Page 5 of 11
The Journal of Organic Chemistry
J = 7.6 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.74 (d, J = 8.0 Hz,
2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-7,9,9-
trimethylsilyl-9-silafluorene (2m) and 3-(4,4,5,5-Tetramethyl-
1,3,2-dioxaborolan-2-yl)-7,9,9-trimethylsilyl-9-sila- fluorene
(2p) (Table 2, entry 2). Following the general procedure using
2,9,9-trimethyl-9-silafluorene (44.8 mg, 0.20 mmol) and L3
(2.1 mg, 10 µmol) provided 54.2 mg (0.15 mmol, 76% yield)
of borylated 9-silafluorenes 2m and 2p as a colorless solid after
purification by flash chromatography on silica gel with hexane
/ EtOAc = 50 / 1 as the eluent. The regioselectivity was deter-
mined to be 2m / 2p = 64 / 36 by 1H NMR analysis of the crude
product. The major isomer 2m was partially separable by puri-
fication using GPC with toluene as the eluent, and the struc-
ture of which was unambiguously determined by X-ray crys-
tallographic analysis (see Figures S1 and Tables S2). 1H NMR
for 2m (400 MHz, CDCl3): d 0.41 (s, 6H), 1.37 (s, 12H), 2.39
(s, 3H), 7.25 (d, J = 8.4 Hz, 1H), 7.45 (s, 1H), 7.75 (d, J = 8.4
Hz, 1H), 7.78 (d, J = 7.6 Hz, 1H), 7.87 (dd, J = 0.8, 7.6 Hz,
1H). 13C{1H} NMR (100 MHz, CDCl3): d 12.2, 13.9, 22.1,
23.5, 35.6, 55.3, 115.1, 118.5, 120.1, 121.8, 126.1, 130.0,
133.2, 137.1, 139.9, 141.2, 148.3, 159.0. HRMS (FAB+): calcd
for C23H33OSi ([M+H]+) 353.2301; found. 353.2300.
1
2
3
4
5
6
7
8
9,9-Dimethyl-2-triisopropylsiloxy-9-silafluorene (3d). Fol-
lowing the general procedure for rhodium-catalyzed silylative
cyclization with dehydrogenation using 2-(dimethylsilyl)-4’-
triisopropylsiloxy-1,1’-biphenyl (692 mg, 1.8 mmol) provided
599 mg (1.6 mmol, 87% yield) of 3d as a colorless solid after
purification by flash chromatography on silica gel with hexane
1
9
as the eluent. mp 36.9-37.5 °C. H NMR (400 MHz, CDCl3):
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
d 0.40 (s, 6H), 1.12 (d, J = 7.6 Hz, 18H), 1.22-1.32 (m, 3H),
6.92 (dd, J = 2.0, 8.0 Hz, 1H), 7.11 (d, J = 2.0 Hz, 1H), 7.20
(dt, J = 1.2, 7.6 Hz, 1H), 7.39 (dt, J = 1.2, 7.6 Hz, 1H), 7.58 (d,
J = 7.6 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 7.6 Hz,
1H). 13C{1H} NMR (100 MHz, CDCl3): d -3.3, 12.7, 18.0,
120.1, 121.4, 121.8, 123.9, 126.2, 130.1, 132.6, 138.3, 140.7,
140.8, 147.9, 155.8. HRMS (FAB+): calcd for C23H34OSi2
([M]+) 382.2148; found. 382.2164.
1
1H), 8.07 (s, 1H). H NMR for 2p (400 MHz, CDCl3): d 0.41
(s, 6H), 1.37 (s, 12H), 2.38 (s, 3H), 7.24 (d, J = 8.4 Hz, 1H),
7.44 (s, 1H), 7.63 (d, J = 7.2 Hz, 1H), 7.70 (d, J = 7.2 Hz, 1H),
7.84 (d, J = 8.4 Hz, 1H), 8.22 (s, 1H). 13C{1H} NMR for 2m
(100 MHz, CDCl3): d -3.2, 21.3, 24.9, 83.7, 119.8, 121.2,
130.9, 133.4, 137.0, 137.5, 137.6, 139.3, 139.8, 145.0, 150.8
(The boron-bound carbon was not detected due to quadrupolar
relaxation). 13C{1H} NMR for 2p (100 MHz, CDCl3): d -3.3,
21.3, 24.9, 83.8, 121.1, 126.4, 130.9, 132.0, 133.2, 133.4,
136.8, 138.7, 142.5, 145.2, 147.2 (The boron-bound carbon
was not detected due to quadrupolar relaxation). HRMS
(FAB+): calcd for C21H28O2SiB ([M+H]+) 351.1952; found.
