Silylative styrenes with vinylsilanes

Marciniec B, Pietraszuk C (1997) Silylation of styrene with vinylsilanes catalyzed by RuCl (SiR3)(CO)(PPh3)(2) and RuHCl(CO)(PPh3)(3). Organometallics 16 4320 326... 

The reactions of trialkoxy- or trisiloxy-substituted vinylsilanes with stjnene in the presence of Grubbs-type Ru-carbene complex (II) proceed stereoselec-tively giving trans-P-(silyl)styrenes with high yield (Eq. 2) [6]. 

Evidence for the migratory insertion of ethylene [121], vinylsilane [122], and styrene [123] into the Ru-Si bond (yielding vinylsilane and two (1,2- and 1,1-bis(silyl)ethene) regioisomers, respectively) showed that in the reaction first reported in 1984 as the metathesis (disproportionation) of vinylsilanes and their co-metathesis with olefins [124], instead of the C=C bond cleavage (formally characterizing alkene metathesis (eq. (8)), a new type of olefin conversion was revealed - silylative coupling of olefins with vinylsilanes. 

Contrary to the previously reported reactions with the M-H and M-Si initial complexes the proposed mechanism of catalysis by [(cod)M(OSiMe3)]2 (where M= Rh, Ir) does not involve highly activated migratory insertion of olefin into the Rh-Si bond (the associative mechanism) since the final step of the product formation occurs via a lower activated step of reductive elimination of product (the dissociative mechanism) (Scheme 4). The reaction under study is conceptually related to dehydrogenative silylation since the basic reaction involves the silylation of a substrate such as styrene by vinylsilane instead in the hydrosilane, equations 17a and 17b. by hydrosilanes... 

Aryl bromides or iodides react with di- or monochloro(vinyl)silanes or triethoxy(vinyl)silane in the presence of Et3N and palladium catalyst to give /S-arylvinylsilanes in moderate to excellent yields. In contrast to the simple silyl substituent, the presence of the electron-withdrawing group on sihcon is essential to avoid the elimination of the silyl group. No silver ion is necessary in these examples . The palladium-catalyzed reaction of aryldiazonium tetrafluoroborates with vinylsilanes gives a mixture of terminal and internal vinylsilanes together with styrenes. ... 

Many ruthenium complexes have been tested in the silylative coupling reaction. In the synthetic procedure the absence of by-products of the homocoupling of vinylsilanes is required so an excess of the olefin has usually been used. However, the screening tests performed at the 1 1 ratio of styrene and phenyldimethylvinylsilane with a variety of ruthenium catalysts have shown that pentacoordinated monocarbonyl bisphosphine complexes appear to be the most active and selective catalysts of which RuHCl(CO)(PCy3)2 has shown high catalytic activity under conditions of catalyst loadings as low as 0.05 mol % [55]. Cuprous salts (chloride, bromide) have recently been reported to be very successful co-catalysts of ruthenium phosphine complexes, markedly increasing the rate and selectivities of all ruthenium phosphine complexes [54]. 

The number of examples of highly selective dehydrogenative silylation is still limited. The most convincing examples are Ru3(CO)i2- and Fe3(CO)i2-cata-lyzed reactions of styrene [106, 114] and vinylsilane [115] with HSiEts, RuH2(H2)2PCy3)2-catalyzed reaction of ethylene with HSiEt3 [116], and cationic rhodium complex-catalyzed dehydrogenative silylation, e.g., [117], as well as the nickel equivalent of the Karstedt catalyst [105]. 

For instance, the reaction of EtaSiH and 2 equiv. of p-methoxystyrene in toluene with 1.0 mol% of 16a afforded at 100°C within 6 h the dehydrogenative silylation product ( )-l-(p-methoxystyryl)-2-(triethyl-silyl)ethylene in 95% yield. The reaction is of high selectivity that neither (Z)-isomers, nor branched dehydrogenative silylation products were seen. Less hydridic silanes, such as triphenylsilane, were less efficient than for instance EtsSiH. Other substituted styrenes such as p-methyl, p-chloro-, and p-fluorostyrene also afforded the corresponding tran -vinylsilanes in high yields and selectivities (up to 98%). In the case of aliphatic alkenes, such as -octene, allyltriethoxysilane, vinylcyclohexane, and ethylene, dehydrogenative silylations were still preferred, but showed less E/Z selectivity. Cyclic olefins, such as cyclooctene, furnished low conversions under the same reaction crmditions. The results are summarized in Scheme 19. 

To distinguish between the non-carbene mechanism of silylative coupling and the carbene mechanism the reactions of a number of vinylsilanes with styrene-dg in the presence of I, II, or III were investigated. In the case of the non-carbene mechanism the formation of silylstyrene-d and ethylene-di is to be expected (Eq. 23). In contrast, the carbene mechanism should afford silylstyrene-de and ethylene-d2 (Eq. 24). 

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