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16
LETTER
A Polymer-supported Iridium Catalyst for the Stereoselective Isomerisation of
Double Bonds
P
I
olymer
-
a
Supported
n
Iridium Catalyst fo
R
r
the
S
tereoselec
.
tive Isomeris
B
ation axendale, Ai-Lan Lee, Steven V. Ley*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
Fax +44(1223)336442; E-mail: svl1000@cam.ac.uk
Received 18 December 2001
In general the isomerisation of electron-rich aromatic
Abstract: A polymer-supported iridium catalyst has been prepared
and used in the isomerisation of the double bonds in aryl allylic de-
rivatives with excellent trans selectivity and without the need for
conventional work-up procedures.
compounds by catalyst 2 results in products with predom-
inantly the trans alkene geometry (entries 1–7; Table).
However, electron-poor derivatives (entries 13 and 14)
are substantially less prone to rearrangement. In fact entry
Key words: iridium, isomerisation, polymer-supported reagents
1
3 was recovered unchanged from the reaction after 12 h.
The iridium catalyst 2 was also examined with various al-
lyl ether derivatives (Table, entries 9–12). Although the
isomerisations proceeded smoothly, these were not trans
selective. However, catalyst 2 may still find an application
in allyl ether deprotections where the trans selectivity is
not relevant.7
The use of solid-supported reagents, catalysts and scaven-
gers is becoming increasingly important for the assembly
of organic compound libraries. In the case of transition
metal catalysts, attachment to a solid-support often offers
the additional advantages of reduced toxicity and air-sen-
sitivity of the species as well as the possibility of recycling
the catalyst.
1
In another experiment we determined whether the isomer-
isations were solely due to 2 or due to a species that may
have leached into the solution from the support during the
course of the reaction. To test this, the reaction with dilla-
piole (Table, entry 6) was stopped after 15 minutes reac-
tion time by filtration, when analysis showed a 60%
conversion to the desired product. The filtrate was then al-
lowed to stir for a further 12 hours. No further reaction
was observed, suggesting that the active catalyst remains
bound on the support during the reaction.
Figure
In an attempt to show that 2 can be recycled, we carried
out a sequence of reactions on allyl benzene. The recov-
ered polymer from the first conversion was reactivated
We became interested in the mild and selective trans
isomerisation of aryl allylic derivatives during the synthe-
8
with hydrogen and reused for a further three runs. The
2
sis of the natural product carpanone. A polymer-support-
conversion dropped from quantitative to 95%, 91% and
ed iridium isomerisation catalyst was developed based on
7
5% in each subsequent run, suggesting some degradation
3
Felkin’s iridium catalyst. A 6:1 molar ratio of polymer-
9
of the catalyst (the cis:trans ratio remained constant).
4
supported triphenylphosphine and [(COD)IrCl] [LG1]
2
As a further investigation the catalytic potential of 2
was tested using the dimethoxy substrate entry 2 (Table).
Employing catalyst 2 (2 mol%) for the isomerisation of
800 mg of the substrate resulted in 77% conversion after
was agitated in THF at room temperature for 24 h. Addi-
tion of ammonium hexafluorophosphate as a solution in
THF and agitation for a further 24 h produced a red poly-
5
mer with the proposed structure 1 (Figure). Elemental
2
4 h. Subsequent monitoring over an additional 3 day pe-
analysis indicated 12.5% Ir, corresponding to a theoretical
loading of 0.65 mmol/g. Hydrogen activation of catalyst 1
produces a chrome yellow polymer with suggested struc-
ture 2 (Figure) which is used to isomerise various aryl al-
riod showed a slow but steady progression to 84% conver-
sion (day 2: 80%; day 3: 82%). The addition of a further
batch of freshly prepared catalyst (1 mol%) allowed
the reaction to proceed to completion over a further 48 h
period.
6
lylic derivatives (Table). The reactions proceed smoothly
in THF at ambient temperature requiring only gentle agi-
tation, work-up is then achieved by simple filtration and We believe that the new polymer-supported catalyst 1 will
solvent evaporation to yield the isomerised products.
add to the rapidly expanding armoury of immobilised re-
agents for use in organic synthesis programmes.
Synlett 2002, No. 3, 04 03 2002. Article Identifier:
1
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