Synthesis, Characterization, and Reactivity of Ruthenium Bis-Allyl Complexes with Chiral Phosphine-Phosphite Ligands

A series of ruthenium bis-allyl complexes of formula
Ru­(η3-2-MeC3H4)2(P-OP) (1) which incorporate chiral phosphine-phosphite
ligands (P-OP)
have been prepared and characterized. Compounds 1 exist
in solution as a mixture of two diastereomers (maj and min) differing
in the configuration at the metal. Both isomers are in equilibrium
in solution, and their ratio varies with the nature of the P-OP ligand.
Along the series, higher values of the maj/min ratio were observed
with complexes bearing less sterically encumbered phosphite groups.
In reactions of compounds 1 with protic reagents removal
of one or two allyl ligands has been observed. Thus, reaction of 1h with pentachlorophenol produces the mixed allyl-phenoxide 3h, whereas reaction of 1a or 1h with tiglic acid produces the corresponding tiglates 4a,h, respectively. In addition, compounds 1 generate active catalysts for the hydrogenation of tiglic and 2-methyl-2-pentenoic
acids. By appropriate optimization of the structure of the P-OP ligand,
enantioselectivities up to 89% ee were obtained in these reactions.
In addition, complexes 1 also catalyze the ROMP reaction
of norbornene, although low initiation rates have been observed for
this process.