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À propos de : Studies of Silyl-Accelerated 1,5-Hydrogen Migrations inVinylcyclopropanes        

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  • Studies of Silyl-Accelerated 1,5-Hydrogen Migrations inVinylcyclopropanes
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  • Thermal 1,5-hydrogen (retro-ene) rearrangements of 1-silylmethylated 2-vinylcyclopropanes havebeen studied. cis-1-Silylmethyl-2-vinylcyclopropanes 17 and 19 undergo facile 1,5-hydrogentransposition upon mild thermolysis in benzene or toluene solution (80−110 °C) to give nearlyquantitative yields of ring-opened 1-silyl-1,4-diene products. These reactions occur at temperaturesat least 100 °C lower than those of the nonsilylated substrates. The silicon center and its ligandsinfluence both the rate and stereoselectivity of diene formation, with the triphenylsilyl substrateproviding the fastest reaction and highest (exclusive) stereoselectivity in forming the diene,regardless of the E/Z geometry of the vinylcyclopropane. The trimethylsilyl and triethoxysilylcompounds (19b and 19c) rearrange more slowly and with lower stereoselectivity. It is proposedthat the rearrangement process takes place via a concerted suprafacial migration by one of twodiastereotopic methylene hydrogens through a transition state having the silyl−carbon bondantiperiplanar to the breaking C−C bond of the cyclopropane ring. This conformational arrangementleads to weakening of the cyclopropane ring bond through orbital hyperconjugation, which facilitatesthe hydrogen transfer. The corresponding trans-1-silylmethyl-2-vinylcyclopropanes are thermallystable under these conditions. In contrast, cis-1-stannylmethyl-2-vinylcyclopropanes 19d,e undergoloss of the stannyl group at room temperature to afford a ring-opened 1,5-diene product 25 througha process that may take place by initial 1,5-stannyl migration.
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