Phosphines with N-Heterocyclic Boryl-Substituents : Ligands for Coordination Chemistry and Catalysis

Abstract

Boryl-substituted phosphines NHB-P(R)Ph (R = H, Ph, NHB = N-heterocyclic boryl substituent) react with Fe-2(CO)(9) to give isolable Fe(CO)(4) complexes, two of which were characterized by single-crystal XRD studies. The electronic and steric properties for a series of the boryl phosphines were further assessed by evaluation of TEPs for in-situ formed complexes [RhCl(NHB-(PRR2)-R-1)(CO)(2)] (R-1, R-2 = H, Ph, Me, NMe2), and calculations of buried volumes for Fe(CO)(4) complexes. The results imply that the NHB-phosphines exhibit due to their conformational flexibility some variability in their steric bulk, and that some specimens may exhibit similar electron releasing power and steric demand as tBu(3)P. Studies of the amination of bromobenzene with 2,6-diisopropylaniline confirmed that these properties can be exploited to promote Pd-catalyzed C-N cross coupling reactions, and that formal replacement of a phenyl by a NHB substituent in the auxiliary phosphine has a beneficial effect on catalyst performance.