What governs nitrogen configuration in substituted aminophosphines?

Abstract

The trigonal planar geometry of the nitrogen atom in commonly used phosphoramidite ligands is not in line with the traditional valence shell electron pair repulsion (VSEPR) model. In this work, the effects governing nitrogen configuration in several substituted aminophosphines, A 2 PNB 2 (A or B ¼ H, F, Cl, Br, Me, OMe, BINOP), are examined using modern computational analytic tools. The electron delocalization descriptions provided by both electron localization function (ELF) and block localized wavefunction analysis support the proposed relationships between conformation and negative hyperconjugative interactions. In the parent H 2 PNH 2 , the pyramidal nitrogen configuration results from nitrogen lone pair electron donation into the s * P-H orbital. While enhanced effects are seen for F 2 PNMe 2 , placing highly electronegative fluorine substituents on nitrogen (i.e., Me 2 PNF 2 ) eliminates delocalization of the nitrogen lone pair. Understanding and quantifying these effects can lead to greater flexibility in designing new catalysts.

Aminophosphines: their chemistry and role as ligands and synthons

“…Alternative ‘(b)’ does not provide clarity for more frequently and readily observed planarization of nitrogen over phosphorus in these types of compounds. Alternative ‘(c)’ appears to have wider scope and hence is more employed nowadays to explain such situations 126. A set of phosphoramidite molecules (‘O 2 NP’ framework) were examined using advanced computational techniques126 and it was concluded that negative hyperconjugation could best explain the observed facts and that the ligands where P is bonded to electron donating groups have never been used in catalysis for obtaining fruitful results.…”

“…Alternative ‘(c)’ appears to have wider scope and hence is more employed nowadays to explain such situations 126. A set of phosphoramidite molecules (‘O 2 NP’ framework) were examined using advanced computational techniques126 and it was concluded that negative hyperconjugation could best explain the observed facts and that the ligands where P is bonded to electron donating groups have never been used in catalysis for obtaining fruitful results. It was also felt that the fundamental electronic properties of phosphoramidite ligands have to be studied in detail for a much better ligand design.…”

Origin of the chemical stability of phosphine-phosphoramidites: structural study of an UPPhos-type crystal and application of UPPhos in the asymmetric hydrogenation of imines

“…The P-N bond is a key segment of the molecule. It is known that the N-atom lone pair can be delocalized on the P-N bond to some extent depending on the electronic nature of the N and P substituents (Wodrich et al, 2009). In the case of PPAs, this n N !…”

“…Thus, the N atom is sp 2 -hybridized, with a nearly trigonal planar geometry. The four substituents of the P and N atoms in (VII) can have orthogonal planar, orthogonal pyramidal or antipyramidal conformations (Mo et al, 1999;Wodrich et al, 2009) (Fig. 2).…”

Synthesis of new N-substituted chiral phosphine–phosphoramidite ligands and their application in asymmetric hydrogenations and allylic alkylations

“…It is known that significant electron donation from the nitrogen lone pair to the amidite phosphorus occurs, resulting in a nearly trigonal-planar molecular geometry of the nitrogen, which is typical of an sp 2 hybridization state-and of an increased P-N bond order, thus the rotation around this bond is restricted. 17 Furthermore, the phosphorus basicity can be finetuned by changing the substituents at the nitrogen, which may have a significant influence on the coordination and steric properties of the ligand and the substrate complexes; hence the catalytic properties can also be fine-tuned. These unique electronic properties prompted us to investigate the N-substituent effect of the ligand in the Rh-catalyzed asymmetric hydrogenation and in the Pd-catalyzed asymmetric allylic alkylation of benchmark substrates.…”