The Chalk-Harrod Mechanism

In the Chalk-Harrod mechanism, oxidative addition of silane occurs to a Pt(0) complex. The ancillary ligands on the active catalyst when reactions are initiated with Speier s catalyst are unknown, but are likely to be the olefin substrate. The ancillary ligands on the active form of Karstedt s catalyst or the related PtfCOD) complex could be the original diene ligands or the olefin substrate. Details on the mechanism of oxidative additions of silanes are provided in Chapter 6. In brief, this reaction occurs by coordination of silane to an open site to form a silane a-complex, followed by cleavage of the Si-H bond to form a silyl hydride species. 

Following this oxidative addition, insertion of the olefin or alkyne occurs into the platinum-hydride bond to form an alkyl or vinyl hydride complex. The regioselectivity of the insertion step and the chemistry of the alkyl silyl complexes control the overall regioselectivity of the hydrosilylation of olefins. Recall that these platinum catalysts form terminal alkylsilane products. This regioselectivity indicates that insertions of terminal olefins occur in order to generate a linear alkylplatinum intermediate. Apparently, the insertions of styrene and acrylic acid derivatives into the platinum hydride in these catalysts occur with the same regiochemistry. 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex.  

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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]. 

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... 

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. 

If we were to assume that the reaction followed the Chalk-Harrod mechanism, then insertion of the olefin into the Pd-hydride bond in all three isomers 8a-c would lead to the correct regioisomer product. Thus, to some degree the regioselectivity of the hydrosilylation in this catalyst system is determined in the styrene coordination step. We will discuss the origin of the regioselectivity in more detail in Section 4. 

The final step of the Chalk-Harrod mechanism that is distinct from the modified Chalk-Harrod mechanism is the reductive elimination to yield the product. For this step, we have only examined the reductive elimination from the most stable ri-allyl complex lOa-anti. 

In the calculations performed with model B the to-7i-complex is destabilized by 1.6 kcal/mol with respect to the corresponding endo isomer. If we were to follow the more stable endo rc-complex isomer through the Chalk-Harrod mechanism, this would lead to the R form of the product as shown on the left-hand-side of Figure 15. Since it is actually the S form of the product that dominates when styrene is the substrate, the formation of the tt-complex cannot be the stereodetermining step of the catalytic cycle. 

As an inversion of enantioselectivity was observed experimentally for 4-(dimethylamino)styrene, (64% R ee) as compared to styrene (64% S ee), we have recalculated the relative thermodynamic stabilities of endo and exo isomers for each step of the catalytic cycle using this second substrate. These calculations allow us to verify the quality of our findings by checking if an inversion in the relative stabilities of the endo and the exo-ri3-silyl-allyl intermediates (with the endo being more stable than the exo) is observed with 4-(dimethylamino)styrene. Using 4-(dimethylamino)styrene as the substrate, the calculated relative stabilities of the intermediates in the Chalk-Harrod mechanism are shown as parenthetic values in Figure 15. 

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 ... 

When the hydrosilylation reactions were reviewed in this series and published in 19893, the Chalk-Harrod mechanism 69 (Scheme 5) was apparently the most widely accepted mechanism for the alkene hydrosilylation, with minor exceptions of photocatalyzed... 

Although the Chalk-Harrod mechanism has been widely accepted,69 some phenomena (include an induction period for many precatalysts and the formation of vinylsilanes) cannot be explained well by the Chalk-Harrod mechanism. An alternative mechanism to the Chalk-Harrod mechanism involves insertion of the alkene into the M-Si bond instead of insertion of the alkene into the M-H bond (Fig. 5).70... 

The first mechanistic proposal for the hydrosilylation reaction where mononuclear homogeneous catalytic intermediates are assumed is known as the Chalk-Harrod mechanism. The catalytic cycle in a slightly modified form is shown in Fig. 7.16. All steps of this catalytic cycle belong to organometallic reaction types that we have encountered many times before. Thus conversions of 7.60 to 7.61, 7.61 to 7.62, and 7.62 to 7.59 are examples of oxidative addition of HSiR3, insertion of alkene into an M-Si bond, and reductive elimination, respectively. 

Scheme 3 The Chalk-Harrod mechanism for hydrosilation and a modified version...

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

Isomerization does not take place on the products. No isomerization of alkenes occurs with the action of chloroplatinic acid alone, but recovered alkenes from a silane-alkene mixture often contain isomers of the alkene. D-H exchange is observed for the recovered hydrosilane from the hydrosilation mixture with DSiCb. These facts suggest the mechanism outlined in Scheme 2, which is known as the Chalk-Harrod mechanism. " The mechanism involves oxidative addition of hydrosilane to the active species of the catalyst, coordination of alkene, followed by <7 — tt conversion, and reductive elimination of the product. 

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. 

The Chalk-Harrod mechanism has been widely accepted with various modifications to account for the hydrosilylation of other multiple bonds (C=C), C=0, C=N), homogeneously catalyzed by various metal complexes. 

Cyclopentadienyl complexes of Rh —> Rh appeared as valuable examples in the mechanistic study of ethylene hydrosilylation [14, 47]. GC/MS and NMR studies as well as deuterium labeling of the substrates allowed an alternative proposal to the Chalk-Harrod mechanism, initiated by a hydrogen shift to form... 

The discovery of the hydrosilation reaction has greatly stimulated research on transition metal-silyl complexes. Currently, there are examples of silyl complexes with almost every transition metal.Silyl hydride complexes are of special interest with respect to the chemistry of the hydrosilation reactions. Such compounds can be seen as the products of oxidative addition into the Si-H bond and thus are intermediates in hydrosilation reactions following the Chalk-Harrod mechanism. 

The classic Chalk-Harrod mechanism is shown in Scheme 16.6, and the modified Chalk-Harrod mechanism " is shown in Scheme 16.7. In the Chalk-Harrod mechanism, oxidative addition of silane occurs to form a silyl hydride complex. Migratory... 

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 theoretical study of [RhCl(PH3)3]-catalyzed hydrosilylation of ethylene by the DFT, MP4 (SDQ), and CCSD(T) methods shows that the rate-determining step in the Chalk-Harrod mechanism is the Si—C reductive elimination (43). 

However, it is worth emphasizing that since the Si—C reductive elimination needs much greater activation energy than the oxidative addition of H—SiMea and ethylene insertion into the Rh—SiMes bond, the modified Chalk-Harrod mechanism is more favorable than the Chalk-Harrod mechanism in the Rh-catalyzed hydrosi-lylation of ethylene (43). 

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