Alkene Hydrosilation and Hydroboration

The Lewis acid BPhj is a useful cocatalyst for the reaction. Such additives are often termed promoters. In this case the promoter improves the selectivity of the system for linear product (it is not clear exactly why) and improves the life of the catalyst. A catalyst deactivates when it loses some or all of its activity by going down an irreversible path that leads to an inactive form of the metal complex. In this case, the formation of the inactive Ni(CN)2 is the principal deactivation step. This can happen in several ways an example is shown here  

Hydrosllatlon This is the addition of a silane RsSi—H across a C=C double bond as illustrated in Eq. 9.25. It is a reaction of some commercial importance for the synthesis of silicon-containing monomers, for use in such products as the self-curing silicone rubber formulations sold for domestic use. 

One of the earliest catalysts (1957), H2PtCl6, or Speier s catalyst, is extremely active 0.1 ppm of catalyst is effective. Commercially, the catalyst is normally not even recovered from the product, even though Pt is a precious metal. There is an induction period before hydrosilation begins, which is attributed to reduction of H2PtCL) to the active catalyst, which was taken to be a Pt(II) species. The mechanism of Chalk and Harrod, shown in Fig. 9.7a, was accepted for many years. Only recently has it been suggested -  

FIGURE 9.7 (a) The Chalk-Harrod mechanism for alkene hydrosilation. (b) An alternative mechanism in which insertion takes place into the M—Si bond. This accounts for the formation of vinylsilane, sometimes seen as a by-product in hydrosilation. 

FIGURE 9.8 Other metal-catalyzed reactions of silanes. 

As in hydroformylation, both linear and branched products can be obtained from a substituted alkene such as RCH=CH2  

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The hydrosilation and hydroboration of alkenes produce a saturated tetraalkyl silicon or boron derivative (Eq. 6.33). In addition dehydrogenative silation or boration derivatives are sometimes encountered as side products. These are substituted alkenes analogous to the Heck reaction products where a new C-Si (or C-B) bond is formed instead of a C-C bond (Eq. 6.34). 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

Silicon substituents can be introduced into alkenes and alkynes by hydrosilation.70 This reaction, in contrast to hydroboration, does not occur spontaneously, but it can be carried out in the presence of catalysts such as H2PtCl6, hexachloroplatinic acid. Other catalysts are also available.71 Halosilanes are more reactive than trialkylsilanes.72... 

The insertion of alkenes and alkynes into M-E (E = SiRa, SnR3, BR2) is not substantially different from the insertion into M-H or M-C bonds. The same orbital considerations hold in these cases (see Section 6.2.1) and theoretical methods have been applied to analyze these processes, generally in the context of mechanistic studies of catalytic reactions where this may be an important step (hydrosilation, hydroboration, diboration, silylstannation, etc.). Table 6.11 contains some calculated activation energies for insertion of ethylene and alkynes into M-ER bonds of comparable systems. The balance of bond breaking/bond making is less favorable for M-Si than for M-C or M-H bonds. Moreover, the directionality of the 0(M-Si) bonds is detrimental for insertion as discussed when... 

Several synthetically very useful processes involve the 1,2 insertion of unsaturated substrates into the M-SiRs bond (hydrosilation [193], dehydrogenative silation [193a,194], and disilation [176]) or the M-BR2 bond (hydroboration [195], dehydrogenative boration [196], and diboration [191,195,197]) Catalytic reactions have been developed lately that comprise the 1,2 addition of two different ER groups to an alkene or alkyne (silylstannation [198], borylsilation... 

One common class of catalytic reactions involves 1,2-addition of a CH bond across a multiple bond (eq. 23). Such is the case for alkene hydrogenation (XY = H2), hydrosilation (XY = RsSi—H), hydroboration (XY = R2B—H), and disilylation (XY = RsSi—SiRs). An alkene isomerization like equation 10 can be considered as intramolecular C—H addition across the 2,3—C=C bond. The Heck reaction (eq. 24) is an example of an addition of a C—Hal bond combined with an elimination of a H—Hal group. Similarly, the Wacker process is effectively an addition of H—OH across the ethylene C—C bond, followed by an elimination of H2. 

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