Abstract
Ruthenium (II) complexes with N-heterocyclic carbenes (NHC) are commonly used as efficient catalysts in hydrogenation of olefins with simultaneous intramolecular C–H activation. Using the DFT approach, we have investigated the entire hydrogenation reaction pathway for four new potential catalysts and ethylene, a model substrate. Our calculations imply that the dissociation of phosphine is the rate-limiting step of hydrogenation, contrary to recent computational results. We also found that catalysts bearing NHCs with aliphatic and aromatic side groups are energetically favorable over other aliphatic cyclohexyl-substituted NHC. To examine how electronic properties of various catalysts influence the energetic barrier in the crucial steps of the reaction, we applied the Noncovalent Interaction analysis, which allowed us to reveal crucial interactions which stabilize/destabilize important intermediates and transition states in the hydrogenation reaction.
Highlights
Even though less than 30 years has passed since the synthesis of the first stable carbene by Arduengo, it is difficult to imagine today’s catalysis without these versatile ligands.[1]
Used extensively both in organocatalysis and transition-metal catalysis, they have become one of the most commonly used ligands for many catalytic processes.[2−6] Historically, the first group of carbenes were N-heterocyclic carbenes (NHCs) and they found numerous uses in a large number of catalytic processes, with the most successful story being the olefin metathesis catalyzed by NHC-ruthenium complexes.[7,8]
The olefin is associated to the ruthenium core of the catalyst and undergoes a series of transformations leading to the interchange of its substituents
Summary
Even though less than 30 years has passed since the synthesis of the first stable carbene by Arduengo, it is difficult to imagine today’s catalysis without these versatile ligands.[1] Used extensively both in organocatalysis and transition-metal catalysis, they have become one of the most commonly used ligands for many catalytic processes.[2−6] Historically, the first group of carbenes were N-heterocyclic carbenes (NHCs) and they found numerous uses in a large number of catalytic processes, with the most successful story being the olefin metathesis catalyzed by NHC-ruthenium complexes.[7,8] During this catalytic reaction, the olefin is associated to the ruthenium core of the catalyst and undergoes a series of transformations leading to the interchange of its substituents. The first example of such system, synthesized in 2001 by the Nolan group, is an active catalyst in hydrogenation of alkenes with TONs up to 24,000 h−1.14 Since other but structurally similar complexes bearing either one or two NHCs have been synthesized and used in hydrogenation (see Figure 1).[15−19]
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