Dehydrogenative silylation reactions

The well-established catalytic hydrosilylation of olefin substrates is in some instances accompanied by a dehydrogenative silylation reaction, yield-... 

Figure 22-4 Mechanism for the hydrosilylation and dehydrogenative silylation of 1-alkenes catalyzed by cationic palladium complexes Pd represents [(phen)Pd]+. The palladium alkene complex A is the resting state of the cycle. Cycle I denotes the hydrosilylation cycle, Cycle II describes the dehydrogenative silylation reaction.

Alkenylsilanes. In a dehydrogenative silylation reaction, a silyl group is introduced to the terminal carbon atom of 1,5-dienes using various hydrosilanes. The major product has an ( )-configuration. 

Subsequent extensive synthetic and catalytic studies have shown Aat silylative coupling of alkenes with vinyl-substituted silicon compounds proceeds (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing M-Si (silicometallics) and M-H bonds (where M = Ru, Rh, Ir, Co, Fe). The insertion of alkene into M-Si bonds and vinylsilanes into M-H bonds, followed by elimination of vinylsilane and ethene, respectively, are the key steps in this new process [9]. 

A breakthrough came when trifluoromethanesulfonic acid (triflic acid, TfOH) was found to catalyze the silylation reaction in toluene. Using TfOH in toluene, silylation is complete within minutes at 25 °C. Other strong acids, FS03H and MsOH gave lower overall yields in the dehydrogenation. 

Falck has recently reported dehydrogenative silylation of heteroarenes with triethylsilane (18) [97]. Coupling with the Si-H bond of triethylsilane, rather than the disilane Si-Si bond, in conjunction with the use of norbomene that presumably acts as a hydrogen acceptor, gives good yields with indoles, thiophenes, and furans, under relatively mild condition (80°C). Unlike the reaction shown in Scheme 7, silylation of indole did not require protection of the N-H group. 

Unlike Pt catalysts, the triad complexes of iron and cobalt catalyze competihvely both dehydrogenative silylation and hydrosilylation [11]. The reaction can proceed via a complex containing the o-alkyl and o-silylalkyl ligands (Scheme 14.1). 

The concurrent P-H transfer from the two ligands to the metal is a key step for two alternative reactions, namely hydrosilylation and/or dehydrogenative silylation [6, 11]. 

A gold monohydride species was also suggested in the report by Ito and Sawamura et al. on the dehydrogenative silylation of alcohols by HSiEt3 and a diphosphine gold(I) complex. Reaction was selective for the silylation of hydroxy groups in the presence of alkyl halides, ketones, aldehydes, alkenes, alkynes and other functional groups [193]. 

Divinylsilane substrates undergo dehydrogenative silylation in the presence of Rh(I) complexes, as shown in Eq. (5).38 For the dehydrogenative reaction to occur, the substrate to silane ratio must be at least 2. 

An interesting variation of the dehydrogenative silylation system involves the platinum complex-catalyzed reaction of 1-alkenes with disilanes to produce vinylsilanes.40 In this system, one H atom and a silyl group are released by the reactants to yield the alkenylsilane product, rather than the two hydrogens released in reactions of hydrosilanes [Eq. (6)]. 

Dehydrogenative silylation has also been observed for terminal alkyne substrates. Doyle and co-workers reported in 1991 that a small amount (6%) of alkynylsilane was observed in the product mixture that results from reaction of phenylacetylene and Et3SiH catalyzed by Rh2(pfb)4.41 The remaining components of the product mixture resulted from hydrosilylation. Crabtree and co-workers have found that in reaction of terminal alkynes with various tertiary silanes, dehydrogenative silylation can become the predominant route, depending on reaction conditions [Eq. (7)].42... 

The reaction of triethoxyvinylsilane, CH2=CHSi(OEt)3, with HSi(OEt)3 catalyzed by Ni(acac)2 gives a complex mixture of products, arising from dehydrogenative silylation, hydrogenation, disproportionation, and dimerization besides the normal hydrosilylation product, (EtO)3SiCH2CH2Si(OEt)356. The dimerization product is a mixture of (EtO)3Si(CH2)4Si(OEt)3 and (EtO)3SiCH(Me)(CH2)2Si(OEt)3. A possible mechanism that can accommodate the formation of these products is proposed, which includes key intermediates such as (EtO)3Si(CH2)2Ni—CH[Si(OEt)3]CH2Si(OEt)3 and Ni[(CH2)2Si(OEt)3]56. 

Dehydrogenative silylation of alkenes is often observed as a side reaction of hydrosilylation (vide supra). However, this reaction becomes a predominant or exclusive process depending on the nature of the catalysts used as well as substrates3,79,80,95-99. 

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