Cyclometalation of aryl-substituted phosphinines through C-H-bond activation: a mechanistic investigation.
A series of 2,4,6-triarylphosphinines were prepared and investigated in the base-assisted cyclometalation reaction using [Cp*IrCl2]2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) as the metal precursor. Insight in the mechanism of the C-H bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time-dependent (31)P{(1)H} NMR spectroscopy. At room temperature, 2,4,6-triarylphosphinines instantaneously open the Ir-dimer and coordinate in an η(1)-fashion to the metal center. Upon heating, a dissociation step towards free ligand and an Ir-acetate species is observed and proven to be a first-order reaction with an activation energy of ΔEA = 56.6 kJ mol(-1) found for 2,4,6-triphenylphosphinine. Electron-donating substituents on the ortho-phenyl groups of the phosphorus heterocycle facilitate the subsequent cyclometalation reaction, indicating an electrophilic C-H activation mechanism. The cyclometalation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction. The cyclometalated products were characterized by means of NMR spectroscopy and in several cases by single-crystal X-ray diffraction. Based on the observed trends during the mechanistic investigation, a concerted base-assisted metalation-deprotonation (CMD) mechanism, which is electrophilic in nature, is proposed.
Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven (The Netherlands).
Leen E E
LE
Broeckx
Sabriye
S
Güven
Frank J L
FJ
Heutz
Martin
M
Lutz
Dieter
D
Vogt
Christian
C
Müller
D001555:Benzene Derivatives / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D006860:Hydrogen Bonding; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D009943:Organophosphorus Compounds / Q000737:chemistry*
D001555:Benzene Derivatives; D056831:Coordination Complexes; D008024:Ligands; D009943:Organophosphorus Compounds; C519090:phosphinine
CH activation; NMR spectroscopy; X-ray diffraction; coordination chemistry; metalacycles; regioselectivity