Guerbet-type β-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex. Issue 15 (23rd June 2022)

• Record Type: Journal Article
• Authors: Narjinari, Himani; Tanwar, Niharika; Kathuria, Lakshay; Jasra, Raksh Vir; Kumar, Akshai
• Is Part Of: Catalysis science & technology. Volume 12:Issue 15(2022)
• Page Start: 4753
• Page End: 4762
• Publication Date: 2022-06-23
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/d2cy00759b
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : β-Alkylation of alcohols has been efficiently accomplished using readily available 3d metal Cr under microwave conditions in air. Well-defined molecular Cr is involved with a KIE of 7.33 and insertion of α-alkylated ketone into Cr–H bond as the RDS. Abstract : The β-alkylation of secondary alcohols has been successfully accomplished using earth abundant, inexpensive and environmentally benign 3 d transition metal chromium salt CrCl3 and its corresponding pincer-Cr complex, ( Ph2 NNN)CrCl3 (0.005 mol%), at 140 °C in air under conventional (3 h) and microwave heating conditions ( ca. 75 W for 1.5 h) in the presence of NaO t Bu (5 mol%). Microwave conditions provided better results (90% yield and 18 000 TON at 12 000 TOh −1 ) in comparison to conventional heating (84% yield and 16 800 TON at 5600 TOh −1 ). While CrCl3 (0.005 mol%) afforded 76% yield (15 200 TON at 10 133 TOh −1 ) of the β-alkylated product under microwave conditions in the presence of 5 mol% NaO t Bu, the corresponding pincer-Cr complex ( Ph2 NNN)CrCl3 (0.005 mol%) resulted in 90% yield (18 000 TON at 12 000 TOh −1 ) of the β-alkylated product under similar conditions. EPR studies suggest the involvement of chromium(ii ) active species when either CrCl3 or its corresponding NNN pincer is used. HRMS analysis, hot-filtration and mercury-drop test provided conclusive evidence for the homogeneous and well-defined molecular nature of the reaction mixture. Apparently, insertion of the α-alkylated ketone into the Cr–H bond is the RDS based on inferences drawn from deuterium labelling studies that demonstrate a KIE of 7.33 and HRMS analysis which provide vital information on the resting-state of the reaction.