Chalk mechanism

Most of the transition metal catalyzed reactions are explained in terms of the hydrometallation mechanism (Harrod-Chalk mechanism) summarized in Scheme 2. The alkene coordinates to the metal (M) to give an alkene complex (5), which oxidatively adds to the Si—H bond of the hydrosilane to give (6). 

A joint that had to be broken was marked with chalk. The mechanic broke another jointthat had an old chalk mark on it and was splashed with a corrosive chemical. The joint should have been marked with a numbered tag. 

An old pipeline, no longer used, was marked with chalk at the point at which it was to be cut. Before the mechanic could start work, heavy rain washed off the chalk mark. The mechanic "remembered" where the chalk mark had been and he was found cutting his way with a hacksaw through a line containing a hazardous chemical. 

Epoxy ester Epoxy esters are a type of alkyd where a high molecular weight resin is reacted with alkyd resin. The curing mechanism remains primarily through the oil-oxidation reaction and their properties are in no way similar to the chemically reacted epoxies. They have similar properties to alkyds although with improved chemical resistance but inferior appearance. They form a reasonably hard, oil-resistant coating, which can sometimes be suitable for machinery enamels, but are primarily for interior use, since they tend to chalk rapidly on exteriors. Their best use is for chemical or water resistance where circumstances dictate that finishes that are more superior cannot be used. 

Table 14. Thermal effect (dH) of interaction of chalk coated with different polymeric materials with the PVC matrix and the mechanical properties of compositions with 40% by mass of chalk [210]...

The proposed mechanism, due to Chalk and Harrod, is outlined in Eqs. (113)-(116) (n = 3 or 4), though it is recognized that this scheme is an oversimplification 54,57) it is probable that, in the absence of CO pressure, the coordinatively unsaturated tricarbonyls, not tetracarbonyls, are the catalytic intermediate 54). 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

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. 

Deuterium-labeling studies pointed to the operation of a nonstandard Chalk-Harrod mechanism for these reactions involving a silyl-alkene insertion step.133... 

A consistently useful theory for a mechanism for homogeneous catalysis of hydrosilation has been expounded by Chalk and Harrod (//-/Id). This theory is accepted, adapted, and relied on to give a qualitatively rational explanation for numerous data in the following discussions. 

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. 

Quickly, it became clear that iridium and rhodium do not cleanly fit the Chalk-Harrod mechanism as does platinum. For electron-rich silanes and relatively unhindered terminal alkynes, the major product is the (Z)-vinylsi-lane (Scheme 3, B) from apparent unusual trans-addition to the alkyne.22 This observation was followed by important and confusing discoveries. First, rhodium, under appropriate conditions, will catalyze the isomerization of the (Z)-vinylsilane product B to the (ft)-vinylsilane product A.23 Second, rhodium can also catalyze the reverse, contra-thermodynamic reaction of the (ft)-vinylsilane A to the (Z)-vinylsilane B.24... 

HYDROSILYLATION MECHANISM CHALK-HARROD VS. MODIFIED-CHALK-HARROD... 

Using quantum chemical molecular modelling tools we have examined the reaction mechanism of palladium catalyzed hydrosilylation of styrene by the precatalyst system, 1, developed by Togni and co-workers. One fundamental question that we have focused on is whether the reaction proceeds by the classical Chalk-Harrod mechanism or by an alternative mechanism such as the modified-Chalk-Harrod mechanism. In this section, the general features of the catalytic cycle are examined. 

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