Concise synthesis of diborylxanthenes

Article abstract of DOI:10.1055/s-2008-1032207

A simple and efficient synthesis of a series of bidentate diborylxanthene derivatives is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1032207

PAPER
859
Concise Synthesis of Diborylxanthenes
Synthesis of
D
ib
a
orylxanthe
k
nes eshi Makino, Ryu Yamasaki, Shinichi Saito*
Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
Fax +81(3)52614631; E-mail: ssaito@rs.kagu.tus.ac.jp
Received 23 October 2007; revised 19 December 2007
trimethylsilane with dilithiated xanthene. Subsequently,
diborylxanthene was synthesized by the ipso-
Abstract: A simple and efficient synthesis of a series of bidentate
diborylxanthene derivatives is described.
6
substitution9 of 5 with boron tribromide and the hydroly-
sis of 6 gave the boronic acid 1a. The pure product was
isolated through these simple procedures in much higher
yield (96%) than those obtained through the reported pro-
cedure (50% yield).7 The reaction of catechol with 6 was
also carried out to afford 1b (Scheme 1).
Key words: boron, cross-coupling, Lewis acids, ipso-substitution,
xanthene
Bidentate organoboron Lewis acids1 have been widely in-
vestigated and utilized as complexing agents for various
anions and bases. Diborylethanes2 and diborylnaph-
thalenes3 bind with small anions such as fluoride, alk-
oxides and hydride. Bidentate organoboranes with
dibenzofuran4 and an anthracene spacer5 have also been
prepared. On the other hand, few bidentate boronic acids
with a xanthene framework have been reported in the lit-
erature.6,7 In this paper, we report the concise synthesis of
a series of bidentate organoboron compounds with a xan-
thene backbone as the spacer (Figure 1).
SiMe3
SiMe3
1) n-BuLi
TMEDA
O
O
BBr3
2) TMSCl
4
5
(86%)
BBr2
BBr2
BR2
BR2
H2O for 1a
or
O
O
catechol for 1b
The results of the synthesis of 1a–c are summarized in
Scheme 1 and Scheme 2. We synthesized 1a in three steps
as shown in Scheme 1. By Ogino’s method,8 silylation
was accomplished in high yield by the reaction of chloro-
1a (96% in 2 steps)
1b (41% in 2 steps)
6
Scheme 1
BR2
BR2
1a R = OH
1b BR2
O
O
O
=
Gabbaï and co-workers reported the synthesis of 1c by the
reaction of 4 with n-BuLi-TMEDA and the subsequent
borylation of the lithiated compound with Mes2BF and
noted that the product was not isolated in pure form.4
Though we, too, were unable to isolate 1c by carrying out
a similar reaction, we succeeded in the synthesis of 1c by
the lithiation of 710 and the subsequent addition of
Mes2BF to the reaction mixture (Scheme 2). The result
suggests that the presence of TMEDA prevents the isola-
tion of 1c in pure form. Although compound 1b was not
stable toward moisture, the decomposition of 1a or 1c was
not observed after exposing the compounds to air for sev-
eral months.
B
1c R = Mes
BR2
BR2
2a R = OH
O
O
2b BR2
=
B
O
2c R = Mes
BR2
BR2
F
F
F
F
F F
BMes2
BMes2
I
I
3a R = OH
3b BR2
1) n-BuLi
2) Mes2BF
O
O
O
O
=
F F
O
B
3c R = Mes
7
1c (50%)
Scheme 2
Figure 1
We introduced spacers between the boron centers and the
xanthene framework which could modulate the steric and
electronic nature of the boron centers. First, phenyl spac-
SYNTHESIS 2008, No. 6, pp 0859–0864
Advanced online publication: 28.02.2008
DOI: 10.1055/s-2008-1032207; Art ID: F19107SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
8

860
T. Makino et al.
PAPER
F
F
F
F
ers were introduced, and the syntheses of 2a–c are sum-
marized in Scheme 3. Diboronic acid 1a reacted with 4-
bromotrimethylsilylbenzene in the presence of Pd(PPh3)4
to give 8.11 Ipso-substitution of 8 with boron tribromide
was carried out and the bis(dibromoboryl) derivative was
hydrolyzed to afford 2a. Compound 2b was synthesized
by a similar method. The reaction of 1a with 4-bromophe-
nyldimesitylborane (9) in the presence of Pd(PPh3)4 af-
forded 2c in moderate yield.
