A Practical and Stereoselective In Situ NHC-Cobalt Catalytic System for Hydrogenation of Ketones and Aldehydes

• Published in: Chem Vol. 5; no. 6; pp. 1552 - 1566
• Main Authors: Zhong, Rui, Wei, Zeyuan, Zhang, Wei, Liu, Shun, Liu, Qiang
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 13.06.2019
• Subjects: cobalt; hydrogenation; ketones; NHC-carbene; SDG12: Responsible consumption and production; SDG9: Industry, innovation, and infrastructure; stereoselectivity; stereoselectivity; SDG12: Responsible consumption and production; cobalt; ketones; NHC-carbene; hydrogenation; SDG9: Industry, innovation, and infrastructure
• ISSN: 2451-9294; 2451-9294
• DOI: 10.1016/j.chempr.2019.03.010

Homogeneous catalytic hydrogenation of carbonyl groups is a synthetically useful and widely applied organic transformation. Sustainable chemistry goals require replacing conventional noble transition metal catalysts for hydrogenation by earth-abundant base metals. Herein, we report how a practical in situ catalytic system generated by easily available pincer NHC precursors, CoCl2, and a base enabled efficient and high-yielding hydrogenation of a broad range of ketones and aldehydes (over 50 examples and a maximum turnover number [TON] of 2,610). This is the first example of NHC-Co-catalyzed hydrogenation of C=O bonds using flexible pincer NHC ligands consisting of a N-H substructure. Diastereodivergent hydrogenation of substituted cyclohexanone derivatives was also realized by fine-tuning of the steric bulk of pincer NHC ligands. Additionally, a bis(NHCs)-Co complex was successfully isolated and fully characterized, and it exhibits excellent catalytic activity that equals that of the in-situ-formed catalytic system. [Display omitted] •A practical and easily accessible in-situ NHC-Co hydrogenation catalyst for C=O bonds•Diastereodivergent Co-catalyzed hydrogenation of cyclohexanone derivatives•Isolation of a catalytically active NHC-Co complex with a non-rigid pincer NHC ligand Catalytic hydrogenation is a powerful tool for the reduction of organic compounds in both fine and bulk chemical industries. To improve sustainability, more ecofriendly, inexpensive, and earth-abundant base metals should be employed to replace the precious metals that currently dominate the development of hydrogenation catalysts. However, the majority of the base-metal catalysts that have been reported involve expensive, complex, and often air- and moisture-sensitive phosphine ligands, impeding their widespread application. From a mixture of the stable CoCl2, imidazole salts, and a base, our newly developed catalytic system that formed easily in situ enables efficient and stereoselective hydrogenation of C=O bonds. We anticipate that this easily accessible catalytic system will create opportunities for the design of practical base-metal hydrogenation catalysts. A practical in situ catalytic system generated by a mixture of easily available pincer NHC precursors, CoCl2, and a base enabled highly efficient hydrogenation of a broad range of ketones and aldehydes (over 50 examples and up to a turnover number [TON] of 2,610). Diastereodivergent hydrogenation of substituted cyclohexanone derivatives was also realized in high selectivities. Moreover, the preparation of a well-defined bis(NHCs)-Co complex via this pincer NHC ligand consisting of a N-H substructure was successful, and it exhibits equally excellent catalytic activity for the hydrogenation of C=O bonds.