A convenient method for the efficient removal of ruthenium byproducts generated during olefin metathesis reactions

[reaction in text] An efficient method for removing ruthenium byproducts generated during olefin metathesis reactions with Grubbs catalysts is described. Treatment of the crude reaction products with triphenylphosphine oxide or dimethyl sulfoxide, followed by filtration through silica gel, was found to be a practical and effective method to remove colored ruthenium byproducts.     

An efficient method for removal of ruthenium byproducts from olefin metathesis reactions

[reaction: see text] Sequential treatment of the ring-closing metathesis reaction products with silica gel, activated carbon (50 equiv wt relative to the crude products), and column chromatography on silica gel efficiently removed dark brown ruthenium byproducts from the reaction mixture. After this treatment, colorless compounds could be obtained with a ruthenium level of 0.06-0.53 microg per 5 mg of product.     

A simple amine protection strategy for olefin metathesis reactions

Acyclic diamines are valuable feedstocks for polyamide synthesis. Ruthenium-alkylidene catalysed cross metathesis of amino alkenes is problematic and acyl derivatisation can result in less efficient syntheses, poor catalyst turnover and isomerisation. Temporary amine masking via stable and soluble ammonium salts delivers cyclic and acyclic aminoalkenes in high yield and purity.     

Asymmetric catalysts for stereocontrolled olefin metathesis reactions

Since the discovery of metathesis as an instrument to reorganize olefinic double bonds, substantial progress has been attained, establishing this method as a versatile and efficient tool for C-C-bond formation. In the last decade fundamental achievements were accomplished in the field of chiral Ru- and Mo-based olefin metathesis, providing an asymmetric access to structures, which are difficult to obtain by alternative routes. The reader is taken behind the scenes of catalyst development, important areas of application are described up to the current state of research; this tutorial review deals with the question, how metathesis is connected to enantioselective synthesis.     

Phosphabicyclononane-containing ru complexes: efficient pre-catalysts for olefin metathesis reactions

The catalytic performances of three Phosphabicyclononane (Phoban)-containing ruthenium-based pre-catalysts have been evaluated for metathesis transformations. A wide screening of substrates in ring-closing metathesis reactions reveals the greater efficiency of pre-catalyst 4. Comparison of the catalytic activities of 4 with Grubbs' first-generation pre-catalyst illustrates the key role of the Phoban ligand. Additionally, a comparative study of three Phoban-containing pre-catalysts has been conducted for the self-metathesis of 1-octene at low catalyst loading (25-100 ppm).     

Efficient removal of ruthenium byproducts from olefin metathesis products by simple aqueous extraction

Simple aqueous extraction removed ruthenium byproducts efficiently from ring-closing metathesis (RCM) reactions catalyzed by a poly(ethylene glycol) (PEG) supported N-heterocyclic carbene-based ruthenium complex.     

Model compounds of ruthenium-alkene intermediates in olefin metathesis reactions

The development of a model system to study ruthenium-olefin complexes relevant to the mechanism of olefin metathesis is reported. Upon addition of 1,2-divinylbenzene to (H2IMes)(py2)(Cl)2Ru=CHPh (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), two ruthenium-olefin adducts are formed. On the basis of 1H NMR spectroscopy experiments and X-ray crystallographic analysis, these complexes are assigned as side-bound isomers in which the olefin and H2IMes ligands are coordinated cis to each other. The dynamic interconversion of these two ruthenium complexes was determined to have a barrier of 19.1 +/- 0.1 kcal/mol.     

Prevention of undesirable isomerization during olefin metathesis

1,4-Benzoquinones have been found to prevent olefin isomerization of a number of allylic ethers and long-chain aliphatic alkenes during ruthenium-catalyzed olefin metathesis reactions. Electron-deficient benzoquinones are the most effective additives for the prevention of olefin migration. This mild, inexpensive, and effective method to block olefin isomerization increases the synthetic utility of olefin metathesis via improvement of overall product yield and purity.     

Ring-closing olefin metathesis reactions; synthesis of iso-β-bisabolol

A ring-closing olefin metathesis is the key step in a synthesis of the rare iso-β-bisabolol found in sandalwood oils.     

Mechanism of ruthenium-catalyzed olefin metathesis reactions from a theoretical perspective

This paper presents a density functional theory study of the ruthenium-catalyzed olefin metathesis reactions. The ligand binding energy has been calculated in the first generation of Grubbs-type (PCy3)2Cl2Ru=CHPh (pre)catalyst, as well as in the heteroleptic (pre)catalytic systems in which a N-heterocyclic carbene, NHC, ligand substitutes a single phosphine. In agreement with experiments PCy3 coordinates more strongly to Ru in the heteroleptic (pre)catalysts than in the Grubbs-type (pre)catalyst. Moreover, ethene coordination and insertion into the Ru-alkylidene bond in the above-mentioned systems, as well as in the Hofmann type catalytic system with a cis-coordinated phosphane ligand, has been studied. The calculated insertion barrier for the NHC systems are lower than that of the (PCy3)2Cl2Ru=CHPh system. This is consistent with the higher activity experimentally observed for the NHC-based system.     