F F
1) n-BuLi
TMEDA
1) n-BuLi
Et2O
2)TMSC6F5
F F
O
4
3) TBAF
53%
2) Me3SnCl
81%
10
SnMe3
F F
SnMe3
BR2
BR2
SiMe3
SiMe3
F
F
F
F
F
F
F F
Br
SiMe3
1) BBr3
F F
O
F F
O
F
F
B(OH)2
B(OH)2
2) H2O for 3a
or
Pd(PPh3)4
Na2CO3
O
O
catechol fo 3b
3a (64%)
3b (42%)
11
8 (76%)
1a
Br
1) BBr3
Scheme 4
BMes2
2) H2O for 2a
or
catechol for 2b
9
BMes2
BMes2
Pd(PPh3)4
Na2CO3
F
F
F
F
F
F
F
F
F F
F F
BMes2
BMes2
BR2
BR2
1) n-BuLi
F F
O
F F
O
2) Mes2BF
66%
O
O
10
3c
Scheme 5
2a (83%)
2b (57%)
2c (69%)
N,N,N¢,N¢-Tetramethylethylenediamine (TMEDA) and chlorotri-
methylsilane (TMSCl) were distilled over CaH2 prior to use. 9,9-
Dimethylxanthene (4),19 4,5-diiodo-9,9-dimethylxanthene (7),10
1-bromo-4-trimethylsilylbenzene,20 and tetrakis(triphenylphos-
phine)palladium21 were prepared according to literature procedures.
Trimethylsilylpentafluorobenzene22 was prepared by the reaction of
pentafluorophenyllithium23 with TMSCl, and purified by distilla-
Scheme 3
Expecting the acidity of the boron centers to be enhanced
by the introduction of electron-withdrawing groups, fluor-
inated spacers were introduced. Compound 10 was isolat-
ed by the nucleophilic substitution of lithiated 4 with tion under reduced pressure from P4O10. Anhydrous Et2O, anhy-
trimethylsilylpentafluorobenzene,12,13 followed by desily-
drous hexane, anhydrous CH2Cl2 and anhydrous toluene were
purchased from Kanto Kagaku Co., Ltd. Other reagents were com-
lation, in good yield. Subsequently, stannylation was ac-
mercially available and used without further purification. NMR
complished by the lithiation of 10 followed by the
spectra were recorded on a Jeol JNM-EX270L FT NMR system
(270 MHz), a Jeol JNM-EX300L FT NMR system (300 MHz), a
Bruker AV400M (400 MHz), Jeol JNM-LA500 FT NMR system
addition of trimethyltin chloride.14 The stannyl compound
11 was reacted with BBr3,15,16 and the product was hydro-
lyzed to afford 3a (Scheme 4).17 Compound 3b was also
(500 MHz), or a Bruker AM600 (600 MHz). Chemical shifts are re-
synthesized in a similar manner.18
ported in delta units (d) relative to CDCl3 (7.24 ppm for 1H NMR
1
and 77.0 ppm for 13C NMR), benzene-d6 (7.15 ppm for H NMR,
Unlike other compounds, the mesitylated borane 3c was
synthesized directly from 10; the reaction of lithiated 10
with dimesitylboron fluoride proceeded smoothly and
compound 3c was isolated in 66% yield (Scheme 5).
1
128.0 ppm for 13C NMR), CD3OD (3.75 ppm for H NMR, 49.3
ppm for 13C NMR), C6F6 (–162.9 ppm for 19F NMR), or BF3·OEt2
(0 ppm for 11B NMR).24,25 IR spectra were recorded on a Jasco FT/
IR-410 (FT-IR). Elemental analyses were carried out on a Yanaco
CHN corder MT-6. High-resolution mass spectra were measured on
a Jeol SX102A (EI) or a Bruker Daltonics microTOF focus (ESI).
Column chromatography was performed with silica gel 60N (spher-
ical, neutral, 63–210 mm, purchased from Kanto Kagaku Co., Ltd).
In summary, we have developed a concise and efficient
method for the synthesis of diborylxanthenes. The acidity
and bulkiness of the boron centers could be modulated by
introducing various boryl groups to the xanthene frame- Gel permeation chromatography (GPC) was carried out on a Japan
Analytical Industry LC-928 Recycling Preparative HPLC,
work. The compounds are promising candidates for che-
equipped with JAIGEL-1H and JAIGEL-2H columns. Thin layer
lation of various anions and bases.
chromatography (TLC) was performed on Merck silica gel 60F-254
Synthesis 2008, No. 6, 859–864 © Thieme Stuttgart · New York

PAPER
Synthesis of Diborylxanthenes
861
plates. All manipulations were carried out under anhydrous and
anaerobic conditions unless otherwise noted.
4,5-Bis(dimesitylboryl)-9,9-dimethylxanthene (1c)
To a solution of diiodoxanthene 710 (0.46 g, 1.0 mmol) in anhyd
THF (3 mL) was added n-BuLi (1.5M in hexane, 1.3 mL, 2.0 mmol)
at 0 °C, and the mixture was stirred for 30 min. The solution was
cooled to –78 °C and dimesitylboron fluoride (0.54 g, 2.0 mmol) in
anhyd THF (5 mL) was added to the solution. The mixture was
slowly warmed to r.t. and stirred overnight. The solvent was re-
moved under reduced pressure and Et2O (30 mL) was added. The
solution was washed with H2O (30 mL) and dried over MgSO4. The
filtrate was concentrated to ~3 mL and the precipitate was collected
by filtration to afford 1c (0.17 g, 24%) as colorless crystals. The res-
idue was purified by column chromatography (silica gel; hexane–
CH2Cl2, 10:1) to afford 1c (0.18 g, 26%).
