Vinylsilanes linear

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... 

The hydrosilation of 1-alkynes can give either Z-or E-isomers (see (E) (Z) Isomers) of the linear vinylsilane or the Markownikow product (equation 5). Reaction of 1-hexyne with trichlorosilane under platinum catalysis conditions gives a mixture of 1- and 2-silyl-l-hexene (78 22). From the E-stereochemistry of the 1-silyl isomer, a yyn-addition mechanism can be assumed. 

This can be illustrated by the acylation of linear and cyclic vinylsilanes. It is quite simple to prepare methylvinylketone (59) or l-methyl-6-TMS-hexa-2,5-diene-4-one (70) In the latter case the starting material is bis(trimethylsilyl)-ethene (62) which can either be achieved via hydrosilylation (vide supra) or by the more classical approaches " Furthermore, because of the hi stereospecifi-ty, compounds like E- cinnamaldehyde (77) are obtainable in good yields and with superb optical purity ... 

Eqs. 53, 54). A platinum complex, PtCl2(AsPh3)2, can also give the linear product with 100% selectivity. The reactions of other vinylsilanes bearing different... 

The amount of each product obtained depends on the catalyst and the nature of R and R, but the linear form generally tends to predominate. The unsaturated vinylsilane, RCH=CHSiR3, is also a product. Although minor in most cases, conditions can be found in which it predominates. The Chalk-Harrod mechanism cannot explain the formation of this dehydrogenative sU-ation product, but the alternate mechanism of Fig. 9.76 in which the alkene inserts into the M—Si bond first does explain it because 3 elimination of the intermediate alkyl leads directly to the vinylsilane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eqs. 7.21 and 7.22, a reaction in which initial insertion of alkyne into the M—Si bond must predominate (>99%). Co2(CO)g also catalyzes a number of other reactions of silanes, as shown in Fig. 9.8. 

Vinyl silanes too can be dimerized using Ziegler-type catalyst systems. For instance, trimethylvinylsilane yielded the linear dimer with the catalyst Ni(acac)2/Et3Al2CVPPh3 [62] and vinylsilanes containing methoxy groups reacted with a catalyst composed of tetrabutoxytitanium, PPh3 and AlEt3 in a 1 1 6 ratio [63] (Equation 45). 

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