Z-Selective olefin metathesis reactions promoted by tungsten oxo alkylidene complexes

Addition of LiOHMT (OHMT = O-2,6-dimesitylphenoxide) to W(O)(CH-t-Bu)(PMe(2)Ph)(2)Cl(2) led to WO(CH-t-Bu)Cl(OHMT)(PMe(2)Ph) (4). Subsequent addition of Li(2,5-Me(2)C(4)H(2)N) to 4 yielded yellow W(O)(CH-t-Bu)(OHMT)(Me(2)Pyr)(PMe(2)Ph) (5). Compound 5 is a highly effective catalyst for the Z-selective coupling of selected terminal olefins (at 0.2% loading) to give product in >75% yield with >99% Z configuration. Addition of 2 equiv of B(C(6)F(5))(3) to 5 afforded a catalyst activated at the oxo ligand by B(C(6)F(5))(3). 5·B(C(6)F(5))(3) is a highly active catalyst that produces thermodynamic products (~20% Z).     

Decomposition of a key intermediate in ruthenium-catalyzed olefin metathesis reactions

Dinuclear ruthenium complex, with a bridging carbide and a hydride ligand, and methyltricyclohexylphosphonium chloride result from thermal decomposition of olefin metathesis catalyst, (IMesH2)(PCy3)(Cl)2Ru=CH2. Involvement of dissociated phosphine in the decomposition is proposed. The dinuclear complex has catalytic olefin isomerization activity, which can be responsible for competing isomerization processes in certain olefin metathesis reactions.     

An artificial metalloenzyme for olefin metathesis

A Grubbs-Hoveyda type olefin metathesis catalyst, equipped with an electrophilic bromoacetamide group, was used to modify a cysteine-containing variant of a small heat shock protein from Methanocaldococcus jannaschii. The resulting artificial metalloenzyme was found to be active under acidic conditions in a benchmark ring closing metathesis reaction.     

A general model for selectivity in olefin cross metathesis

In recent years, olefin cross metathesis (CM) has emerged as a powerful and convenient synthetic technique in organic chemistry; however, as a general synthetic method, CM has been limited by the lack of predictability in product selectivity and stereoselectivity. Investigations into olefin cross metathesis with several classes of olefins, including substituted and functionalized styrenes, secondary allylic alcohols, tertiary allylic alcohols, and olefins with alpha-quaternary centers, have led to a general model useful for the prediction of product selectivity and stereoselectivity in cross metathesis. As a general ranking of olefin reactivity in CM, olefins can be categorized by their relative abilities to undergo homodimerization via cross metathesis and the susceptibility of their homodimers toward secondary metathesis reactions. When an olefin of high reactivity is reacted with an olefin of lower reactivity (sterically bulky, electron-deficient, etc.), selective cross metathesis can be achieved using feedstock stoichiometries as low as 1:1. By employing a metathesis catalyst with the appropriate activity, selective cross metathesis reactions can be achieved with a wide variety of electron-rich, electron-deficient, and sterically bulky olefins. Application of this model has allowed for the prediction and development of selective cross metathesis reactions, culminating in unprecedented three-component intermolecular cross metathesis reactions.     

Cis-trans isomerization of polybutadiene double bonds during olefin metathesis

The cis-trans isomerization of polybutadiene double bonds during metathesis degradation using WCl₆/(CH₃)₄Sn catalyst system has been estimated kinetically along with productive metathesis. The isomerization was followed both for noncrosslinked and for crosslinked polybutadiene. Ninety-six percent cis-1, 4 units are found to isomerize into ca. 75% trans-1, 4 units. The rate of stereomutation is found to be different in the presence and absence of a low-molecular-weight olefin. The results are explained with the help of a stereo model originally proposed by Katz (Advances in Organometallic Chemistry, Academic, New York, 1977, Vol. 16.)     

Dichlorotetracarbonyltungsten as a catalyst for olefin metathesis

The photolysis of hexacarbonyltungsten (I) in carbon tetrachloride yields a catalytic mixture for olefin metathesis. One of the principal products of this transformation is dichlorotetracarbonyltungsten (II). Chemical preparation of II from I and chlorine was carried out. Authentic II was shown to give catalysis of 2-pentene metathesis by either heating in chlorobenzene or chloroform or irradiation in chlorobenzene or carbon tetrachloride. The Dubois report of phosgene formation in the original irradiated mixture of I in carbon tetrachloride was shown to be an artifact.