A Highly Selective Rearrangement of a Housane-Derived Cation Radical: An Electrochemically Mediated Transformation

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Abstract
To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14 •+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained σ framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.

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