Chalk-Harrod process

A possible mechanism for the hydrosilylation reaction based on the Chalk-Harrod process is shown in Scheme 10 [16]. Initially, reduction of the phebox-Rhflll) with hydrosilane proceeds to give the Rh(I) intermediate 16. Subsequent oxidative addition of hydrosilane gives a phebox-Rh(H)(SiR3) species 17. Coordination of an aUtene to 17 produces two possible isomers 18a and 18b, which tmdergo insertion into the Rh-H bond followed by reductive elimination to give the a-silyl-adduct 14 and the p-silyl-adduct 15. The absolute configuration of the ot-silyl-adduct can be... 

Following olefin coordination, the Chalk-Harrod mechanism proceeds by olefin insertion into the M-H bond, whereas with the modified Chalk-Harrod mechanism, olefin coordination is followed by insertion into the M-Si bond. This step distinguishes the two mechanisms. Thus, the coordination of styrene to the hydridosilyl complex to form an olefin 7t-complex may be the first step of the catalytic cycle that discriminates between the two mechanisms. We have examined this coordination process as well as the relative energies of the many isomers of the 7i-complex that are possible. 

Figure 13. Calculated reaction profile for the silylmetallation process required for the modified-Chalk-Harrod hydrosilylation mechanism.

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

Detailed mechanistic study on these intramolecular hydrosilylation of allylic O-silyl ethers 59 and allylic A -silylamincs 63 using deuterium labeling techniques shows that 5-endo cyclization giving 60 or 64 proceeds via a Chalk-Harrod type hydrometalation catalytic cycle, while 4-exo cyclization process yielding 61 or 65 includes a Seitz-Wrighton type silylmetalation mechanism89. 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

A recent detailed theoretical study of the platinum-catalyzed hydrosilylation of ethylene [15] led to a conclusion that this process proceeds through the Chalk-Harrod mechanism. The rate-determining step in this mechanism is the isomerization of the Pt(silyl)(alkyl) complex formed by ethylene insertion into the Pt-H bond, and the activation barrier of this step is 23 kcal moP for R = Me and -26 kcal mol for R = Cl). In the modified Chalk-Harrod mechanism, however, the rate-determining step is ethylene insertion into the Pt-SiRa bond and its barrier is 44 kcal moP for R = Me and 60 kcal moP for R = Cl. 

Two different mechanisms can be envisioned for these reactions that differ in the insertion step. Since both processes are very similar we will only discuss the silation procesess which have been more extensively studied. The so called Chalk-Harrod mechanism (Scheme 6.62, a) was first proposed for the hydrosilation of alkenes, and involves insertion of the alkene into the M-H bond formed by oxidative addition of the silane to the metal, followed by reductive elimination of a silyl alkyl. A competing route (the modified Chalk-Harrod mechanism. Scheme 6.62, b) derives from insertion of the alkene into the M-SiRs bond to... 

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. " ... 

It is now accepted that alkene hydrosilylation catalyzed by several systems occur by the modified Chalk-Harrod medTanism. Wrighton s studies on tlie insertion of ethylene into the M-Si bond of (CO) CoSiMe3 led to die conclusion that this photocatalytic process occurs by the mechanism in Scheme 16.8. Conversion of the p-silylalkyl product of insertion to the free organosilane is thought to occur by a sequence of oxidative addition of silane... 

The hydrosilylation of alk5mes can also occur by Chalk-Harrod or modified Qialk-Harrod mechanisms, as shown in Scheme 16.10. Reactions forming products from cis addition of the silane across the alkyne would occur by one of these mechanisms. The origin of the regioselectivity of these hydrosilylations has not been well established experimentally, However, based on computational studies, it has been argued that the imusual Markovnikov regiochemistry of the ruthenium-catalyzed process occurs by insertion of the alkyne into the M-Si bond to form an T -vinyl intermediate. These T -vinyl intermediates are shown in Scheme 16.10. 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum > rhodium > ruthenium = iridium > osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267]... 

The results summarized under the headings 1 and 2 rather conclusively indicate that, under the conditions used, the reaction must proceed via hydropaUadation. For the other steps, a combination of oxidative addition and reductive elimination, as in the Chalk-Harrod mechanism, is plausible, but other possibilities involving metathetical processes may not be ruled out. It should be noted that the two reactions employed by Brookhart and co-workers " and Hayashi and co-workers are substantially different. It is therefore very likely that the mechanistic conclusions made in their studies are both fundamentally correct and that, as in many other reactions of organopalladium compounds, more than one mechanism operates for a given type or class of organopalladium reactions. 

The results of the extensive ah initio calculations performed by Sakaki s group using MO/MP2-MP4(SDQ) and CCD methods, for Pt-catalyzed Si-H addition to ethylene, can be summarized as follows (1) Oxidative addition of Si-H to Pt is facile as is ethylene coordination. (2) The rate determining step for a Chalk-Harrod pathway is the coordinated-ethylene-assisted cis-trans isomerization of the Si-Pt-H complex, with a barrier of ca. 22-26 kcal/mol. The isomerization is proposed to occur through a distorted square-pyramidal intermediate via the Berry pseudo-rotation process. (3) For a modified Chalk-Harrod mechanism, the... 

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). 

To account for these results, the hydrosilylation reaction in the presence of Wilkinson s catalyst was proposed to occur via a cis addition process (according to the Chalk and Harrod mechanism, cf. Sect. IV-A-l-a) with concomitant isomerization taking place, the catalyst retaining the olefinic product in its coordination sphere. Conversion of the more stable trans to cis product is proposed to be directed by steric interactions in an intermediate o-alkylrhodium complex. 

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