Electrophilic attack at the y-position

Seyferth showed in 1959 that allyllithium, which is not available from allyl halides because of competing substitution reactions, can be prepared from the reaction of allylstannanes with organolithium reagents (XM = R Li Section 22.1). These reactions involve nucleophilic attack of R upon tin to form a pentacoordinate intermediate. Lewis acids, MX, on the other hand, react by electrophilic attack at the y-position. 

---