Metathesis Step Research Articles

Mechanistic Studies of Olefin Metathesis by Ruthenium Carbene Complexes Using Electrospray Ionization Tandem Mass Spectrometry

The olefin metathesis reaction of the Grubbs ruthenium carbene complexes has been investigated in the gas phase by electrospray ionization tandem mass spectrometry. Relative rates of reaction for substituted ruthenium benzylidenes and alkylidenes after removal of one phosphine ligand were interpreted with the aid of linear free energy analysis and kinetic isotope effects. The experimental observations are consistent with a reaction profile in which the metallacyclobutane structure is a transition state rather than an intermediate, although alternative explanations cannot be wholly ruled out. Electron withdrawal on the carbene moiety is found to accelerate the metathesis reaction when only the metathesis step itself is examined. Quantum chemical calculations at a variety of levels were performed to check for the consistency of the interpretation.

Synthesis of a trans-Fused Tricyclic Ether Using a Novel Differentially Protected Glucal

The construction of the trans-fused 7,6,6-ABC fragment (3) of ciguatoxin 3C, starting from the novel carbohydrate derivative 3-O-benzyl-4-O-tert-butyldimethylsilyl-6-O-trityl-d-glucal, is reported. The formation of the A- and C-rings can be effected by executing two individual ring-closing metathesis steps in a sequential or a one-pot procedure.

Total Synthesis of Epothilone E and Analogues with Modified Side Chains through the Stille Coupling Reaction

Ring-closing metathesis (step a in the scheme below), deprotection (b), Stille coupling (c), and epoxidation (d) are the key steps in the first total synthesis of the naturally occurring epothilone E (1). An advanced intermediate, the product of the olefin metathesis, allows access to a range of epothilone E analogues having different aromatic moieties in the side chain. TBS = tBuMe2Si.

Total Synthesis of Epothilone E and Analogues with Modified Side Chains through the Stille Coupling Reaction

Ring-closing metathesis (step a in the scheme below), deprotection (b), Stille coupling (c), and epoxidation (d) are the key steps in the first total synthesis of the naturally occurring epothilone E (1). An advanced intermediate, the product of the olefin metathesis, allows access to a range of epothilone E analogues having different aromatic moieties in the side chain. TBS = tBuMe2Si.

Reactions of symmetrical and unsymmetrical disilanes in the presence of Cp 2MCl 2/ nBuLi (M = Ti, Zr, Hf)

The reactions of disilanes with catalytic quantities of Cp 2MCl 2/ n BuLi (M = Ti, Zr) and cis-cyclooctene in a disilane/cyclooctene/metal ratio of approximately 30: ≥ 30: 1 exhibited rapid formation of monosilanes and trisilanes. The product distributions produced from H(PhMeSi) 2H, H(BuMeSi) 2H (symmetrical disilanes) and from HPh 2SiSiPhMeH, HPhMeSiSiMe 2H, HPhMeSiSiPrMeH and HPh 2SiSiPhH 2 (unsymmetrical disilanes) were determined by GCMS in all cases and the trisilanes from HPh 2SiSiPhMeH and HPhMeSiSiMe 2H were isolated and characterized spectroscopically. Major and minor isomers of Ph 4Me 2Si 3H 2 and Ph 5MeSi 3H 2 (formed from HPh 2SiSiPhMeH) and of PhMe 5Si 3H 2 and Ph 2Me 4Si 3H 2 (formed from HPhMeSiSiMe 2H) were identified. Analysis of the product distribution in these reactions indicated that both SiSi bond cleavage and SiH dehydrocoupling of the starting disilane occurs. Rationalization of the product distributions and apparent isomer preferences through both σ-bond metathesis steps and metal silylene intermediates is presented. The rapid reaction of the SiSi bond in H(SiPhMe) x H was found to be characteristic of the disilane and occurred only to a minor extent in the trisilane and was absent in the tetrasilane.

Aromatization of propylene and butadiene over MoO3−SiO2 and WO3−SiO2 catalysts

The activity of various MoO3−SiO2 and WO3−SiO2 catalysts for the aromatization of propylene and butadiene has been investigated. The results obtained show a comparatively high aromatization activity of the catalysts for propylene to benzene and toluene. The direct conversion of butadiene to ethylbenzene, using WO3 on acid treated silica, is considered to be an alternative reaction pathway to the known metathesis step to ethylene and benzene.