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Issue 2, 2004
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Keteneacetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

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Abstract

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio (G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH2 and NMe2) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol−1 for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for π-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol−1 for X = NMe2. In contrast, the 1,3-shifts of CH3 and H have very high barriers.

Graphical abstract: Ketene–acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

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Publication details

The article was received on 11 Aug 2003, accepted on 11 Nov 2003 and first published on 21 Nov 2003


Article type: Paper
DOI: 10.1039/B309549E
Citation: Org. Biomol. Chem., 2004,2, 195-199
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    Keteneacetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

    R. Koch and C. Wentrup, Org. Biomol. Chem., 2004, 2, 195
    DOI: 10.1039/B309549E

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