S. De et al. / Journal of Organometallic Chemistry 694 (2009) 2295–2298
2297
Table 1
Rhodium catalyzed transmetallation/Diels-Alder/hydrolysis reactions of maleimides.
Entry
1a
Dienophile
Diene
Catalyst (mol%)
None
Ligand (%)
None
Solvent-system
Temp. (°C)
Time [h]
Conversionc,d
100c
6a
1
Toluene:water (9:1)
Wet acetone
100–105
100–105
Room temp.
Room temp.
Room temp.
Room temp.
50–55
50–55
50–55
65–70
50–55
50–55
50–55
50–55
50–55
50–55
50–55
22
2
7
2a
6b
6b
6a
6a
6b
6b
6b
6b
6b
6b
6b
6b
6b
6b
6b
1
1
1
1
1
1
1
1
1
1
1
1
5
5
4
None
B (10)
A (10)
A (10)
B (10)
None
A (10)
A (10)
B (10)
B (10) + 20% AgSbF
B (10)
B (10)
B (10)
None
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Dioxane:water (4:1)
Toluene:water(4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Toluene:water (4:1)
Tracec
Tracec
Tracec
a
3
BINAP (11)
BINAP (11)
BINAP (11)
BINAP (11)
None
BINAP (11)
BINAP (11)
DPPF (11)
BINAP (11)
7
a
4
27
48
16.5
16.5
16
16
16
16
16
16
16
16
16
a
c
5
Trace
62
a
d
6
a
c
7
Trace
b
d
8
34
22
58
63
68
71
54
20
b
d
9
d
1
1
1
1
1
1
1
0
1
2
3
4
5
6
d
6
d
H
2
then BINAP (11)
d
none
d
BINAP(11)
BINAP (11)
BINAP (11)
d
A(10)
B (10)
d
70
2 2 2
Catalyst used A = [Rh(COD) ]BF4ÁxH O, B = [Rh(COD) ]OTf.
a
Reactions were conducted in sealed tube.
Under refluxing conditions.
Determined by 1H NMR analysis.
Isolated yield.
b
c
d
at 300.1 MHz or a Bruker Avance 500 MHz spectrometer operating
grayish white to greenish black. The mixture was heated to reflux
for an additional 30 min after completion of the addition. The Grig-
nard reagent thus obtained was immediately added dropwise to a
solution of trimethoxyborane (8.5 mL, 75 mmol) in toluene
(50 mL) using a double-ended needle. The addition was controlled
in such a way that the internal temperature of the mixture was
maintained below À60 °C all the time. After completion of the
addition, stirring was continued for 30 min at –60 °C, and then
the solution was allowed to warm to room temperature and stirred
for a further 30 min. The reaction mixture was cooled back to 0 °C
and concentrated sulfuric acid (5 mL) was added and allowed to
stir for an additional hour. To this white solution, saturated sodium
hydroxide (5–10 mL) was added and a white precipitate crashed
out of solution. The solid was collected by filtration, washed with
petroleum ether (2 Â 20 mL) and allowed to dry overnight under
1
3
at 500.1 MHz. C NMR spectra were obtained using a Bruker
1
1
Avance 300 MHz spectrometer operating at 75.5 MHz. B and
1
9
F NMR spectra were recorded on a Bruker Avance 300 MHz spec-
trometer at 96.3 and 282.4 MHz, respectively. 1H and C NMR
spectra were referenced to the residual proton or carbon signals
of the respective deuterated solvents. In the 13C NMR spectrum,
13
the signal of the quaternary carbon
a to the tetravalent boron
was not observed in organoboron compounds due to quadrupolar
broadening as expected [15]. 11B NMR chemical shifts were refer-
enced to external BF
were referenced to external CFCl
3
Á OEt
(0.0 ppm). All 19F NMR chemical shifts
2
3
(0.0 ppm). All elemental analyses
were performed by Atlantic Microlabs Inc., Norcross, GA. High-res-
olution mass spectrometry was performed at the UNC Mass Spec-
trometry Facility, Chapel Hill, NC.
1
All reactions were carried out under an atmosphere of nitrogen.
Tetrahydrofuran (THF) was degassed with argon and then passed
through two 4 Â 36 in. columns of anhydrous neutral A-2 alumina
vacuum affording 4 (3.09 g, 44.6 mmol, 45% yield). H NMR
(500 MHz, DMSO d 6.43 (m, 1H), 5.8 (bd, J = 18.3, 1H), 5.08 (bs,
1
3
4H), 4.83 (bs, 1H), 4.43 (bd, J = 11.3 Hz, 1H) C NMR (75.4 MHz,
1
3
(
8 Â 14 mesh; activated under a flow of Ar at 350 °C for 3 h) to re-
DMSO d 143.91, 119.12, 113.98. In the C NMR spectrum, the sig-
nal of the quaternary carbon to the tetravalent boron was not ob-
move water. Toluene (PhMe) was degassed with N
2
. Water was
a
1
1
+
deionized and distilled. Deuterated solvents were purchased from
Cambridge Isotope Laboratories and dried over molecular sieves.
Sodium sulfate, sodium hydroxide, magnesium small turnings,
served. B NMR (96.3 MHz, DMSO d 2.77 (bs). MALDI LRMS M
Calcd for C BO Na: 138.046. Found: 138.131.
4
H
8
3
1
,2-dibromoethane, potassium hydrogen fluoride, were purchased
3.3. Optimized Rh catalyzed Diels-Alder/hydrolysis protocol for
from Aldrich Chemical Company and used as received. Trimethylb-
orate 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.
2-Phenyl-3a,4,7,7a-tetrahydro-isoindole-1,3-dione (7b)
A sealed tube equipped a magnetic stirring bar was charged
with potassium 1,3-dienyl trifluoroborate (1) (1.2 equiv, 96 mg,
0.6 mmol) [Rh(COD)
11 mol%, 34 mg, 0.055 mmol) and N-phenylmaleimide (87 mg,
0.5 mmol, under nitrogen. Degassed toluene/water mixture
2.0:0.5 mL) was added at room temperature and the tube was
2
]OTf (10 mol%, 23 mg, 0.05 mmol), S-BINAP
(
3.2. Synthesis of 1,3-butadienyl-2-sodium trihydroxyborate (4)
(
A mixture of magnesium (2.0 g, 82.2 mmol), 1,2-dibromoethane
0.5 mL), and THF (2 mL) was heated to reflux under nitrogen for
5 min to activate the magnesium. To the mixture anhydrous zinc
chloride (0.3 g) in THF (25 mL) was added and reflux was contin-
ued for another 15 min. 2-Chloro-1,3-butadiene (9.7 mL, 50 mmol)
density 0.915 g/mL, 50% in xylene) and 1,2-dibromoethane (1.9 g,
0 mmol) in THF (25 mL) were added dropwise over a period of
0 min. This addition was controlled so as to bring the mixture into
a gentle reflux. The color of the contents changed gradually from
closed with a cap. The tube was placed in a pre-heated oil bath
at 50–55 °C for 16.5 h. After cooling the vessel at room tempera-
ture the reaction mixture was passed through a short pad of celite,
eluting with ethyl acetate (25 mL). The solvents were removed un-
der reduced pressure. Chromatographic purification over silica gel
(EtOAc:Hexane) afforded 2-Phenyl-3a,4,7,7a-tetrahydro-isoindole-
1,3-dione (7b) as a white solid (0.070 g, 0.31 mmol, 62%) identical
(
1
(
1
3
1
13
by GCMS, H NMR and C NMR comparison to previously reported
material [16].