4,5-Bis(trimethylsilyl)-9,9-dimethylxanthene (5)
To a solution of 9,9-dimethylxanthene (4; 25 g, 120 mmol) and
TMEDA (36 mL, 240 mmol) in a mixture of anhyd Et2O (480 mL)
and anhyd hexane (360 mL) at r.t., was added n-BuLi (1.6 M in hex-
ane, 180 mL, 288 mmol) diluted with anhyd hexane (360 mL) over
10 min, and the mixture was heated to 40 °C for 3 h. The deep-red
solution was cooled to 0 °C and TMSCl (37 mL, 288 mmol) in an-
hyd hexane (240 mL) was added over 10 min. The mixture was
warmed to r.t. and a colorless precipitate was formed. After stirring
for 13 h, H2O (200 mL) was added and the organic layer was sepa-
rated. The aqueous layer was extracted with hexane (2 × 50 mL)
and the combined organic layer was dried over MgSO4 and concen-
trated. The residue was subjected to short column chromatography
(silica gel; hexane), and further purified by recrystallization from
EtOH to afford 5.
Colorless solid; mp 289.0–291.5 °C.
IR (KBr): 2965, 2915, 1604, 1445, 1396, 1223, 846 cm–1.
1H NMR (600 MHz, CDCl3): d = 1.62 (s, 24 H), 1.68 (s, 6 H), 2.23
(s, 12 H), 6.82 (dd, J = 7.2, 1.2 Hz, 2 H), 6.95 (t, J = 7.2 Hz, 2 H),
7.48 (dd, J = 7.2, 1.2 Hz, 2 H).
Yield: 36 g (86%); colorless crystals; mp 84.5–84.8 °C.
13C NMR (150 MHz, CDCl3): d = 21.3, 23.3 (br), 33.7, 34.2, 123.2,
128.0, 128.4 (br), 129.9, 133.6, 137.2, 138.9 (br), 141.4 (br), 153.8.
IR (KBr): 3058, 2963, 2899, 1605, 1567, 1385, 1247, 1216, 1194,
1145, 1124, 953, 841, 787, 758, 745, 682, 629, 484 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.40 (s, 18 H), 1.61 (s, 6 H), 7.08
(t, J = 7.2 Hz, 2 H), 7.36 (dd, J = 7.2, 1.8 Hz, 2 H), 7.46 (dd, J = 7.2,
1.8 Hz, 2 H).
Anal. Calcd for C51H56B2O: C, 86.69; H, 7.99. Found: C, 86.87; H,
8.06.
4,5-Bis(4-trimethylsilylphenyl)-9,9-dimethylxanthene (8)
A mixture of diboronic acid 1a (1.5 g, 5.0 mmol), 1-bromo-4-tri-
methysilylbenzene (2.1 mL, 11 mmol), tetrakis(triphenylphos-
phine)palladium (0.35 g, 0.30 mmol) and Na2CO3 (1.2 g, 11 mmol)
was dissolved into toluene (25 mL), EtOH (minimum volume to
dissolve diboronic acid 1a, ~5 mL) and H2O (5.5 mL). After reflux-
ing for 30 h, the organic layer was separated and the aqueous layer
was extracted with CH2Cl2 (3 × 10 mL). The combined organic so-
lution was dried over MgSO4 and concentrated. The residue was
subjected to short column chromatography (silica gel; hexane–
CH2Cl2, 20:1), and the product was recrystallized from hexane to af-
ford 8.
13C NMR (75 MHz, CDCl3): d = 0.9, 32.8, 34.1, 122.6, 126.3, 127.6,
129.6, 134.0, 155.6.
Anal. Calcd for C21H30OSi2: C, 71.12; H, 8.53. Found: C, 70.95; H,
8.35.
9,9-Dimethylxanthene-4,5-diboronic Acid (1a)
To a solution of 5 (3.5 g, 10 mmol) in anhyd CH2Cl2 (30 mL) was
added BBr3 (2.3 mL, 24 mmol), and the mixture was stirred at r.t.
overnight. To the resulting solution of 6 was added H2O (3 mL), and
a colorless precipitate was formed. The precipitate was collected by
filtration and washed with H2O (3 × 30 mL) and CH2Cl2 (10 mL).
The product was dried under reduced pressure to afford 1a (2.9 g,
Yield: 1.9 g (76%); colorless crystals; mp 170.0–170.2 °C.
IR (KBr): 2954, 2894, 1422, 1246, 875, 851, 825, 788, 749 cm–1.
