A compound of formula I or formula I ': or a pharmaceutically acceptable salt thereof, wherein: Ar 1 is a 5-6 membered aromatic monocyclic ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, wherein said ring is optionally fused to a 5-12 membered monocyclic or bicyclic, aromatic, partially unsaturated, or saturated ring, wherein each ring contains 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, wherein Ar 1 has m substituents, each independently selected from -WR W; W is a bond or is an optionally substituted C 1 -C 6 alkylidene chain wherein up to two methylene units of W are optionally and independently replaced by O, -CO-, -CS-, -COCO-,-CONR'-, -CONR'NR'-, -CO 2 -, -OCO-, -NR'CO 2 -, -O-, -NR'CONR'-, -C NR'-, -OCONR'-,-NR'NR', -NR'NR'CO-, -NR'CO-, -S-, -SO, -SO 2 -, -NR'-, -SO 2 NR'-, NR'SO 2 -, or-NR'SO 2 NR'-; R W is independently R', halo, cyano, CF 3, CHF 2, OCHF 2, Me, Et, CH(Me) 2, CHMeEt, n-propyl, t-butyl, OH, OMe, OEt, OPh, O-fluorophenyl, O-difluorophenyl, O-methoxyphenyl, O-tolyl, O-benzyl, SMe, SCF 3, SCHF 2, SEt, CH 2 CN, NH 2, NHMe, N(Me) 2, NHEt, N(Et) 2, C CH 3, C Ph, C NH 2, SPh, SO 2 -(amino-pyridyl), SO 2 NH 2, SO 2 Ph, SO 2 NHPh, SO 2 -N-morpholino, SO 2 -N-pyrrolidyl, N-pyrrolyl, N-mopholino, 1-piperidyl, phenyl, benzyl, (cyclohexyl-methylamino)methyl, 4-Methyl-2,4-dihydro-pyrazol-3-one-2-yl, benzimidazol-2yl, furan-2-yl, 4-methyl-4H-[1,2,4]triazol-3-yl, 3-(4'-chlorophenyl)-[1,2,4]oxadiazol-5-yl, NHC Me, NHC Et, NHC Ph, or NHSO 2 Me; m is 0-5; k is 0-2; each of R 1 is independently -X-R X; X is a bond or is an optionally substituted C 1 -C 6 alkylidene chain wherein up to two methylene units of X are optionally and independently replaced by -CO-, -CS-, -COCO-,-CONR'-, -CONR'NR'-, -CO 2 -, -OCO-, -NR'CO 2 -, -O-, -NR'CONR'-, -OCONR'-, -NR'NR',-NR'NR'CO-, -NR'CO-, -S-, -SO, -SO 2 -, -NR'-, -SO 2 NR'-, NR'SO 2 -, or -NR'SO 2 NR'-; R X is independently R', halo, NO 2, CN, CF 3, or OCF 3; R 2 is hydrogen; R 3 is hydrogen; R 4 is hydrogen or a C 1-6 aliphatic group optionally substituted with -X-R X; R' is independently selected from hydrogen or an optionally substituted group selected from a C 1- C 8 aliphatic group, a 3-8-membered saturated, partially unsaturated, or fully unsaturated monocyclic ring having 0-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or an 8-12 membered saturated, partially unsaturated, or fully unsaturated bicyclic ring system having 0-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur; or two occurrences of R are taken together with the atom(s) to which they are bound to form an optionally substituted 3-12 membered saturated, partially unsaturated, or fully unsaturated monocyclic or bicyclic ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur. Ar 1 is a 5-6 membered aromatic monocyclic ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, wherein said ring is optionally fused to a 5-12 membered monocyclic or bicyclic, aromatic, partially unsaturated, or saturated ring, wherein each ring contains 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur, wherein Ar 1 has m substituents, each independently selected from -WR W; W is a bond or is an optionally substituted C 1 -C 6 alkylidene chain wherein up to two methylene units of W are optionally and independently replaced by O, -CO-, -CS-, -COCO-,-CONR'-, -CONR'NR'-, -CO 2 -, -OCO-, -NR'CO 2 -, -O-, -NR'CONR'-, -C NR'-, -OCONR'-,-NR'NR', -NR'NR'CO-, -NR'CO-, -S-, -SO, -SO 2 -, -NR'-, -SO 2 NR'-, NR'SO 2 -, or-NR'SO 2 NR'-; R W is independently R', halo, cyano, CF 3, CHF 2, OCHF 2, Me, Et, CH(Me) 2, CHMeEt, n-propyl, t-butyl, OH, OMe, OEt, OPh, O-fluorophenyl, O-difluorophenyl, O-methoxyphenyl, O-tolyl, O-benzyl, SMe, SCF 3, SCHF 2, SEt, CH 2 CN, NH 2, NHMe, N(Me) 2, NHEt, N(Et) 2, C CH 3, C Ph, C NH 2, SPh, SO 2 -(amino-pyridyl), SO 2 NH 2, SO 2 Ph, SO 2 NHPh, SO 2 -N-morpholino, SO 2 -N-pyrrolidyl, N-pyrrolyl, N-mopholino, 1-piperidyl, phenyl, benzyl, (cyclohexyl-methylamino)methyl, 4-Methyl-2,4-dihydro-pyrazol-3-one-2-yl, benzimidazol-2yl, furan-2-yl, 4-methyl-4H-[1,2,4]triazol-3-yl, 3-(4'-chlorophenyl)-[1,2,4]oxadiazol-5-yl, NHC Me, NHC Et, NHC Ph, or NHSO 2 Me; m is 0-5; k is 0-2; each of R 1 is independently -X-R X; X is a bond or is an optionally substituted C 1 -C 6 alkylidene chain wherein up to two methylene units of X are optionally and independently replaced by -CO-, -CS-, -COCO-,-CONR'-, -CONR'NR'-, -CO 2 -, -OCO-, -NR'CO 2 -, -O-, -NR'CONR'-, -OCONR'-, -NR'NR',-NR'NR'CO-, -NR'CO-, -S-, -SO, -SO 2 -, -NR'-, -SO 2 NR'-, NR'SO 2 -, or -NR'SO 2 NR'-; R X is independently R', halo, NO 2, CN, CF 3, or OCF 3; R 2 is hydrogen; R 3 is hydrogen; R 4 is hydrogen or a C 1-6 aliphatic group optionally substituted with -X-R X; R' is independently selected from hydrogen or an optionally substituted group selected from a C 1- C 8 aliphatic group, a 3-8-membered saturated, partially unsaturated, or fully unsaturated monocyclic ring having 0-3 heteroatoms independently selected from nitrogen, oxygen, or sulfur, or an 8-12 membered saturated, partially unsaturated, or fully unsaturated bicyclic ring system having 0-5 heteroatoms independently selected from nitrogen, oxygen, or sulfur; or two occurrences of R are taken together with the atom(s) to which they are bound to form an optionally substituted 3-12 membered saturated, partially unsaturated, or fully unsaturated monocyclic or bicyclic ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur.