ABSTRACT
Silica-supported cationic Mo-imido alkylidene N-heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ-donating property of the NHC ancillary ligand, which disfavors the formation of the parent square-planar metallacyclobutane, an off-cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.
ABSTRACT
An extensive series of the first ionic Mo- and W-based Schrock-type catalysts based on pyridinium and phosphonium tagged aryloxide ligands were prepared. Bisionic complexes of the general formula Mo(Imido)(CHR)(OR')2 (OTf)2 and monoionic monoaryloxide pyrrolide (pyr) (MAP-type) catalysts [M(Imido)(CHR)(OR')(pyr)+ ][A- ] were successfully employed and tested in various olefin metathesis benchmark reactions under both, homogeneous and biphasic conditions using pyrrole and, for the first time with Schrock-type catalysts, ionic liquids as the polar phase. Productivities under biphasic conditions up to several thousand turn overs were achieved and are comparable to those obtained in reactions carried out in chlorobenzene or toluene. Metal contamination in the nonpolar product-containing heptane phase was <2â ppm.
ABSTRACT
New stereodynamic diphosphinites derived from 2,2'-biphosphole, were synthesised by introduction of a linker obtained from chiral diols between the two phosphorus atoms and used for catalytic hydrogenation through a dual chirality control induced by Rh-coordination. The application of these ligands in hydrogenation of dimethyl itaconate shows that the enantioselectivity strongly depends on steric and electronic properties of the chiral linker whereas the sense of enantioselection is determined by the configuration of these stereocentres. These stereodynamic diphosphinites induce higher enantioselectivities than the analogous stereodynamic diphosphanes derived from 2,2-biphosphole.
ABSTRACT
Diphosphine sulfides derived from 2,2'-biphosphole have been efficiently synthesized in an enantiomerically pure form by a four step synthetic sequence. These S,S-ligands were used for the first time in Pd-catalyzed asymmetric allylic alkylation. Good yields and enantiomeric excess up to 73% were obtained.
