We report that a regioselective reductive transposition of primary allylic bromides is catalyzed by a biphilic organophosphorus (phosphetane) catalyst. Spectroscopic evidence supports the formation of a pentacoordinate (σ5-P) hydridophosphorane as a key reactive intermediate. Kinetics experiments and computational modeling are consistent with a unimolecular decomposition of the σ5-P hydridophosphorane via a concerted cyclic transition structure that delivers the observed allylic transposition and completes a novel PIII/PV redox catalytic cycle. These results broaden the growing repertoire of reactions catalyzed within the PIII/PV redox couple and suggest additional opportunities for organophosphorus catalysis in a biphilic mode.

A small-ring phosphacycle is found to catalyze the deoxygenative condensation of α-keto esters and carboxylic acids. The reaction provides a chemoselective catalytic synthesis of α-acyloxy ester products with good functional group compatibility. Based on both stoichiometric and catalytic mechanistic experiments, the reaction is proposed to proceed via catalytic PIII/PV═O cycling. The importance of ring strain in the phosphacyclic catalyst is substantiated by an observed temperature-dependent product selectivity effect. The results point to an inherent distinction in design criteria for organophosphorus-based catalysts operating via PIII/PV═O redox cycling as opposed to Lewis base (nucleophilic) catalysis.

A commercial phosphorus-based reagent (P(NMe2)3) mediates umpolung alkylation of methyl aroylformates with benzylic and allylic bromides, leading to either Barbier-type addition or ylide-free olefination products upon workup. The reaction sequence is initiated by a two-electron redox addition of the tricoordinate phosphorus reagent with an α-keto ester compound (Kukhtin–Ramirez addition). A mechanistic rationale is offered for the chemoselectivity upon which the success of this nonmetal mediated C–C bond forming strategy is based.

Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N–H oxidative addition to a planar mononuclear σ3-phosphorus compound (1). The pentacoordinate phosphorane products (1·[H][NHR]) are structurally robust, permitting full characterization by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Isothermal titration calorimetry was employed to quantify the enthalpy of the N–H oxidative addition of n-propylamine to 1 (nPrNH2 + 1 → 1·[H][NHnPr], ΔHrxn298 = −10.6 kcal/mol). The kinetics of n-propylamine N–H oxidative addition were monitored by in situ UV absorption spectroscopy and determination of the rate law showed an unusually large molecularity (ν = k[1][nPrNH2]3). Kinetic experiments conducted over the temperature range of 10–70 °C revealed that the reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (ΔH⧧ = −0.8 ± 0.4 kcal/mol, ΔS⧧ = −72 ± 2 cal/(mol·K)) indicate that the cleavage of strong N–H bonds by 1 is entropy controlled due to a highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred, proceeding by initial amine-assisted N–H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-controlling N → P proton transfer.

The synthesis and reactivity of geometrically constrained tricoordinate phosphorus (σ3-P) compounds supported by tridentate triamide chelates (N[o-NR-C6H4]23–; R = Me or iPr) are reported. Studies indicate that 2 (P{N[o-NMe-C6H4]2}) adopts a Cs-symmetric structure in the solid state. Variable-temperature NMR studies demonstrate a low-energy inversion at phosphorus in solution (ΔG⧧exptl298 = 10.7(5) kcal/mol), for which DFT calculations implicate an edge-inversion mechanism via a metastable C2-symmetric intermediate. In terms of reactivity, compound 2 exhibits poor nucleophilicity, but undergoes oxidative addition at ambient temperature of diverse O–H- and N–H-containing compounds (including alcohols, phenols, carboxylic acids, amines, and anilines). The resulting pentacoordinate adducts 2·[H][OR] and 2·[H][NHR] are characterized by multinuclear NMR spectroscopy and X-ray crystallography, and their structures (which span the pseudorotation coordinate between trigonal bipyramidal and square planar) are evaluated in terms of negative hyperconjugation. At elevated temperatures, the oxidative addition is shown to be reversible for volatile alcohols and amines.

Z-Enediynes are prepared by a vicinal dialkynylation of triaryl(arylethynyl)phosphonium cations. The method, which proceeds under mild transition metal-free conditions, can be conducted on multigram scale as a one-pot, phosphine-mediated synthetic cycle giving enediyne products with excellent control of configuration.

We report that pyramidal inversion of trivalent phosphines may be catalyzed by single electron oxidation. Specifically, a series of P-stereogenic (aryl)methylphenyl phosphines are shown to undergo rapid racemization at ambient temperature when exposed to catalytic quantities of a single electron oxidant in solution. Under these conditions, transient phosphoniumyl radical cations (R3P•+) are formed, and computational models indicate that the pyramidal inversion barriers for these open-shell intermediates are on the order of ∼5 kcal/mol. The observed 1020-fold rate enhancement over uncatalyzed pyramidal inversion opens new opportunities for the dynamic stereochemistry of phosphines and may hold additional implications for the configurational stability of P-stereogenic phosphine ligands on high-valent oxidizing transition metals.

A reductive homocondensation of E-benzylidenepyruvate esters mediated by P(NMe2)3 is described. The transformation, initiated by the Kukhtin–Ramirez addition of the phosphorus reagent to the vinyl-substituted α-dicarbonyl substrate, proceeds via a resonance delocalized oxyphosphonium dienolate intermediate to provide access to diverse oxygenated heterocycles as a function of the substituent.

The preparation of densely functionalized unsymmetrical 1,4-dicarbonyl structural motifs by a phosphorus(III)-mediated reductive condensation of α-keto esters and enolizable carbon pronucleophiles is described. The reaction, which is initiated by Kukhtin–Ramirez addition of commercially available tris(dimethylamino)phosphine to the α-keto ester substrate, proceeds rapidly under mild conditions.

A method for the synthesis of α-amino acids by direct reductive carboxylation of aromatic imines with CO2 is described. The protocol employs readily available commercial reagents and serves as a one-step alternative to the Strecker synthesis.

A planar, trivalent phosphorus compound is shown to undergo reversible two-electron redox cycling (PIII/PV) enabling its use as catalyst for a transfer hydrogenation reaction. The trivalent phosphorus compound activates ammonia-borane to furnish a 10-P-5 dihydridophosphorane, which in turn is shown to transfer hydrogen cleanly to azobenzene, yielding diphenylhydrazine and regenerating the initial trivalent phosphorus species. This result constitutes a rare example of two-electron redox catalysis at a main group compound and suggests broader potential for this nonmetal platform to support bond-modifying redox catalysis of the type dominated by transition metal catalysts.

A method for the synthesis of α-amino acids by direct reductive carboxylation of aromatic imines with CO2 is described. The protocol employs readily available commercial reagents and serves as a one-step alternative to the Strecker synthesis.

Z-Enediynes are prepared by a vicinal dialkynylation of triaryl(arylethynyl)phosphonium cations. The method, which proceeds under mild transition metal-free conditions, can be conducted on multigram scale as a one-pot, phosphine-mediated synthetic cycle giving enediyne products with excellent control of configuration.