1236584-20-5Relevant articles and documents
Dihydrogen activation by antiaromatic pentaarylboroles
Fan, Cheng,Mercier, Lauren G.,Piers, Warren E.,Tuononen, Heikki M.,Parvez, Masood
, p. 9604 - 9606 (2010)
Facile metal-free splitting of molecular hydrogen (H2) is crucial for the utilization of H2 without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H2 in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons α to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H2 provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.
Mechanistic studies on the metal-free activation of dihydrogen by antiaromatic pentarylboroles
Houghton, Adrian Y.,Karttunen, Virve A.,Fan, Cheng,Piers, Warren E.,Tuononen, Heikki M.
, p. 941 - 947 (2013/03/13)
The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H-H bond cleavage and formation of boracyclopentene products. The mechanism of this reaction has been studied experimentally through evaluation of the kinetic properties of the slower reaction between 2 and H2. The reaction is first-order in both [borole] and [H2] with activation parameters of ΔH? = 34(8) kJ/mol and ΔS? = -146(25) J mol-1 K-1. A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H-H cleavage in the rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring- closing mechanism is proposed and supported by the separate synthesis of a proposed intermediate and its observed conversion to product. Furthermore, extensive DFT mapping of the reaction mechanism supports the plausibility of this proposal. The study illustrates a new mechanism for the activation of H2 by a strong main group Lewis acid in the absence of an external base, a process driven in part by the antiaromaticity of the borole rings in 1 and 2.
