Lanthanum-Catalyzed Double Hydrophosphinylation of Nitriles

A new lanthanum-based
catalyst was shown to be effective for the
double hydrophosphinylation of unactivated nitriles under very mild
conditions. Surprisingly, the lanthanum catalyst gave two regioisomeric
products depending on the nature of the starting nitrile. Primary
alkyl nitriles undergo 1,1-addition to give products with a new P–C–P
linkage and concomitant formation of a primary amine. Under the same
conditions, secondary alkyl and aryl nitriles instead produced 1,2-addition
products, where 1 equiv of the phosphine oxide was added to the carbon,
while a second equivalent added to the nitrogen of the nitrile, resulting
in a P–C–N–P framework. Further investigation
of the catalytic cycle yielded evidence that all nitriles first undergo
1,1-addition (deemed the kinetic product) that then undergoes isomerization
to the final unsymmetric addition product (the thermodynamic product).
All catalytic reactions were run neat or with very little solvent,
required little workup, and had high to moderate yields.