1
96%) as a colorless solid. H NMR and 13C NMR were in good
agreement with the literature.7
1H NMR (600 MHz, CDCl3): d = 0.29 (s, 18 H), 1.69 (s, 6 H), 7.13
(t, J = 7.8 Hz, 2 H), 7.18 (dd, J = 7.8, 3.0 Hz, 2 H), 7.32 (d, J = 8.4
Hz, 4 H), 7.39 (d, J = 8.3 Hz, 4 H), 7.42 (dd, J = 7.8, 3.0 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = –0.8, 31.3, 34.7, 123.1, 124.5,
129.1, 129.8, 131.5, 132.9, 138.3, 138.4, 148.4.
11B NMR (96 MHz, DMSO-d6): d = 20.8.
4,5-Bis(1,3,2-dioxaborolyl)-9,9-dimethylxanthene (1b)
To a solution of 5 (3.5 g, 10 mmol) in anhyd CH2Cl2 (30 mL) at r.t.,
was added BBr3 (2.3 mL, 24 mmol). The solution turned yellow.
After stirring at r.t. overnight, all volatiles were removed under re-
duced pressure and the yellow-white residue was dissolved in anhyd
toluene (100 mL). Catechol (2.2 g, 20 mmol) was added and the
mixture was stirred under reflux for 4 h. The solvents were removed
under reduced pressure and the resulting residue was extracted with
hot hexane (100 mL). The filtrate was concentrated and the residue
was recrystallized from toluene–hexane to afford 1b.
Anal. Calcd for C33H38OSi2: C, 78.20; H, 7.56. Found: C, 78.31; H,
7.57.
4,5-Bis(4-dihydroxyborylphenyl)-9,9-dimethylxanthene (2a)
To a solution of 8 (253 mg, 0.50 mmol) in CH2Cl2 (3 mL) was added
BBr3 (0.12 mL, 1.2 mmol), and the mixture was stirred at r.t. over-
night. All volatiles were removed under reduced pressure then H2O
(1 mL) was added to the residue. After stirring the suspension for 1
h, the precipitate was collected by filtration and washed with
CH2Cl2 (2 × 5 mL) to afford 2a.
Yield: 1.8 g (41%); colorless crystals; mp 184–185 °C.
IR (KBr): 3530, 3336, 3059, 2979, 2954, 1622, 1473, 1415, 1396,
1370, 1336, 1239, 756, 736 cm–1.
1H NMR (300 MHz, C6D6): d = 1.36 (s, 6 H), 6.74–6.81 (m, 8 H),
6.94 (t, J = 7.4 Hz, 2 H), 7.23 (dd, J = 7.4, 1.7 Hz, 2 H), 7.83 (dd,
J = 7.4, 1.7 Hz, 2 H).
Yield: 187 mg (83%); colorless solid; mp >300 °C.
IR (KBr): 3245, 2962, 1714, 1430, 1367, 1244, 1203, 1102, 1017
cm–1.
13C NMR (75 MHz, C6D6): d = 31.7, 34.3, 112.9, 122.7, 123.4,
129.8, 130.4, 134.9, 149.2, 155.5.
1H NMR (600 MHz, CD3OD): d = 2.15 (s, 6 H), 7.60–7.66 (m, 6 H),
7.80 (br, 4 H), 7.95–7.96 (m, 4 H).
11B NMR (96 MHz, C6D6): d = 31.8.
HRMS-EI: m/z [M+] calcd for C27H20B2O5: 446.1497; found:
13C NMR (150 MHz, CD3OD): d = 32.1, 36.1, 124.6, 126.3, 129.2,
130.0, 130.3, 130.8, 131.7, 133.0, 134.8, 149.6.
11B NMR (96 MHz, CD3OD): d = 31.0.
446.1505.
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T. Makino et al.
PAPER
HRMS-ESI: m/z [M + Na]+ calcd for C27H24B2O5Na: 473.1711;
found: 473.1710.
1H NMR (600 MHz, CDCl3): d = 1.71 (s, 6 H), 1.87 (s, 24 H), 2.31
(s, 12 H), 6.76 (s, 8 H), 7.14 (t, J = 7.2 Hz, 2 H), 7.20 (dd, J = 7.2,
1.2 Hz, 2 H), 7.27 (d, J = 7.8 Hz, 4 H), 7.44–7.46 (m, 6 H).
4,5-Bis[4-(1,3,2-dioxaborolyl)phenyl]-9,9-dimethylxanthene
(2b)
13C NMR (150 MHz, CDCl3): d = 21.3, 23.7, 32.1, 34.5, 123.2,
125.2, 128.1, 129.2, 129.5, 130.7, 131.1, 135.8, 138.6, 141.1, 141.7,
141.8, 144.7, 148.0.
To a solution of 8 (0.50 g, 1.0 mmol) in CH2Cl2 (3 mL) was added
BBr3 (0.23 mL, 2.4 mmol) and the mixture was stirred at r.t. over-
night. A colorless precipitate formed. All volatiles were removed
under reduced pressure and the residue was suspended in toluene
(10 mL). Catechol (0.22 g, 2 mmol) was added to the suspension
and the mixture was stirred under reflux for 4 h. The mixture was
cooled to r.t. and the precipitate was collected by filtration. The pre-
cipitates were washed with toluene and hexane to afford 2b.
