P–N Cooperative Borane Activation and Catalytic Hydroboration by a Distorted Phosphorous Triamide Platform

Abstract

Studies on 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 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−1s−1 at 25 °C. The B–H bond activation process produces a 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 catalyst and HBpin as 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.

Catalytic Hydroboration of Organic Nitriles Promoted by Aluminum Complex

“…[9][10][11][12][13][14] As a result, innumerable procedures have been evolved to accomplish the addition of boranes to nitriles. All these procedures require more or less effective catalysts based on transition-metal complexes such as Co, Fe, Ru, [15][16][17][18][19][20][21][22][23] alkaline earth metal (Mg), [24][25][26][27][28][29][30][31][32][33][34][35] but the processes culminate in frustrated Lewis pairs. [36][37][38] On the other hand, while the reduction of aryl and alkyl nitriles can be achieved using stoichiometric quantities of maingroup reducing agents such as LiAlH 4 and NaBH 4 , [39] the combustible nature of these reagents and large quantities of inorganic waste by-products they generate render the process unattractive, and hence reductive hydroboration is preferable in order to provide further functionality to the resultant amine.…”

Validating the Biphilic Hypothesis of Nontrigonal Phosphorus(III) Compounds

“…Indeed, we have shown that nontrigonal s 3 -P compounds (see 1 and 2,F igure 1a)e volve to pentacoordinate phosphorus (s 5 -P) products through oxidative addition of EÀHb onds (E = OR, NHR, Ru), [25][26][27][28] in some cases reversibly. [29][30][31][32][33][34][35] Although applications of the biphilic character of nontrigonal s 3 -P compounds such as 1 and 2 have emerged in reactivity,adirect experimental demonstration of how the C 3v !C s structural deformation controls s 3 -P frontier orbital energies has not been previously presented. With av iew toward establishing arational schema for biphilic s 3 -P design, we provide here the first explicit spectroscopic evidence validating the descent-in-symmetry hypothesis of biphilicity at s 3 -P.T hrough phosphorus K-edge X-ray absorption nearedge structure (XANES) spectroscopy and supporting (TD)DFT demonstrations,w eq uantify the impact of nontrigonal distortion (C 3v !C s !C 2v )onthe electronic structure of as eries of compositionally related compounds and show that nontrigonal s 3 -P compounds present inherently contracted frontier orbital energy gaps that colocalize electrondonor and electron-acceptor behavior at as ingle s 3 -P site, engendering biphilic character.O verall, these results codify how structural modifications control s 3 -P electronic structure and provide aforward-looking blueprint for the development of next-generation nontrigonal P III catalysts and ligands with exceptional biphilic reactivity.…”

A Redox‐Confused Bismuth(I/III) Triamide with a T‐Shaped Planar Ground State

“…Tethered triamido ligands have recently been applied to achieve nonclassical geometries and reactivity paradigms at B III and P III centres . In the P III case, Radosevich and co‐workers found that the ground state is a distorted‐pyramidal 8‐P‐3 geometry that undergoes unusually facile inversion at phosphorus because of the availability of a transient 10‐P‐3 intermediate along the inversion coordinate (Scheme d) ,. However, despite its purported importance in unlocking a wide range of valuable reactivity, a planar 10‐E‐3 intermediate has never been experimentally realized for any group 15 triamide, further motivating our quest for a 10‐Bi‐3 environment within the (R)N‐N‐N(R) framework.…”