H₂ activation by a highly electron-deficient aralkylated organoborane

Abstract

The electron-deficient and sterically bulky trialkylborane derivative tris[bis(pentafluorophenyl)methyl]borane [1, B(CH(C6F5)2)3], has been synthesised and comprehensively characterised; detailed (1)H and (19)F NMR studies reveal two dynamic bond rotational processes in the solution phase. Despite conventional probes (Gutmann-Beckett and Childs methods) implying that the compound has a very limited Lewis acidity, it was used to generate frustrated Lewis pairs capable of heterolytically activating H2 in ethereal solutions, which suggests that the hydridophilicity of 1 is comparable to the potent Lewis acid B(C6F5)3.

Optimizing the Energetics of FLP-Type H2 Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

“…This effect has been observed experimentally with the elevated temperature required for the H 2 activation with LAs 6 and 11. 36,41 Hence, we see that the bulkiness and structural congestion around the boron center increase the barrier height. Note that the opposite effect is seen for the LBs; e.g., the bulky tBu 3 P shows with all LA partners a lower barrier for H 2 splitting than Me 3 P. We note that, although highly Lewis acidic boranes with strong H − affinity provide facile H 2 activation, e.g., 1 and 2, the resultant B−H bonds (borohydride anions) are consequently poor hydride donors that can limit the scope of substrates to be reduced in the hydride transfer step.…”

“…According to previous experimental investigations, considerable steric shielding of the boron center imparted by the large CH(C 6 F 5 ) 2 ligands hinders access to the Lewis base and results in a weak borane LA. 41 The H – affinity of 11 is nevertheless stronger than that of the LAs of class B. This means that the C 6 F 5 rings can induce their electronegativity to the B atom despite the extra C–H groups and make it more electrophilic than the class B LAs.…”

“…This is in agreement with the experimental observation that borane LAs with fewer F atoms are less reactive toward H 2 activation. 38 , 41 One can conclude that structures 7 and 8 in group B are the most efficient LAs in combination with ethereal solvent molecules since they have a moderate barrier of H 2 activation (not too high) and the H – affinity is also not too strong to prohibit further hydride donation for the subsequent C=O hydrogenation. This is in line with the experimental observation that only 7 and 8 in group B can produce alcohol from an olefinic ketone.…”

“…According to previous experimental investigations, considerable steric shielding of the boron center imparted by the large CH(C 6 F 5 ) 2 ligands hinders access to the Lewis base and results in a weak borane LA. 41 The Lewis basicity of the LBs can be estimated from the strength of the dative bond to 12.…”

“…For comparison, the well-known P-based LBs t Bu 3 P and Me 3 P have also been included. All LAs have previously been shown to activate H 2 experimentally. ,,,− We have examined, using density functional theory (DFT) calculations with a continuum solvation model, the structure and interaction energy of the initial adduct between the LA and LB and the energies of the TS and the product ion pair. The reaction energy of the hydrogen activation is based on the reaction Here, we focus our attention on this first step and therefore do not simulate the entire hydrogenation reaction process, including substrate molecules, because this step is common to all hydrogenation reactions.…”