Anal. Calcd for C63H64B2O: C, 88.11; H, 7.51. Found: C, 87.82; H,
7.52.
4,5-Bis(2,3,5,6-tetrafluorophenyl)-9,9-dimethylxanthene (10)
To a solution of 9,9-dimethylxanthene (4; 3.2 g, 15 mmol) and
TMEDA (4.5 mL, 30 mmol) in a mixture of anhyd Et2O (60 mL)
and anhyd hexane (45 mL), was added n-BuLi (1.6 M in hexane, 23
mL, 36 mmol) diluted with anhyd hexane (45 mL) over 10 min. The
mixture was heated to 40 °C for 3 h, then the deep-red solution was
cooled to 0 °C and trimethylsilylpentafluorobenzene (7.3 mL, 38
mmol) in anhyd hexane (40 mL) was added. The mixture was
warmed to r.t. and stirred for 2 h then TBAF (1 M in THF, 76 mL,
76 mmol) was added and the mixture was stirred for 1 h. H2O (100
mL) was added and the organic layer was separated and washed
with aq HCl (2M, 100 mL), dried over MgSO4, filtered and evapo-
rated. The residue was purified by column chromatography (silica
gel; hexane–CH2Cl2, 20:1) to afford 10.
Yield: 0.34 g (57%); colorless wool-like solid; mp 269.5–270.0 °C.
IR (KBr): 1611, 1474, 1427, 1395, 1378, 1337, 1240, 728, 658
cm–1.
1H NMR (600 MHz, CDCl3): d = 1.73 (s, 6 H), 6.82–6.88 (m, 8 H),
7.16 (t, J = 7.8 Hz, 2 H), 7.21 (dd, J = 7.8, 1.2 Hz, 2 H), 7.33 (d,
J = 7.8 Hz, 4 H), 7.46 (dd, J = 7.8, 1.2 Hz, 2 H), 7.75 (d, J = 7.8 Hz,
4 H).
13C NMR (150 MHz, CDCl3): d = 31.4, 34.8, 112.1, 122.4, 123.2,
125.2, 128.6, 129.3, 129.9, 131.2, 134.5, 141.5, 147.9, 148.3.
11B NMR (96 MHz, CDCl3): d = 32.6.
HRMS-EI: m/z [M+] calcd for C39H28B2O5: 598.2123; found:
Yield: 4.0 g (53%); colorless solid; mp 149.5–151.0 °C.
IR (KBr): 3064, 1502, 1464, 1448, 1426, 1391, 1288, 1248, 1174,
1147, 952, 940, 910, 886, 845, 793, 750, 699 cm–1.
598.2122.
1H NMR (500 MHz, CDCl3): d = 1.71 (s, 6 H), 6.92–6.98 (m, 2 H),
7.13 (br d, J = 6.6 Hz, 2 H), 7.19 (t, J = 7.8 Hz, 2 H), 7.54 (dt,
J = 7.8, 1.2 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = 31.9, 34.6, 104.8 (t, JC–F = 21.8
Hz), 115.3, 117.6 (t, JC–F = 17.6 Hz), 123.2, 127.4, 129.5, 130.8,
142.8–143.0 (m), 144.5–144.8 (m), 146.3–146.4 (m), 147.7.
4-Bromophenyldimesitylborane (9)
To a solution of 1,4-dibromobenzene (0.94 g, 4.0 mmol) in anhyd
Et2O (10 mL) was added n-BuLi (1.6 M in hexane, 2.4 mL, 4 mmol)
at –78 °C. After stirring for 1 h at r.t., the solution was cooled to
–78 °C then dimesitylboron fluoride (1.07 g, 4.4 mmol) in anhyd
Et2O (5 mL) was added. The mixture was stirred for 17 h at r.t. then
the solvent was removed under reduced pressure. The residue was
extracted with CH2Cl2 (3 × 10 mL) and the organic layer was dried
over MgSO4. The filtrate was concentrated and the product was pu-
rified by column chromatography (silica gel; hexane–CH2Cl2, 30:1)
to afford 9.
19F NMR (470 MHz, C6D6): d = –141.0 (m, 2 × F), –139.5 (m, 2 ×
F).
Anal. Calcd for C27H14F8O: C, 64.04; H, 2.79; F, 30.01. Found: C,
64.05; H, 2.91; F, 30.04.
Yield: 619 mg (84%); colorless crystals; mp 180.0–182.5 °C.
IR(KBr): 2912, 1605, 1573, 1422, 1213, 1067, 1010, 848, 812 cm–1.
