Angewandte
Chemie
[
a]
atmospheric pressure, 2) high yields and turnover numbers
Table 1: Reactions of aromatic diamines with CO catalyzed by A.
2
(
TONs), and 3) applicability to a wide range of substrates.
Entry Substrate
CO2
Product
Yield
A benchmark reaction of 1,2-phenylenediamine (1a) with
[MPa]
[%]
CO to give 2-benzimidazolone (2a) in N-methylpyrrolidone
2
1
2
0.1
2
91
90
(
NMP) was carried out (Figure 1b and see Table S2 in the
Supporting Information). Since reactivities of less nucleo-
philic arylamines with CO are very low, which is in sharp
2
3
4
0.1
2
85
99
[6]
contrast with those of alkylamines, 2a has not yet been
obtained in the absence of catalysts even under harsh reaction
[
7]
conditions (6 MPa of CO pressure at 423 K). Notably, the
5
6
0.1
2
78
98
2
A-catalyzed reaction proceeded efficiently with CO2 at
atmospheric pressure (0.1 MPa) to give 2a in 91% yield.
Reactions hardly proceeded in the absence of A or in the
presence of the other POMs such as TBA [W O ], TBA -
7
8
0.1
2
87
94
2
6
19
4
[
W O ], and TBA [a-SiW O ], and is in accord with the
10 32 4 12 40
9
0.1
2
50
76
À1
computational results. The reaction rate (10 mm min ) of A
was 48-fold faster compared with those (< 0.01–
0
10
À1
.21 mm min ) of typical inorganic and organic strong bases
11
0.1
2
45
70
such as Cs CO , TBAOH, 1,8-diazabicyclo[5.4.0]undec-7-ene
12
2
3
(
DBU), and 1,1,3,3-tetramethylguanidine (TMG). To the best
of our knowledge, no successful example of either stoichio-
metric or catalytic synthesis from aromatic diamines and CO2
has been reported.
13
14
0.1
2
56
83
The A-catalyzed system could be applied to various kinds
of structurally diverse aromatic diamines (Table 1). Various
kinds of 1,2-phenylenediamines (1a–1g) having electron-
donating as well as electron-withdrawing substituents were
converted into the corresponding 2-benzimidazolones 2a–2g
1
16
5
0.1
2
87
97
[
[
b]
b]
1
1
7
8
0.1
2
85
94
in moderate to high yields using CO at atmospheric pressure
2
(
Table 1, entries 3, 5, 7, 9, 11, and 13). 1,8-Diaminonaphtha-
lene (1h) and 3,3’-diaminobenzidine (1i) were also effectively
converted into the corresponding cyclic urea derivatives (2h
and 2i) in 87 and 85% yields, respectively (Table 1, entries 15
and 17). For the reactions of various aromatic diamines at
[a] Reaction conditions: Substrate (1 mmol), A (20 mmol for entries 2, 4,
6, 16, and 18), (50 mmol for entries 8 and 10, 100 mmol for entries 3, 5,
1
(
1
2, 14, and 15, and 150 mmol for entries 1, 7, 9, 11, 13, and 17), NMP
1 mL), 413 K, 24 h. Yields were determined by GC (entries 1–10, 15, and
6) or LC (entries 11–14) analysis. [b] Substrate (0.5 mmol). Yields are
2
MPa of CO , high to excellent yields of the corresponding
2
those of the isolated products.
urea derivatives could be obtained even with low catalyst
loadings (2–10 mol%; Table 1, entries 2, 4, 6, 8, 10, 12, 14, 16,
and 18). The 4 mmol scale reaction of 1b with CO (2 MPa),
catalyzed by A (0.5 mol% with respect to 1b) at 413 K for
d = 1.7 ppm) upon addition of 1a (Figures 2d,e). The hydro-
gen bond could weaken the NÀH bond and facilitate the
2
9
6 h, gave 2b in 93% yield. The TON got up to 186, and the
nucleophilic attack of the NH group in 1a on the carbon
2
value was much higher than those (0–52) reported for the
base-catalyzed reactions of amines to urea derivatives (see
atom of CO2.
When the DMF solution of A was exposed to CO
(0.1 MPa), a new C signal appeared at d = 163.6 ppm (Fig-
ure 2c and see Figure S1 in the Supporting Information) with
satellites [coupling constants ( JW-C, 14% satellite intensities)
of 6.7 Hz], thus suggesting the presence of a W-O-C bond.
The a-carbon atoms of the terminal O-iPr ligands in W R -
2
[
3]
13
Table S1 in the Supporting Information).
2
À
Despite the weaker basicity of [WO4] (pK of conjugate
a
2
acid in water: 3.5) compared with those of the inorganic and
2
À
organic strong bases such as [CO ] (10.3), DBU (12.0), and
3
[
8]
TMG (13.6), A showed much higher catalytic activity. To
2
2
elucidate the role of A, the reactivity of A with 1a and CO
(R’CꢀCR’’) (O-iPr) (R = CH Ph, CH SiMe , nPr, and C H -
2
2
4
2
2
3
6
4
1
13
183
were investigated by using H, C, and W NMR spectros-
p-Me; R’CCR’’ = MeCꢀCMe, MeCꢀCEt, and EtCꢀCEt)
1
2 [10]
copy. The H NMR spectrum of 1a showed the downfield shift
show similar coupling constants
J
(4–12 Hz).
The
W-C
of the signal of the NH protons from d = 4.22 to 4.76 ppm by
chemical shift was different from those of free CO (d =
2
2
addition of one equivalent of A (relative to 1a; Figures 2a,b,
and Figure S1 in the Supporting Information), thus indicating
the hydrogen-bonding interaction between A and 1a de-
scribed in reference [9] (Figure S2). The downfield shift of the
125.3 ppm) and TBAHCO (d = 160.7 ppm). Therefore, the
new C signal at d = 163.6 ppm could arise from the A/CO2
3
1
3
adduct, which is similar to Lewis base/CO adducts such as
2
amidines, guanidines, and N-heterocyclic carbenes (Fig-
[
11]
signal for the NH protons (Dd =+ 0.54 ppm) with A was
ure S2). These base/CO adducts are key carrier intermedi-
2
2
much larger than that (Dd =+< 0.01 ppm) with DBU, thus
suggesting the stronger interaction of 1a with A. The signal at
d = 16.4 ppm in the W NMR spectrum of A also shifted (to
ates for accomplishing the fixation through nucleophilic
[
1,12]
incorporation of the O=C=O unit.
Upon introduction of
1
83
183
CO (0.1 MPa), a new W signal appeared at d = 57.8 ppm
2
Angew. Chem. Int. Ed. 2012, 51, 6700 –6703
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6701