9
58
N. Lewis et al.
LETTER
Thus the titanium containing polymers in the Table were thylamino ethanol gave no transesterification products
then evaluated in the key transesterification reaction with catalysts C and D. These substrates were found to de-
shown in the Scheme.
activate the catalysts, presumably owing to their ability to
form tight chelates with titanium.
In conclusion we have identified two highly active, insol-
uble polymer-based transesterification catalysts particu-
larly useful for the formation of functionalised
methacrylates. It is anticipated that these catalysts will be
useful for the preparation of a range of important meth-
acrylate monomers and other esters containing sensitive
functionality. The polymers may also be useful catalysts
in other titanium catalysed processes.
Scheme
Catalyst A was found to be totally inert as a transesterifi-
cation catalyst despite containing the highest loading of ti-
tanium. Presumably the titanium species bound in the
polymer is highly ligated, stopping the facile ligand ex-
change required for transesterification activity. Catalysts
B and E were found to act as transesterification catalysts,
although they were only weakly active. The most active
catalysts for the preparation of 2 were catalysts C and D
References and Notes
(1) Seebach, D.; Hungerbuehler, E.; Naef, R.; Schnurrenberger,
P.; Weidmann, B.; Zueger, M. Synthesis, 1982, 138.
(
2) Akelah, A. Synthesis, 1981, 413. Akelah, A.; Sherrington, D.
C. Chem. Rev. 1981, 81, 557. Manecke, G.; Storck, W. Angew.
Chem. Int. Ed. Engl. 1978, 17, 657.
(3) Deleuze, H.; Schultze, X.; Sherrington, D.C. Polymer, 1998,
39, 6109.
1
0
where transesterification was rapid (4 hours). In both
cases high conversion to the desired methacrylate mono-
mer 2 was obtained (92-97%). The product 2 was isolated
in 87% yield and high purity (97%) simply by filtration of
the catalyst and removal of excess methyl methacrylate by
distillation. In addition both catalysts C and D could be re-
used at least four times, although some reduction in rate of
reaction was observed on re-use. It was later found that
catalyst C was moisture sensitive and this accounted for
the loss in activity since no precautions to protect the cat-
alysts from atmospheric moisture were taken prior to re-
use. Indeed a fresh batch of catalyst C exposed to air over-
night prior to use gave only 40% conversion to 2 in 6h
compared with 97% conversion in 4h for freshly made
catalyst C. Comparing the activity of catalysts B to E, the
low transesterification rate with E was attributed to the
low level of titanium incorporation. Although catalyst B
had an equivalent level of titanium incorporation and
cross-linking to C, the lower activity was presumably due
to the titanium being too tightly ligated (as with A) to en-
able efficient transesterification to occur.
(4) Typical experimental for catalyst C: Polyethylene glycol
600 bound to cross-linked polystyrene (Fluka, 10 g)
containing 0.35 m eq g ligand was treated with
chlorotriisopropoxytitanium (1.0 g, 0.004 mol) and toluene
-1
(100ml) and heated at reflux under nitrogen for 18h. The
mixture was cooled and the resulting polymer beads filtered
and washed with toluene. The polymer beads were then
repeatedly slurried with dichloromethane and filtered until the
filtrate showed no colour change when treated with a 1%
solution of hydroquinone in acetone. The resultant polymer
beads were dried in vacuo at 75°C to give 9.6 g of catalyst
C as white beads.
(
5) Typical experimental for Catalyst D: Acetyl acetone
functionalised polymethylstyrene was prepared by suspension
polymerisation of a mixture of 3-(3- and 4-vinylbenzyl)-
pentane-2,4-dione (29.9 g, 0.15 mol) and divinylbenzene (2
6
ml, 0.014 mol). The monomers were suspended in a mixture
of water (300ml) and polyvinylalcohol (0.2 g). Dibenzoyl
peroxide (1.0 g, 0.004 mol) was added and the mixture heated
at reflux for 2h. The mixture was cooled, filtered and the
product washed with methanol (2x150 ml), filtered, washed
with acetone and dried to give 23 g of white beads. 10g of the
resulting polymer was treated with
chlorotriisopropoxytitanium (30ml, 0.13 mol), Hünigs base
(6.5 g, 0.05 mol) and 1,2-dichloroethane (100ml) and heated
at reflux under nitrogen for 19h. The mixture was cooled and
the bright orange beads filtered. The polymer beads were then
repeatedly slurried with dichloromethane and filtered until the
filtrate showed no colour change when treated with a solution
of 1% hydroquinone in acetone. The resultant polymer beads
were dried in vacuo at 65°C to give 11 g of polymer D.
6) Linden, G.L.; Farona, M.F. J.Catal. 1977, 48, 284.
7) Benson, G.M.; Hickey, D.M.B.H. Patent WO 9118027, 1991;
Chem. Abstr. 1992, 116, 158913. Benson, G.M.; Alston, D.R.;
Bond, A.N; Gee, A.; Haynes, C.; Hickey, D.M.B.; Iqubal, S.;
Jackson, B.; Jaxa-Chamiec, A.A.; Johnson, M.R.; Roberts,
M.G.; Slingsby, B.P.; Whittaker, C.M.; Suckling, K.E.
Atherosclerosis, 1993, 101, 51.
(
(
Having discovered two active transesterification catalysts
for the preparation of 2, we briefly explored their utility
using different alcohols. The acetyl acetone-based cata-
lyst D was evaluated and found to be very active catalyst
for n-butanol and the sterically more hindered 2-ethylhex-
an-1-ol giving n-butyl methacrylate 4 and 2-ethylhex-1-yl
methacrylate 5 in 85% and 95% isolated yields respective-
ly in 4-6h. Attempts to use ethylene glycol or N,N-dime-
(
8) Lewis, N.J.; Wells, A.S. Patent WO 9109005,1991; Chem.
Abstr. 1991, 115, 184134.
9) Wells, A.S. Synth.Commun. 1996, 26, 1143.
(
(
10) Typical procedure for transesterification: 11-
Bromoundecanol (3, 5 g, 0.02 mol) was dissolved in methyl
Synlett 1999, S1, 957–959 ISSN 0936-5214 © Thieme Stuttgart · New York