351.1953.
2-Methoxy-3,9,9-trimethyl-9-silafluorene (3e). Following
the general procedure for rhodium-catalyzed silylative cycliza-
tion with dehydrogenation using 2-(dimethylsilyl)-4’-
methoxy-3’-methyl-1,1’-biphenyl (462 mg, 1.8 mmol) provid-
ed 293 mg (1.2 mmol, 64% yield) of 3e as a colorless solid
after purification by flash chromatography on silica gel with
hexane as the eluent. mp 111.7-112.2 °C. 1H NMR (400 MHz,
CDCl3): d 0.42 (s, 6H), 2.29 (s, 3H), 3.91 (s, 3H), 7.07 (s, 1H),
7.20 (t, J = 7.2 Hz, 1H), 7.39 (dt, J = 1.2, 7.2 Hz, 1H), 7.58 (d,
J = 7.2 Hz, 1H), 7.61 (s, 1H), 7.71 (d, J = 7.2 Hz, 1H).
13C{1H} NMR (100 MHz, CDCl3): d -3.1, 16.8, 55.5, 113.4,
120.0, 123.5, 126.1, 128.8, 130.1, 132.5, 137.5, 138.3, 140.4,
148.0, 157.7. HRMS (FAB+): calcd for C16H18OSi ([M]+)
254.1127; found. 254.1134.
General Procedure for Hypervalent Iodine-Promoted
Bromination of 2-Alkoxy-9-silafluorene Derivatives (Table
3 and Figure 3): A flame-dried sealed tube was charged with
LiBr (31.3 mg, 0.36 mmol), 2-alkoxy-9-silafluorenes 3 (0.30
mmol), (diacetoxyiodo)benzene (58.0 mg, 0.18 mmol) and
THF (1.0 mL), and stirred at 0 °C for 30 min. Additional io-
dobenzene diacetate (58.0 mg, 0.18 mmol) was added at 0 °C,
and stirred for 30 min. After stirring at 25 °C for further 1 h,
the solvent was removed under the reduced pressure. The resi-
due was subjected to flash column chromatography on silica
gel with hexane / EtOAc as the eluent to afford brominated
9,9-dimethyl-9-silafluorenes 4.
2-Bromo-7-methoxy-9,9-dimethyl-9-silafluorene (4a). Fol-
lowing the general procedure using 3a (71.8 mg, 0.30 mmol)
provided 67.0 mg (0.21 mmol, 71% yield) of 4a as a colorless
solid after purification by flash chromatography on silica gel
with hexane / EtOAc = 20 / 1 as the eluent. mp 64.5-70.5 °C.
1H NMR (400 MHz, CDCl3): d 0.44 (s, 6H), 3.87 (s, 3H), 6.96
(dd, J = 2.4, 8.4 Hz, 1H), 7.15 (d, J = 2.4 Hz, 1H), 7.51 (dd, J
= 2.0, 8.4 Hz, 1H), 7.57 (d, J = 8.4 Hz, 1H), 7.69 (d, J = 2.0
Hz, 1H), 7.70 (d, J = 8.4 Hz, 1H). 13C{1H} NMR (100 MHz,
CDCl3): d -3.3, 55.4, 115.7, 117.8, 120.8, 121.7, 122.0, 133.0,
135.3, 139.5, 140.4, 141.1, 146.5, 159.4. IR (KBr / cm-1):
2963, 2938, 2837, 1599, 1566, 1458, 1410, 1217, 1070, 1059,
1038, 868. HRMS (FAB+): calcd for C15H15BrOSi ([M]+)
318.0076; found. 318.0066.
