in the literature, there is good agreement with our results, even
though different approaches have been taken to the analysis.10–16
In conclusion, we can state on the basis of this study that the
solvent effects that are seen in these Diels–Alder reactions are a
function of both the solvent and the solute. For a solvent effect
to be seen, the solute must have a complimentary character. For
instance, when investigating both the selectivities and rates of these
reactions, it was found that the solvent hydrogen bond donation
ability (a) was important in the reactions of acrolein and methyl
acrylate, but not of acrylonitrile.
filled with nitrogen before adding the reactants. The second-order
kinetic constants are reported in Table 5.
Reaction of cyclopentadiene with methyl acrylate. In a typical
experiment, methyl acrylate (172 ll, 1.9 mmol) was added to 2.1 ml
ionic liquid in a capped vial under a nitrogen atmosphere and
the mixture was stirred for 15 min before adding a standard,
bromobenzene (0.1 ml, 9.4 mmol). Then, the reaction was initiated
by injection of freshly cracked cyclopentadiene (150 ll, 1.9 mmol).
The solution was magnetically stirred for 6 h 30 min under a
nitrogen atmosphere. Aliquots (50 ll) were removed from the
reaction mixture at various times, products and reagents were
extracted using diethyl ether or other solvents (0.5 ml), and the
extracts analyzed by GC. The kinetic constants were determined
following the disappearance of the dienophile over time.
Experimental
Dicyclopentadiene (90%), acrolein (90%), acrylonitrile (90%),
mesitylene (≥99.8%) and bromobenzene (≥99.5%) were pur-
chased from Fluka, methyl acrylate (99%) from Lancaster and
anhydrous methanol (99.8%) from Aldrich. Methanol, mesity-
lene, acrylonitrile and bromobenzene were used without further
purification; the ionic liquids were synthesized as reported in the
ESI† and dried by heating in vacuo to 65 ◦C for 3 hours prior
to use; the rest of solvents were dried by heating under reflux and
distilled following standard drying procedures. Dicyclopentadiene
was cracked before use; during the cracking process the collecting
flask was kept in liquid nitrogen to avoid dimerization. Methyl
acrylate was purified by several washings with an aqueous solution
containing 5% sodium hydroxide and 20% sodium chloride and
dried afterwards with anhydrous calcium chloride. The dried
solution was distilled and stored over calcium hydride in a
refrigerator. Before using, the monomer was distilled on a vacuum
line. Acrolein was distilled from CuSO4 at low pressure, placing
the receiver flask in liquid nitrogen to avoid polymerization. All
liquids were manipulated under a nitrogen atmosphere and syringe
techniques were employed in their transfer.
Reaction of cyclopentadiene with acrolein. In a typical experi-
ment, the cell containing 2 ml of the solvent was thermostatted at
the appropriate temperature ( 0.1 ◦C) for 10 min. Acrolein (20 ll,
299 lmol) was then added, and the solution was mixed manually
until homogeneous. Then, cyclopentadiene (24.8 ll, 299 lmol)
was added. The second-order rate constants were determined by
monitoring the decrease of absorbance of acrolein at appropriate
wavelengths (normally 250–450 nm) in a 1 cm quartz UV cell. For
all the reactions, the cuvettes were sealed with a septum to prevent
evaporation of the reactants; evaporation of cyclopentadiene
or/and acrolein could seriously hamper the kinetic measurements
and lead to large errors in the determination of the rate constants.
Reaction of cyclopentadiene with acrylonitrile. The second-
order rate constants for the reaction of acrolein with cyclopentadi-
ene were determined by monitoring the decrease of absorbance of
acrylonitrile at 240 nm using a 0.1 cm quartz UV cell. Typically, the
initial concentration of cyclopentadiene was ∼10−2 M and that of
acrylonitrile 20–100 times higher. In some cases the cut-off wave-
length of the solvent did not allow the monitoring of the reaction
by UV-vis spectroscopy, and therefore gas chromatography was
used. When gas chromatography was used, the kinetic experiments
were performed under second-order conditions (7.60 × 10−1 M),
following the disappearance of the dienophile, using mesitylene as
an internal standard.
GC measurements for the reaction of methyl acrylate and
acrolein with cyclopentadiene were performed using a RH-WAX
GC column (30 m × 0.25 mm ID × 50 lm). The conditions used for
◦
all runs were: injector temperature 250 C; detector temperature
250 ◦C; oven temperature 120 ◦C and 100 ◦C respectively; total run
time 15 min. GC measurements for the reaction of acrylonitrile
were carried using a HP-Wax crosslinked polyethylene glycol
column (30 m × 0.25 mm ID × 0.25 lm). The conditions used for
◦
all runs were: injector temperature 250◦ C; detector tempe◦rature
Determination of the endo/exo ratio in the reaction of
cyclopentadiene with methyl acrylate, acrolein and acrylonitrile in
ionic liquids and molecular solvents
250 ◦C; oven 70 ◦C for 15 min, then 10 C min−1 up to 120 C.
Equation coefficients and statistical parameters of LSER cor-
relations were obtained by multilinear correlation analysis using
Minitab 14 or SPSS statistical software.
In a typical procedure to measure the endo/exo ratio, the
dienophile (175 ll, 1.9 mmol) and freshly cracked cyclopentadiene
(150 ll, 1.9 mmol) were added to 2.1 ml ionic liquid or molecular
solvent in a sealed vial. The single solution was stirred at 25 ◦C for
24 h under nitrogen atmosphere. A sample of 50 ll was taken from
the reaction and the product mixture and reagents were extracted
from the ionic liquid, preferentially with diethyl ether (0.5 ml),
but sometimes another molecular solvent (0.5 ml). Extracts were
analyzed by GC to obtain the endo/exo ratios reported in Table 1.
Determination of the kinetic constants in ionic liquids and
molecular solvents
The Diels–Alder reactions between cyclopentadiene and methyl
acrylate or acrolein and were carried out under second-order
conditions using UV-vis or GC techniques. In contrast, the kinetics
of acrylonitrile were studied by UV under pseudo-first-order
conditions, using an excess of the dienophile (20–100-fold), and
by GC under second-order conditions with equal concentrations
of reagents. All reactions were contained within 1 or 0.1 cm
path length cuvettes, using a Perkin Elmer Lambda 2 or a Cary
2200 UV-vis spectrophotometer. Quartz cuvettes were sealed and
Acknowledgements
We would like to thank the EPSRC for a studentship (VLM) and
the University of Pisa for financial support.
2528 | Org. Biomol. Chem., 2008, 6, 2522–2529
This journal is
The Royal Society of Chemistry 2008
©
Bini, Riccardo
