Styrenes dehydrogenative silylations

In a more recent report from Seki and Murai, Fe3(CO)i2 is shown to exhibit complete selectivity in the catalytic dehydrogenative silylation of styrenes.32 No products resulting from hydrosilylation are observed with the iron complex catalyst, in comparison to the minor amounts of hydrosilylated... 

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). 

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. 

Basic methods for their production involve the hydrosilylation of alkylacetylenes catalyzed by platinum complexes [8] and the dehydrogenative silylation of olefins, e.g. styrene [9], 1-hexene [10,11], are catalyzed by rhodium [10], ruthenium [9,12, 13] and iridium [11] complexes and photocatalyzed by iron and cobalt [14,15] carbonyls. 

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... 

In most cases, group R (Scheme 4) depicts an electronegative substituent in such olefins as styrene, substituted styrenes, trifluoropropene, and vinyl trisub-stituted silanes. Complexes of iron and cobalt triads have appeared extremely favorable catalysts of the dehydrogenative silylation, but Ni, Pd, and Pt complexes have also recently been reported as active catalysts of these olefins conversions [for reviews see References (3,11,13,18). 

A general scheme of the dehydrogenative silylation catalysis of styrene by M complexes is given in (Scheme 5). 

Over the last two decades, Wilkinson complex and related phosphine complexes of rhodium(I) have been used in numerous reactions for synthetic purposes, such as in the hydrosilylation of styrene and vinylcyclo-propene to yield ring-opening products of vinylamines. The [ (dippe)Rh 2(/u.-H)2] complex [where dippe = l,2-bis(diisopropylphosphino)ethane] is active in the hydrosilylation of olefins by diphenylsilane (4). Rhodium complexes were extremely favorable catalysts for dehydrogenative silylation of alkenes and divinyldiorganosilanes (4,13). 

The reactions of styrene, 4-chlorostyrene, 4-methoxystyrene, and methyl acrylate with l,2-bis(dimethylsilyl)benzene (14) catalyzed by Pt(CH2=CH2)(PPh3)2 give the corresponding dehydrogenative double silylation products 15 in good to high yields (equation 7)14. When dimethyl maleate is employed, benzo-l,4-disilacyclohexene 16 (R1 = R2 = C02Me) is obtained as the major product (15/16 = 22/78) (equation 7). In the reactions of 1-alkenes, i.e., ethylene and 1-octene, the formation of monosilylated products is also observed (13-57% yield). On the basis of the fact that no deuterium is incorporated into the products when l,2-dideuterio-l,2-bis(dimethylsilyl)benzene is used, disilyl-Pt metallacycle 17 is proposed to be the key intermediate of this process (equation 7). 

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