828 Bull. Chem. Soc. Jpn., 75, No. 4 (2002)
Dehydrogenative Borylation of Vinylarenes
2 (2.0 mmol) were added successively, and the mixture was stirred
at room temperature for 4 h. The reaction mixture was diluted
with toluene, washed with water, and dried over MgSO4. The sol-
vent was evaporated, and the alkenylboronate 3 was isolated by
distillation with Kugelrohr or by chromatography over silica gel.
The following compounds were prepared by the above proce-
dure.
4,4,5,5-Tetramethyl-2-[(E)-styryl]-1,3,2-dioxaborolane (3a):
1H NMR (CDCl3) δ 1.32 (s, 12H), 6.16 (d, J = 18.5 Hz, 1H), 7.3–
7.6 (m, 6H); 13C NMR (CDCl3) δ 24.84, 83.35, 127.09, 128.55,
128.88, 137.59, 149.52; HRMS (m/z) for C14H19BO2 calcd
230.1478, found 230.1477.
2-[(E)-2-(4-Methoxyphenyl)ethenyl]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (3b): 1H NMR (CDCl3) δ 1.31 (s, 12H),
3.81 (s, 3H), 6.01 (d, J = 18.6 Hz, 1H), 6.87 (d, J = 8.5 Hz, 2H),
7.35 (d, J = 18.6 Hz, 1H), 7.44 (d, J = 8.5 Hz, 2H); 13C NMR
(CDCl3) δ 24.81, 55.28, 83.23, 113.97, 128.48, 130.43, 149.07,
160.30; HRMS (m/z) for C15H21BO3 calcd 260.1583, found
260.1584.
4,4,5,5-Tetramethyl-2-[(E)-2-p-tolylethenyl]-1,3,2-dioxabo-
rolane (3c): 1H NMR (CDCl3) δ 1.31 (s, 12H), 2.34 (s, 3H),
6.11 (d, J = 18.6 Hz, 1H), 7.14 (d, J = 7.9 Hz, 2H), 7.37 (d, J =
18.6 Hz, 1H), 7.38 (d, J = 7.9 Hz, 2H); 13C NMR (CDCl3) δ
21.33, 24.82, 83.28, 127.03, 129.29, 134.81, 138.96, 149.48;
HRMS (m/z) for C15H21BO2 calcd 2441635, found 244.1638.
2-[(E)-2-(4-Chlorophenyl)ethenyl]-4,4,5,5-tetramethyl-1,3,
2-dioxaborolane (3c): 1H NMR (CDCl3) δ 1.31 (s, 12H), 6.13
(d, J = 18.6 Hz, 1H), 7.30 (d, J = 8.2 Hz, 2H), 7.33 (d, J = 18.6
Hz, 1H), 7.41 (d, J = 8.5 Hz, 2H); 13C NMR (CDCl3) δ 24.82,
83.47, 128.24, 128.81, 134.63, 135.99, 148.03; HRMS (m/z) for
C14H18B35ClO2 calcd 264.1088, found 264.1113.
Methyl 4-[(E)-2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-
yl)-ethenyl]benzoate (3d): 1H NMR (CDCl3) δ 1.32 (s, 12H),
3.91 (s, 3H), 6.28 (d, J = 18.5 Hz, 1H), 7.41 (d, J = 18.5 Hz, 1H),
7.53 (d, J = 8.4 Hz, 2H), 8.01 (d, J = 8.4 Hz, 2H); 13C NMR
(CDCl3) δ 24.82, 52.13, 83.58, 126.91, 129.92, 130.15, 141.73,
148.14, 166.80; HRMS (m/z) for C16H21BO4 calcd 288.1533,
found 288.1513.
Fig. 1. Proposed catalytic cycle for dehydrogenative boryla-
tion (M = Rh, Ru).
both the phosphine-free rhodium complex and the ruthenium
catalyst having a less electron-donating phosphine may be fa-
vorable for coordination of the second alkene. On the other
hand, a catalyst having an electron-donating ligand, such as the
phosphine (Table 1, entries 2 and 14) and the aliphatic alkene
(Scheme 2), may be more favorable for the reductive elimina-
tion of 4 or 5 from the monoalkyl intermediate.
