Communications
[
a]
Table 1: Reaction of benzene, anisole, or chlorobenzene with various CO sources in the presence of mpg-C N as catalyst.
2
3
4
[
b]
[c]
CO source
Solvent
Coreactant
t [h]
Conversion [%]
Product
2
1
2
3
4
5
6
7
8
9
1
1
1
–
benzene
benzene
benzene
heptane
anisole
chlorobenzene
benzene
benzene
benzene
benzene
benzene
benzene
triethylamine (200 mg)
–
–
benzene (100 mg)
–
–
–
triethylamine (200 mg)
triethylamine (200 mg)
pyridine (200 mg)
triethylamine (200 mg), phenol (1 g)
triethylamine (200 mg)
20
20
20
20
20
20
24
12
24
48
48
24
0
–
[
[
[
d]
d]
e]
NaHCO
NaHCO (100 mg)
NaHCO (400 mg)
NaHCO (200 mg)
NaHCO (200 mg)
CO (10 bar)
CO (10 bar)
CO (3 bar)
3
(200 mg)
46
100
100
0
phenol (100%)
phenol (100%)
phenol (100%)
–
–
–
phenol (65%), biphenyl (35%)
phenol (29%), biphenyl (71%)
–
biphenyl (100%)
–
3
3
3
0
3
0
2
[
f]
f]
20
13
0
2
[
2
0
1
2
CO (10 bar)
–
CO (10 bar)
2
5
0
[
g]
2
[
a] In a typical reaction mpg-C N (100 mg) was added to the desired solvent (either heptane or the pure aromatic compound; 10 mL) in a 100-mL
3 4
stainless steel autoclave. The CO source was added and the closed autoclave was heated to 1508C. [b] Conversion rates were determined by GC–MS
2
with an internal standard (toluene) as the ratio between the formed products and the initial amount of limiting reactant. [c] The percentage in
parentheses is the amount of the specified product in the reaction mixture, as determined by GC–MS with an internal standard. [d] Conversion
calculatedwith respect to the initial amount of NaHCO . [e] Conversion calculatedwith respect to the amount of benzene. [f] Conversion calculated
3
with respect to the amount of base. [g] Reference experiment without catalyst.
sources, such as the neutralization enthalpy of the phenol, the
hydration of CO, or other subsequent reactions. In this
(Table 1, entry 5)or chlorobenzene (Table 1, entry 6)failed to
yield any detectable product even at higher temperatures (up
to 1808C for chlorobenzene and 2208C for anisole). This
result is consistent with previous observations that anisole is
far less reactive than benzene in mpg-C N -catalyzed Friedel–
Crafts reactions. The most straightforward explanation is
that steric hindrance prevents effective adsorption of sub-
stituted arenes on the tri-s-triazine units.
reaction pathway, the very stable component CO would be
2
converted into a more reactive species, which would then be
available for further chemical reactions. As previously
3
4
[
14]
described metal-free activation of CO resulted only in the
2
formation of organic carbonates, this pathway would then
lead to new, previously unknown CO chemistry. Incidentally,
2
such a reaction could also provide an alternative to the usual
cumene process for phenol production or even to more
advanced syntheses relying on the direct oxidation of benzene
A careful FTIR investigation of the catalysts after the
reaction indicated the formation of a carbamate species, as a
new IR bands appear at 1419 cm in the spectrum of the
À1
[
18]
with oxygen or nitrous oxide.
powder. These observations are in good qualitative agree-
ment with the formation of a carbamate on an adenine
We thus undertook to test these reactions with various
CO2 sources and various aromatic compounds (benzene,
anisole, and chlorobenzene)with mpg-C N as catalyst. The
[
12]
derivative reported by Ratnasamy and co-workers. We thus
postulated that the first step of our catalytic process involves
the formation of a carbamate species, presumably on surface
primary or secondary amino groups of mpg-C N (Scheme 3).
3
4
reactions were performed at 1508C in a 100-mL stainless steel
autoclave fitted with a teflon mantel with 100 mg of mpg-
3
4
[
15]
C N . NaHCO or gaseous CO (3–10 bar)was used as the
The existence of such amino groups, which result from
incomplete condensation of the tri-s-triazine units, was
3
4
3
2
carbon dioxide source. Either heptane or the aromatic
substrate itself (10 mL)was used as the solvent. After the
reaction was complete, the reaction mixture was neutralized
and then analyzed by GC–MS. To identify and quantify the
[
19]
previously demonstrated by Lotsch and Schnick.
formed carbamate would then be well positioned to react
The
[
14]
with an aromatic molecule activated by the catalyst. The
hypothetical [2+2] addition of an aromatic CÀH bond of the
benzene to the C=O double bond would result in the
1
13
products, they were isolated and analyzed by H and C NMR
spectroscopy (Table 1). The extents of conversion were
calculated on the basis of the limiting reactant (benzene, the
formation of a hemiacetal, which could easily eliminate
phenol to yield a formamide. The latter can eliminate CO, as
CO source, or the base).
2
[
20]
Surprisingly, the second reaction, namely, the oxidation of
reported for the thermolysis of formamide.
benzene to phenol, takes place whenever a CO source and a
DFT calculations were undertaken to support the first
steps of the proposed reaction mechanism. The geometry of a
model tri-s-triazine unit was optimized (see the Supporting
Information for computational details)along with the geo-
metries of the corresponding carbamate (6)and adduct with
benzene (7, Scheme 3). The first step is slightly endothermic
2
sufficiently strong base are present (Table 1, entries 2–4, 8, 9).
The reaction products are free of benzoic acid, the product of
direct carboxylation of benzene. The only side reaction
observed is the formation of biphenyl in the presence of a
large excess of benzene (Table 1, entries 6 and 7). This
product probably results from the arylation of benzene with
phenol, as indicated by the fact that under similar conditions
with the same catalyst a mixture of benzene and phenol
yielded 5% biphenyl (Table 1, entry 11). Unfortunately,
reactions with other aromatic compounds such as anisole
À1
(8.9 kcalmol ), which is again consistent with the fact that
CO is stable and carbamates eliminate CO , except under
2
2
alkaline conditions (the neutralization enthalpy is À13.34 kcal
À1
mol ). DFT calculations also reveal that the adsorption of
benzene on the model tri-s-triazine unit is weakly exothermic
2
718
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 2717 –2720