Vinylsilyl groups

The ability of these macromonomers to participate in free-radical copolymerization was not tested. They were used28b) for the synthesis of tri- and multiblock copolymers by means of hydrosilylation, i.e. an addition reaction between vinylsilyl groups and the —Si — H functions at the chain end of poly(dimethylsiloxane)mole-cules. 

The enyne system 223 with an o-iodophenyl starter and a vinylsilyl group terminator underwent 5-exo-dig and 5-exo-trig cyclizations to yield 225. No further cyclization of 225 took place. Instead the jS-H elimination gave the vinylsilane 226 as the final product when PPh3 was used. No 6 7r-electrocyclization of 226 occurred. Also the reaction was terminated by elimination of the y3-TMS group to give the olefin 227, when BINAP, which suppresses jS-H elimination, was used [64]. 

Reactions involving cobalt catalysis have been used with the vinylsilyl group to form cyclized products. A Co-catalyzed intramolecular hydroacylation reaction was performed with a vinylsilyl aromatic aldehyde 49 to form the cyclized sUylether product 50 in quantitative conversion (eq 23). ... 

When 2-iodoalkyl vinylsilyl ethers 231 (Z=Si) were reacted with 224 in the presence of the Co(dppb)Cl2 catalyst, silatetrahydrofurans 232a (Z=Si) were isolated [294], When substrates 231 with substituted allyl groups (R3 and/or R4=alkyl) were treated with catalytic amounts of Co(dppe)Cl2 and 224 or phenylmagnesium bromide, a 5-exo cyclization proceeded and tetrahydrofurans 232c with exocyclic alkenyl units were isolated in 58-89% yield. For substrates with disubstituted alkene acceptors (R3=alkyl, R4=H), the formation of 18% of reduced tetrahydrofuran 232b was also observed [280, 295],... 

The introduction of vinylsilyl end groups into a polymer chain has been achieved by several techniques. Since vinylsilyl functions are sensitive to the attack by strong nucleophiles 27), it was considered appropriate to decrease the nucleophilicity of the styryl carbanions by the addition of oxirane before reacting them with either chloromethyldimethylvinylsilane or chlorodimethylvinyl silane 28a). The following structures are formed ... 

The first example of the vinylsilylation of unactivated alkynes was also reported by the same group [110]. The reaction of the carbon tethered alkynyl vinylsilane 112 in the presence of 0.2 equiv. EtAlCl2 in CH2CI2 at -78 °C gave the fram-carbosilylation product 113 regio- and stereoselectively in 92 % yield (Sch. 73). 

A plausible mechanism for the Lewis acid-catalyzed fran -vinylsilylation is shown in Sch. 74. The coordination of a Lewis acid to the triple bond of 112 would form ji-complex 114 and the a-carbon of the vinylsilane would attack the electron-deficient triple bond from the side opposite to the Lewis acid to produce an aluminum ate complex 115 stereoselectively. The migration of the trimethylsilyl group to the aluminate center would afford 113 and regenerate the Lewis acid catalyst. 

The selective cleavage of one terminal trimethylsilyl group [2] with potassium tert-butoxide is a superior protocol for the synthesis of silyl anions. Also in the case of vinylsilanes, the method proved to be equally selective and efficient (Eq. 2), providing quantitative yield of the first vinylsilyl anions (4a, 4b). 

Sterically demanding dialkylvinylsilanes enabled the control of the endolexo selectivity of the Diels-Alder reaction. Three vinylsilyl-tethered precursors 27 were investigated in an intramolecular Diels-Alder reaction to furnish the exo product 28 over the endo product 29. As the size of the R group was increased, the selectivity for 28 increased, with the di-r-butyl group giving the highest selectivity (eq 14). ... 

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