Carbodiimides, hydrophosphination

Scheme 23 Proposed mechanism for calcium-catalyzed hydrophosphination of carbodiimides (for simplicity, coordinated THF is not shown in 12 or related intermediates)...

Hydrophosphination of carbodiimides to yield phosphaguanidines was catalyzed by several related metal complexes. For example, after carbodiimide coordination by the calcium phosphide complex 12 (see Scheme 18 above), P-C bond formation involving a four-centered transition state and rearrangement was proposed to yield the N,N-bound intermediate 13. Addition of the P-H bond across the Ca-N bond, followed by dissociation of the product, then regenerates the phosphide intermediate (Scheme 23) [37]. Related chemistry was reported recently using the calcium phosphide complex Ca(PPh2)2(THF)4 [38]. 

Organolanthanide complexes are known to be highly active catalysts for a variety of organic transformations, which can be either intramolecular or intermolecular in character. Successful intramolecular transformations include hydroelementation processes, which is the addition of a H-E (E = N, O, P, Si, S, H) bond across unsaturated C-C bonds, such as hydroamination, hydroalkoxylation, and hydrophosphination. Intermolecular transformations include a series of asymmetric syntheses, the amidation of aldehydes with amines, Tishchenko reaction, addition of amines to nitriles, aUcyne dimerization, and guanylation of terminal aUcynes, amines, and phosphines with carbodiimides. 

Scheme 14 Reaction scope for the group 2-catalyzed hydrophosphination of carbodiimides...

Reaction mechanism Based uprm a series of stoichiometric reactions, and in contrast to the previously discussed hydrophosphinatiOTi reactions (vide supra), it was postulated that unsaturated homoleptic heavier alkaline-earth phosphides are unlikely to be long-lived intermediates in the catalytic hydrophosphination of carbodiimides conducted in non-coordinating solvents. Rather, it was proposed that the phosphaguanidine product was acting as a ligand for the Lewis acidic metal center [114]. 

Busacca and co-workers have prepared menthol-derived phosphinite boranes 101 and 102, by direct addition of anionic secondary phosphine borane 100 to carbodiimides, yielding the chiral phosphinite boranes under ambient temperature conditions (Scheme 25). These and other derivatives, prepared by hydrophosphination of carbodiimides, were used in the synthesis of enantiomerically enriched phosphaguanidines with potential use as ligands in asymmetric catalysis. 

---