Atroposelective PIII/PV=O Redox Catalysis for the Isoquinoline-Forming Staudinger–aza-Wittig Reaction

Herein, we describe the feasibility of atroposelective P^III/P^V=O redox organocatalysis by the Staudinger–aza-Wittig reaction. The formation of isoquinoline heterocycles thereby enables the synthesis of a broad range of valuable atropisomers under mild conditions with enantioselectivities of up to 98 : 2 e.r. Readily prepared azido cinnamate substrates convert in high yield with stereocontrol by a chiral phosphine catalyst, which is regenerated using a silane reductant under Brønsted acid co-catalysis. The reaction provides access to diversified aryl isoquinolines, as well as benzoisoquinoline and naphthyridine atropisomers. The products are expeditiously transformed into N-oxides, naphthol and triaryl phosphine variants of prevalent catalysts and ligands. With dinitrogen release and aromatization as ideal driving forces, it is anticipated that atroposelective redox organocatalysis provides access to a multitude of aromatic heterocycles with precise control over their configuration.     

Csp3−PIII Bond Formation via Cross-Coupling of Umpolung Carbonyls with Phosphine Halides Catalyzed by Nickel

A novel method for the formation of Csp³−P^III bonds via the nickel-catalyzed cross-coupling of Umpolung carbonyls and phosphine chlorides is reported herein. This process leads to a series of alkylphosphines, which are characterized as sulfides or borane-phosphine complexes after undergoing further transformation with moderate to good yields. Invaluable free alkylphosphines can be easily obtained by desulfurization or deboration of the products. A possible mechanistic pathway is also discussed. This report represents the first example of using renewable carbonyls as latent organometallic reagent surrogates for cross-coupling with heteroatom electrophiles.     

Chelating Phosphorus–An O,C, O-Coordinating Pincer-Type Ligand Coordinating PIII and PV Centres

The sequence of reactions of the phosphorus-containing aryllithium compound 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂]₂C₆H₂Li (ArLi) with Ph₂PCl, KMnO₄, elemental sulfur and elemental selenium, respectively, gave the aryldiphenylphosphane chalcogenides 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂]₂C₆H₂P(E)Ph₂ ( 1 , E=O; 2 , E=S; 3 , E=Se). Compound 1 partially hydrolysed giving [5-t-Bu-1-{(P(O)(O-i-Pr)₂}-3-{(P(O)(OH)₂}C₆H₂]P(O)Ph₂ ( 4 ). The reaction of ArLi with PhPCl₂ provided the benzoxaphosphaphosphole [1(P), 3(P)-P(O)(O-i-Pr)OPPh-6-t-Bu-4-P(O)(O-i-Pr)₂]C₆H₂P ( 5i ) as a mixture of the two diastereomers. The oxidation of 5i with elemental sulfur gave the benzoxaphosphaphosphole sulfide [1(P), 3(P)-P(O)(O-i-Pr)OP(S)Ph-6-t-Bu-4-P(O)(O-i-Pr)₂]C₆H₂ ( 5 ) as pair of enantiomers P1(R), P3(S)/P1(S), P3(R) of the diastereomer (RS/SR)- 5 ( 5b ). The aryldiphenylphosphane 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂]₂C₆H₂PPh₂ ( 6 ) was obtained from the reaction of the corresponding aryldiphenylphosphane sulfide 2 with either sodium hydride, NaH, or disodium iron tetracarbonyl, Na₂Fe(CO)₄. The oxidation of the aryldiphenylphosphane 6 with elemental iodine and subsequent hydrolysis yielded the aryldiphenyldioxaphosphorane 9-t-Bu-2,6-(OH)-4,4-Ph₂-3,5-O₂-2,6-P₂-4λ⁵-P-[5.3.1.0]-undeca-1(10),7(11),8-triene ( 7 ). Both of its diastereomers, (RR/SS)- 7 ( 7a ) and (RS/SR)- 7 ( 7b ), were separated as their chloroform and i-propanol solvates, 7a ⋅2CHCl₃ and 7b ⋅i-PrOH, respectively. DFT calculations accompanied the experimental work.     

Studies on the Efficient Generation of Phosphorus?Carbon Bonds via a Rearrangement of PIII Esters Catalysed by Trimethylhalosilanes

Unprecedented C? P systems : Me₃SiX (X=Br, I) catalyses rearrangements of P^III esters R′R′′P? OR into the corresponding phosphoryl systems, providing a simple, mild and efficient route to a variety of structures containing P? C bonds. The mechanism has been found to be fundamentally different from that of the Arbuzov–Michaelis reaction and includes three definite steps a, b and c (see scheme).

     

Catalytic Transfer Hydrogenation by a Trivalent Phosphorus Compound: Phosphorus‐Ligand Cooperation Pathway or PIII/PV Redox Pathway?

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.     

Catalytic Transfer Hydrogenation by a Trivalent Phosphorus Compound: Phosphorus‐Ligand Cooperation Pathway or PIII/PV Redox Pathway?

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.