 The primary advantage of using Lewis acids versus fluoride catalysis is site selectivity—fluoride activation tends to generate allyl anions, which can then react at either the α or γ position.Use of Lewis-acid-activated electrophiles leads to reaction at only the γ position of allylsilanes.Alkyl substitution can be achieved when alkyl halides are used in conjunction with a Lewis acid; however, only activated (allylic, benzylic) or highly substituted ("tert"-butyl) halides are useful in this context.