Gas-phase electrophilic attack of a double bond exhibits stereoselectivity.
Protonated acetaldehyde (ion 1) reacts with allyltrimethylsilane (allyl-TMS) in the gas phase to yield cis-piperylene (cis-1,3-pentadiene) as the major product. The cis isomer predominates over trans by a factor >/=15:1, a degree of stereoselectivity that is unprecedented in a reaction where the double bond geometry has not been specified in the reactant. The neutral products were assessed by creating tritiated 1 via decay of a tritium nucleus in gaseous ethanol molecules labeled with >1 tritium atom. The radioactive C5H8 products must result from addition of the electrophilic ion to the allyl group followed by an elimination. Deprotonation of C5H9+ cannot account for the product stereochemistry. One possible explanation is that addition of the electrophile to the double bond is followed by elimination of Me3SiOH2+ on a time scale faster than that by which the initially formed adduct ion can change its conformation.
Mayer
Philip S
PS
Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
Morton
Thomas Hellman
TH