Hydrophosphorylation of Alkynes Catalyzed by Palladium: Generality and Mechanism

Abstract

We carried out a comprehensive study on the generality, scope, limitations, and mechanism of the palladium-catalyzed hydrophosphorylation of alkynes with P(O)-H compounds (i.e., H-phosphonates, H-phosphinates, secondary phosphine oxides, and hypophosphinic acid). For H-phosphonates, Pd/dppp was the best catalyst. Both aromatic and aliphatic alkynes, with a variety of functional groups, were applicable to produce the Markovnikov adducts in high yields with high regioselectivity. Aromatic alkynes showed higher reactivity than aliphatic alkynes. Terminal alkynes reacted faster than internal alkynes. Sterically crowded H-phosphonates disfavored the addition. For H-phosphinates and secondary phosphine oxides, Pd/dppe/PhP(O)OH was the catalyst of choice, which led to highly regioselective formation of the Markovnikov adducts. By using Pd(PPh) as the catalyst, hypophosphinic acid added to terminal alkynes to give the corresponding Markovnikov adducts. Phosphinic acids, phosphonic acid, and its monoester were not applicable to this palladium-catalyzed hydrophosphorylation. Mechanistic studies showed that, with a terminal alkyne, (RO)P(O)H reacted, like a Brønsted acid, to selectively generate the α-alkenylpalladium intermediate via hydropalladation. On the other hand, Ph(RO)P(O)H and PhP(O)H gave a mixture of α- and β-alkenylpalladium complexes. In the presence of PhP(O)OH, hydropalladation with this acid took place first to selectively generate the α-alkenylpalladium intermediate. A subsequent ligand exchange with a P(O)H compound gave the phosphorylpalladium intermediate which produced the Markovnikov adduct via reductive elimination. Related intermediates in the catalytic cycle were isolated and characterized.

Rhodium Complexes in P−H Bond Activation Reactions

“…Among the different approaches to this reaction, one of the most popular involves proton transfer from phosphanes or phosphane oxides to an internal base, that is, a proton acceptor group coordinated to the metal. Relevant examples include protonolysis at alkyl, and acetate palladium complexes, nickel silanolates and silylamides, and iron complexes with an Fe–CH 2 SiMe 3 motif . In addition, metals with formal d 0 electron count such as lanthanides, early transition metals and some actinides engage in σ‐bond metathesis as reported for complexes with alkyls, silylamides, amides, and more recently alkoxy groups …”

Palladium‐Catalyzed Direct Decarbonylative Phosphorylation of Benzoic Acids with P(O)–H Compounds

“…On the basis of previous literatures, we proposed a plausible catalytic circle as shown in Scheme . [16a], [16l], [16m], [16n], , Pd(OAc) 2 is first reduced to generate an active Pd 0 species A , which oxidatively adds to a mixing anhydride B generated in situ from (Boc) 2 O and the starting carboxylic acids,[16m] yielding an complex C . The resulting C undergoes decarbonylation,[13a], , followed by ligand exchange to give intermediate E , , .…”

“…, entries[16][17][18][19][20][21][22]. When dppb[1,4-bis(diphenylphosphino)butane] was used, 39 % yield of 3a was obtained.…”

Facile Substituent Exchange at H‐Phosphonate Diesters Limiting an Effective Synthesis of D‐Phosphonate Diesters

“…H ‐phosphonate diesters are useful synthons in a variety of reactions . Selected specific examples include, reactants in the catalytic cross coupling of P−N, P−O−P, P−P, P−F, P−C, P−O−C, P−Se and P−S bonds, base‐catalyzed hydrophosphination of azobenzene to form N−N−P units, organophosphorus ligands in metal complexes, in addition to precursors to phosphines, P−Cl, and biologically active phosphorus compounds . Furthermore, H ‐phosphonate diesters are reactants in transesterification, alkylation reactions and in P−O−Si polymer synthesis .…”