posted on 2017-04-11, 00:00authored byYi-Chun Lin, Emmanuel Hatzakis, Sean M. McCarthy, Kyle D. Reichl, Ting-Yi Lai, Hemant P. Yennawar, Alexander T. Radosevich
Studies
of the stoichiometric and catalytic reactivity of a geometrically
constrained phosphorous triamide 1 with pinacolborane
(HBpin) are reported. The addition of HBpin to phosphorous triamide 1 results in cleavage of the B–H bond of pinacolborane
through addition across the electrophilic phosphorus and nucleophilic
N-methylanilide sites in a cooperative fashion. The kinetics of this
process of were investigated by NMR spectroscopy, with the determined
overall second-order empirical rate law given by ν = −k[1][HBpin], where k = 4.76
× 10–5 M–1 s–1 at 25 °C. The B–H bond activation process produces P-hydrido-1,3,2-diazaphospholene
intermediate 2, which exhibits hydridic reactivity capable
of reacting with imines to give phosphorous triamide intermediates,
as confirmed by independent synthesis. These phosphorous triamide
intermediates are typically short lived, evolving with elimination
of the N-borylamine product of imine hydroboration with regeneration
of the deformed phosphorous triamide 1. The kinetics
of this latter process are shown to be first-order, indicative of
a unimolecular mechanism. Consequently, catalytic hydroboration of
a variety of imine substrates can be realized with 1 as
the catalyst and HBpin as the terminal reagent. A mechanistic proposal
implicating a P–N cooperative mechanism for catalysis that
incorporates the various independently verified stoichiometric steps
is presented, and a comparison to related phosphorus-based systems
is offered.