ChemInform Abstract: Reactions of Halides of Tervalent Phosphorus with Imines and α‐Diimines.

Recent Developments in the Chemistry of N-Heterocyclic Phosphines

“…[72] alternation is associated with a general deterioration of cyclic p -electron delocalization [8,18,39] . The structures of P-chloro-NHPs invited attention due to the very early discovery of species with exceptionally long P-Cl bonds [19,22,53] which stimulated discussions about a possible ionic nature of these compounds. This perception was later contradicted by a survey of a series of crystal structure analyses which revealed that the observed P-Cl bond lengths (2.24-2.70 Å), even though being much longer than a normal single bond (<2.18 Å), remain still in a range suggesting significant covalent interaction [55] .…”

“…The reaction is applicable to both N -alkyl and N -aryl substituted diimines derived from glyoxal. Diimines derived from 1,2-diketones react via [4 + 1] cycloaddition to yield heterocyclic chlorophosphonium salts [22] which may be converted into NHPs after subsequent reduction [21] . The two-electron reduction of a -diimines to prepare the required starting materials for a subsequent condensation is usually achieved by reaction with lithium but other alkaline (Na) or alkaline earth (Mg) metals should be useful as well.…”

“…The most widely applied precursors for the synthesis of monocyclic NHPs are a -diimines which can be converted to the target heterocycles either in a two-step reaction sequence involving two-electron reduction of the diimine to an enediamide, enediamine, or a -aminoamine and subsequent condensation with PCl 3 [18][19][20] or a dichlorophosphine RPCl 2 [21] , or via direct base-promoted reaction with PCl 3 [20,22] . The latter reaction involves addition of a P-Cl bond to each imine moiety followed by base-promoted elimination of hydrogen chloride leading to 2,4-dichloro-1,3,2-diazaphospholenes 9 (Scheme 3 ) which were isolated in moderate to good yields after crystallization.…”

2‐Halogeno‐1,3,2‐diazaarsolenes and 2‐halogeno‐1,3,2‐diazastibolenes: Examples for spontaneous EX bond heterolysis or not?

“…It is known for some time that the mesomeric stabilization in a phosphenium ion can be further enhanced by incorporating the dicoordinate phosphorus atom into the framework of a 1,3,2-diazaphospholenium ring, thus giving rise to cyclic cations II (E = P) [14][15][16][17] with the same valence electron count, and a closely similar electronic structure, as neutral imidazoyl carbenes (III). We have shown recently that, as a consequence of the enhanced π-stabilization, diazaphospholenium cations are more stable and less Lewis acidic than other structural types of phosphenium ions [17], but retain their capability to act as ambiphilic ligands and form carbene-analogue transition metal complexes [18].…”

Stability and Electrophilicity of Phosphorus Analogues of Arduengo Carbenes—An Experimental and Computational Study

“…[13,16] On the other hand, compound 3 b (C 6 H 11 cyclohexyl; PÀCl 2.42 ) [17] was unmistakably addressed as a covalent species, and the bond lengthening with respect to compounds such as (Me 2 N) 2 PCl (2.180(4) ) [18] was attributed to hyperconjugation between the nitrogen lone pairs and the s*(P À Cl) orbital. [17] Even for 2 a[Cl], the large deviation of the 31 P NMR chemical shift from that of ionic hexafluorophosphates or tetrachlorogallates 2 a[X] (X BF 4 , GaCl 4 ) suggested that in solution covalently interacting ion pairs rather separated ions are present. [13] In a recent theoretical study of stability and electrophilicity of phosphenium ions we have established that the energetic effect of p delocalization in 1,3,2-diazaphospholenium ions is of comparable magnitude to inductive substituent effects.…”