Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity.

The development of novel Lewis acids derived from bipyridinium and phenanthrolinium dications is reported. Calculations of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis acidity at the ortho and para positions. This arises in part from extensive LUMO delocalization across the aromatic backbones. Species [C10 H6 R2 N2 CH2 CH2 ]2+ (R=H [1 a]2+ , Me [1 f]2+ , tBu [1 g]2+ ), and [C12 H4 R4 N2 CH2 CH2 ]2+ (R=H [2 a]2+ , Me [2 b]2+ ) were prepared and evaluated for use in the initiation of hydrodefluorination (HDF) catalysis. Compound [2 a]2+ proved highly effective towards generating catalytically active silylium cations via Lewis acid-mediated hydride abstraction from silane. This enabled the HDF of a range of aryl- and alkyl- substituted sp3 (C-F) bonds under mild conditions. The protocol was also adapted to effect the deuterodefluorination of cis-2,4,6-(CF3 )3 C6 H9 . The dications are shown to act as hydride acceptors with the isolation of neutral species C16 H14 N2 (3 a) and C16 H10 Me4 N2 (3 b) and monocationic species [C14 H13 N2 ]+ ([4 a]+ ) and [C18 H21 N2 ]+ ([4 b]+ ). Experimental and computational data provide further support that the dications are initiators in the generation of silylium cations.

Regio- and Stereospecific Cyclopolymerization of α,ω-Diynes by Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

Both solvent-free and acetonitrile-containing cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR')(CHCMe2 R)(NHC)(X)+ A- ] (R' = 2,6-Cl2 -C6 H3 , tBu, 2-CF3 -C6 H4 , 2-tBu-C6 H4 , 2,6-iPr2 -C6 H3 , 2,6-Me2 -C6 H3 ; R = Me, Ph; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-di-iPr-imidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene); X = CF3 SO3 , C6 F5 O, OCH(CF3 )2 , OC(CF3 )3 , pyrrolide, C6 F5 COO, 2,6-(CF3 )2 -C6 H3 COO; A- = B(ArF )4 - , Al(OC(CF3 )3 )4 - ), have been investigated for their propensity to cyclopolymerize 4,4-disubstituted 1,6-heptadiynes. All metal complexes contain a stereogenic (chiral) metal center, which accounts for the high reactivity and high regioselectivity of insertion (>99%) that are observed for all metal complexes, leading to highly conjugated, α-insertion-derived polyenes that are based on a highly regular polymer backbone and that show absorption maxima close to 600 nm. With the chiral monomer 4-(ethoxycarbonyl)-4-(1S,2R,5S)-(-)-menthoxycarbonyl-1,6-heptadiyne, high syndiospecifity (>99% syndiotactic) is observed. A mechanism explaining the high regio- and stereoselectivity is presented. Thus, α-addition of the monomers proceeds chain-end-controlled trans to the NHC and is preferred over ß-addition through intramolecular Mo-O chelation. Insertion of the monomers entails double inversion at the stereogenic metal center in the course of one complete monomer insertion.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts.

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo-, cross and ring-opening cross metathesis reactions. The catalysts remain active even in 2-PrOH and are applicable in ring-opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3-dimesitylimidazol-2-ylidene NHC ligand was found essential for reactive and yet robust catalysts.

Olefin Metathesis in Confined Geometries: A Biomimetic Approach toward Selective Macrocyclization.

The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.55, corresponding to 35% MMC product obtained with the homogeneous catalyst, up to 1.49, corresponding to 60% MMC product. A correlation between the MMC/O ratio and the substrate-to-pore-size ratio was successfully established. Modification of the inner pore surface with dimethoxydimethylsilane allowed fine-tuning the effective pore size and reversing surface polarity, which resulted in a further increase of the MMC/O ratio up to 2.2, corresponding to >68% MMC product. Molecular-level simulations in model pore geometries help to rationalize the complex interplay between spatial confinement, specific (substrate and product) interaction with the pore surface, and diffusive transport. These effects can be synergistically adjusted for optimum selectivity by suitable surface modification.

Mono- and Bisionic Mo- and W-Based Schrock Catalysts for Biphasic Olefin Metathesis Reactions in Ionic Liquids.

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 <2 ppm.

Latent and Air-Stable Pre-Catalysts for the Polymerization of Dicyclopentadiene: From Penta- to Hexacoordination in Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

The pentacoordinated, 16-valence electron (VE) Mo imido alkylidene N-heterocyclic carbene (NHC) complexes I1-I5 and the hexacoordinated 18-VE Mo imido alkylidene NHC complexes 1-4, 8, 10 and 12 containing a chelating ligand have been prepared and used as thermally latent catalysts in the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Both 10 and 12 are the first Mo imido alkylidene complexes with a chelating alkylidene featuring a carboxylate group. Complexes I1-I3 and I5 as well as 1-4 proved to be fully thermally latent in the presence of DCPD. With the changes in both the electronic and steric situation at the imido ligand provided by these pre-catalysts, different temperatures of the onset of polymerization (Tonset =65-140 °C) and for the exothermic maximum of the curing curve (Texo,max =98-183 °C) of DCPD were achieved. Also, the degree of crosslinking was successfully varied as indicated by swelling experiments in toluene, which revealed degrees of swelling between 0 and 50 %. While the introduction of a chelating alkylidene increases Tonset , the introduction of more electron-donating anionic ligands (tert-butoxide, phenoxide) resulted in a drastic reduction in Tonset , underlining the high flexibility of these systems. The hexacoordinated high-oxidation state molybdenum imido alkylidene NHC complexes 2, 3 and 4 were stable under air for at least twelve hours in the solid state.

