Do Antarafacial Cycloadditions Occur? Cycloaddition of Heptafulvalene with Tetracyanoethylene |
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Authors: | Anthony R Izzotti Prof Dr James L Gleason |
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Institution: | Department of Chemistry, McGill University, 801 Sherbrooke W., Montreal, QC, H3A 0B8 Canada |
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Abstract: | The cycloaddition of heptafulvalene ( 1 ) with tetracyanoethylene (TCNE) was previously described as an example of an antarafacial cycloaddition, a π14a+π2s] process that afforded only the trans cycloadduct by virtue of the edge-to-face approach of TCNE, facilitated by the S shape of 1 . The reaction has been investigated in depth and found not to be a concerted antarafacial process. At low temperature, the reaction is observed to give a mixture of cis and trans cycloadducts as well as a 4+2] cycloadduct. The mixture of products is converted to the trans cycloadduct by equilibration upon warming to room temperature. Studies with diethyl 2,3-dicyanofumarate and -maleate confirmed the formation of cis cycloadducts. DFT studies at the M06-2X/6-311+G(2d,p) SCRF=acetone level of theory show that the originally proposed edge-to-face approach of TCNE to 1 is highly disfavored, whereas a stepwise mechanism involving the addition of TCNE at C2 to form a zwitterion followed by collapse at either C2′ or C7′ is energetically accessible. The Diels-Alder adduct is also formed in a stepwise reaction by competitive addition of TCNE at C4 of 1 . These studies suggest that edge-to-face interactions are prohibitive in even the most favorable cases. |
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Keywords: | antarafacial reactions computational chemistry cycloaddition heptafulvalene Woodward-Hoffmann rules |
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