L. Wang, M.E. Welker / Journal of Organometallic Chemistry 723 (2013) 15e18
17
Scheme 3.
deuterated solvents. In the 13C NMR spectrum, the signal of the
quaternary carbon to the tetravalent boron was not observed in
allowed to warm to room temperature quickly. The cloudy gray
colored reaction mixture was stirred for 1 h. To the resulting
mixture at room temperature, 0.5 M HCl (100 mL) was added. The
reaction mixture was extracted with Et2O (2 ꢁ 75 mL). The
combined colorless clear organic layers were dried over MgSO4, and
the volatiles were removed by rotovap (30 ꢀC, 20 Torr) to yield
diene boronic acid (2).
a
organoboron compounds due to quadrupolar broadening as
expected [21]. All elemental analyses were performed by Atlantic
Microlabs Inc., Norcross, GA. High-resolution mass spectrometry
was performed at the UNC Mass Spectrometry Facility, Chapel Hill,
NC and Mass Spectrometry Lab, School of Chemical Sciences,
University of Illinois at Urbana-Champaign.
All reactions were carried out under an atmosphere of nitrogen.
Tetrahydrofuran (THF) was degassed with argon and then passed
through two 4 ꢁ 36 inch columns of anhydrous neutral A-2 alumina
(8 ꢁ 14 mesh; activated under a flow of Ar at 350 ꢀC for 3 h) to
remove water. Toluene (PhMe) was degassed with N2. Water was
deionized and distilled. Deuterated solvents were purchased from
Cambridge Isotope Laboratories and dried over molecular sieves.
Sodium sulfate, sodium hydroxide, magnesium small turnings,
1,2-dibromoethane were purchased from Aldrich Chemical
Company and used as received. Trimethylborate and all rhodium (I)
species used were purchased from Strem Chemicals and used as
received. 2-Chloro-1,3-butadiene, 50% in xylene (Chloroprene) was
purchased from Pfaltz & Bauer Inc and used as received. Diene 3
was prepared as reported previously [10].
3.3. Preparation of sodium 1-(buta-1,3-dien-2-yl)-4-methyl-2,6,7-
trioxa-1-borabicyclo(2,2,2) octan-1-uide (4)
Follow the general procedure to obtain diene boronic acid (2).
The dried diene boronic acid was added at once to a solution of
2-(hydroxymethyl)-2-methylpropane-1,3-diol (0.7 eq, 2.10 g,
17.5 mmol) dissolved in THF (100 mL). Sodium hydride (1.5 eq,
37.5 mmol, 0.086 g) was added slowly to the mixture at room
temperature. The solvent was removed by rotovap after 2 h. Dry
acetone (3 ꢁ 100 mL) was added to dissolve and wash the solid. The
solid was removed by filtration and the solvent was removed by
rotary evaporation and high vacuum to obtain the white solid
product (10) (2.68 g, 13.13 mmol, 75%): 1H NMR (300 MHz, DMSO)
d
6.18 (dd, J ¼ 17.1, 10.5 Hz, 1H), 5.40 (dd, J ¼ 17.1, 3.8 Hz, 1H), 4.90
(m, 2H), 4.65 (dd, J ¼ 10.5, 3.8 Hz, 1H), 3.49 (s, 6H), 0.43 (s, 3H); 13C
NMR (300, MHz, DMSO) d 144.5, 118.0, 113.0, 73.1, 34.4, 16.3; HRMS
3.2. General procedure for preparing boron substituted dienes
calcd for C9H14BNaO3 [M þ H]þ ¼ 205.1012, found: 205.1012.
A mixture of magnesium (1.0 g, 41.1 mmol), 1,2-dibromoethane
(0.5 mL, 5.3 mmol), and THF (10 mL) was refluxed under nitrogen
for 15 min to activate the magnesium. To the mixture anhydrous
zinc chloride (0.6 g, 4.1 mmol) in THF (60 mL) was added and reflux
was continued for another 15 min. 2-Chloro-1,3-butadiene (4.9 mL,
25 mmol) (density 0.915 g/mL, 50% in xylene) and
1,2-dibromoethane (0.95 g, 5 mmol) in THF (30 mL) were added
dropwise over a period of 30 min. This addition was controlled so as
to bring the mixture into a gentle reflux. The color of the contents
changed gradually from grayish white to greenish black. The
mixture was heated to reflux for an additional 30 min after
completion of the addition. The Grignard reagent thus obtained
was immediately added dropwise to a solution of trimethoxybor-
ane (4.25 mL, 38.5 mmol) in THF (25 mL) using a double-ended
needle. The addition was controlled in such a way that the
internal temperature of the mixture was maintained below ꢂ60 ꢀC
all the time. After completion of the addition, the solution was
3.4. Preparation of 2-phenyl-3a, 4, 7, 7a-tetrahydro-1H-isoindole-
1,3(2H)-dione (6a) using Cu(OTf)2
Diene 4 (102 mg, 0.5 mmol) was added to a N2 flushed thick wall
tube with Cu(OTf)2 (19.9 mg, 0.055 mmol, 11%) and (S)-BINAP
(20 mg, 0.03 mmol, 6%) in toluene (10 mL) at rt for 30 min.
N-phenyl-maleimide (87 mg, 0.5 mmol) was added to the mixture
followed by t-butanol (0.5 mL). The tube was sealed and heated up
to 55 ꢀC for 24 h then cooled to room temperature. The solution was
filtered through a silica gel pad to remove catalysts. The filtrate was
quenched with water (30 mL) and extracted with Et2O (3 ꢁ 50 mL).
The combined organic layers were dried over MgSO4 and volatiles
were removed by rotary evaporation. The resulting residue was
purified by chromatography (ethyl ether:hexane ¼ 1:4). The final
product was obtained as a white powder (0.079 g, 0.35 mmol, 70%).
1H NMR data was identical to that previously reported [8].