Hydrophosphination imines

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. 

Recently, intermolecular hydrophosphination of alkynes was also reported with ytterbium-imine complex catalyst precursors [20]. Aromatic alkynes react at room temperature to afford ( )-isomers, while aliphatic ones require heating at 80 °C and, quite surprisingly, (Z)-isomers (trans-addition products) are formed preferentially (Table 4). In this respect the ytterbium-catalyzed reactions are different from the radical process, in which the ( )-isomer formed initially isomerizes to the (Z)-isomer. 

A mechanism that involves ytterbium phosphide species has been proposed, similarly to the foregoing intramolecular hydrophosphination. Generation of the phosphide species is supported by the formation of Ph2CDNHPh (after aqueous quench) upon treatment of the imine complex with Ph2PD (Scheme 15). Lanthanide phosphide is known to react with THF, forming a 4-diphenylphosphino-l-butoxyl species [21], which was indeed found as a side product in the catalytic hydrophosphination of disubstituted aliphatic alkynes run in THF, supporting further the ytterbium-phosphide intermediate (Scheme 16). 

Organolanthanide-catalyzed intermolecular hydrophosphination is a more facile process than intermolecular hydroamination. The reaction of alkynes, dienes, and activated alkenes with diphenylphosphine was achieved utilizing the ytterbium imine complex 9 (Fig. 8) as catalyst [185-188]. Unsymmetric internal alkynes react regioselectively, presumably due to an aryl-directing effect (48) [186]. 

Ytterbium/imine complex [Yb( / -Ph2CNPh)(HMPA)6] was reported as a catalyst for hydrophosphination of alkynes (Scheme 8.55) [137]. From 52% to quantitative product yields were found in the reaction. Regio- and stereoselective transformation was observed in case of internal alkynes, but the reaction with terminal alkynes gave a mixture of a- and -adducts with the former being... 

Scheme 8.82 BHs-mediated transition-metal-free hydrophosphination of aldehydes and imines...

Takaki K, Takeda M, Koshoji G, Shishido T, Takehira K (2001) Intermolecular hydrophosphination of alkynes and related carbon—carbon multiple bonds catalyzed by ytterbium-imine complexes. Tetrahedron Lett 42 6357-6360... 

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