Hydrogenation of Simple Aromatic Molecules: A Computational Study of the Mechanism

Abstract

Quantum chemistry calculations have been used to study the metal-free hydrogenation reactions of a variety of simple aromatic, heteroaromatic, and related linear conjugated systems. We find that the barrier for uncatalyzed 1,4-hydrogenation is always substantially lower (by approximately 200 kJ mol-1) than that for 1,2-hydrogenation, despite similar reaction enthalpies. The presence of hydrogen fluoride as a catalyst is found to decrease the 1,2-hydrogenation barriers but, in most cases, to slightly increase the 1,4-hydrogenation barriers when a constrained geometric arrangement is employed. These qualitative observations are consistent with orbital symmetry considerations, which show that both the uncatalyzed 1,4-hydrogenation and the catalyzed 1,2-hydrogenation are formally symmetry-allowed processes. An extreme example of the catalyzed 1,2-hydrogenation of benzene is provided by the involvement of a second molecule of hydrogen, which leads to a substantial lowering of the barrier. The effect of catalysis was further investigated by applying a selection of additional catalysts to the 1,2- and 1,4-hydrogenation of benzene. A decreasing barrier with increasing catalyst acidity is generally observed for the catalytic 1,2-hydrogenation, but the situation is more complex for catalytic 1,4-hydrogenation. For the uncatalyzed 1,4-hydrogenation of aromatic systems containing one or more nitrogen heteroatoms, the barriers for [C,C], [C,N], and [N,N] hydrogenations are individually related to the reaction enthalpies by the Bell-Evans-Polanyi principle. In addition, for a given reaction enthalpy, the barriers for [C,C] hydrogenation are generally lower than those for [C,N] or [N,N] hydrogenation. Finally, we find that the distortion experienced by the reactants in forming the transition structure represents a secondary factor that influences the reaction barrier. The correlation between these quantities allows the 1,4-hydrogenation barriers to be predicted from a ground-state property.

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

“…The barrier for the uncatalyzed proton-transfer reaction recalculated by us at the B3LYP/6-311+G(d,p) level of the theory is 45.3 kcal mol −1 in agreement with the barrier of 43.5 kcal mol −1 calculated by Chalk and Radom with the modified G2 method (15). Thus, the H 2 catalyst reduces the barrier by more than 30 kcal mol −1 (see Table 4).…”

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

“…A related mechanism has been described for 11. [48] Calculations reported by the Tamm group [37] showed the transition state for the activation of H 2 by carbene and borane gave rise to a carbene-borane "encounter complex" similar to that proposed by Pµpai et al…”

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

“…Eine ähnliche Reaktionsweise ist auch für 11 beschrieben worden. [48] DFT-Rechnungen von Tamm et al [37] [14] Bei der Spaltungsreaktion von H 2 lässt sich der stereochemische Verlauf nicht verfolgen, allerdings sind solche Informationen bei der verwandten B(C 6 F 5 ) 3 -katalysierten Hydrosilylierung von Ketonen [50] und verwandten Substraten erhältlich. [51] Piers et al hatten schon gezeigt, dass diese Reaktion über die Aktivierung des Silans und nicht der Carbonylverbindung durch die starke LewisSäure B(C 6 F 5 ) 3 [14] verläuft.…”