723-738 11.4 Inglese

Carbon monoxide (CO) is an important building block in industrial organic chemistry. Organic substrates containing a carbon-carbon unsaturation may undergo an addition of CO, normally resulting in the formation of a new carbon-carbon sequence, where one carbon atom is provided by carbon monoxide, the other ones originating from the unsaturated substrate, e.g. an olefin or an acetylenic derivative. Carbonylation reactions perhaps most appropriately exemplify how the nature of the metal catalyst can influence the formation of the products. Coordinative addition of the reagents to the metal catalyst is believed to be an essential step on the way to products. Selectivity can originate from either the intrinsic thermodynamic stability of the product or from the preferential coordination of the substrates within the activated metal centre. For example, the formation of consecutive carbon-carbon bonds from CO forming a polyketone such as [ C(O) C(O) ] is believed to be thermodynamically less favoured than the sequence [ CH2 CH2 C(O) ] , the latter being therefore the preferred one. On the other hand, selectivity induced by the metal is evidenced by the absence of olefin polymerization when a CO/olefin mixture is used, e.g. in the hydroformylation reaction which consists of converting an olefin into the higher aldehyde in the presence of molecular hydrogen. As it will be discussed in the following sections, only a detailed study of the coordination properties of transition elements will help to explain the specificity of catalytic processes. Carbon monoxide, which is required for preparing binary metal carbonyls, is the product of partial combustion of carbon (Df H°= 110.4 kJ/mol) and can be prepared by the syn-proportionation reaction (DG°= 120.1 kJ/mol) between carbon dioxide and carbon, the equilibrium to CO being favoured at high temperature: