Supported chiral Mo-based complexes as efficient catalysts for enantioselective olefin metathesis

Syntheses and catalytic activities of seven new polymer-supported chiral Mo-based complexes are disclosed. Four of the complexes are polystyrene-based, and three involve polynorbornene supports. Studies concerning the ability of the polymer-bound chiral complexes to promote an assortment of asymmetric ring-closing (ARCM) and ring-opening (AROM) metathesis reactions are detailed. In many instances, levels of reactivity and enantioselectivity are competitive with those of the analogous homogeneous catalysts. The positive effect of lower cross-linking within the polymer backbone on reaction efficiency and asymmetric induction is detailed. The optically enriched products obtained through the use of the supported complexes, after simple filtration and removal of the supported Mo catalysts, contain significantly lower levels of metal impurities as compared to products synthesized with the corresponding homogeneous catalysts.     

Bidentate NHC-based chiral ligands for efficient Cu-catalyzed enantioselective allylic alkylations: structure and activity of an air-stable chiral Cu complex

Cu-catalyzed addition of alkylzinc reagents to a range of allylic phosphates is promoted efficiently and with high enantioselectivity to afford tertiary as well as quaternary carbon centers (up to 98% ee). Reactions proceed to completion with 0.5-5 mol % catalyst loading and are best promoted by commercially available CuCl2.2H2O. The X-ray structure of the chiral NHC-Ag(I) complex used in the study as well as that of a catalytically active NHC-Cu(II) complex are also reported; both complexes are air-stable and are formed in >/=95% isolated yield. The isolated Cu complex, which can be handled in air, is catalytically active. The present report provides the first precedent for efficient Cu-catalyzed allylic alkylations with chiral NHC ligands.     

A chiral Ag-based catalyst for practical, efficient, and highly enantioselective additions of enolsilanes to alpha-ketoesters

A Ag-based chiral catalyst promotes efficient and highly enantioselective aldol additions of ketone-derived enolsilanes to alpha-ketoesters in the presence of a readily available amino acid-based ligand and commercially available AgF2. alpha-Ketoester substrates may bear alkyl, alkenyl, and aryl substituents; reactions proceed to >98% conversion to afford the desired tertiary alcohols in 61->98% isolated yield and 60-96% ee. In contrast to previously reported approaches, highest enantioselectivities are observed with sterically demanding substrates, and reactions can be carried out in undistilled solvent, in air with as little as 1 mol % catalyst.     

A ROMP-derived, polymer-supported chiral Schrock catalyst for enantioselective ring-closing olefin metathesis

A permanently polymer-immobilised version of Schrock's molybdenum catalyst was realised via polymerisation of chiral 5,5'-bis(norborn-5-ene-2-ylmethyleneoxymethyl)-3,3'-di-tert-butyl-6,6'-dimethylbiphen-2,2'-diol followed by reaction with the catalyst precursor Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OSO2CF3)2(CH3OCH2CH2OCH3); using this 5-, 6- and 7-membered heterocycles were synthesised via ring-closing metathesis (RCM) from (pro-)chiral substrates in high yields and ee with short reaction times; the catalyst shows low loss of molybdenum during RCM, and was easily separated and recycled.     

First chiral phosphoroamidite-phosphite ligands for highly enantioselective and versatile Pd-catalyzed asymmetric allylic substitution reactions

[reaction: see text] A series of phosphite-phosphoroamidite ligands, derived from readily available d-xylose, has been successfully applied for the first time in the Pd-catalyzed allylic substitution of several substrates with different steric and electronic properties, with high enantioselectivities (ee's up to 98) and activities in standard conditions.     

N-heterocyclic carbene and phosphine ruthenium indenylidene precatalysts: a comparative study in olefin metathesis

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.     

Novel and efficient chiral sulfideoxathiane ligands for palladium-catalyzed asymmetric allylic alkylation

Easily prepared, chiral sulfideoxathiane ligands are described which give excellent enantioselectivity (up to 99% ee) in the Pd-catalyzed allylic alkylation of 1,3-diphenyl-2-propenyl acetate with a range of alkyl malonate nucleophiles.     

A highly efficient, recyclable catalyst for C-C coupling reactions in ionic liquids: pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II)

[reaction: see text] A hemilabile pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II) has been synthesized. It is an excellent catalyst for Heck and Suzuki cross-coupling reactions in ionic liquids.     

