Within the past few decades, organoboron compounds have become essential
intermediates in organic and medicinal chemistry due to their unique advantages
and versatility. Many synthetic approaches have been utilised to prepare these
useful compounds, including several catalytic systems and diverse hydroborating
agents. The hydroboration of terminal alkynes is one of the more common routes
that provides alkenylboron reagents, allowing regio- and stereo-selective
installation of the boron.
Alcoholysis of phosphine borane complex to generate a new metastable
hydroborating agent has been studied. The effect of the alcohol and solvent used in
the solvolysis reaction have been determined. The use of halogenated solvents
removed the irreproducibility observed when an ethereal solvent is used on its own.
Thus having a mixture of both solvents, produces an ideal media to stabilise
dialkoxyborane species. All kinetic data obtained are consistent with a direct
SN2-like mechanism. A plausible mechanism has been proposed where an
inconsistent generation of an ionic active species is likely to be causing the
irreproducibility in the kinetic stability of the intermediate HB(OEt) 2.
The dicyclohexylborane-catalysed hydroboration of 4-fluoropheny-lacetylene with
pinacolborane was used as a benchmark. The kinetic profile for all components,
including intermediates and side-products, have also been established by using in
situ 19F NMR kinetic analysis and simulation. Isotope entrainment and
stoichiometric studies employing isotopic labelling were used to characterise key
steps of the borane-catalysed hydroboration reaction. The experimental evidence
used to propose a thorough mechanism were also backed up by density functional
theory (DFT) calculations. The mechanism includes the first true transborylation
step, responsible of the regeneration of the catalyst (HBCy2) via B-C-B transfer of
the alkenyl group with retention of (E)-configuration
