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>Guerbet-type beta-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex
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Guerbet-type beta-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex
The beta-alkylation of secondary alcohols has been successfully accomplished using earth abundant, inexpensive and environmentally benign 3d transition metal chromium salt CrCl3 and its corresponding pincer-Cr complex, ((NNN)-N-Ph2)CrCl3 (0.005 mol%), at 140 degrees C in air under conventional (3 h) and microwave heating conditions (ca. 75 W for 1.5 h) in the presence of (NaOBu)-Bu-t (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 beta-alkylated product under microwave conditions in the presence of 5 mol% (NaOBu)-Bu-t, the corresponding pincer-Cr complex ((NNN)-N-Ph2)CrCl3 (0.005 mol%) resulted in 90% yield (18 000 TON at 12 000 TOh(-1)) of the beta-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 alpha-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.
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