Alkenes dehydrogenative silylation

Scheme 14.1 Organometallic intermediate competitive hydrosilylation and dehydrogenative silylation of alkenes.

Use of a hydrosilane instead of molecular hydrogen in combination with a transition metal has opened the door leading to hydrosilylation and dehydrogenative silylation of unsaturated bonds. Thus, replacement of hydrogen by a hydrosilane is a reasonable strategy to improve serious issues in hydroformylation of alkenes. Along this line, some types of silylcarbonylation were extensively studied by Murai and his co-workers. However, the silicon moiety always attaches to the oxygen atom of incorporated GO molecule (Scheme 1). 

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]. 

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 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. 

Two different mechanisms have been proposed for this dehydrogenative silylation process. The first mechanism proposed by Oro, Esteruelas and coworkers includes the oxidative addition of 1-alkyne to the Ir—Si bond, followed by the reductive elimination of 151 (equation 61)117,118. The proposed mechanism is supported by the identification of [IrH(C=CPh)( j2-( -Pr)2PCH2CH20Me)]BF4 in stoichiometric as well as catalytic conditions by 31P 1H NMR analyses118. The other mechanism proposed by Jun and Crabtree includes the insertion of 1-alkyne into the Ir—Si bond, followed by isomerization and /J-hydride elimination (equation 62)113, which is consistent with the mechanism proposed for the highly selective formation of (Z)-l-silyl-l-alkenes (see Section IILB)115. 

Vinylsilanes and allylsilanes are prepared by dehydrogenative silylation of alkenes catalysed by Rh and other complexes [221]. A particularly effective catalyst for alkenylsilanes is Ru3(CO)12 [222], Using excess 1-hexene, 1-silyl-1-hexene 575 was... 

As we have already mentioned, ruthenium complexes predominantly catalyze the dehydrogenative silylation of alkenes but competitively with the hydrosilylation so the reaction usually gives a mixture of the dehydrogenative silylation and hydrosilylation products. Ru3(CO)12 appears to be a very active catalyst for the dehydrogenative silylation of styrene, para-substituted styrenes [ 19, 20],trifluoropropene and pentafluorostyrene [21] by trialkyl-, phenyldialkyl-silanes (but also triethoxysilane) (Eq. 10). 

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.

HRu3(CO)n)] 174 are employed in the hydrosilylation of alkenes. The selectivity of the reaction depends on the reaction conditions as well as the structure of alkenes, hydrosilanes and catalysts. For the rationale of the competitive occurrence of dehydrogenative silylation and hydrosilylation, reasonable mechanisms have been proposed... 

Triphenylsilyl ethers are typically prepared by the reaction of the alcohol with triphenylsilyl chloride (mp 92-94 °C) and imidazole in DMF at room temperature. The dehydrogenative silylation of alcohols can be accomplished with as little as 2 mol% of the commercial Lewis acid tris(pentaf1uorophenyl)borane and a silane such as triphenylsilane or triethylsilane [Scheme 4.98]. Primary, secondary, tertiary and phenolic hydroxyls participate whereas alkenes, alkynes, alkyl halides, nitro compounds, methyl and benzyl ethers, esters and lactones are inert under the conditions. The stability of ether functions depends on the substrate. Thus, tetrahydrofurans appear to be inert whereas epoxides undergo ring cleavage. 1,2- and 1,3-Diols can also be converted to their silylene counterparts as illustrated by the conversion 983 98.4. Hindered silanes such as tri-... 

---