Hydrophosphonylation mechanism

Scheme 5-18 Stoichiometric reactions relevant to the proposed mechanism for palladium-catalyzed hydrophosphonylation of alkynes...

Scheme 5-27 Proposed mechanism for titanium alkoxide-catalyzed asymmetric hydrophosphonylation of arylaldehydes...

Scheme 5-33 LLB-catalyzed asymmetric hydrophosphonylation of aldehydes and a proposed mechanism LLB = La/Li/BINOL...

Scheme 5-46 Proposed mechanism for LPB-catalyzed asymmetric hydrophosphonylation ofimines...

The proposed mechanism for this catalytic asymmetric hydrophosphonylation is shown in Figure 35. The first step of this reaction is the deprotonation of dimethyl phosphite by LPB to generate potassium dimethyl phosphite. This potassium phosphite immediately coordinates to a lanthanoid to give I due to the strong oxophilicity of lanthanoid metals. The complex I then reacts (in the stereochemistry-determining step) with an imine to give the potassium salt of the a-aminophosphonate. A proton-exchange reaction affords the product... 

The reaction between trialkyl phosphites and a,P-unsaturated nitriles has been less thoroughly investigated. Whereas the hydrophosphonylation of unsaturated nitriles with diakyl phosphites proceeds smoothly, the addition of trialkyl phosphites requires more severe conditions. It involves the nucleophilic attack by the trivalent phosphorus reagent at the tenninal carbon atom of the conjugated system followed by valency expansion of phosphorus in agreement with the Michaelis-Arbuzov mechanism (Scheme 6.27). - ... 

Scheme 29 Proposed mechanism for Ti-catalyzed hydrophosphonylation of activated alkenes...

Scheme 39 Proposed mechanism for enantioselective hydrophosphonylation of enones catalyzed by a dinuclear zinc complex...

The mechanism and stereochemistry of hydrophosphonylation of a-ketoesters by dimethylphosphonate [H-P(=0)(0Me)2l has been studied theoretically by the ONIOM method, for catalysis by cinchona-thioureas. Deprotonation of the phosphonate 0 is rate determining. It is followed by C-P bond formation (the stereo-controlhng step) via nucleophilic addition, and then reprotonation (regenerating the catalyst). Multiple hydrogen bonds activate the substrates, facilitate charge transfer and stabihze transition states. 

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