imperial
College, London, SW7 2AZ, UK.
(2) For reviews, see: (a) Krossing, I.; Raabe, I. Angew. Chem.
Int. Ed. 2004, 43, 2066. (b) Strauss, S. H. Chem. Rev. 1993,
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(20) Purification of Laboratory Chemicals, 3rd ed.; Perrin, D.
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Synthesis 2008, No. 2, 245–252 © Thieme Stuttgart · New York