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Metal‐Free Reduction of CO 2 with Hydroboranes: Two Efficient Pathways at Play for the Reduction of CO 2 to Methanol
Author(s) -
Das Neves Gomes Christophe,
Blondiaux Enguerrand,
Thuéry Pierre,
Cantat Thibault
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201400349
Subject(s) - chemistry , adduct , hydride , methanol , catalysis , reagent , formate , medicinal chemistry , nucleophile , reductive elimination , ene reaction , methyl formate , reactivity (psychology) , combinatorial chemistry , metal , organic chemistry , alternative medicine , pathology , medicine
Guanidines and amidines prove to be highly efficient metal‐free catalysts for the reduction of CO 2 to methanol with hydroboranes such as 9‐borabicyclo[3.3.1]nonane (9‐BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD), 7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (Me‐TBD), and 1,8‐diazabicycloundec‐7‐ene (DBU), are active catalysts for this transformation and Me‐TBD can catalyze the reduction of CO 2 to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h −1 (25 °C), respectively. Formate HCOOBR 2 and acetal H 2 C(OBR 2 ) 2 derivatives have been identified as reaction intermediates in the reduction of CO 2 with R 2 BH, and the first CH‐bond formation is rate determining. Experimental and computational investigations show that TBD and Me‐TBD follow distinct mechanisms. The NH bond of TBD is reactive toward dehydrocoupling with 9‐BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO 2 molecule and form the stable adduct 2 , which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me‐TBD promotes the reduction of CO 2 through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.