Molybdenum and Tungsten Imido Alkylidene N-Heterocyclic Carbene Catalysts Bearing Cationic Ligands for Use in Biphasic Olefin Metathesis.

Ionic Mo- and W-imido alkylidene N-heterocyclic carbene (NHC) olefin metathesis catalysts, [Mo{N-2,6-(Me2 )C6 H3 }(CHCMe2 Ph)(IMesH2 )(OTf)(PPS)]OTf (3), [Mo(N-2,6-(Me2 )C6 H3 )(CHCMe2 Ph)(IMesH2 )(OC6 F5 )(PPS)][B(ArF )4 ] (5), [Mo(NtBu)(CHCMe2 Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)(OTf)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][OTf] (9), [Mo(NtBu)(CHCMe2 Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][B(ArF )4 ]2 (10, PPS=pyridiniumpropanesulfonate, IMesH2 =1,3-dimesitylimidazolin-2-ylidene, OTf=CF3 SO3 , B(ArF )4 =tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) were prepared from betaine-type ligands. Also, the first bis-NHC and a nitron-based bis(amido) bistriflate imido alkylidene complex, [Mo(NtBu)(CHCMe2 Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)2 (THF)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][OTf]2 (11) and [Mo(N-2,6-Me2 -C6 H3 )(CHCMe2 Ph)(N-{1,4-diphenyl-1,3,4-triazol-2-ylium}-N-phenyl-amido)2 ][OTf]2 (14) along with ionic [W(N-2,6-iPr2 -C6 H3 )(CHCMe2 Ph)(N-2,5-Me2 C4 H2 )(IiPr)(2,6-tBu-4-PPh3 -phenolate)] (17, IiPr=1,3-diisopropylimidazol-2-ylidene) are reported. With these new catalysts, the first biphasic reaction setup with Group 6 metal alkylidene NHC complexes was successfully established using a pyrrole/heptane mixture as a liquid phase. Productivities under biphasic conditions were comparable to those of reactions in 1,2-dichloroethane or toluene. Metal concentrations of <2 ppm migrated into the nonpolar heptane phase as measured by inductively coupled plasma-optical emission spectroscopy.

High Oxidation State Molybdenum N-Heterocyclic Carbene Alkylidyne Complexes: Synthesis, Mechanistic Studies, and Reactivity.

The first synthetic protocol to high oxidation state molybdenum(VI) N-heterocyclic carbene (NHC) alkylidyne complexes (NHC=1,3-diisopropylimidazol-2-ylidene, 1,3-dimethyl-4,5-R2 -imidazol-2-ylidene, R2 =H, Cl, CN) is reported. Steric limitations of the NHCs and the benzylidyne are described. All novel complexes were characterized by single crystal X-ray diffraction and solution NMR techniques. It was shown that all complexes presented here show activity in the self-metathesis of 1-phenyl-1-propyne at room temperature. To identify mechanistic differences, an experimental sequence to detect dissociation of ligands was developed. Results reveal dissociation of less electron-donating NHCs in course of the reaction. Mechanistic and reactivity differences were attributed to electronic and steric effects through Tolman's electronic parameter and the percentage of buried volume. Furthermore, Mo-1 containing the 1,3-dimethylimidazol-2-ylidene ligand showed good activity in self-metathesis reactions of p-substituted 1-phenyl-1-propynes with electron-donating moieties at room temperature.

9-BBN and chloride catalyzed reduction of chlorophosphines to phosphines and diphosphines.

The commercially available Lewis acid, 9-BBN and Lewis basic [Et4N]Cl are used as catalysts for the reduction of chlorophosphines R2PCl in the presence of phenylsilane. Aryl-chlorophosphines afford primarily diphosphines (P2R4) while secondary phosphines predominate for alkyl-substituted precursors. Use of the combined catalysts leads to reduced reaction time and temperature, providing a rapid, scalable, and facile protocol for the preparation of diphosphines or secondary phosphines.

Probing B-X to B-H conversions and applications in C-F bond activation catalysis.

Herein we exploit a catalytic amount of [Ph3C]+ to initiate B-X to B-H bond conversion with Et3SiH. This was applied to 6 haloboranes. However, 9-X-9-borabicyclo[3.3.1]nonane (B-X-9-BBN, X = F, Br) reacts directly with silane. Thus, C-F bond activation of benzyl fluorides in the presence of arenes afforded the Friedel-Crafts (FC) products using B-H-9-BBN in the presence of Et3SiH. This catalysis was probed with a range of arenes and several benzyl fluoride derivatives. The protocol is simple, cheap and a convenient route to 1,1-diarylmethanes from benzyl fluorides in good to excellent yields (up to 99%) under mild conditions.