4,5-Bis(4-trimethylstannyl-2,3,5,6-tetrafluorophenyl)-9,9-di-
methylxanthene (11)
To a solution of 10 (1.5 g, 3.0 mmol) in anhyd Et2O (15 mL) was
added n-BuLi (1.6 M in hexane, 3.8 mL, 6 mmol) at –78 °C. After
stirring for 2 h at –78 °C, trimethyltin chloride (1.2 g, 6.0 mmol) in
anhyd Et2O (10 mL) was added. The mixture was slowly warmed to
r.t. and stirred overnight. The precipitate was removed by filtration
and the filtrate was concentrated by rotary evaporation. The residue
was purified by recrystallization from toluene–hexane to afford 11.
1H NMR (270 MHz, CDCl3): d = 1.96 (s, 12 H), 2.28 (s, 6 H), 6.80
(s, 4 H), 7.34 (d, J = 8.4 Hz, 2 H), 7.47 (d, J = 8.4 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = 21.2, 23.4, 127.4, 128.2, 131.3,
137.8, 138.9, 140.7.
11B NMR (96 MHz, CDCl3): d = 74.2.
Anal. Calcd for C24H26BBr: C, 71.14; H, 6.47. Found: C, 71.14; H,
6.25.
Yield: 2.0 g (81%); colorless crystals; mp 181.0–183.0 °C.
IR (KBr): 2982, 2958, 2919, 1418, 1358, 1292, 1281, 1121, 1087,
948, 892, 881, 781, 754, 700, 542, 515 cm–1.
1H NMR (600 MHz, CDCl3): d = 0.49 [s, 117Sn (7.7%) and 119Sn
(8.4%) satellites, 2J = 58.2 Hz, 18 H], 1.70 (s, 6 H), 7.10 (d, J = 7.2
Hz, 2 H), 7.17 (t, J = 7.2 Hz, 2 H), 7.53 (d, J = 7.2 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = –7.1 [117Sn (7.7%) and 119Sn
(8.4%) satellites, J = 378, 362 Hz], 31.7, 34.6, 116.2 [117Sn (7.7%)
and 119Sn (8.4%) satellites, J = 93.4 Hz], 118.0 (t, JC–F = 18.5 Hz),
123.0, 126.9, 130.3, 131.1, 143.1 (ddd, JC–F = 247.8, 17.6, 8.1 Hz),
147.7–147.9 (m), 148.3, 149.3–149.5 (m).
4,5-Bis[4-(dimesitylboryl)phenyl]-9,9-dimethylxanthene (2c)
A mixture of 1a (298 mg, 1.0 mmol), 9 (891 mg, 2.2 mmol),
Pd(PPh3)4 (69 mg, 0.06 mmol) and Na2CO3 (233 mg, 2.2 mmol) was
dissolved in toluene (5 mL), EtOH (minimum volume to dissolve
1a, ~1 mL) and H2O (1.1 mL). The solution was heated to reflux and
stirred for 28 h. H2O (10 mL) was added and the product was ex-
tracted with CH2Cl2 (3 × 10 mL). The organic layer was dried over
MgSO4 and the filtrate was concentrated. The product was purified
by recrystallization from toluene–hexane to afford 2c.
Yield: 593 mg (69%); colorless crystals; mp >300 °C.
IR (KBr): 2972, 1604, 1420, 1387, 1240, 1209 cm–1.
19F NMR (470 MHz, C6D6): d = –139.2 (m, 2 × F), –123.3 (m, 2 ×
F).
Anal. Calcd for C33H30F8OSn2: C, 47.64; H, 3.63. Found: C, 47.69;
H, 3.50.
Synthesis 2008, No. 6, 859–864 © Thieme Stuttgart · New York

PAPER
Synthesis of Diborylxanthenes
863
4,5-Bis(4-dihydroxyboryl-2,3,5,6-tetrafluorophenyl)-9,9-di-
methylxanthene (3a)
IR (KBr): 2962, 2920, 1606, 1450, 1419, 1306, 1244 cm–1.
1H NMR (600 MHz, CDCl3): d = 1.70 (s, 6 H), 1.99 (s, 24 H), 2.25
(br, 12 H), 6.73 (br, 8 H), 7.08 (br d, J = 7.2 Hz, 2 H), 7.16 (t,
J = 7.8 Hz, 2 H), 7.54 (dd, J = 7.8, 1.2 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = 21.3, 22.7, 32.8, 34.5, 115.5,
119.9 (t, JC–F = 18.7 Hz), 123.0, 124.2 (t, JC–F = 24.1 Hz), 127.6,
128.5, 130.8, 131.2, 140.1 (br), 141.0 (br), 141.7 (br), 143.8 (dd,
JC–F = 250.2, 16.4 Hz), 146.2 (dt, JC–F = 246.0, 12.1 Hz), 147.9.
19F NMR (376 MHz, CDCl3): d = –141.4 (dd, J = 23.3, 12.4 Hz,
2 × F), –132.1 (dd, J = 21.5, 12.0 Hz, 2 × F).