A recyclable chiral Ru catalyst for enantioselective olefin metathesis. Efficient catalytic asymmetric ring-opening/cross metathesis in air

The synthesis and structure of a new chiral bidentate imidazolinylidene ligand and a derived chiral Ru-based carbene are disclosed. The Ru complex is stereogenic at the metal center; it can be prepared in >98% diastereoselectivity and purified by silica gel chromatography with undistilled solvents. The air-stable Ru complex efficiently catalyzes ring-closing and ring-opening metathesis and is recyclable. The chiral complex is highly effective (0.5-10 mol % loading) in promoting enantioselective ring-opening/cross metathesis reactions (up to >98% ee). These enantioselective transformations can be effected in air, with unpurified solvent and with substrates that would only polymerize with Mo-based catalysts.     

A highly efficient olefin metathesis initiator: improved synthesis and reactivity studies

The synthesis of ligand 8, required for the preparation of catalyst 4c has been optimised. Ligand exchange studies indicate that biphenyl-based alkylidene 4c initiates considerably faster than its unsubstituted analogue 4a. The performance of 4c in ring-opening cross metathesis reactions involving substrates containing unprotected chelating atoms is also reported.     

On the ligand properties of the P- versus the N-heterocyclic carbene for a Grubbs catalyst in olefin metathesis

The 1,4-diphospha-2-azol-5-ylidene is a homologue to the Ender’s type carbene. It is a possible candidate for a ligand in the metathesis reaction of olefins. Based on density functional calculations the differences between the electronic structures of both systems are evaluated. The NHC (N-heterocyclic carbene) possesses a larger singlet–triplet energy separation than the PHC (P-heterocyclic carbene) analogue. Thus the latter exerts a larger Lewis acidity than the former. In comparison with, the donor-ability (σ-basicity) in both systems is similar. As a consequence for the PHC carbene a Ru-fragment as a ligand for catalysis is stronger bound. This causes in the olefin metathesis a lower dissociation energy (compared to the NHC analogue) with respect to the formation of the catalyst active 14el species. As a consequence, the olefin will be weaker bound as well. This can be overcome by attaching sterically demanding substituents such as mesityl or super-mesityl to the phosphorus atoms. They induce mutual steric hindrance with concomitant increase of the S–T separation of the free carbene. Thus the Lewis acidity of the carbene is reduced. On this basis for the PHC’s with larger S–T energy separations the dissociation energy of the phosphine fragment is raised and the adding olefin fragment will be stronger bound to the transition metal. While these effects describe the electronic situation in the reactive species, steric effects at the ligand carbene mediate the stabilities of the individual intermediates in the metathesis reaction by exertion of inter- and intra-ligand repulsion.     

A new cationic,chiral catalyst for highly enantioselective Diels-Alder reactions

[reaction: see text] The Diels-Alder reaction of cyclopentadiene and 2-methacrolein is catalyzed by a chiral Lewis acid to form the exo adduct in 96% yield and 96% ee.     

PEG-modified N-heterocyclic carbene ligands for highly efficient and recyclable Pd-catalyzed Heck reaction in water

A water-soluble and air-stable Pd(OAc)₂/mPEG _n MeImI system was found to be a highly efficient and reusable catalyst for the coupling of aryl halides and styrene in neat water using K₂CO₃ as base. After extraction, the catalytic system could be reused several times with only a slight decrease in its activity.     

Two-fold amino acid-based chiral aminophosphine-oxazolines and use in asymmetric allylic alkylation

Chiral aminophosphine-oxazoline ligands derived from two different amino acids (tetrahydroisoquinoline carboxylic acid and proline) have been synthesized and examined as chiral auxiliaries in asymmetric allylic alkylation of three substrates. Stereoisomers 1a and 2a are providing the highest enantioselectivities (up to 94% ee) in the allylation of diphenylpropenyl acetate.     

Palladium/chiral phosphine–olefin complexes: X-ray crystallographic analysis and the use in catalytic asymmetric allylic alkylation

X-ray crystallographic studies on π-allylpalladium complexes coordinated with a chiral phosphine–olefin ligand (−)-1b demonstrate that the phosphine ligand shows a larger trans-influence than the π-bound olefin. The palladium/chiral phosphine–olefin complex efficiently catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with 96% enantioselectivity.