1261153-19-8Relevant articles and documents
Imidazoliophosphines are true N-heterocyclic carbene (NHC)-phosphenium adducts
Abdellah, Ibrahim,Lepetit, Christine,Canac, Yves,Duhayon, Carine,Chauvin, Remi
, p. 13095 - 13108 (2010)
Whereas the external nucleophilic reactivity of α-amidiniophosphines has been previously illustrated by their complexation to transition-metal centers, their internal electrophilic reactivity is herein investigated by using BIMIONAP (BIMIONAP=N-methylated BIMINAP cation, BIMINAP=formal contraction of the acronyms BIMIP=2,2′-bis(diphenylphosphino)-1,1′-bibenzimidazole and BINAP=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl). Reaction of tetraethylammonium chloride with free BIMIONAP is found to induce heterolytic cleavage of the N2C-P bond to give chlorodiphenylphosphine and a transient phosphine-N-heterocyclic carbene (NHC) species that is trapped in situ by protonation to the corresponding phosphine-benzimidazolium cation. When the chloride anion reacts with the cationic [Pd(η2-BIMIONAP)Cl 2] complex, the same cleavage occurs and the phosphine-NHC moiety is trapped in the corresponding [PdCl2(η2-phosphine-NHC)] complex. When the chloride anion reacts with the dicationic [Pd(π-allyl)(η2-BIMIONAP)]+ complex, allyldiphenylphosphine is produced, and the [PdCl(η2-phosphine- NHC)(PPh2CH2CH=CH2)]+ complex is obtained. Reaction of free BIMIONAP with the harder n-butyllithium nucleophile also induces heterolytic cleavage of the N2C-P bond, from which the phosphine-NHC moiety is trapped by hydrolysis of the benzimidazole ring or by P,C-sulfurization. Cleavage of a C-P bond with the weak Cl- nucleophile to release the reactive NHC moiety (according to the unusual scheme C-P+Cl-→C:+Cl-P) is a definite experimental indication of the dative nature of the N2C-P bond of amidiniophosphines, which are, therefore, better described as NHC→phosphenium adducts. This interpretation is supported by the calculation, at the DFT level, of a heterolytic dissociation mode of the N2C-P bond lower in energy than the homolytic one. A mesomeric description of the NHC→phosphenium entity is also proposed on the basis of electron localization function (ELF) and atoms in molecules (AIM) analyses. Finally ELF and AIM-based Fukui indices, molecular orbitals, and MESP analyses show that the initial attack of Cl- takes place at the carbenic atom of BIMIONAP. Addition of anionic nucleophiles (Cl-, nBu-) to free BIMIONAP and BIMIONAP-containing palladium complexes results in selective cleavage of the N2C-P bond, from which the released N-heterocyclic carbene (NHC) fragment can be trapped by protonation, hydrolysis, sulfurization, or coordination to PdII centers (see scheme).