HRMS-ESI: m/z [M + Na]+ calcd for C63H56B2F8ONa: 1025.4301;
A mixture of 11 (416 mg, 0.50 mmol) and BBr3 (0.95 mL, 10 mmol)
was stirred for 11 h. All volatiles were removed under reduced pres-
sure and the organotin compounds were removed by sublimation
under reduced pressure. H2O (1 mL) was added to the residue and
the mixture was stirred for 1 h. The precipitate was collected by fil-
tration and washed with CH2Cl2 (2 × 5 mL) to afford 3a.
Yield: 190 mg (64%); colorless solid; mp >300 °C.
IR (KBr): 3388, 2970, 1652, 1472, 1424, 1346, 1252, 1040, 961
cm–1.
1H NMR (300 MHz, CD3OD): d = 1.79 (s, 6 H), 7.23 (br d, J = 6.9
Hz, 2 H), 7.31 (t, J = 7.8 Hz, 2 H), 7.74 (dd, J = 7.8, 1.8 Hz, 2 H).
found: 1025.4301.
13C NMR (150 MHz, CD3OD): d = 32.5, 36.0, 117.0, 119.7 (t,
JC–F = 17.6 Hz), 124.9, 131.1, 132.7, 143.4 (dd, JC–F = 247.7, 16.3
Hz), 147.5 (br), 149.1 (br), 149.3.
19F NMR (470 MHz, CD3OD): d = –140.4 (br s, 2 × F), –132.1 (br
s, 2 × F).
11B NMR (96 MHz, CD3OD): d = 28.1.
HRMS-ESI: m/z [M + Na]+ calcd for C27H16B2F8O6Na: 617.0958;
References
(1) Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.;
Wiley-VCH: Weinheim, 2000.
(2) Schriver, D. F.; Biallas, M. J. J. Am. Chem. Soc. 1967, 89,
1078.
(3) (a) Katz, H. E. J. Org. Chem. 1985, 50, 5027. (b) Katz, H.
E. J. Am. Chem. Soc. 1985, 107, 1420. (c) Katz, H. E.
Organometallics 1987, 6, 1134.
found: 617.0949.
(4) Wang, H.; Gabbaï, F. P. Organometallics 2005, 24, 2898.
(5) Katz, H. E. J. Org. Chem. 1989, 54, 2179.
(6) Okamura, R.; Wada, T.; Aikawa, K.; Nagata, T.; Tanaka, K.
Inorg. Chem. 2004, 43, 7210.
(7) Hirotsu, M.; Ohno, N.; Nakajima, T.; Ueno, K. Chem. Lett.
2005, 34, 848.
4,5-Bis[4-(1,3,2-benzodioxaborolyl)-2,3,5,6-tetrafluorophenyl]-
9,9-dimethylxanthene (3b)
Compound 11 (0.42 g, 0.50 mmol) was dissolved in BBr3 (0.47 mL,
5.0 mmol) and the mixture was stirred at r.t. overnight. All volatiles
were removed under reduced pressure and the residue was washed
with hexane (2 × 5 mL). The residue was dissolved in toluene (4
mL) and catechol (0.11 g, 1.0 mmol) was added. The mixture was
heated to reflux for 4 h then cooled to r.t. and the precipitate was
collected by filtration and washed with toluene (5 mL) and hexane
(2 × 5 mL) to afford 1c.
(8) Tobita, H.; Hasegawa, K.; Minglana, J. J. G.; Luh, L. S.;
Okazaki, M.; Ogino, H. Organometallics 1999, 18, 2058.
(9) (a) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 1772. (b) Qin, Y.; Cheng, G.;
Sandararaman, A.; Jäkle, F. J. Am. Chem. Soc. 2002, 124,
12672. (c) Zhao, Z.; Snieckus, V. Org. Lett. 2005, 7, 2523.
(10) McWilliams, K.; Kelly, J. W. J. Org. Chem. 1996, 61, 7408.
(11) Sharp, M. J.; Cheng, W.; Snieckus, V. Tetrahedron Lett.
1987, 28, 5093.
(12) (a) Fujita, M.; Obayashi, M.; Hiyama, T. Tetrahedron 1988,
44, 4135. (b) Ishihara, K.; Hasegawa, A.; Yamamoto, H.
Angew. Chem. Int. Ed. 2001, 40, 4077.
(13) The synthesis of a Brønsted acid with a similar framework
has been previously reported, see: Hasegawa, A.; Ishikawa,
T.; Ishihara, K.; Yamamoto, H. Bull. Chem. Soc. Jpn. 2005,
78, 1401.
Yield: 160 mg (42%); colorless wool-like solid; mp 242.5–
243.0 °C.
IR (KBr): 1469, 1428, 1399, 1362, 1330, 1279, 1239, 976, 966, 809,
737 cm–1.
