Temperature-dependent isomerisation versus net fragmentation of secondary allylic alcohols with Grubbs’ catalyst

Secondary allylic alcohols with 10 mol% of Grubbs catalyst in refluxing toluene/1,2-dichloroethane undergo isomerisation to ethyl ketones whereas with 100 mol% of Grubbs catalyst at room temperature, a net fragmentation reaction with the loss of a carbon atom occurs, to provide a methyl ketone. Probable mechanisms are described.