Fabia Grisi

The Pivotal Role of Symmetry in the Ruthenium-Catalyzed Ring-Closing Metathesis of Olefins

The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.

Syndiotactic Polymerization of Propylene: Single‐Site Vanadium Catalysts in Comparison with Zirconium and Nickel

The syndiotactic-specific polymerization of propylene can be promoted by single-site catalysts based on vanadium, nickel and group IV metals. The polymerization mechanisms are reasonably well established for nickel and group IV metals. For vanadium catalysts the polymerization mechanisms reported in the literature are still debatable, since the stereochemistry of the monomer insertion is well known, but the active species have never been isolated. In this paper the mechanisms concerning Ni and group IV metal catalysts are briefly summarized and proposed mechanisms concerning vanadium are compared within one another.

Group 4 Cs Symmetric Catalysts and 1-Olefin Polymerization

Abstract Polymerization of propene and 1-butene promoted by group 4 Cs symmetric metallocene precursors has been tested in different reaction conditions. The stereochemical structure of the polymer is affected by the radius of the transition metal, the substituent of the olefin and in some cases the reaction temperature and the monomer concentration. The results are tentatively rationalized considering the possible catalytic cycles that may produce either syndiotactic or isotactic polymers. A sample of syndiotactic polypropylene melting at 170°C is also reported.

Probing the Relevance of NHC Ligand Conformations in the Ru‐Catalysed Ring‐Closing Metathesis Reaction

Probing the Relevance of NHC Ligand Conformations in the Ru-Catalysed Ring-Closing Metathesis Reaction Cat power : Judicious backbone substitution of N-heterocyclic carbenes (NHCs) leads to stable Ru metathesis catalysts with frozen NHC conformations. This finding not only permits the isolation of complexes that are among the most active catalysts in the ringclosing metathesis of hindered olefins (see graphic; Ts= p-toluenesulfonyl), but also provides fundamental mechanistic insights on the role of N-aryl substituent conformations on catalyst activity.

Synthesis, characterization and catalytic behaviour of a palladium complex bearing a hydroxy-functionalized N-heterocyclic carbene ligand

The synthesis of a new stable palladium(II) complex (3) featuring an unsymmetrical substituted N-heterocyclic carbene (NHC) ligand with a pendant hydroxy-functionalized group was successfully accomplished via transmetalation of the corresponding bis-NHC silver(I) complex (2). Solid-state structures of both 2 and 3 were determined by single-crystal X-ray diffraction. The catalytic behaviour of 3 in the direct regioselective arylation of furan and thiophene derivatives by using challenging aryl halides was studied, revealing that 3 was capable of promoting these environmentally attractive coupling reactions to afford arylated heterocycles in moderate to good yields.

Activity and stereoselectivity of Ru-based catalyst bearing a fluorinated imidazolinium ligand

AbstractGrubbs II generation catalyst (3), bearing a fluorinated imidazolinium ligand, was investigated in cross metathesis (CM), ring closing metathesis (RCM) and ring opening polymerization metathesis (ROMP) for a variety of substrates. Kinetic studies showed reduced stability of the catalyst in methylene chloride following the first 15 minutes of reaction preventing a higher efficiency despite the very high activity. Beneficial solvent effects on the catalyst stability were observed by performing RCM in C6F6.

Highly stereoregular polymerization of 1,3-cyclohexadiene in the presence of Cp2Ni-MAO catalyst

Polymerizations of 1,3-cyclohexadiene, in the presence of Cp 2 Ni (Cp = cyclopentadienyl) activated by methylaluminoxane (MAO), were carried out at different temperatures and Ni-MAO mole ratios. The polymers obtained are stereoregular with a 1-4 structure, and are highly crystalline with a melting point close to 315°C. Copolymerizations of 1,3-cyclohexadiene and butadiene with the title catalyst give polymers with a nearly random distribution of the comonomeric units.

Chemoselective mechanism of (Z)-1,3-pentadiene polymerization in the presence of cyclopentadienyltitanium trichloride and methylaluminoxane

Comparative polymerizations of 4,5,5,5-d4-(Z)-1,3-pentadiene with (Z)-1,3-pentadiene at different temperatures in the presence of CpTiCl 3 -MAO have shown that the chemoselectivity and its temperature dependence (1,2 or 1,4 polymerization at low or high temperatures, respectively) are not influenced by deuteration of the monomer. This suggests the absence of any relevant role of H-agostic interactions between the growing chain-end and the metal, in determining chemoselectivity and rate of polymerization. Copolymerizations of (Z)-1,3-pentadiene with ethylene at -20°C give random copolymers containing substantial amounts of cis-1,4-pentadiene units adjacent to ethylene. This indicates a relevant role of the growing chain-end in determining the chemoselectivity of the insertion of the incoming monomer. A polymerization mechanism involving a back-biting coordination (a simultaneous coordination of the allyl terminal and of the last double bond) of the growing chain is proposed.

Stereolability of chiral ruthenium catalysts with frozen NHC ligand conformations investigated by dynamic-HPLC.

The stereolability of chiral Hoveyda-Grubbs II type ruthenium complexes bearing N-heterocyclic carbene (NHC) ligands with Syn-phenyl groups on the backbone and Syn- or Anti-oriented o-tolyl N-substituents was studied by resorting to dynamic high-performance liquid chromatography (D-HPLC). A complete chromatographic picture of the involved stereoisomers (four for Anti- and two for Syn-complexes) was achieved at very low temperatures (-53°C and -40°C respectively), at which the NHC-Ru bond rotations were frozen out. Inspection of the chromatographic profiles recorded at higher temperatures revealed the presence of plateau zones between the couples of either Syn or Anti stereoisomers, attesting to the active interconversion between the eluted species. Such dynamic chromatograms were successfully simulated through procedures based on both theoretical plate and classical stochastic models. The good superimposition achieved between experimental and simulated chromatographic profiles allowed determination of the related isomerization energy barriers (ΔGisom (#) ), all derived by rotation around the NHC-Ru bond. The obtained diastereomerization barriers between the Anti isomers were found in very good agreement with those previously measured by experimental nuclear magnetic resonance (NMR) and assessed through Density Functional Theory (DFT) calculations. With the same approach, for the first time we also determined the enantiomerization barrier of the Syn isomer. Focused changes to the structure of complex Syn, studied by a molecular modeling approach, were found suitable to strongly reduce the stereolability arising from rotation around the NHC-Ru bond.

Polymerization of styrene and conjugated diolefins in the presence of nickelocenes‐based catalysts

Several nickelocenes activated by methylaluminoxane have been investigated in the polymerization of styrene obtaining isotactic polystyrene. The most stereospecific system is Cp 2 Ni-MAO, which is also able to polymerize some conjugated diolefins producing polymers with 1,4 structure.