text: The ratio of the two possible carbonyl products (I and II) obtained is determined by the relative migratory abilities of the carbonyl substituents (R and R).  In general, the R-group most capable of stabilizing the partial positive charge formed during the rearrangement migrates preferentially.  A prominent exception to this general rule is hydride shifting.  The migratory preferences of the carbonyl R-groups can be heavily influenced by solvent and diazoalkane choice.  For example, methanol has been shown to promote aryl migration.  As shown below, if the reaction of piperanol (IV) with diazomethane is carried out in the absence of methanol, the ketone obtained though a hydride shift is the major product (V).  If methanol is the solvent, an aryl shift occurs to form the aldehyde (VI), which cannot be isolated as it continues to react to form the ketone (VII) and the epoxide (VIII) products.
  The diazoalkane employed can also determine relative yields of products by influencing migratory preferences, as conveyed by the reactions of o-nitropiperonal with diazomethane and diazoethane.  In the reaction between o-nitropiperonal (IX) and diazomethane, an aryl shift leads to production of the epoxide (X) in 9 to 1 excess of the ketone product (XI).  When diazoethane is substituted for diazomethane, a hydride shift produces the ketone (XII), the only isolable product.