title: Mechanistic studies on the metal-free activation of dihydrogen by antiaromatic pentarylboroles.
abstract: 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 H(2). The reaction is first-order in both [borole] and [H(2)] 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 H(2) 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.
authors.affiliation: Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4.
authors.forename: Adrian Y
authors.initials: AY
authors.lastname: Houghton
authors.forename: Virve A
authors.initials: VA
authors.lastname: Karttunen
authors.forename: Cheng
authors.initials: C
authors.lastname: Fan
authors.forename: Warren E
authors.initials: WE
authors.lastname: Piers
authors.forename: Heikki M
authors.initials: HM
authors.lastname: Tuononen