Mechanism and stereoselectivity of the stepwise 1,3-dipolar cycloadditions between a thiocarbonyl ylide and electron-deficient dipolarophiles: a computational investigation.
The 1,3-dipolar cycloadditions of a thiocarbonyl ylide to dimethyl 2,3-dicyanofumarate and 2,3-dicyanomaleate have been studied with DFT computations. The first C-C bond is formed via an anti attack to produce a very polar, zwitterionic diradical. The low stereoselectivity of the reaction was found to arise from rotations about single bonds in the intermediates that compete with cyclization. A distortion-interaction energy analysis was performed to compare the stepwise and concerted mechanisms, and to explain why the stepwise mechanism is favored in this unusual case.
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States.
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