Cationic silane complexes

Mn quadrupolar broadening. On heating these compounds decompose via hetero-lytic cleavage of the p -H-Si bond, typical behavior for a cationic silane complex because the silicon center becomes electron deficient upon coordination of the H-Si bond to metal and thus activated toward nucleophilic attack. 

Cationic silane complexes are normally stable only in solution at low temperature and are rarely isolable (103). Thermally unstable bis(silane) adducts such as [IrHaCr -HSiEts PPhs SbFe (16) were reported by Crabtree to be detectable by NMR at low temperature and show intriguing reactivity, structures, and dynamics (103). 

Cationic palladium complex 121 reductively coupled enynes (Eq. 20) using trichlorosilane as the stoichiometric reductant [71]. This combination of catalyst and silane afforded silylated methylenecyclopentanes such as 122 in good yield from enynes such as 123. Attempts to develop an enantioselective version of this reaction were not successful [71]. When enediyne 124 was cyclized in the presence of trichlorosilane, the reaction favored enyne cycli-zation 126 by a 3 1 ratio over diyne cyclization to 125 (Eq. 21). In contrast, when the more electron-rich dichloromethylsilane was used as the reductant, diyne cyclization product 125 was preferred in a ratio of 4 1 [71]. Selectivities of up to 10 1 for enyne cyclization were observed, depending on the substrate employed [72],... 

Rhodium complexes facilitate the reductive cydization of diyne species in good yield, although the product olefin geometry depends on the catalysts used. Moderate yields of -dialkylideneclopentane 169 resulted if a mixture of diyne 146 and trialkylsilane was added to Wilkinson s catalyst ClRh[PPh3]3 (Eq. 33) [101]. If, however, the diyne followed by silane were added to the catalyst, a Diels-Alder derived indane 170 was produced (Eq. 34). Cationic Rh complex, (S-BINAP)Rh(cod) BF4, provides good yields of the Z-dialkylidenecyclopentane derivatives, although in this case, terminal alkynes are not tolerated (Eq. 35) [102]. 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

Brookhart and co-workers found that the cationic a-silane complex [CpFe(CO)(PR3)(HSiEt3)]+ was observable by NMR at RT but only in the presence of excess silane (sacrificial removal of trace H20 as Et3SiOH) and could not be isolated as a solid (122). The [CpFe(CO)(PR3)] fragment catalyzed silane alcoholysis in the presence of the BArF counterion (123). Although rapid deactivation of the catalyst occurred with ethanol as substrate, phenol reacted continuously with turnover numbers up to 80 min-1. It was proposed (Scheme 9)... 

The Pt-catalyzed hydrosilylation of trimethyl silane and alkenols or alkenyl-polyethers lead to nonionic silane surfactants, whereas the addition of allylglycidyl ether to trimethylsilane results in a precursor for ionic derivatives. The epoxy group is highly reactive towards nucleophilic agents and can be easily transformed into quaternary ammonium, betaine, or sulfonate complexes. Additionally, cation-anion complexes can be formed by the transformation of two equivalents of epoxy silane with one equivalent of trialkyl ammonium hydrogen sulfite. The reaction of hydroxyalkyltrimethylsilane... 

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