Alkyne Concentration Research Articles

The reactions of a number of chromium carbene complexes (CO) 5CrC(OMe)R (R = Ph, o-OMePh, p-OMePh, o-O-tBuPh, 1-C 6H 9, 1-C 5H 7O) were examined with a variety of acetylenes (R′CCR 2, R 1, R 2 = H, Me, Et, n-Pr, Ph, SiMe 3) in solvents ranging from low to high coordinating ability. The high selectivity for the benzannulated product from the reactions of simple α,β-unsaturated complexes is not affected by changes in solvent or substituents on the acetylene. The reactions of aryl complexes with acetylenes are quite sensitive to the nature of the solvent and the acetylene. The highest selectivities and efficiencies for the benzannulated product from the reactions of aryl complexes are with solvents of low coordinating ability. Solvents with intermediate coordinating ability and small size give high selectivity for cyclobutenone formation for reactions with disubstituted acetylenes. Solvents with high coordinating ability give the least selective reactions and a considerable amount of indene products. An o-methoxy group on the aryl substituent of the carbene complex can chelate to the metal center during the course of the benzannulation reaction and, in acetonitrile, alter the product distribution in favor of the cyclobutenone product. An amino substituted complex, (CO) 5 CrC(NMe 2)Ph, was found to react with diethylacetylene in THF to give indene products. The reactions of aryl complexes with alkynes can be accelerated by ultraviolet irradiation which results in high selectivity for the benzannulated product at as low a temperature as −78°C, and high selectivity for indene products for those complexes having a chelating group (methoxy) on the aryl ring. Finally, the product distribution from the reaction of the o-methoxyphenyl complex with diethylacetylene was found to be dependent on alkyne concentration, a phenomenon not previously observed for the reactions of carbene complexes and acetylenes.

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