1. ALKYL CARBONATE SYNTHESIS FROM CO2 AND ALCOHOL
J. Cámpora*, P. Palma*, Luis M. Martínez-Prieto and Eleuterio Álvarez
Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla
Isla de la Cartuja, Avda. Américo Vespucio, Isla de la Cartuja, Sevilla, Spain
Introduction:
Mononuclear alkoxo complexes of late transition metals (Ni and Pd) are unusual species characterized by very high
basicity and nucleophilicity. These compounds readily add carbon dioxide to give carbonate derivates. This reaction is
interesting for its relation with some important catalytic processes, such as the synthesis of organic carbonate derivatives.
Using CO2 as a readily available renewable carbon source for carbonate synthesis, avoiding the use of phosgene, is
potentially an atractive kind of environmentally friendly industrial process. In the present investigation, we have used
mononuclear hydroxo and alkoxo derivates of nickel and palladium stabilized by PCP pincer-type ligands as model systems,
because these highly chelating ligands make it possible to stabilize and identify reaction intermediates.
Synthesis of Alkoxo Complexes
Carboxylation of Methoxo Complexes
We are working mainly in three synthetic routes.
The insertion of CO2 into M-OR bonds of transition-metal alkoxo complex is known for late transition metals. The reaction usually occurs under mild conditions and it is found to be reversible (1).The CO2 insertion may have a practical interest, because the resulting alkyl carbonate may generate the organic carbonate. By drying the compound under vacuum , the alkyl carbonate converts to a carbonate complex.
(1)Santosh, M.K. Organometallics 1993, 12, 1714.
Three-step Process
for Alkyl Carbonate
Reversible Carboxylation of Ni and Pd Hydroxides
In the literature there are complexes
capable of catalysing this process
but not very efficiently. The key problem
is finding suitable catalysts and
eliminate water. Dehydration may be
the most powerful methodology to shift
the reaction equilibrium to the carbonate
side. However, it is difficult to dehydrate
a reaction mixture containing a large amount
of alcohol, such as methanol, at a high
temperature or under a high CO2 pressure.
Ortep perspectives of bicarbonate and carbonate complex
This reaction is interesting because it constitutes a useful gauge of the reactivity of the complexes. We observed that mononuclear hydroxides readily react with carbon dioxide . Interestingly, these reactions are reversible and the products can be decarboxylated back to starting materials. For example, the hydroxides react with an excess of CO2 giving rise to labile mononuclear bicarbonate products. In the absence of CO2, facile decarboxylation reactions ensue, to give the corresponding binuclear carbonates. Theese are much more stable, but these can be also decarboxylated back to the corresponding hydroxides under THF reflux in the presence of water. Both, the carbonate and bicarbonate complexes, have been structurally characterized.