ABSTRACT
The kinetics of enyne metathesis were studied by IR spectroscopy for a variety of alkyne-alkene combinations. The rate law was determined for alkyne-ethylene and alkyne-1-hexene cross metathesis. In the cases examined, greater substitution on the alkyne accelerates the rate of metathesis, and chelation by propargylic esters was ruled out through rate comparison with hydrocarbon alkynes. The findings are discussed in terms of an alkylidene-first reaction mechanism, phosphine-bound ruthenium carbene resting states, and the rate-determining turnover of vinyl carbene intermediates (for alkyne-1-hexene metatheses).
Subject(s)
Alkenes/chemistry , Alkynes/chemistry , Alkenes/chemical synthesis , Kinetics , Structure-Activity RelationshipABSTRACT
A two-step, three-component coupling of an alkyne, enol ether, and vinyl diazoester was accomplished by use of successive metal carbene-catalyzed transformations. This efficient approach to cycloheptadienes is both diastereo- and enantioselective. Kinetic resolution was accomplished on dienol ethers bearing a racemic chiral center at the propargylic position. A model is offered which explains the observed selectivity and accounts for the reactivity difference between trans- and cis-dienol ethers.
ABSTRACT
The stereoselectivity of diene bond formation in the ruthenium-carbene mediated intermolecular enyne metathesis was studied. Initial reaction between an alkyne and 1-hexene gave mixtures of E- and Z-isomers in the newly formed 1,3-diene. However, over time the mixtures equilibrated to form mostly the diene of the E-configuration. To evaluate individual reactivity of the E- and Z-dienes, they were independently synthesized. The E-diene was found to be kinetically-stable under nominal metathesis conditions while the Z-diene isomerized to the E-isomer. The Z-isomeric dienes were found to react with other alkenes to produce a new diene of E-configuration. A secondary metathesis mechanism involving ruthenium alkylidene intermediates was invoked to explain the dynamic stereochemistry observed in this study.
ABSTRACT
[Reaction: see text] The intermolecular enyne metathesis between alkynes and styrene derivatives was developed to study electronic effects in enyne metathesis. A Hammett plot for the overall reaction, catalyst initiation and vinyl carbene turnover was determined with the second generation Grubbs ruthenium carbene catalyst.
Subject(s)
Alkynes/chemistry , Styrenes/chemistry , Catalysis , Hydrocarbons/chemistry , Kinetics , Methane/analogs & derivatives , Methane/chemistry , Molecular Structure , Ruthenium/chemistryABSTRACT
[reaction: see text] Intermolecular enyne metathesis between functional group-rich alkynes and vinyl ethers was promoted by ethylene cometathesis. The concentration of ethylene was optimized to suppress the competing formation of butadiene through background ethylene metathesis. The role of ethylene appears to be both protective and rate enhancing.
Subject(s)
Alkadienes/chemical synthesis , Alkynes/chemistry , Ethylenes/chemistry , Catalysis , Ethers/chemistry , Models, Chemical , Molecular Structure , Ruthenium Compounds/chemistry , Vinyl Compounds/chemistryABSTRACT
[reaction: see text] A new intermolecular metathesis between a variety of alkynes and alkyl enol ethers is described. The best results were obtained with the second generation Grubbs' precatalysts. The reaction occurred with a high degree of regioselectivity and provided electron-rich dienes, which underwent a variety of [4 + 2] cycloadditions.
Subject(s)
Alkynes/chemical synthesis , Cyclization , Electrons , Ethers/chemical synthesis , Indicators and Reagents , Magnetic Resonance Spectroscopy , Ruthenium Compounds/chemistry , StereoisomerismABSTRACT
The powerful Buchwald-Hartwig amination was utilized for the construction of the benzimidazole nucleus with the substituted nitrogen atom bearing a chiral substituent. A successive amination/imination was followed by an acid-catalyzed ring closure step to give the benzimidazole ring. The products were deprotonated and acylated at the C2 position and could be alkylated on nitrogen to give chiral benzimidazolium salts.
