Use of a compound of formula I: wherein R is selected from the group consisting of substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted 3- to 20-membered heteroaryl, substituted or unsubstituted C7-C25 alkyl-aryl, substituted or unsubstituted C1-C5 alkyl-3- to 20- membered heteroaryl, substituted or unsubstituted C3-C20 cycloalkyl, and substituted or unsubstituted 3- to 20- membered heterocyclic group; wherein the heteroaryl or heterocyclic group has 1 to 5 heteroatoms selected from the group consisting of N, O and S; the cycloalkyl or heterocyclic group may be a monocyclic, polycyclic, spiro or bridged ring structure; R a is selected from (a) or (b): (a) OR'; wherein R' is selected from the group consisting of substituted or unsubstituted C1-C6 alkyl; or (b) N(R") 2; wherein each R" is independently selected from the group consisting of H, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted 3- to 20- membered heteroaryl, substituted or unsubstituted C7-C25 alkyl-aryl, substituted or unsubstituted C1-C5 alkyl-3- to 20- membered heteroaryl, substituted or unsubstituted C3-C20 cycloalkyl, and substituted or unsubstituted 3- to 20- membered heterocyclic group; wherein the heteroaryl or heterocyclic group has 1 to 5 heteroatoms selected from the group consisting of N, O and S; the cycloalkyl or heterocyclic group may be a monocyclic, polycyclic, spiro or bridged ring structure; R b is selected from the group consisting of H, and C1-C6 alkyl; or R b and R, together with adjacent nitrogen atom, form a substituted or unsubstituted 3-to 20- membered heteroaryl, or substituted or unsubstituted 3- to 20- membered heterocyclic group; the term "substituted" means that one or more hydrogen atoms on the group is substituted by a substituent selected from the group consisting of halogen, C1-C6 alkyl, halogenated C1-C6 alkyl, C1-C6 alkoxy, C6-C10 aryl, C6-C10 aryl-oxy, C2-C10 ester group (alkyl-COO-), C2-C10 acyl-alkoxy (alkyl-OOC-), C2-C10 acyl (alkyl-CO-), C2-C10 acyl amino (alkyl/aryl-NHC -), -COOH, nitro, hydroxy, amino, and amino substituted by one or two C1-C6 alkyl; wherein the compound of formula I is used as a ligand for copper-catalyzed coupling reaction of aryl chloride. wherein R is selected from the group consisting of substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted 3- to 20-membered heteroaryl, substituted or unsubstituted C7-C25 alkyl-aryl, substituted or unsubstituted C1-C5 alkyl-3- to 20- membered heteroaryl, substituted or unsubstituted C3-C20 cycloalkyl, and substituted or unsubstituted 3- to 20- membered heterocyclic group; wherein the heteroaryl or heterocyclic group has 1 to 5 heteroatoms selected from the group consisting of N, O and S; the cycloalkyl or heterocyclic group may be a monocyclic, polycyclic, spiro or bridged ring structure; R a is selected from (a) or (b): (a) OR'; wherein R' is selected from the group consisting of substituted or unsubstituted C1-C6 alkyl; or (b) N(R") 2; wherein each R" is independently selected from the group consisting of H, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted 3- to 20- membered heteroaryl, substituted or unsubstituted C7-C25 alkyl-aryl, substituted or unsubstituted C1-C5 alkyl-3- to 20- membered heteroaryl, substituted or unsubstituted C3-C20 cycloalkyl, and substituted or unsubstituted 3- to 20- membered heterocyclic group; wherein the heteroaryl or heterocyclic group has 1 to 5 heteroatoms selected from the group consisting of N, O and S; the cycloalkyl or heterocyclic group may be a monocyclic, polycyclic, spiro or bridged ring structure; (a) OR'; wherein R' is selected from the group consisting of substituted or unsubstituted C1-C6 alkyl; or (b) N(R") 2; wherein each R" is independently selected from the group consisting of H, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted 3- to 20- membered heteroaryl, substituted or unsubstituted C7-C25 alkyl-aryl, substituted or unsubstituted C1-C5 alkyl-3- to 20- membered heteroaryl, substituted or unsubstituted C3-C20 cycloalkyl, and substituted or unsubstituted 3- to 20- membered heterocyclic group; wherein the heteroaryl or heterocyclic group has 1 to 5 heteroatoms selected from the group consisting of N, O and S; the cycloalkyl or heterocyclic group may be a monocyclic, polycyclic, spiro or bridged ring structure; R b is selected from the group consisting of H, and C1-C6 alkyl; or R b and R, together with adjacent nitrogen atom, form a substituted or unsubstituted 3-to 20- membered heteroaryl, or substituted or unsubstituted 3- to 20- membered heterocyclic group; the term "substituted" means that one or more hydrogen atoms on the group is substituted by a substituent selected from the group consisting of halogen, C1-C6 alkyl, halogenated C1-C6 alkyl, C1-C6 alkoxy, C6-C10 aryl, C6-C10 aryl-oxy, C2-C10 ester group (alkyl-COO-), C2-C10 acyl-alkoxy (alkyl-OOC-), C2-C10 acyl (alkyl-CO-), C2-C10 acyl amino (alkyl/aryl-NHC -), -COOH, nitro, hydroxy, amino, and amino substituted by one or two C1-C6 alkyl; wherein the compound of formula I is used as a ligand for copper-catalyzed coupling reaction of aryl chloride.