472
J. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 470–474
Table 1
Addition of phenylboronic acid to 4-chlorobenzaldehyde catalyzed by the NHC–Rh complexa
Entry
Imidazolium salt
Metal salt
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
a
4
–
4
4
4
4
4
4
4
4
4
4
4
1
2
3
RhCl3 Æ 3H2Ob
DME–H2O
DME–H2O
DME–H2O
DME–H2O
DME–H2O
DME–H2O
H2O
THF–H2O
1,4-dioxane–H2O
n-BuOH–H2O
1,4-dioxane–H2O
1,4-dioxane–H2O
1,4-dioxane–H2O
1,4-dioxane–H2O
1,4-dioxane–H2O
1,4-dioxane–H2O
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-t
KOBu-tf
Cs2CO3
KOHf
KOBu-tf
KOBu-tf
KOBu-tf
0
0
[Rh(COD)Cl]2
c
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
21
31
70
41
58
36
76
<5
95
26
89
81
90
86
d
e
f
All reactions were carried out using metal salt (1.5 mol%), imidazolium salt (1 mol%), and a base (1 equiv. to the aldehyde) in mixture solvent
(V:V = 4:1) at 80 ꢁC for 16 h in argon unless stated otherwise. Yields represent isolated yield based on p-chlorobenzaldehyde.
b
3 mol% RhCl3 Æ H2O.
0.5 mol% [Rh(COD)Cl]2.
1 mol% [Rh(COD)Cl]2.
3 mol% [Rh(COD)Cl]2.
c
d
e
f
2 equiv. of base to aldehyde were used.
reacted very cleanly with various arylboronic acids in excel-
lent yields (Table 2, entries 2, 3, 6–10).
DMSO-d6): d = 138.6, 136.9, 123.1, 122.8, 47.5, 35.9.
HRMS (ESI): m/z calcd for C36H48Br5N12 [M À Br]+:
1043.0042. Found 1043.0036.
3. Experimental
Hexakis[(3-phenylimidazolio)methyl]benzene hexabro-
mide (2): yield: 93%. m.p. 248–250 ꢁC (methanol). 1H
NMR (400 MHz, DMSO-d6): d = 9.89 (s, 6H, NCHN),
8.31 and 8.27 (s, 2 · 6H, phenyl-CH), 7.87 (s, 12H,
NCHCHN), 7.54–7.50 (m, 18H, phenyl-CH), 6.10 (s,
12H, NCH2). 13C NMR (100 MHz, DMSO-d6): d = 138.8,
135.6, 134.4, 130.0, 124.2, 122.3, 120.6, 48.8. HRMS
(ESI): m/z calcd for C66H60Br4N12 [M À 2Br]2+: 668.0899.
Found 668.0893.
3.1. General
Reactions were carried out under argon to exclude oxy-
gen from the reaction systems. The complex [Rh(COD)Cl]2
[34] and hexakis(bromomethyl)benzene [35] were prepared
according to the literature methods. All other reagents
were used as they were received without any purification
unless noted otherwise. 1H NMR (400 MHz) and 13C
NMR (100 MHz) were recorded on a Varian INOVA
400 MHz NMR spectrometer. Mass spectra were obtained
by using Bruker Daltonics Data Analysis 3.2.
Hexakis{[3-(p-methylphenyl)imidazolio]methyl}
ben-
zene hexabromide (3): yield: 95%. m.p. 255–257 ꢁC (meth-
1
anol). H NMR (400 MHz, DMSO-d6): d = 9.80 (s, 6H,
NCHN), 8.26 and 8.21 (s, 2 · 6H, phenyl-CH), 7.71 and
7.70 (s, 2 · 6H, NCHCHN), 7.30 and 7.28 (s, 2 · 6H, phe-
nyl-CH), 6.06 (s, 12H, NCH2), 2.38 (s, 18H, CH3). 13C
NMR (100 MHz, DMSO-d6): d = 139.7, 138.8, 135.2,
132.0, 130.3, 124.1, 122.0, 120.7, 48.7, 20.7. HRMS (ESI):
m/z calcd for C72H72Br4N12 [M À 2Br]2+: 710.1368. Found
710.1363.
3.2. Procedure for the preparation of hexadentate
imidazolium salts
1-Substituted imidazole (13.2 mmol) and hexakis(bro-
momethyl)benzene (2 mmol) were stirred in THF (15 ml)
at reflux for 24–48 h. The precipitate was filtered and
washed with dry THF (3 · 15 ml) to afford the product
as a white powder. Pure sample was obtained after recrys-
tallization from an appropriate solvent.
Hexakis[(3-methylimidazolio)methyl]benzene hexabro-
mide (1): yield: 96%. m.p. 160–163 ꢁC (ethanol). 1H
NMR (400 MHz, DMSO-d6): d = 9.32 (s, 6H, NCHN),
7.78 and 7.66 (s, 2 · 6H, NCHCHN), 5.78 (s, 12H,
NCH2), 3.84 (s, 18H, CH3). 13C NMR (100 MHz,
Hexakis{[3-(2,4,6-trimethylphenyl)imidazolio]methyl}
benzene hexabromide (4): yield: 90%. m.p. 262–263 ꢁC
1
(methanol). H NMR (400 MHz, DMSO-d6): d = 9.77 (s,
6H, NCHN), 8.09 and 7.99 (s, 2 · 6H, NCHCHN), 7.10
(s, 12H, phenyl-CH), 6.14 (s, 12H, NCH2), 2.32 (s, 18H,
CH3), 2.07 (s, 36H, CH3). 13C NMR (100 MHz, DMSO)
d = 140.4, 139.1, 138.1, 134.9, 131.2, 129.4, 123.8, 123.0,
48.4, 20.8, 18.2. HRMS (ESI): m/z calcd for C84H96Br4N12
[M À 2Br]2+: 794.2307. Found 794.2302.