On the theme of amidiniophosphines: Synthesis and reactivity of benzo-, diimidazolo-tris-annelated p-aminodiazaphosphepines
Résumé
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.