ORGN 9 |
| The electrocyclic ring-closing reactions and the ring-closing metathesis (RCM) reactions both provide excellent tools for constructing functionalized carbo- and heterocyclic compounds. Herein, we report ring-closing reactions of allene-enes and allene-ynes, which are promoted by a metal substituent and a metal catalyst. The thermal electrocyclic ring-closing reaction of the cis isomer of conjugated vinylallene having a boryl substituent at the vinylic terminus (4-phenyl-5-borylpenta-1,2,4-triene) giving the corresponding methylenecyclobutene (4-boryl-3-methylene-1-phenylcyclobutene) proceeded unidirectionally, and more importantly, more than 20 times faster than that of the trans isomer of the vinylallene. The dramatic acceleration observed with the cis isomer can be ascribed to electronic participation of the vacant boron p orbital in the transition state SHOMO. A RCM reaction of allene-ynes (octa-1,2-dien-7-ynes) occurred at room temperature in the presence of a catalytic amount of the Schrock's molybdenum alkylidene complex to give ring-closed cyclic alkenes having allenyl substituents. The vinylallene skeletons are constructed by a metathesis-type reaction between the alkyne moiety and the proximal carbon–carbon double bond of the allene moiety. A molybdenum vinylidene species is assumed as the catalytically active species. |
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Allenes in Synthesis
8:00 AM-12:00 PM, Sunday, 26 March 2006 Georgia World Congress Center -- C303/304/305, Oral
Division of Organic Chemistry |