November 1998
SYNLETT
1211
Cycloisomerisation of 1,6-Dienes in the Presence of Cationic Palladium Catalysts
Andreas Heumann* and Maati Moukhliss
Université d'Aix-Marseille, Faculté de St-Jérôme, ENSSPICAM, UMR-CNRS 6516, F 13397 Marseille Cedex 20, France
fax: +33 4 91 02 77 76; e-mail: heumann@spi-chim.u-3mrs.fr
Received 6 August 1998
Abstract: Mono- and dicationic palladium efficiently catalyses the
cycloisomerisation of bis allyl substrates with high chemo-, regio- and
enantioselectivities (60% ee).
When the catalyst precursor PdCl2-5 was treated with 2 equivalents of
silver salt, presumably a dicationic palladium complex is formed,
which, in the presence of bis pyrazole ligands leads to the formation of
the isomeric exomethylene cyclopentane 3 (75%) accompanied with
circa 20% of 2 (60°C, 4 hours, ratio determined by 1H NMR). A similar
result is obtained with commercial (MeCN)4Pd(BF4)2 (39% yield, 8
hours) at room temperature. The latter catalyst is less stable and
decomposes slowly during the reaction. In addition the lack of
heterocyclic ligands decreases the selectivity and isomeric, but non
cyclised alkadienes are observed (Table 1). The formation of endocyclic
cyclopentene 2 from 1 (vide supra) follows a different reaction pathway,
than the isomerisation to exomethylene-3 though 2 could isomerise to 3
(and vice versa). Thus, when heating 2 with [Pd2+] (7 hours, CHCl3) no
modification of 2 is observable in the NMR spectra. The situation is
more complicated with the kinetic product 3 which does not form
exclusively. Treatment of 3 (product mixture from entry 5) with
[PdCl2+] (50°C, 22 hours, CHCl3) effectively leads to isomerisation (2
45%, 3 53%). This kind of experiment was the sole case where the
symmetrical cyclopentene 4 could be detected.9
Unsaturated cycloalkanes are an integral part of many natural products
and their preparation is of permanent interest in organic synthesis. One
important method under the conditions of homogeneous catalysis
concerns the (atom economically) interesting cycloisomerisation1 of
alkynes2 and 1,3-dienes.3 In this context, the cycloisomerisation of 1,6-
dienes (diallylmalonates, e.g. 1) described by Grigg and coworkers4 has
been shown to lead to unsaturated five-membered rings with late
transition metals, and, more precisely, palladium acetate in chloroform
containing HCl gave selectively 1,2-dimethylcyclopent-2-enes
2
whereas rhodium(I) chloride triphenylphosphine complexes catalyse the
formation of 1-methyl-2-methylenecyclopentanes 3. It seemed to us
interesting to develop this reaction for several reasons. First of all, the
starting material is easily available and it is possible to introduce
heteroatoms (N, O, S, Si, and others) between the allyl groups, allowing
an access to carbo- and heterocylic five-membered rings. On the other
hand, the generation of a chiral centre is an opportunity to study
enantioselective catalytic transformations, provided the system tolerates
chiral ligands. Now, we are able to show that palladium complexes can
catalyse both reactions efficiently with high regioselectivity and, more
importantly, that the addition of nitrogen centered chelating ligands does
not alter the reactivity thus permitting enantioselective palladium
catalysis with chiral complexes.
Apart from its atom economical1 aspect this reaction is remarkable for
different reasons. First of all, the ring closure is highly chemo- and
regioselective and the position of the double bond depends on the
charge on the Pd-catalyst. Secondly, symmetrical compounds are absent
and products form with creation of a stereogenic center. For this reason
it is important that selectivity and yields are improved with heterocyclic
amine-coordinated palladium. These three conditions are favorable
prerequisites for the elaboration of catalytic enantioselective
transformations.10
It is well known that cationic palladium complexes favour the
dimerisation of ethylene compounds.5 When
1 was treated in
chloroform at 60°C with cationic [(MeCN)3PdCl]BF4 generated in situ
from bisacetonitrile palladium(II) chloride and AgBF4, 4,4-
biscarboxylato-1,2-dimethylcyclopent-2-ene 2 was formed in more than
79% yield, after distillation. The compound is isomerically pure, and
only trace amounts of 3 or symmetrical 4 could be detected.
Scheme 2
Both cycloisomerisation reactions of 1 occur with mono- and dications
of a Pd complex coordinated to 5. In the case of monocationic Pd the
transformation seems slightly more efficient. Dicationic Pd is less stable
and yields are lower. Variable amounts of cyclopentene 2 are also
formed as a side product, and sometimes, non-cyclised, isomerised
dienes. The sterically more demanding ligand 6 reduces the reactivity
and raises reaction times even at 50°C. The control of the different
isomerisation processes will certainly require more profound studies of
the reaction conditions, nonetheless we were pleased to observe
appreciable chiral inductions with bis-oxazoline and (-)-sparteine
palladium(II). Thus, under 'dicationic' conditions a mixture of scalemic
2 (23 to 37%ee) and 3 (60 %ee) form after 7 hours at 50°C. The
enantioselectivies in the formation of 2 are variable and differ notably
according to the catalysts. Thus, the PdCl+ system is rather unselective
(7% ee, sparteine) whereas the Pd2+ catalysis generates better, but still
modest inductions of 23-37%ee (entries 3, 6 and 7).
Scheme 1 a (MeCN)2PdCl2 (0.05 equiv.), MeCN, AgBF4 (0.045 equiv.)
20°C, filtration and CHCl3, 60°C, 18h,
2 79% (distilled); b
(MeCN)4Pd(BF4)2 (0.05 equiv.), CHCl3, 20°C, 8h, 3 (39%) (NMR).
The reaction course and the yield of 2 is not notably altered when the
cationic catalyst is coordinated to polypyrazoles, e.g. the methano bis
pyrazole (CH3)2C(pz)2 5, prepared from (MeCN)2PdCl2 in
dichloromethane.6 Though the exact mechanism for the formation of 2
is not yet clear,4 it is trivial that at least two hydride migrations must be
involved. It has been shown that dicationic palladium species favour
carbocationic rearrangements of substituted olefins7 but, at the same
time, are highly selective dimerisation catalysts of styrene when
coordinated to pyridine ligands.8 In the latter reaction no double bond
migration was observed.
Heumann, Andreas
