Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands |
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| Authors: | Dr M Victoria Jiménez Dr Javier Fernández‐Tornos Dr F Javier Modrego Prof Jesús J Pérez‐Torrente Prof Luis A Oro |
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| Affiliation: | Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea‐ISQCH, Universidad de Zaragoza‐CSIC, C/Pedro Cerbuna, 50009‐Zaragoza (Spain) |
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| Abstract: | The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex Ir(NCCH3)(cod)(MeIm(2‐ methoxybenzyl))]BF4] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0) of 1283 h?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde. |
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| Keywords: | alcohols alkylation heteroditopic NHC ligands iridium N‐heterocyclic carbenes oxidation |
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