Metal‐Free Reduction of CO₂ with Hydroboranes: Two Efficient Pathways at Play for the Reduction of CO₂ to Methanol

Abstract

Guanidines and amidines prove to be highly efficient metal-free catalysts for the reduction of CO2 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 CO2 to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h(-1) (25 °C), respectively. Formate HCOOBR2 and acetal H2C(OBR2)2 derivatives have been identified as reaction intermediates in the reduction of CO2 with R2BH, and the first C-H-bond formation is rate determining. Experimental and computational investigations show that TBD and Me-TBD follow distinct mechanisms. The N-H bond of TBD is reactive toward dehydrocoupling with 9-BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO2 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 CO2 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.

Implications of CO2 Activation by Frustrated Lewis Pairs in the Catalytic Hydroboration of CO2: A View Using N/Si+ Frustrated Lewis Pairs

“…Although the catalytic performances are modest, these results are interesting as neither the parent guanidine base TBD nor the dimeric boron analogue III, nor the N/B FLP-CO 2 adduct V (Scheme 2) show any catalytic activity with catBH and pinBH. 27 The same trends in reactivity are observed for catBH and 9-BBN when catalyst 1 is replaced with iPr 1, Ph Table 1). All together, these results suggest that the formation of a CO 2 -adduct (e.g., 2) facilitates the catalytic hydroboration of CO 2 with 9-BBN and catBH.…”

“…CO 2 -adducts are often described as key-intermediates in the catalytic cycle of the CO 2 reduction processes, 25,57 and both adducts IV and V were shown to react with an electrophilic borane (9-BBN) to yield formate species. 27 In light of these data and the reaction chemistry of 1 described above, activation of CO 2 by 1 is a reasonable starting point for the hydroboration of CO 2 with 1 (Scheme 4). DFT calculations, performed at the M05-2X/ PCM = THF/6-31+G* (6-311++G** for the active hydride) level of theory confirm that the formation of a CO 2 adduct [2 + ,Cl − ] from 1 is indeed exergonic by −6.1 kcal/mol with a low activation barrier of only 10.6 kcal/mol (Scheme 4).…”

Catalyst‐Free Hydroboration of CO2 With a Nucleophilic Diborane(4)

“…Cantat et al. reported hydroboration of CO 2 with a guanidine base . On the basis of mechanistic studies, it was proposed that guanidine bonding to the borane enables nucleophilic attack at the CO 2 .…”

Thermal-reductive transformations of carbon dioxide catalyzed by small molecules using earth-abundant elements

“…Cantat et al [45] showed that nitrogenous bases can catalyze the reduction of CO 2 to methoxide with hydroboranes as the reductant. Specifically, guanidines and amidines such as TBD and Me-TBD are efficient catalysts for the hydroboration of CO 2 .…”