A Rh(III)-catalyzed C-H bond direct alkylation between 2-arylphthalazine-1,4-diones and α-Cl ketones, which are sp 3 -carbon synthons, under mild conditions has been disclosed. The corresponding phthalazine derivatives are readily obtained in moderate to excellent yields with a wide range of substrates and high functional group tolerance. The practicality and utility of this method are demonstrated by the derivatization of the product.