Chemoselective Staudinger Reactivity of Bis(azido)phosphines Supported with a π-Donating Imidazolin-2-iminato Ligand

Synthesis and characterization of new P(III) and P(V) bis(azido)phosphines/phosphoranes supported by an N,N '-bis(2,6-diisopropylphenyl) imidazolin-2-iminato (IPrN) ligand and their reactivity with various secondary and tertiary phosphines result in the formation of chiral and/or asymmetric mono(phosphinimino)azidophosphines via the Staudinger reaction. The reaction of IPrNP(N-3)(2) (2) or IPrNP(S)(N-3)(2) (4S) with an excess of tertiary phosphine resulted in the chemoselective formation of IPrNP(N-3)(NPMe3) (7) or IPrNP(S)N-3(NPR3) (5(R)), respectively. The chemoselective Staudinger reactivity was also observed in reactions using a secondary phosphine (HPCy2) to produce IPrNP(S)N-3[NP(H)Cy-2] (6a), which exists in equilibrium with a tautomeric IPrNP(S)N-3[N(H)PCy2] form (6b), as confirmed by P-31-P-31 nuclear Overhauser effect spectroscopy (NOESY). Density functional theory (DFT) calculations point to a combination of energetically unfavorable lowest unoccupied molecular orbitals (LUMOs) and the accumulation of increasing negative charge at the terminal azido-nitrogen upon a single azide-to-phosphinimine conversion that gave rise to the observed chemoselectivity.