Chiral [Iminophosphoranyl]ferrocenes: Synthesis, coordination chemistry, and catalytic application

A series of new chiral [iminophosphoranyl]ferrocenes, {η⁵- C₅H₄-(PPh₂=N-2,6-R₂-C ₆H₃)}Fe{η⁵-C₅H ₃-1-PPh₂-2-CH(Me)NMe₂} (1: R = Me, ⁱPr), {η⁵-C₅H₄-(PPh ₂=N-2,6-R₂¹-C₆H₃)} Fe{η⁵-C₅H₃-1-(PPh₂=N-2,6-R ₂-C₆H₃)-2-CH(Me)R²} (2: R ¹ = Me, ⁱPr; R² = NMe₂, OMe), and (η⁵-C₅H₅)Fe{η⁵-C ₅H₄-1-PR₂-2-CH(Me)N=PPh₃} (3: R = Ph, C₆H₁₁) have been prepared from the reaction of [1,1′-diphenylphosphino-2-(N,N-dimethyl-amino)ethyl]ferrocene with arylazides (1 & 2) and the reaction of phosphine dichlorides (R ₃PCl₂) with [1,1′-diphenylphosphino-2-aminoethyl] ferrocene (3), respectively. They form palladium complexes of the type [Pd(C₃H₅)(L)]BF₄ (4-6: L = 1-3), where the ligand (L) adopts an η²-N,N (2) or η²-P,N (3) as expected. In the case of 1, a potential terdentate, an η²-P,N mode is realized with the exclusion of the -P=NAr group from the coordination sphere. Complexes 4-6 were employed as catalysts for allylic alkylation of 1,3-diphenylallyl acetate leading to an almost stoichiometric product yield with modest enantiomeric excess (up to 74% ee). Rh(I)-complexes incorporating 1-3 were also prepared in situ for allylic alkylation of cinnamyl acetate as a probe for both regio- and enantioselectivities of the reaction. The reaction exhibited high regiocontrol in favor of a linear achiral isomer regardless of the ligand employed.