Scope, Limitation, and Mechanism of the Homoconjugate Electrophilic Addition of Hydrogen Halides

Article abstract of DOI:10.1021/jo00208a027

Hydrogen halides (HCl, HBr, HI) add by a homoconjugate 1,5 mechanism to cyclopropanes carrying certain electron-withdrawing substituents.When the substituent is COCH3, COC6H5, CO2H, or CN, the reaction gives the 1,3-disubstituted propane in high yield.Addition of DCl gives a product with deuterium only in the position α to the substituent.The order of rates is not in agreement with a mechanism whereby the cyclopropane ring is protonated initially, since the rate of such a process should be slowed by electron-withdrawing groups.The ketones, however, react much more rapidly than benzylcyclopropane, a model for the direct protonation mechanism.The homoconjugate mechanism involves rapid protonation of the side chain, followed by nucleophilic attack on the cyclopropane ring.The reaction is limited to substrates that can be protonated on the side chain to produce an intermediate with charge ajacent to the cyclopropane ring.This charge must be able to be transmitted by resonance to the unsubstituted ring positions in order to facilitate the nucleophilic step.