Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles

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.