Phosphorus(III) compounds are widely used in organic chemistry as catalysts to promote a wide range of reactions. Frequently, these reactions rely on the strength of the phosphorus-oxygen bonds to drive the reaction to completion, resulting in the generation of stoichiometric amounts of phosphorus(V) oxides which can complicate purification of desired products. Recent work in this field has focused on the <i>in-situ </i>regeneration of the active phosphorus(III) catalyst by employing silanes as a chemoselective reductant. Recently, we disclosed the discovery of a novel silane reducing agent, 1,3-diphenyldisiloxane (<b>DPDS</b>), which presents unparalleled chemoselectivity with mild conditions, placing it at the forefront of green, industrially-desired phosphine oxide reductions. However, during our efforts to compare this novel reduction system with previously-reported silane-mediated reductions, we noted that the kinetic schemes governing these reductions did not correlate at all with the purported mechanism (Scheme 1A). Instead, we propose an alternative mechanism that proceeds through a lower-energy, 6-membered transition state, featuring a stabilizing hydrogen bond between the phosphine oxide and the reducing agent, in addition to the hydride transfer typical of these reactions (Scheme 1B). To further probe the mechanistic underpinnings of this reaction, empirical and computational experiments were conducted to identify key electronic and energetic effects. Herein we report kinetic and thermodynamic evidence that corroborates experimentally-observed trends and supports our proposed mechanism.

Empirical and computational studies drive complete understanding of the mechanism of silane-mediated reductions of phosphine oxides

255th National Meeting (2018)

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