Imines hydrophosphorylation

Because organophosphorus compounds are important in the chemical industry and in biology, many methods have been developed for their synthesis [1]. This chapter reviews the formation of phosphorus-carbon (P-C) bonds by the metal-catalyzed addition of phosphorus-hydrogen (P-H) bonds to unsaturated substrates, such as alkenes, alkynes, aldehydes, and imines. Section 5.2 covers reactions of P(lll) substrates (hydrophosphination), and Section 5.3 describes P(V) chemistry (hydrophosphorylation, hydrophosphinylation, hydrophosphonylation). Scheme 5-1 shows some examples of these catalytic reactions. 

The pathway of the Kabachnik-Fields reaction depends on the nature of the substrates. The amine and hydrophosphoryl compound form a complex in which either one of the partners may react with the carbonyl compound. Often, the basicity of the amine determines the reaction pathway. Weakly basic amines such as anilines, which can act as proton donors, favour the formation cf an imine, whereas alkylamines such as cyclohexylamines do not form imines ... 

Phosphorylated aminomethylindoles 98 were obtained from 3-formylindoles 97a,b, amines, and hydrophosphoryl compounds [99-101] in the Kabachnik-Fields reaction [233-235]. As shown by DTA, 3-(hydroxyamino)methylindole is formed initially in the case of secondary amines and then it reacts with dialkylphosphorous acid. In the case of ammonia and primary amines imine is formed at the first stage and then it adds dialkylphosphorous acid. 

Thiourea effectively catalyses hydrophosphorylation of imine [28] (Table 9.10). The products can be deprotected under mild hydrogenolysis conditions to afford the corresponding optically active amino phosphoric acids. 

Types of reaction C-P bond formation Reaction conditions Dichloromethane, room temperature Synthetic strategy Hydrophosphorylation of imines Catalyst p-Tosyl chloride (p-TsCl)... 

Keywords Imines, diethyl phosphate, p-TsCl, dichloromethane, room temperature, hydrophosphorylation, a-aminophosphonates... 

Excellent results were reported in asymmetric hydrophosphorylation of imines by heterobimetallic catalysts (Scheme 8.77) [193]. Although the yield and enantiomeric excess were affected by nature of the imine, solvent, temperature and catalyst, as high as 96% e.e. was achieved. The catalyst with potassium ions showed the best performance under studied conditions. This synthetic approach is of potential interest for industrial applications in the synthesis of chiral a-aminophosphonic acids [194]. 

Catalytic hydrophosphorylation of imines was studied by Katsuki et al. with optically active salalen ligands (the same system was used for hydrophosphony-lation of aldehydes, see Scheme 8.73) [186]. High yields (up to 93%) and good enantiomeric excess (up to 87%) were found. 

Scheme 8.78 Proposed mechanism of hydrophosphorylation of imines using (R)-LPB catalyst...

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