Enyne metathesis, efficient methods for the synthesis of 1,3-dienes, scope and mechanism

Enyne cross metathesis with a variety of alkene and alkyne sources have been developed. The enyne metathesis reaction provides an efficient method for the synthesis of substituted 1,3-dienes. Employing the use of ruthenium carbene catalysts the reaction scope was explored with a variety of heteroatom functionality. The developed synthetic methodology was coupled with kinetics data to assist in the investigation of the enyne metathesis mechanism.;The mechanism of enyne metathesis was explored. Catalyst initiation rate was explored with alkene and alkyne substrates. Initiation was much faster with 1-alkenes than with 1-alkenes. The overall reaction rate was determined by infrared spectroscopy. Styrene-alkyne cross metathesis was developed. The reaction rate for a series of substituted styrenes was determined. A Hammett study was used to evaluate the overall reaction, catalyst initiation and catalyst turnover. The Hammett study indicated that the enyne metathesis reaction rate is accelerated with the use of electron-withdrawing substituents on the alkene. However, there is a limit to the observed acceleration. Highly E-selective enyne cross metathesis was explored. Investigation of the reaction course indicated that the reaction initially produced a kinetic mixture of products which resolved to the E isomer over time. The Z-isomer was found to be unstable to the reaction conditions, and crossover of the Z-diene was achieved with internal and terminal alkenes. Catalyst initiation with 3,4-functionalized 1,5-hexadienes was studied by proton NMR. Catalyst initiation was found to produce ruthenium methylidene, not the expected ruthenium alkylidene.;The scope of enyne metathesis was expanded. The reaction of sulfur-containing alkynes was achieved with ethylene at elevated pressures. Enol ethers were found to be competent substrates for enyne metathesis even with in situ generation of 'stable' Fischer carbene catalysts. The reaction was explored for the scope of alkyne substrates and found to be tolerant of various functionality and substitution patterns. In the exploration of the enol ether metathesis reaction it was found that reaction with sulfur-containing alkynes was difficult. Utilizing an atmosphere of ethylene was found to assist catalysis for these troublesome substrates.