J . Org. Chem. 2000, 65, 3231-3232
3231
P r ep a r a tion of a Resin -Bou n d
Ar en e-Ru th en iu m Com p lex a n d
Assessm en t of Its Use in En ol F or m a te
Syn th esis a n d Olefin Cyclop r op a n a tion
Nicholas E. Leadbeater,*,† Kathryn A. Scott,‡ and
Lucy J . Scott
Department of Chemistry, University of Cambridge,
Lensfield Road, Cambridge CB2 1EW, United Kingdom
Received December 16, 1999
In tr od u ction
Recent interest in the development of environmentally
benign synthesis has evoked a renewed interest in
developing polymer-bound metal catalysts and reagents
for organic synthesis that maintain high activity and
selectivity.1 Advantages of attaching a catalyst to a
polymer support include ease of separation from the
product mixture at the end of a reaction and the fact that
attaching a metal complex to a polymer can reduce the
toxicity and air sensitivity of the species considerably.
In addition, as the catalyst is easily removed from the
reaction mixture, it can be reused in subsequent reac-
tions. As transition metal complexes are often expensive,
attaching the species to a polymer support also has
economic implications.
In this paper, we report the preparation and synthetic
versatility of the polymer-supported arene ruthenium
complex 1. This and other arene ruthenium complexes
are used frequently in metal-mediated organic synthesis
for reactions as diverse as transfer hydrogenation,2
Diels-Alder chemistry,3 olefin cyclopropanation,4 and
enol formate formation.5 For the purposes of our studies,
we have focused attention on enol formate synthesis and
cyclopropanation as representative reactions for compar-
ing the activity of 1 with its nonsupported analogue [Ru-
(p-cymene)Cl2(PPh3)] (2). Our attention has particularly
been focused on leaching, if any, of the metal complex
from the polymer support and also on the effect of
catalyst recycling.
F igu r e 1. Preparation of polymer-bound catalyst 1.
cross-linked with 2% divinylbenzene; 3 mmol P/g resin).
The immobilized complex 1 was prepared by thermolysis
of the dimer [Ru(p-cymene)Cl2] with the functionalized
resin in dichloromethane/toluene (1:2) (Figure 1). Sub-
sequent filtration, washing, and drying of the polymer
gave a deep red powder that was characterized as 1 on
comparison of spectroscopic data with that of the previ-
ously reported complex 2,6 which is also deep red in color.
The assignment was further confirmed by elemental
analysis, this also allowing us to determine the catalyst
loading at 2.5 mmol per gram of resin. By varying the
molar ratio of phosphine functionalized polymer to
ruthenium arene dimer the P/Ru ratio could be varied
but not increased above the threshold of 6:5. The polymer
bound complex formed is stable in air, no decomposition
being noted over the period of 3 months at room tem-
perature.
Use of 1 a s a Ca ta lyst for En ol F or m a te Syn th esis
a n d Olefin Cyclop r op a n a tion . In the presence of a
catalytic amount of 1, the regioselective addition of formic
acid to a range of terminal alkynes and diynes led to the
formation of the corresponding enol formates in good
yield as shown in Figure 2. For comparative purposes,
reported yields for the analogous reactions using 2 are
also shown.5,7 From these results, it is clear that the
attachment of the metal complex to the polymeric support
has little effect on the yields of reaction compared to the
homogeneous analogue. Reactions were performed in
toluene as this led to optimum yields even though
swelling of the beads is not as marked as in other solvents
such as dichloromethane or thf.
Resu lts a n d Discu ssion
P r ep a r a t ion of P olym er -Su p p or t ed Ca t a lyst 1.
The polymer support chosen for immobilization of the
ruthenium arene complex was commercially available
“polymer-supported triphenylphosphine” (polystyrene
† Current address: Department of Chemistry, King’s College Lon-
don, Strand, London WC2R 2LS, United Kingdom.
‡ AstraZeneca funded vacation project student.
In an attempt to show that 1 can be recycled, the
reaction of phenylacetylene with formic acid was repeated
five times using the same batch of supported catalyst.
As seen in Table 1, the yields remain around 90%, clearly
illustrating the reusability of the catalyst. The entire
crude reaction mixture in each case was dissolved in
CDCl3 and analyzed by 1H NMR spectroscopy. There
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10.1021/jo991921z CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/19/2000