Compounds for inhibiting mitotic progression

A compound of formula (I):

• or a pharmaceutically acceptable salt thereof, wherein:
• Ra is selected from the group consisting of C1-3 aliphatic, C1-3 fluoroaliphatic, —R1, -T-R1—R2, and -T-R2;
• T is a C1-3 alkylene chain optionally substituted with fluoro;
• R1 is an optionally substituted aryl, heteroaryl, or heterocyclyl group;
• R2 is selected from the group consisting of halo, —C≡C—R3, —CH═CH—R3, —N(R4)2, and —OR;
• R3 is hydrogen or an optionally substituted aliphatic, aryl, heteroaryl, or heterocyclyl group;
• each R4 independently is hydrogen or an optionally substituted aliphatic, aryl, heteroaryl, or heterocyclyl group;
• or two R4 on the same nitrogen atom, taken together with the nitrogen atom form an optionally substituted 5- to 6-membered heteroaryl or 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms selected from N, O, and S;
• R5 is hydrogen or an optionally substituted aliphatic, aryl, heteroaryl, or heterocyclyl group;
• and
• Rb is selected from the group consisting of fluoro, chloro, —CH3, —CF3, —OH, —OCH3, —OCF3, —OCH2CH3, and —OCH2CF3.