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Intramolecular Cycloadditions of Cyclobutadiene with Dienes:  Experimental and Computational Studies of the Competing (2 + 2) and (4 + 2) Modes of Reaction

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journal contribution
posted on 31.12.2003, 00:00 by John Limanto, Kelli S. Khuong, K. N. Houk, Marc L. Snapper
Highly functionalized, cyclobutene-containing adducts are afforded through intramolecular cycloadditions between cyclobutadiene and tethered dienes. The cycloaddition displays the following reactivity trend:  cyclobutadiene serves as a dienophile in intramolecular reactions when it is connected to the diene through a four-atom tether. In cases where a three-atom linker separates the two reaction partners, the cyclobutadiene can function as both a diene and dienophile, affording a mixture of vinylcyclobutane (2 + 2) and cyclohexene-containing cycloadducts (4 + 2). Theoretical studies provide insight into the factors influencing the various pericyclic pathways operative in this system. In cases where cyclobutadiene functions as a diene to generate vinylcyclobutanes, these (2 + 2) adducts can be converted into the corresponding (4 + 2) cyclohexenyl products through a [3,3]-sigmatropic rearrangement.