Studies of silyl-accelerated 1,5-hydrogen migrations in vinylcyclopropanes

Thermal 1,5-hydrogen (retro-ene) rearrangements of 1-silylmethylated 2-vinylcyclopropanes have been studied. cis-1-Silylmethyl-2-vinylcyclopropanes 17 and 19 undergo facile 1,5-hydrogen transposition upon mild thermolysis in benzene or toluene solution (80-110 degrees C) to give nearly quantitative yields of ring-opened 1-silyl-1,4-diene products. These reactions occur at temperatures at least 100 degrees C lower than those of the nonsilylated substrates. The silicon center and its ligands influence both the rate and stereoselectivity of diene formation, with the triphenylsilyl substrate providing the fastest reaction and highest (exclusive) stereoselectivity in forming the diene, regardless of the E/Z geometry of the vinylcyclopropane. The trimethylsilyl and triethoxysilyl compounds (19b and 19c) rearrange more slowly and with lower stereoselectivity. It is proposed that the rearrangement process takes place via a concerted suprafacial migration by one of two diastereotopic methylene hydrogens through a transition state having the silyl-carbon bond antiperiplanar to the breaking C-C bond of the cyclopropane ring. This conformational arrangement leads to weakening of the cyclopropane ring bond through orbital hyperconjugation, which facilitates the hydrogen transfer. The corresponding trans-1-silylmethyl-2-vinylcyclopropanes are thermally stable under these conditions. In contrast, cis-1-stannylmethyl-2-vinylcyclopropanes 19d,e undergo loss of the stannyl group at room temperature to afford a ring-opened 1,5-diene product 25 through a process that may take place by initial 1,5-stannyl migration.