Catalytic Transfer Hydrogenation by a Trivalent Phosphorus Compound: Phosphorus‐Ligand Cooperation Pathway or P^III/P^V Redox Pathway?

Abstract

Main-group-element catalysts are a desirable alternative to transition-metal catalysts because of natural abundance and cost. However, the examples are very limited. Catalytic cycles involving a redox process and E-ligand cooperation (E=main-group element), which are often found in catalytic cycles of transition-metal catalysts, have not been reported. Herein theoretical investigations of a catalytic hydrogenation of azobenzene with ammonia-borane using a trivalent phosphorus compound, which was experimentally proposed to occur through P(III) /P(V) redox processes via an unusual pentavalent dihydridophosphorane, were performed. DFT and ONIOM(CCSD(T):MP2) calculations disclosed that this catalytic reaction occurs through a P-O cooperation mechanism, which resembles the metal-ligand cooperation mechanism of transition-metal catalysts.

On the Redox Reactivity of a Geometrically Constrained Phosphorus(III) Compound

“…29(14), O2-P1-Br2 86.82 (12),N1-P1-Br1 107.88 (15), N1-P1-Br2 146.11(16), Br1-P1-Br2 105.99 (5). 4:P 1-N1 1.700(3), P1-O1 1.672(2), P1-O21 .670(2), P1-I1 2.464(1), P1-I2 2.501(1);O 1-P1-O2 162.77(14),O 1-P1-N1 89.07 (12), O1-P1-I19 8.92 (10), O1-P1-I2 86.32 (9), O2-P1-N1 89.31 (12), O2-P1-I1 98.00 (9), O2-P1-I2 85.95 (9), N1-P1-I1 106.30 (11), N1-P1-I2 147.86 (11), I1-P1-I2105.84(3). the 19 FNMR spectrum at À55.7 ppm.…”

“…Furthermore, 2,2-bipyridine, au nimolecular (albeit slightly less strongly do-nating)m odel of the two DMAP ligands also fails to displace a chloridef rom 2,a lthough the reaction can be promoted by adding an equivalent of ac hloride abstracting agent such as Na [BAr F 4 ]t od eliver green solutionso fc ation 9,w hich exhibits as inglet resonance at À77.2 ppm. Layering concentratedd ichloromethane solutions of [9][BAr F 4 ]w ith hexane affords large crystalso ft he product and the structure could then be confirmed by subsequent X-ray diffraction studies ( Figure 6). In analogy to the structure of 7,t he phosphorus centre exhibits an octahedral geometry,w ith the chloride substituent sitting perpendicular to the planar pincerl igand.…”

“…The reactivity of such species towards common nucleophiles and electrophiles seems to highlight that such reactions are unlikely to proceed in a concerted fashion at the phosphorus centre, but rather that the ligand framework is intimately involved in such reactions . Several theoretical studies have also corroborated this hypothesis …”

“…[7,8] Severalt heoretical studies have also corroboratedt his hypothesis. [9,10] This class of compounds forms part of ar apidly increasing family of main group element-containing species that have been shown to activate small molecule substrates, [11] the most notable of which are alkyl(amino)c arbenes (AACs), [12] lowo xidationstate compounds of the triel and tetrel elements, [13,14] and frustratedL ewisp airs (FLPs). [15] An umber of such species have even been shown to be activeincatalytic processes,such as the hydrogenationo fi mines,a lkenes, alkynes, aldehydes and ketones, [16] making the prospecto fc arrying out catalytic transformations using elements with ah igh crustal abundance an increasingly attractive target.…”

Sterically Constrained Bicyclic Phosphines: A Class of Fascinating Compounds Suitable for Application in Small Molecule Activation and Coordination Chemistry

“…The phosphorus coordination sphere in the solid state structure was reported to be intermediate between a trigonal‐bipyramidal and a square‐pyramidal arrangement of the substituents. Two mechanisms were suggested by quantum‐chemical calculations, which both identify ring cleavage similar to Scheme b as the key step at the beginning of the reaction, but differ with respect to the mechanism of ring closure for completion of the hydrogenation process (H 3 B←NH 3 versus ammonia contribution). Ring opening as an essential reaction pathway has been confirmed experimentally by treatment of the diphosphorus compound 4 with H 3 B←NH 3 .…”