1H NMR (600 MHz, C6D6): d = 1.28 (s, 6 H), 6.54–6.57 (m, 4 H),
6.64–6.66 (m, 4 H), 6.90 (t, J = 7.2 Hz, 2 H), 7.01 (d, J = 7.2 Hz,
2 H), 7.10 (d, J = 7.2 Hz, 2 H).
13C NMR (150 MHz, C6D6): d = 28.4, 31.6, 109.8, 112.6, 112.7,
118.2, 118.8 (t, JC–F = 17.6 Hz), 120.2, 120.7, 126.6, 128.8, 141.4
(dd, JC–F = 243.5, 14.3 Hz), 145.03, 146.2–146.4 (m), 147.9–148.1
(m).
(14) (a) Frohn, H.-J.; Lewin, A.; Bardin, V. V. J. Organomet.
Chem. 1998, 570, 255. (b) Bardin, V. V.; Pressman, L. S.;
Furin, G. G. J. Fluorine Chem. 1991, 53, 213. (c) Fild, M.;
Glemser, O.; Christoph, B. Angew. Chem., Int. Ed. Engl.
1964, 3, 801.
19F NMR (470 MHz, C6D6): d = –140.4 (dd, J = 23.0, 14.1 Hz, 2 ×
F), –121.3 (dd, J = 21.6, 15.0 Hz, 2 × F).
11B NMR (96 MHz, C6D6): d = 30.8.
(15) Since the silicon–boron exchange reaction did not take place
on silylpolyfluoroarenes, the use of the stannylpolyfluoro-
arene was essential. See: Frohn, H.-J.; Franke, H.; Fritzen,
P.; Bardin, V. V. J. Organomet. Chem. 2000, 598, 127.
(16) We chose the trimethylstannyl group instead of the more
common tributylstannyl group as the substituent, since the
removal of the organotin derivatives, which were formed by
the borylation reaction, could be easily achieved by
sublimation. See: Britovsek, G. J. P.; Ugolotti, J.; White, A.
J. P. Organometallics 2005, 24, 1685.
(17) The boronic acid 3a was also prepared in 35% yield by the
lithiation (n-BuLi) of 10, the reaction of the aryllithium with
B(OMe)3 and the hydrolysis of the aryltrimethoxyborate.
See: Frohn, H.; Adnin, N. Y.; Bardin, V. V.; Starichenko, V.
F. Z. Anorg. Allg. Chem. 2002, 628, 2827; the solubility of
3a, however, was very low in common organic solvents and
HRMS-EI: m/z [M+] calcd for C39H20B2F8O5: 742.1369; found:
742.1368.
4,5-Bis(4-dimesitylboryl-2,3,5,6-tetrafluorophenyl)-9,9-dimeth-
ylxanthene (3c)
To a solution of 10 (506 mg, 1.0 mmol) in anhyd Et2O (10 mL), was
added n-BuLi (1.6 M in hexane, 1.3 mL, 2 mmol) at –78 °C and the
mixture was stirred for 2 h. Mes2BF (563 mg, 2.1 mmol) in anhyd
Et2O (10 mL) was added to the solution at –78 °C. The mixture was
warmed to r.t. and stirred for 19 h. H2O (30 mL) was added and the
product was extracted with Et2O (2 × 10 mL). The organic layer
was dried over MgSO4, filtered and concentrated. The product was
purified by column chromatography (silica gel; hexane–EtOAc,
50:1) to afford 3c.
Yield: 663 mg (66%); colorless amorphous solid; mp 132.0–
133.0 °C.
Synthesis 2008, No. 6, 859–864 © Thieme Stuttgart · New York

864
T. Makino et al.
PAPER
(22) (a) Frohn, H.-J.; Lewin, A.; Bardin, V. V. J. Organomet.
it was necessary to use a large amount of CH2Cl2 (~900 mL
for 1 mmol scale) to extract 3a from the reaction mixture.
Chem. 1998, 570, 255. (b) Bardin, V. V.; Pressman, L. S.;
Furin, G. G. J. Fluorine Chem. 1991, 53, 213. (c) Fild, M.;
Glemser, O.; Christoph, B. Angew. Chem., Int. Ed. Engl.
1964, 3, 801.
(18) The reaction of lithiated 10 with B-bromocatecholborane
gave 3b in low yield (10%).
(19) Nowick, J. S.; Ballester, P.; Ebmeyer, F.; Rebek, J. Jr. J. Am.
Chem. Soc. 1990, 112, 8902.
(23) Fearon, F. W. G.; Gilman, H. J. Organomet. Chem. 1967, 10,
(20) Itami, K.; Terakawa, K.; Yoshida, J.; Kajimoto, O. J. Am.
Chem. Soc. 2003, 125, 6058.
(21) Coulson, D. R. Inorg. Synth. 1972, 13, 121.
409.
(24) The 11B NMR chemical shifts of 1c, 2c and 3c were not
observed, probably because the signals were too broad.
(25) It is not adequate to discuss the Lewis acidity by the values
of 11B NMR chemical shifts. See ref. 16.
Synthesis 2008, No. 6, 859–864 © Thieme Stuttgart · New York