title: Non-redox metal ion promoted oxidative coupling of indoles with olefins by the palladium(ii) acetate catalyst through dioxygen activation: experimental results with DFT calculations.
abstract: Developing new catalytic technologies through C-H bond activation to synthesize versatile pharmaceuticals has attracted much attention in recent decades. This work introduces a new strategy in catalyst design for Pd(ii)-catalyzed C-H bond activation in which non-redox metal ions serving as Lewis acids play significant roles. In the oxidative coupling of indoles with olefins using dioxygen, it was found that Pd(OAc)2 alone as the catalyst is very sluggish at ambient temperature which provided a low yield of the olefination product, whereas adding non-redox metal ions to Pd(OAc)2 substantially improves its catalytic efficiency. In particular, it provided bis(indolyl)methane derivatives as the dominant product, a category of pharmacological molecules which could not be synthesized by Pd(ii)-catalyzed oxidative coupling previously. Detailed investigations revealed that the reaction proceeds by heterobimetallic Pd(ii)/Sc(iii)-catalyzed oxidative coupling of an indole with an olefin followed by Sc(iii)-catalyzed addition with a second indole molecule. DFT calculations disclosed that the formation of heterobimetallic Pd(ii)/Sc(iii) species substantially decreases the C-H bond activation energy barrier, and shifts the rate determining step from C-H bond activation of indole to the olefination step. This non-redox metal ion promoted Pd(ii)-catalyzed C-H bond activation may offer a new opportunity for catalyst design in organic synthesis, which has not been fully recognized yet.
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Sicheng
authors.initials: S
authors.lastname: Zhang
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Zhuqi
authors.initials: Z
authors.lastname: Chen
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Shuhao
authors.initials: S
authors.lastname: Qin
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Chenlin
authors.initials: C
authors.lastname: Lou
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Senan
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Rong-Zhen
authors.initials: RZ
authors.lastname: Liao
authors.affiliation: School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China and Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, PR China. gyin@hust.edu.cn rongzhen@hust.edu.cn.
authors.forename: Guochuan
authors.initials: G
authors.lastname: Yin