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Robert H Grubbs

Publications and source records attributed to Robert H Grubbs.

At least 19 recordsLinked to original sources
PubMed2006

Ruthenium metallacycles derived from 14-electron complexes. New insights into olefin metathesis intermediates.

Ruthenium(IV) metallacycles derived from both ethylene and propene are reported. The propene-derived metallacycles represent the first observed examples of substituted ruthenacyclobutanes and offer new insight into the preferred stereochemical orientation about metathesis intermediates. In addition, a metallacycle possessing an unsymmetrical N-heterocyclic carbene (NHC) ligand was prepared and investigated to ascertain the dynamics of the NHC relative to the metallacycle ring. Metallacycles investigated were found to possess exchange cross-peaks between the alpha- and beta-positions in the 2D NMR, indicating a dynamic structure. The implications of these results to the mechanism of ruthenium-catalyzed olefin metathesis are discussed.

Alkenes↗
PubMed2006

Template-directed one-step synthesis of cyclic trimers by ADMET.

A trifurcated template, containing three secondary dialkylammonium ion recognition sites for encirclement by a dibenzo [24]crown-8-containing acyclic diene, is used to promote acyclic diene metatheses (ADMET) catalyzed by ruthenium-alkylidene complexes, affording a cyclic trimer in 55% yield. Following this one-step, threefold ADMET reaction, the resulting cyclic trimer was isolated by preparative HPLC and characterized by NMR spectroscopy and mass spectrometry.

Catalysis↗
PubMed2006

On the mechanism of the conversion of methanol to 2,2,3-trimethylbutane (triptane) over zinc iodide.

Methanol is converted to a mixture of hydrocarbons by reaction with zinc iodide at 200 degrees C with one highly branched alkane, 2,2,3-trimethylbutane (triptane), being obtained in surprisingly high selectivity. Mechanistic studies implicate a two-stage process, the first involving heterogeneously catalyzed formation of a carbon-carbon-bonded species, probably ethylene, that undergoes homogeneously catalyzed sequential cationic methylation to higher hydrocarbons. The first stage can be bypassed by addition of olefins, higher alcohols, or arenes, which act as initiators. Rationales for the particular activity of zinc iodide and for the selectivity to triptane are proposed.

Journal Article↗
PubMed2006

Rhenium-catalyzed 1,3-isomerization of allylic alcohols: scope and chirality transfer.

The scope of the triphenylsilyl perrhennate (O3ReOSiPh3, 1) catalyzed 1,3-isomerization of allylic alcohols has been thoroughly explored. It was found to be effective for a wide variety of secondary and tertiary allylic alcohol substrates bearing aryl, alkyl, and cyano substituents. Two general reaction types were found which gave high levels of product selectivity: those driven by formation of an extended conjugated system and those driven by selective silylation of a particular isomer. The efficiency of chirality transfer with various substrates was investigated, and conditions were found in which secondary and tertiary allylic alcohols could be formed with high levels of enantioselectivity. Consideration of selectivity trends with respect to the nature of the substituents around the allylic system revealed that this is a reliable and predictable method for allylic alcohol synthesis.

Alcohols↗
PubMed2006

Rate acceleration in olefin metathesis through a fluorine-ruthenium interaction.

The synthesis, structure, and performance of new ruthenium-based olefin metathesis catalysts, featuring fluorinated NHC ligands are presented. The introduction of halogen atoms into the N-heterocyclic carbene ligand profoundly alters the catalytic activity and can afford a more efficient catalyst. Structural investigations suggest that a fluorine-ruthenium interaction is responsible for this increased activity.

Journal Article↗
PubMed2006

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.

Alkenes↗
PubMed2006

Self-assembly with block copolymers through metal coordination of SCS-Pd(II) pincer complexes and pseudorotaxane formation.

Poly(norbornene)-based block copolymers containing side chains of palladated pincer complexes/dibenzo[24]crown-8 or palladated pincer complexes/dibenzylammonium salts were synthesized. Noncovalent functionalization was accomplished with their corresponding recognition units through simple 1:1 addition with association constants (Ka) greater than 10(5) m(-1). The self-assembly processes were monitored by using both 1H NMR spectroscopy and isothermal titration calorimetry. In all cases, we found that the self-assembly of the recognition units along each polymer block does not preclude the self-assembly processes along the other block.

Journal Article↗
PubMed2006

Highly active water-soluble olefin metathesis catalyst.

A novel water-soluble ruthenium olefin metathesis catalyst supported by a poly(ethylene glycol) conjugated saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand is reported. The catalyst displays improved activity in ring-opening metathesis polymerization, ring-closing metathesis, and cross-metathesis reactions in aqueous media.

Alkenes↗
PubMed2006

Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis.

The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3-5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a-4a (to form 2b-4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of < or =1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.

Alkenes↗
PubMed2005

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.

Journal Article↗
PubMed2005

Template-directed olefin cross metathesis.

[reaction: see text] A template containing two secondary dialkylammonium ion recognition sites for encirclement by olefin-bearing dibenzo[24]crown-8 derivatives has been used to promote olefin cross metatheses with ruthenium-alkylidene catalysts. For monoolefin monomers, the rates of metatheses and yields of the dimers are both amplified in the presence of the template. Likewise, for a diolefin monomer, the yield of the dimer is enhanced in the presence of the template under conditions where higher oligomers are not formed.

Alkenes↗
PubMed2005

Magic ring catenation by olefin metathesis.

[reaction: see text]. Olefin metathesis has been employed in the efficient syntheses of a [2]catenane with the templation being provided by the recognition between a secondary ammonium ion and a crown ether. In one approach, a crown ether precursor has been clipped around an NH2+ center situated in a macrocyclic ring, yielding the mechanically interlocked compound. In the other approach, the reversible nature of olefin metathesis allows for a magic ring synthesis to occur wherein two free macrocycles can be employed as the stationary materials, leading to the formation of the same [2]catenane.

Journal Article↗
PubMed2005

Highly selective 1,3-isomerization of allylic alcohols via rhenium oxo catalysis.

Two reaction strategies are developed to promote the highly selective 1,3-isomerization of a variety of allylic alcohols using O3ReOSiPh3 as a catalyst. The first strategy utilizes substrates whose 1,3-regioisomer contains a conjugated alkene, which relies on thermodynamics to obtain high selectivity. The second strategy employs N,O-bis(trimethylsilyl)acetamide as an additive to selectively and irreversibly remove the product from the reaction equilibrium and works well for the isomerization of tertiary allylic alcohols into primary allylic alcohols containing trisubstituted alkene components. High stereoselectivity is also observed in the 1,3-isomerization of enantioenriched allylic alcohols.

Alcohols↗
PubMed2004

A stable four-membered N-heterocyclic carbene.

The synthesis of the first four-membered N-heterocyclic carbene is described. Depending on the substituents on the nitrogen atoms, it is possible to characterize at room temperature the carbene dimer or the free carbene. Crystallographic analyses are provided for these carbene species.

Journal Article↗