a reaction of grignards with oxygen in presence of an alkene makes an ethylene extended alcohol. these are useful in synthesizing larger compounds. this modification requires aryl or vinyl grignard reagents. adding just the grignard and the alkene does not result in a reaction, showing that the presence of oxygen is essential. the only drawback is the requirement of at least two equivalents of grignard reagent in the reaction. this can addressed by using a dual grignard system with a cheap reducing grignard reagent such as n-butylmagnesium bromide.