In the case of 1,5-hexadiene 1i, the reaction gave 5i predom-
inantly in the presence of PPh3 (Table 2, entry 12). Thus, the
phosphine-containing rhodium catalysts also induced hydride
migration. This regioselectivity was consistent with that of
phosphinite- or amide-directed hydroboration.16 On the other
hand, the regioselectivity was more economically accommo-
dated by a mechanism involving the migration of boron during
the course of the phosphine-free rhodium-catalyzed hydrobo-
ration process (entry 11).
One-Pot Synthesis of Stilbenes. The potential versatility
of the present borylation was demonstrated by a one-pot syn-
thesis of stilbenes. Although the dehydrogenative borylation
of 1 provided a mixture of the major alkenylboronates and a
small amount of alkylboronates, the resulting solution could be
used directly for the next Suzuki–Miyaura reaction, since alky-
lboronates, such as 4 and 5, were not capable of palladium-cat-
alyzed cross-coupling.1,17 Thus, the cross-coupling of 3c pre-
pared from 1c (3 mmol) with 4-fluoroiodobenzene (1.0 mmol)
at 90 °C for 8 h in the presence of [PdCl2(dppf)] (0.03 mmol)
and 3 M aq KOH (1 mL) provided the corresponding stilbene
in a 79% isolated yield based on the iodoarene employed.
[(E)-2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ethen-
yl]ferrocene (3f): 1H NMR (CDCl3) δ 1.30 (s, 12H), 4.10 (s,
5H), 4.27 (t, J = 1.8 Hz, 2H), 4.42 (t, J = 1.8 Hz, 2H), 7.53 (d, J
= 18.2 Hz, 1H), 8.01 (d, J = 18.2 Hz, 1H); 13C NMR (CDCl3) δ
24.81, 67.58, 69.44, 69.60, 83.07, 149.44; HRMS for
C18H23BFeO2 calcd 338.1141, found 338.1139.
Olefin-Directed Hydroboration of 1,5-Hexadiene.
To a
mixture of [RhCl(cod)]2 (0.005 mmol) and a ligand (0.04 mmol)
in toluene (4 mL), pinacolborane 2 (1.0 mmol) and 1,5-hexadiene
1i (4.0 mmol) were added; the resulting solution was stirred for 4
h at room temperature. Two regio isomers were separated by col-
umn chromatography on silica gel; the spectral data are described
below.
2-(5-Hexenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4i):
1H NMR (CDCl3) δ 0.78 (t, J = 7.3 Hz, 2H), 1.24 (s, 12H), 1.4–
1.5 (m, 4H), 2.04 (dt, J = 6.1, 7.3 Hz, 2H), 4.92 (dd, J = 1.5, 9.8
Hz, 1H), 4.99 (dd, J = 1.5, 17.1 Hz, 1H), 5.81 (ddt, J = 6.1, 9.8,
17.1 Hz, 1H); 13C NMR (CDCl3) δ 23.55, 24.80, 31.66, 33.57,
82.86, 114.03, 139.19; HRMS (m/z) for C12H23BO2 calcd
210.1792, found 210.1799.
Experimental
Synthesis of Vinylboronates. Typical Procedure.
was charged with [RhCl(cod)]2 (0.005 mmol) and toluene (4 mL)
under an argon flow. Pinacolborane 1 (1.0 mmol) and vinylarenes
A
flask
4,4,5,5-Tetramethyl-2-(1-methyl-4-pentenyl)-1,3,2-dioxa-
borolane (5i): 1H NMR (CDCl3) δ 0.97 (d, J = 6.7 Hz, 3H),
1.04 (tq, J = 6.7, 7.9 Hz, 1H), 1.24 (s 12H), 1.38 (dt, J = 7.3, 7.9