Palladium-catalysed formation of maleic anhydrides from CO, CO2 and
alk-1-ynes
Bartolo Gabriele,*a Giuseppe Salerno,*a Mirco Costab and Gian Paolo Chiusolib
a
Dipartimento di Chimica, Universita` della Calabria, 87030 Arcavacata di Rende, Cosenza, Italy.
E-mail: b.gabriele@unical.it; g.salerno@unical.it
Dipartimento di Chimica Organica e Industriale, Universita` di Parma, Viale delle Scienze, 43100 Parma, Italy
b
Received (in Liverpool, UK) 8th April 1999, Accepted 20th May 1999
Carbon dioxide causes palladium-catalysed synthesis of
unsaturated g-lactones from alk-1-ynes and CO to shift
towards maleic anhydrides.
The effect of CO2 can be rationalised in the following way
(Scheme 3; anionic iodide ligands are omitted for simplicity).
An I-Pd-CO2H species,2 stabilised by iodide ligands, is first
generated from PdI2, CO and H2O and then inserts the alkyne
and CO to give intermediate I. At this point two pathways are
possible. Formation of furanones requries the intervention of a
palladium hydride species,3 which must derive from decarbox-
ylation4 of I-Pd-CO2H, as shown in Scheme 3. Hydride
exchange on complex I by H-Pd-I5 followed by reductive
cyclization of intermediate II leads to allylpalladium complex
III, whose protonolysis affords 1 with regeneration of the
catalytically active species [path (a)]. On the other hand,
anhydrides can be formed via tautomerization of acylpalladium
intermediate I6 followed by elimination of Pd0 and HI or I-Pd-H
[path (b)]. This reaction is stoichiometric and reoxidation is
needed to start a new cycle.6,7 However, in the presence of an
excess of CO2 I-Pd-H can be reconverted into a catalytically
active form. In fact, CO2 may insert into the palladium hydride
bond8 with formation of either a I-Pd-CO2H species (which can
directly start a new cycle) or a Pd-O(CO)H species, whose
protonolysis by HI would afford PdI2. In both cases no
reoxidation is needed and the anhydride cycle can go on.
Moreover, in the presence of added CO2, the decarboxylation
equilibrium is shifted to the left and the anhydride cycle
becomes competitive with the reduction pathway leading to
furanones.
We recently showed that working in dioxane–water at 80 °C
under a 10 atm pressure of CO in the presence of PdI2 and 10
equiv. of KI leads to catalytic reductive carbonylation of alk-
1-ynes with formation of furan-2(5H)-ones (Scheme 1).1
Oxidation of CO to CO2 accounts for the stoichiometry of the
process.
Scheme 1
We now find that in the presence of added CO2 another
catalytic reaction takes place, consisting of the formation of
maleic anhydrides according to Scheme 2.
R
Pd cat.
RC CH
+
CO
+
CO2
O
O
(R = alkyl, aryl)
O
2
In summary, we have shown that under our conditions CO2
strongly influences the distribution of products. In the presence
of an excess of added CO2 the resulting concentration of
palladium hydride species is reduced in favour of Pd-CO2H
and/or Pd-O(CO)H species, thus minimising furanone forma-
tion and allowing the anhydride cycle to run.
The reaction reported here is the first example in which CO
and CO2 are used together for the catalytic formation of
unsaturated cyclic anhydrides. The stoichiometric formation of
an anhydride from a metallacyclic complex, CO and CO2 in two
steps was described some years ago.9 In the recently reported
Scheme 2
Table 1 reports the results obtained with and without
additional CO2 pressure. By working under the same reaction
conditions which selectively lead to furanones but under an
additional 40 atm pressure of CO2, the product distribution was
clearly altered in favour of maleic anhydrides, although the
overall reaction rate was decreased.
Both the presence of small amounts of water and the nature of
PdII counterion were essential to the process, only traces of
products being obtained under anhydrous conditions or using
PdCl2 and 10 equiv. of KCl as catalyst. Decomposition to Pd
metal occurred only to a limited extent. In any case, Pd metal
was not a catalytically active species for the present reaction, as
shown by control experiments.
Table 1 Reactions of alk-1-ynes (100 equiv.) with CO (10 atm), CO2 and
H2O (200 equiv.) in dry dioxane in the presence of PdI2 (1 equiv.) and KI
(10 equiv.), substrate conc.: 0.5 mmol ml21 dioxane, T = 80 °C
Yield (%)a
P(CO2)/
Substrate
atm
t/h
1
2
BuC·CH
BuC·CH
PhC·CH
PhC·CH
—
40
—
40
15
64
15
24
77b
32c
67
7
47
traces
34
30
a
b
Based on starting alk-1-yne, by GLC.
(8%) was also detected in the reaction mixture. c 4-Butyl-2(4H)-furan-2-one
4-Butyl-2(5H)-furan-2-one
(10%) was also present in the reaction mixture.
Scheme 3
Chem. Commun., 1999, 1381–1382
1381
Gabriele, Bartolo
