Chalk-Harrod hydrosilylation

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

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

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. 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

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. 

Figure 5. General schemes for the Chalk-Harrod (right) and the modified-Chalk-Harrod (left) mechanisms for the metal catalyzed hydrosilylation of olefins.

The fact that isomers 8a, 8b and 8c are the lowest energy styrene coordinated complexes have potentially important ramifications that concern the modified-Chalk-Harrod mechanism and the regioselectivity observed in the hydrosilylation. With the modified-Chalk-Harrod mechanism olefin insertion into the M-Si bond follows styrene coordination. However, in all three isomers depicted in Figure 8, the coordinated hydride lies between the... 

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. 

According to our simulations, hydrosilylation reaction proceeds through the classic Chalk-Harrod mechanism as depicted on the right-hand-side of Figure 5. The modified or anti-Chalk-Harrod mechanism is hindered by a rather large silylmetallation barrier which is calculated to be 46 kcal/mol for the minimal QM model A. 

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 ... 

The mechanism of the intramolecular hydrosilylation catalyzed by Rh and Pt complexes was investigated by using deuterated silanes, which indicated the operation of both the traditional Chalk-Harrod hydrometallation and silylmetallation pathways accompanied by rapid P-hydride elimination [65]. This intramolecular reaction was applied to the syntheses of natural products [66],... 

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... 

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. 

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. 

A widely accepted cycle for olefin hydrosilylation is the so-called Chalk-Harrod mechanism, where it is assumed that H migration to the alkene is faster than silyl migration, followed by reductive elimination from a silyl-alkyl intermediate,... 

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... 

This Chalk-Harrod mechanism includes the insertion of an olefin into a hydrogen-metal bond (step iii). However, it is also conceivable that an olefin can insert into a silicon-metal bond, and this type of mechanism was found to be operative when Fe(CO)5, M3(CO)12 (M = Fe, Ru, Os) and R3SiCo(CO)4 were used as catalysts for the photocatalyzed hydrosilylation of alkenes104. 

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