N ‐Phosphanylformamidines (phosfam) R 2 ′N–C(H)=N–PR 2 : One‐Pot Synthesis and Versatile Protonation Reaction

A straightforward synthesis of unprecedented N ‐phosphanylformamidines (phosfam), 3a , b has been developed. The single‐crystal X‐ray study of 3a revealed an E ‐formamidine stereoisomer. The structural parameters show a strong localization of the C1–N1 double bond in the formamidine pattern. Versatile protonation reactions with HCl on 3a and 3b are reported, leading to P–N cleavage vs. prototropy. Experimental studies and DFT calculations have evidenced that the imino nitrogen atom is the basic center of phosfams 3a and 3b . DFT calculations show that the isomers and rotamers of the N ‐ and P ‐protonated forms of 8a / 9a and 8b / 9b are energetically close, which prevents conclusions being drawn on the existence of thermodynamic and/or kinetic products. The accessibility of the anti bonding PN orbital (σ* P1N1 ) is partly responsible for the cleavage of the PN bond in 8a ; 8b possesses a less energetically accessible σ* P1N1 orbital which is consistent with the preservation of the PN bond and the quantitative formation of the corresponding phosphonium compound 9b· Cl.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)