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

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 CC 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 CO 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.