Ž
.
Journal of Organometallic Chemistry 553 1998 473–476
Priority communication
Modelling intermediates in the catalytic carbonylation of CH2 I2 to
malonate esters; Evidence for a ketene pathway
)
William S. Weston , Philip Lightfoot, David J. Cole-Hamilton
School of Chemistry, UniÕersity of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland, UK
Received 29 July 1997; received in revised form 26 August 1997
Abstract
From model intermediates and from labelling studies, a ketene-based mechanism is proposed for the double carbonylation of CH2I2 to
.Ž . x
w
Ž
malonate esters catalysed by RhX CO PEt3
and for the deactivation of the catalyst. q 1998 Elsevier Science S.A.
2
Keywords: Catalytic carbonylation; Rhodium complexes; Labelling studies; Mechanism
Malonate esters are important intermediates in a vari-
ety of organic transformations and there are only a very
few reports of their production from the catalytic car-
We reasoned, therefore, that since the C–Cl bond is
stronger than the C–I bond it might be possible to
isolate an analogue of the putative iodoacyl intermediate
from oxidative addition of ClCH 2 COCl to
w
x
bonylation of dihaloalkanes. 1–7 We have recently
w
Ž
.Ž
. x
reported that complexes of the form RhX CO PEt3 ,
2
w
Ž
.Ž
. x
RhCl CO PEt3 . This reaction proceeds smoothly to
2
Ž
.
w
Ž
.Ž .Ž
. x
2
XsOAc, Cl or I can catalyse the double carbonyla-
give RhCl2 COCH2Cl CO PEt3 in high yield. This
Ž . w x
Ž
.
tion of CH2 I2 as in Eq. 1 8 .
complex has the expected structure see Fig. 1 with
trans-phosphines and the chloroacyl group trans to Cl.
We know of only one chloroacyl complex to have been
structurally characterised 10 , Co COCH 2
Cl CO PPh3 , and, despite the rather low precision
w
Ž
.Ž
.2x
RhX CO PEt3
CH2 I2 qCO
™
CH2 CO Et
1
Ž .
Ž
.
2
2
EtOH
w
x
w
Ž
The reaction only proceeds in low yield so we were
interested to determine the mechanism in order to im-
prove the process.
.Ž . Ž
.x
3
of our structure arising from disorder in the PEt3 lig-
ands, the C–Cl bond in the rhodium complex does not
1
w x
Our initial studies 9 showed that the oxidative
appear to be especially weakened relative to that in the
cobalt complex.
Assuming that the ketene complex does form from
the iodoacyl species, it is possible to propose a new
mechanism for the double carbonylation of CH2 I2
w
Ž
.Ž .2 x
addition of CH2 I2 to RhCl CO PEt3 proceeds
w
Ž .Ž
.Ž .Ž
.
x
smoothly to give RhCl I CH2 I CO PEt3 , which
2
was structurally characterised, but that reaction of CO
with this complex did not give the expected insertion
X
w
X Ž .Ž
.
2 x
Ž
product but rather RhX3 CO PEt3
X3 is a mixture
Ž
.
Scheme 2 . This involves, in the last step, the reductive
.
of Cl and I . Further attempts to produce an iodoacyl
intermediate by oxidative addition of ICH2COCl to
Ž
.
elimination of ICOCCH2COOEt from a rhodium III
intermediate. We have modelled this intermediate by
the oxidative addition of ClCOCH 2 COOEt to
w
Ž
.Ž
.
2 x
w
X Ž .Ž
. x
also produced RhX3 CO PEt3 ,
2
RhCl CO PEt3
but diketene was detected as a product, suggesting that
the iodoacyl complex may be unstable with respect to
cleavage of the C–I bond and formation of an ionic
w
Ž
.Ž .2 x
RhCl CO PEt3 to form the monodentate ethyl-
malonyl complex, the structure of which is shown in
Ž
.
ketene complex Scheme 1 .
1 Because the precision of the structure determinations is low, the
data has not been deposited at the Cambridge Crystallographic
Database, but full structural parameters are available from the au-
thors.
)
Corresponding author. Johnson Matthey, Orchard Rd., Royston,
Herts, SG8 5HE.
0022-328Xr
98r
$19.00 q
1998 Elsevier Science S.A. All rights reserved.