Do Antarafacial Cycloadditions Occur? Cycloaddition of Heptafulvalene with Tetracyanoethylene

The cycloaddition of heptafulvalene (1) with tetracyanoethylene (TCNE) was previously described as an example of an antarafacial cycloaddition, a [pi(14)(a)+pi(2)(s)] 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.

Automated summary

The previously proposed edge-to-face cycloaddition mechanism was found to be not a concerted process. At low temperature, a mixture of cis and trans cycloadducts as well as other cycloadducts was observed. Upon warming to room temperature, the mixture of products converted to trans cycloadducts. The formation of cis cycloadducts was confirmed by additional studies.