On the Theme of Amidiniophosphines: Synthesis and Reactivity of Benzo-, Diimidazolo-Tris-Annelated P-Aminodiazaphosphepines

GRAPHICAL ABSTRACT

Abstract

After a brief overview of the current development of carbeniophosphine ligands for applications in catalysis, in particular of imidazoliophosphines, a focus is given on the reference class of moderately flexible benzo-,diimidazolo-2,3,4,5,6,7-tris-annelated 3,6-diazaphosphepine derivatives with various exocyclic P–R substituents. The cases R = NMe₂, NⁱPr₂ are addressed by comparison to the known cases R = Ph, ^tBu. After selective N,N′-dimethylation of the imidazole nitrogen atoms with MeOTf, further P-oxidation of the resulting dication with m-CPBA is shown to occur readily for R = NⁱPr₂, as previously reported for R = ^tBu. In contrast, the same oxidative treatment of a parent neutral P-aminophosphepine is found to afford a cis/trans stereoisomeric mixture of the P-oxide. On the basis of line shape analyses of ¹H and ³¹P NMR spectra between 313 and 373 K, the free enthalpy of activation of the stereo-conversion process is shown to be ca 17 kcal/mol. ROESY ¹H NMR experiments show that the major isomer exhibits a cis orientation of the NiPr₂ group and the phenylene bridge. The corresponding boat conformation of the diazaphosphepine ring is confirmed by X-ray crystallography. As in the R = ^tBu series, the same boat conformation is observed in the crystal state of the dicationic oxide and neutral and dicationic P(III) parents.

Keywords:

• Amidiniophosphines
• carbeniophosphines
• cationic ligands
• imidazolophosphines
• phosphepines
• phosphine oxides