N -Heterocyclic Carbene-Coordinated M(II) (M = Yb, Sm, Ca) Bisamides: Expanding the Limits of Intermolecular Alkene Hydrophosphination.

A series of NHC-stabilized amido compounds (NHC) n M[N(SiMe 3 ) 2 ] 2 (M = Yb(II), Sm(II), Ca(II); n = 1, 2) showed remarkable catalytic efficiency in addition of PhPH 2 and PH 3 to alkenes under mild conditions and low catalyst loading. The effect of σ-donor capacity of NHCs on catalytic activity in hydrophosphination of styrene with PhPH 2 and PH 3 was revealed. For the series of three-coordinate complexes 1-4M , a tendency to increase the catalytic activity with growth of σ-donating strength of the carbene ligand was clearly demonstrated. The complex (NHC) 2 Sm[N(SiMe 3 ) 2 ] 2 (NHC = 1,3-diisopropyl-2 H -imidazole-2-ylidene) ( 5Sm ) proved to be the most efficient catalyst, which enabled hardly realizable transformations such as PhPH 2 addition across internal C═C bonds of norbornene and cis - and trans -stilbenes, providing the highest reaction rate for addition of PH 3 to styrene. Excellent regio- and chemoselectivities of alkylation of PH 3 with styrenes allow for a selective and good-yield synthesis of desired organophosphines─either primary, secondary, or tertiary. Stepwise alkylation of PH 3 with various substituted styrenes can be efficiently applied as an approach to nonsymmetric secondary phosphines. The rate equation of the addition of styrene to PH 3 promoted by 5Sm was found: rate = k [styrene] 1 [5Sm] 1 .