Terblans et al.
855
Scheme 1. Ring opening of butene oxide with methanol.
OCH3
OH
Al(CF SO )
3 3
O
3
HO
CH O
3
+
CH OH
3
1-Methoxy-2-butanol
2-Methoxy-1-butanol
alcohols (8), as well as for the transesterification of
carboxylic acid esters with a slight excess of alcohols via the
use of trimethylsilyl chloride (TMSCl) as a co-catalyst (9).
Other organic base triflates that have successfully been
employed in esterification reactions include 4-(benzylamino-
carbonyl)diphenylammonium triflate (BDPAT) (10) and
polymer-supported 4-aminoforoyldiphenylammonium triflate
Scheme 2. Esterification of acetic acid with ethanol or n-
propanol.
O
O
Al(CF SO )
3
3 3
OH
OR
ROH
Ester
(
PS-AFDPAT) (11).
IR thermography has been used to screen the catalytic
ROH = CH CH OH or CH CH CH OH
3
2
3
2
2
acitivity of rare earth triflates (e.g., Eu(CF SO ) , La(CF SO ) ,
3
3 3
3
3 3
Lu(CF SO ) , Nd(CF SO ) , Sm(CF SO ) , Sc(CF SO ) ,
3
3 3
3
3 3
3
3 3
3
3 3
Y(CF SO ) , and Ce(CF SO ) ) for the ring opening of
3
3 3
3
3 4
action and ∆H = +12.8 kcal/mol for the potassium methoxide
f
epoxides by alcohols as well as the Baeyer–Villiger oxida-
tion of cyclobutanones with hydrogen peroxide. In the case
of the ring opening of epoxides, scandium triflate was found
to be an exceptionally active catalyst, whereas in the case of
the Baeyer–Villiger oxidation, it was again the most active,
followed by neodymium triflate (12). Furthermore, it has
been reported that Yb(CF SO ) catalyzes intermolecular
ring-opening reaction. It appears that the differences in cata-
lyst concentrations required for the reactions under investi-
gation are in line with the differences in the formation
energies of the active intermediate.
Experimental
3
3 3
ring-opening reactions of epoxides with alcohols to give β-
alkoxy alcohols in good to high yields with high regio- and
stereoselectivity (13).
Materials
Unless otherwise stated, all chemicals used in this study
were purchased from Sigma-Aldrich and used without fur-
ther purification. Aluminium triflate was prepared as de-
scribed in the literature (16). For the ring-opening reactions,
a stock solution of 500 ppm Al(CF SO ) was prepared in
Aluminium triflate has been used as a catalyst in organic
synthesis for reactions such as the ring opening of epoxides
in the presence of alcohols or amines (14–16), Friedel–
Crafts acylation and alkylation (17, 18), the epoxidation of
olefins with iodosylbenzene (19), and the preparation of es-
ters and lactones (20). It has also been used as an initiator
for cationic polymerization (21) and as co-catalyst in the
polymerization of olefins (22).
3
3 3
methanol and the desired quantity thereof was weighed for
every reaction. In the case of the esterification reactions, a
fresh catalyst solution (of the desired concentration) was
prepared in the alcohol substrate prior to each reaction.
We now report on the considerable difference in the re-
Kinetic reactions
quired concentration of Al(CF SO ) vs. KOCH as catalysts
3
3 3
3
Kinetic reactions were carried out in a Parr autoclave. The
catalyst concentration was varied between 0 and 10 ppm for
the ring-opening reactions and between 10 and 5000 ppm for
the esterification reactions. In most cases, the esterification
reactions were allowed to proceed to equilibrium, since none
of the reaction products (including water) were removed.
Gas chromatography was performed on a Shimadzu GC-
for the ring opening of butene oxide (BuO) with methanol
MeOH) (Scheme 1) on the one hand, and the similar con-
centration requirements of Al(CF SO ) and p-TSA as cata-
(
3
3 3
lysts for the esterification of n-propanol (n-PrOH) or ethanol
EtOH) with acetic acid (Scheme 2) on the other hand. Low
Al(CF SO ) concentrations (e.g., ~5 ppm) are sufficient to
(
3
3 3
obtain excellent conversions and high reaction rates when
performing the ring-opening reaction of butene oxide with
methanol, whereas much higher catalyst concentrations (e.g.,
1
7A, equipped with a flame ionization detector and a PONA
capillary column (50 m × 0.21 mm × 0.5 µm). Each sample
was weighed into a GC vial and heptane was used as an in-
ternal standard.
~
1000 ppm or higher) are needed to obtain similar results
for the KOCH catalysed reactions. A related observation
3
was that both acid (p-TSA) and Al(CF SO ) catalysed
esterification reactions of propanol and (or) ethanol with
acetic acid require similar catalyst concentrations of
Typical experimental procedure for the ring opening of
butene oxide
Methanol (192.25 g, 6.00 mol) and butene oxide (72.11 g,
1.00 mol) were combined in a 300 mL Parr reactor. On clos-
ing, the reactor was degassed and flushed with nitrogen to
remove all the air present. This flushing procedure was re-
peated three times.
3
3 3
~
1000 ppm to get reasonable activities. A molecular model-
ing study was undertaken with a view to rationalizing the ex-
perimental observations and to postulate a reasonable
explanation for the low levels of Al(CF SO ) needed in the
3
3 3
ring-opening reactions. These calculations showed that ∆Hf
values for the formation of the catalytically active species in
the potassium methoxide and aluminium triflate catalyzed
A catalyst bomb was placed under vacuum prior to the
run. The stock solution was weighed out (2.644 g, 5 ppm)
and transferred via a syringe to this bomb, which was then
connected to the inlet of the autoclave. A high-pressure ni-
trogen line was connected to the opposite side of the catalyst
ring-opening reactions differ significantly, with ∆H
=
f
–
3.3 kcal/mol (1 cal = 4.184 J) for the aluminium triflate re-
©
2005 NRC Canada