The present invention provides novel imidazolidinonyl aminopyrimidine compounds believed to have clinical use for treatment of cancer through inhibiting Plk1. Formula I wherein: R1 is aminomethyl, (C1-C3 alkyl)aminomethyl, di(C1-C2 alkyl)aminomethyl, N-ethyl-N-methyl-aminomethyl, 1-aminoethyl, 1-((C1-C2 alkyl)amino)-ethyl, 3,3,3-trifluoro-propylaminomethyl, ethynyl, 2-hydroxy-ethoxy, 2-hydroxyethylaminomethyl, 2-cyanoethylaminomethyl, mopholin-4-ylmethyl, methoxymethoxymethyl, cyclopropyl, 1-azetidinylmethyl, 1-pyrrolidinylmethyl, or 1,3-dioxolan-2-yl; R2 is hydrogen or halo; R3 is hydrogen or halo; provided that at least one of R2 and R3 is hydrogen; R4 is hydrogen, methyl, or halo; and is a single bond that is either present or absent, or pharmaceutically acceptable salt thereof.