2-Bromo-7-methoxy-9,9-dipentyl-9-silafluorene (4c). Fol-
lowing the general procedure using 3c (106 mg, 0.30 mmol)
provided 106 mg (0.21 mmol, 82% yield) of 4c as a colorless
oil after purification by flash chromatography on silica gel
(eluent: hexane / EtOAc = 20 / 1) and HPLC (eluent: hexane).
1H NMR (400 MHz, CDCl3): d 0.82 (t, J = 7.2 Hz, 6H), 0.89-
0.94 (m, 4H), 1.21-1.28 (m, 8H), 1.30-1.36 (m, 4H), 3.86 (s,
3H), 6.95 (dd, J = 2.4, 8.0 Hz, 1H), 7.12 (d, J = 2.4 Hz, 1H),
7.49 (dd, J = 2.4, 8.4 Hz, 1H), 7.57 (d, J = 8.4 Hz, 1H), 7.64
(d, J = 1.6 Hz, 1H), 7.69 (d, J = 8.4 Hz, 1H). 13C{1H} NMR
(100 MHz, CDCl3): d 12.1, 13.9, 22.1, 23.4, 35.5, 55.4, 115.3,
118.6, 120.7, 121.7, 122.0, 132.8, 135.6, 139.4, 140.1, 147.0,
General Procedure of Iridium-Catalyzed Borylation of
9-Silafluorene (Table 1). A flame-dried sealed tube was
charged with [Ir(OMe)(cod)]2 (3.3 mg, 5.0 µmol), ligand (10
µmol),
9,9-dimethyl-9-silafluorenes
(0.20
mmol),
bis(pinacolato)diboron (101.5 mg, 0.40 mmol), 1,4-dioxane
(0.20 mL), and stirred at 100 °C for 24 h. The residue was
directly subjected to flash column chromatography on silica
gel with hexane / EtOAc as the eluent to afford borylated 9,9-
dimethyl-9-silafluorenes 1 or 2 as mixture of regioisomers.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dimethylsilyl-9-silafluorene (1mm
)
and 2,6-Bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dimethylsilyl-9-
silafluorene (1mp) and 3,6-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dimethylsilyl-9-silafluorene (1pp) (Ta-
ble 1, entry 1). Following the general procedure using 9,9-
dimethyl-9-silafluorene (42.0 mg, 0.20 mmol) and dtbpy (2.7
mg, 10 µmol) provided 86.1 mg (0.19 mmol, 96% yield) of the
mixture of diborylated 9-silafluorenes 1mm, 1mp, and 1pp as a
colorless solid after purification by flash chromatography on
silica gel with hexane / EtOAc = 10 / 1 as the eluent. 1H NMR
for 1mm (400 MHz, CDCl3): d 0.42 (s, 6H), 1.39 (s, 24H), 7.88
1
(d, J = 7.8, 2H), 7.90 (d, J = 7.8 Hz, 2H), 8.10 (s, 2H). H
NMR for 1mp (400 MHz, CDCl3): d 0.42 (s, 6H), 1.39 (s, 24H),
7.67 (d, J = 7.8 Hz, 1H), 7.75 (d, J = 7.8 Hz, 1H), 7.89 (d, J =
7.8 Hz, 1H), 7.96 (d, J = 7.8 Hz, 1H), 8.09 (s, 1H), 8.29 (s,
1
1H). H NMR for 1pp (400 MHz, CDCl3): d 0.42 (s, 6H), 1.39
(s, 24H), 7.65 (d, J = 7.8 Hz, 2H), 7.74 (d, J = 7.8 Hz, 2H),
8.37 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3): d -3.4, -3.3, -
3.2, 24.9, 83.75, 83.78, 83.9, 120.5, 120.6, 127.0, 127.2,
132.0, 132.1, 133.7, 134.1, 136.90, 136.94, 137.6, 138.6,
139.25, 139.29, 142.5, 143.5, 146.9, 147.2, 150.4, 150.7 (The
boron-bound carbon was not detected due to quadrupolar re-
laxation). HRMS (FAB+): calcd for C26H36O4SiB2 ([M]+)
462.2569; found. 462.2556.
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