Chalk-Harrod mechanism described

Hydrosilylation turned out to be a unique method in organosilicone chemistry, but in some cases it suffers from severe side reactions. An explanation is provided by the generally accepted reaction mechanism known as "Chalk-Harrod mechanism" described elsewhere [7]. Included in this series of reaction steps is an insertion of olefmic ligands into a platinum-hydrogen bond. Since the metal may be bonded to either of the unsaturated carbon atoms and the reaction is also an equilibrium, alkenes may result which are in fact isomerized starting material. Isomeric silanes are to be expected as well (Eq. 1), along with 1-hexylsilane, which is by far, the main compound produced. 

Rhodium complexes also catalyze the hydrosilylation of olefins, and one of the earliest soluble catalysts used for hydrosilylation was Wilkinson s catalyst. - As is described in more detail below, the mechanisms for hydrosilylation catalyzed by rhodium complexes differ from those catalyzed by platinum complexes. Olefin insertion occurs into different ligands in the two mechanisms. The rhodium-catalyzed processes occiu by a so-called modified Chalk-Harrod mechanism. " Rhodium complexes were also among tlie first complexes used for the asymmetric hydrosilylation of ketones. " ... 

The mechanism of hydrosilylation involves a sequence of elementary reactions described in the earlier chapters of the book. The most commonly cited mechanism for hydrosilylation was first described by Chalk and Harrod and involves oxidative addition of the silane, insertion of an olefin into the metal-hydride bond, and reductive elimination to form the silicon-carbon bond in the organosilane product. More recently, a related but distinct mechanism involving insertion of the olefin into the silyl group has been recognized, and this mechanism is often called the modified Chalk-Harrod mechanism. Before these steps are described, some of the mechanistic issues regarding the specific systems of Speier s catalyst and Karstedt s catalyst are described briefly. 

The mechanism of hydrosilation can be rationalized as described by Chalk and Harrod (1 l-lId). In this view, all the chemical changes take place within the coordination sphere of a transition metal. Other reactions of unsaturated molecules are explained in much the same way. Hydrogenation, the oxo-process, and the Ziegler process are important examples. 

The mechanism of the reaction was first described by Harrod and Chalk [10], It involves the general mechanism of H-X additions to unsaturated organic compounds, starting with an oxidative addition of HX to a zerovalent platinum complex. The process is the same as that of addition of HCN to double bonds (Chapter 11). 

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

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