4500
Organometallics 2005, 24, 4500-4502
Insertion of Carbon Dioxide into (PCP)PdII-Me Bonds
Roger Johansson, Martin Jarenmark, and Ola F. Wendt*
Organic Chemistry, Department of Chemistry, Lund University, P.O. Box 124,
S-221 00 Lund, Sweden
Received July 4, 2005
Summary: A (PCP)PdMe complex has been synthesized
and structurally characterized, displaying a very long
Pd-Me bond. It is reactive toward CO2 insertion, giving
the corresponding acetate in quantitative yield. The
methyl complex can be regenerated using ZnMe2, and
catalytic carboxylation is possible in benzene.
There is no information about a stoichiometric insertion
step, but such a step seems likely.6 Our earlier inves-
tigations of Pd-Me complexes with the methyl group
trans to nitrogen or phosphorus show that they are
either inert or, in the presence of small amounts of
water, give carbonate complexes.7 We therefore decided
to try to labilize the Pd-C bond, and our main focus
has been on PCP-type complexes which combine high
thermal stability with a reactive site trans to the trans-
labilizing in-plane aryl ring. PCP complexes of pal-
ladium have been shown to catalyze a large number of
C-C bond forming reactions.8 Here we report the
synthesis of (PCP)Pd complexes with hydrocarbyl ligands
in the fourth position and their reactivity toward CO2.
We also report the structural characterizations of
(PCP)Pd methyl and acetate complexes.
With the increasing awareness of the world’s finite
petroleum reserves, carbon dioxide has become interest-
ing as a feedstock for the synthesis of useful organic
molecules.1 The formation of C-C bonds would be of
particular value, but of course this poses difficulties,
since CO2 is an inert molecule that does not react readily
under most conditions. There are examples of a number
of transition-metal carbon bonds that undergo inser-
tion,2 and there are also examples of catalytic reactions
that form new C-C bonds.3
To develop new reactions for catalysis, we decided to
investigate the insertion into Pd-C σ-bonds. Palladium
is known to activate C-H bonds in hydrocarbons,4
which are interesting substrates to react with CO2 in a
C-C bond-forming reaction. Also, palladium-allyl com-
plexes are known to undergo insertion reactions with
CO2;5 however, there are no known examples of inser-
tions into Pd-C σ-bonds. Furthermore, Fujiwara and
co-workers have reported palladium-catalyzed direct
carboxylations of arenes, albeit with low conversions.
The free (PCP)H (1) ligands were synthesized accord-
ing to literature methods9 and reacted with Pd(TFA)2
to give the TFA complexes 2 and 4 (cf. Scheme 1). These
Scheme 1
* To whom correspondence should be addressed. E-mail: ola.wendt@
organic.lu.se.
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10.1021/om0505561 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/19/2005