Kim et al.
TABLE 1. Effect of Amine on the Isomerization of VNBa
b
amine
temp (°C)
yield (%)
ethylenediamine
diethylenetriamine
25
25
25
25
96.3
96.2
95.8
n.r.c
80.5
n.r.
n.r.
n.r.
n.r.
1
1
1
1
1
,2-diaminopropane
,2-diaminocyclohexane
,2-diaminocyclohexane
,3-diaminocyclohexane
,4-diaminocyclohexane
100
25, 100
25, 100
25, 100
25, 100
ethylamine
cyclohexylamine
a
Reaction conditions: reaction time ) 1 h, molar ratio of NaH/amine/
b
VNB ) 1:30:50. ENB yield (%) ) (moles ENB produced/initial moles
VNB) × 100. n.r. ) no reaction.
c
TABLE 2. Effect of Alkali Metal Hydride on the Isomerization of
FIGURE 1. Effect of the molar ratio of EDA/NaH on the isomeriza-
tion.
a
VNB
b
alkali metal hydride
temp (°C)
yield (%)
The isomerization was greatly affected by the alkali metal
hydride employed. As shown in Table 2, KH produced ENB
almost quantitatively at room temperature, but LiH gave ENB
in very low yield (e.g., about 4%) at room temperature. The
activity of the alkali metal hydride increased with the increasing
size of alkali metal: KH > NaH > LiH. It seems that the
formation of the active species and the evolution of hydrogen
gas are more facilitated in the presence of a hydride of larger-
sized alkali metal because the dissociation energy of alkali metal
hydride decreases with the increasing size of alkali metal.
The influence of the molar ratio of EDA/NaH on the VNB
isomerization was also examined. The yield of ENB increased
with an increasing molar ratio of EDA/NaH up to 30 and
remained nearly constant thereafter on further increases in the
molar ratio (Figure 1). This phenomenon can be ascribed to
the polar character of the active species produced from the
reaction of EDA with NaH. The active species formed from
the interaction of NaH and EDA is soluble in EDA but
completely insoluble in VNB or ENB. Therefore, at a lower
molar ratio of EDA/NaH, the majority of active species exists
as a heterogeneous oily state in the reaction mixture, thereby
limiting the interaction with VNB. However, with an increasing
molar ratio of EDA/NaH, the oily active species becomes
soluble in the bottom EDA layer and thus the interaction of the
active species with the VNB layer is more facilitated when
stirring is applied.
LiH
LiH
NaH
KH
25
100
25
4.0
92.6
96.3
99.9
25
a
Reaction time ) 1 h. Molar ratio of MH/EDA/VNB ) 1:30:50. b ENB
yield (%) ) (moles ENB produced/initial moles VNB).
Even though various catalysts have been developed, some
of which have been successfully used in the commercial
production of ENB, the mechanistic details on the isomerization
of VNB have rarely been investigated. For this reason, we have
undertaken a mechanistic study on the isomerization process
with a hope that understanding the mechanism might help to
develop isomerization catalysts with better performance. Among
the catalysts reported, we are particularly interested in the liquid
base catalysts consisting of an alkali metal hydride and a
polyamine because they are highly active and relatively easy
to handle for the mechanistic investigation.
In this paper, we report our observations on the mechanistic
aspects of the VNB isomerization conducted in the presence of
a catalytic system based on a polyamine and an alkali metal
hydride.
Results and Discussion
Effects of Amines and Alkali Metal Hydrides. The isomer-
ization reactions of VNB were performed using a catalytic
system consisting of an amine and an alkali metal hydride.
Tables 1 and 2 show the effects of amines and alkali metal
hydrides on the isomerization of VNB, respectively. As can be
seen in Table 1, ENB was produced in high yields at room
temperature in the presence of a di- or triamine including
ethylenediamine (EDA), 1,2-diaminopropane, and diethylene-
triamine. The isomerization also proceeded in the presence of
UV-Vis and EPR Experiments. In the course of investigat-
ing the isomerization reaction with a catalytic system consisting
of an alkali metal hydride and an amine, we have found that
the activity of the catalytic system is strongly related to the
color of the solution. The active solution containing EDA and
NaH exhibited a purple color at room temperature and showed
a strong peak centered at 581 nm in the UV-vis spectrum
(Figure 2). However, neither the color nor the absorption band
at visible regions was observed for the solution consisting of
NaH and an inactive amine such as 1,3-cyclohexane diamine.
The intensity of the absorption band at 581 nm was affected by
the activity of alkali metal hydride. For instance, the mixture
of EDA and LiH gave much lower intensity than that of EDA
and NaH, demonstrating the importance of the peak at 581 nm.
Such an UV absorption band was also observed in many
delocalized radical species containing nitrogen atoms in the
1
1
1
,2-diaminocyclohexane but at a much higher temperature of
00 °C. Interestingly, however, no ENB was produced even at
00 °C when 1,3- or 1,4-diaminocyclohexane was used instead
of 1,2-diaminocyclohexane, supporting the importance of the
locations of two amino groups. It is likely that the formation of
the active species from a 1,2-diamine and NaH is more
facilitated through resonance stabilization when the two amino
groups of the diamine are located at the vicinal positions. As
expected from the above results, monoamines such as ethyl-
amine and cyclohexylamine were not effective for the isomer-
ization.
1
3,14
range 400-800 nm.
Interestingly, the solution containing 1,2-diaminocyclohexane
and NaH showed a purple color only when the solution was
heated to 100 °C. Once the purple color persisted, the isomer-
(
12) Ishihara, T. JP Patent 6-40956, 1994.
912 J. Org. Chem., Vol. 71, No. 3, 2006