A compound of formula I: or a pharmaceutically acceptable salt thereof, wherein R 1 is H or C 1-6 alkyl; L 1 is C 1-4 alkylene; Ar 1 is a C 6 - 12 aryl or 5-10 membered heteroaryl having 1-4 heteroatoms selected from N, O, and S, each of which is optionally substituted with 1-3 R a groups; each R a is independently H, halogen, C 1-4 alkyl, C 1-4 alkoxy, C 1-4 haloalkyl, C 1-4 haloalkoxy, -SO 2 R a1 , or -NR a1 R a2 ; R a1 and R a2 are each independently H or C 1-4 alkyl; or R a1 and R a2 when attached to a nitrogen atom are combined to form a 3-6 membered heterocycle having 1-2 heteroatoms selected from N, O, and S, which is optionally substituted with 1-2 R a3 ; each R a3 is independently H, halogen, C 1-4 alkyl, or C 1-4 alkoxy; Ar 2 is a C 6 - 12 aryl or 5-10 membered heteroaryl having 1-4 heteroatoms selected from N, O, and S, wherein the aryl is substituted with 1-4 R b groups, and wherein the heteroaryl is optionally substituted with 1-4 R b groups; each R b is independently halogen, CN, hydroxy, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxy, C 1-4 alkoxy-C 1-4 alkyl, C 1-4 hydroxyalkyl, C 1-4 haloalkyl, C 1-4 haloalkoxy, -OR b4 , -NR b1 R b2 , -C(O)R b1 , -C(O)OR b1 , -OC(O)R b1 , -C(O)NR b1 R b2 , - NR b1 C(O)R b2 , -SO 2 R b1 , -SO 2 NR b1 R b2 , or C 3-6 cycloalkyl; alternatively, two R b groups on adjacent ring atoms can be combined to form a C 5-8 cycloalkyl or a 5-8 membered heterocycle having 1-2 heteroatoms selected from N, O, and S; R b1 and R b2 are each independently H or C 1-4 alkyl; or R b1 and R b2 when attached to a nitrogen atom are combined to form a 3-6 membered heterocycle having 1-2 heteroatoms selected from N, O, and S, which is optionally substituted with 1-2 R b3 ; each R b3 is independently H, halogen, C 1-4 alkyl, or C 1-4 alkoxy; and each R b4 is independently C 1-4 hydroxyalkyl, C 1-4 alkoxy-C 1-4 alkyl, C 3-6 cycloalkyl or C 3-6 cycloalkyl-C 1-4 alkyl. R 1 is H or C 1-6 alkyl; L 1 is C 1-4 alkylene; Ar 1 is a C 6 - 12 aryl or 5-10 membered heteroaryl having 1-4 heteroatoms selected from N, O, and S, each of which is optionally substituted with 1-3 R a groups; each R a is independently H, halogen, C 1-4 alkyl, C 1-4 alkoxy, C 1-4 haloalkyl, C 1-4 haloalkoxy, -SO 2 R a1 , or -NR a1 R a2 ; R a1 and R a2 are each independently H or C 1-4 alkyl; or R a1 and R a2 when attached to a nitrogen atom are combined to form a 3-6 membered heterocycle having 1-2 heteroatoms selected from N, O, and S, which is optionally substituted with 1-2 R a3 ; each R a3 is independently H, halogen, C 1-4 alkyl, or C 1-4 alkoxy; Ar 2 is a C 6 - 12 aryl or 5-10 membered heteroaryl having 1-4 heteroatoms selected from N, O, and S, wherein the aryl is substituted with 1-4 R b groups, and wherein the heteroaryl is optionally substituted with 1-4 R b groups; each R b is independently halogen, CN, hydroxy, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxy, C 1-4 alkoxy-C 1-4 alkyl, C 1-4 hydroxyalkyl, C 1-4 haloalkyl, C 1-4 haloalkoxy, -OR b4 , -NR b1 R b2 , -C(O)R b1 , -C(O)OR b1 , -OC(O)R b1 , -C(O)NR b1 R b2 , - NR b1 C(O)R b2 , -SO 2 R b1 , -SO 2 NR b1 R b2 , or C 3-6 cycloalkyl; alternatively, two R b groups on adjacent ring atoms can be combined to form a C 5-8 cycloalkyl or a 5-8 membered heterocycle having 1-2 heteroatoms selected from N, O, and S; R b1 and R b2 are each independently H or C 1-4 alkyl; or R b1 and R b2 when attached to a nitrogen atom are combined to form a 3-6 membered heterocycle having 1-2 heteroatoms selected from N, O, and S, which is optionally substituted with 1-2 R b3 ; each R b3 is independently H, halogen, C 1-4 alkyl, or C 1-4 alkoxy; and each R b4 is independently C 1-4 hydroxyalkyl, C 1-4 alkoxy-C 1-4 alkyl, C 3-6 cycloalkyl or C 3-6 cycloalkyl-C 1-4 alkyl. <EOS>