Mechanisms and Origins of Periselectivity of the Ambimodal [6 + 4] Cycloadditions of Tropone to Dimethylfulvene.

The mechanisms and selectivities of the cycloadditions of tropone to dimethylfulvene have been investigated with M06-2X and B3LYP-D3 density functional theory (DFT) calculations and quasi-classical direct molecular dynamics simulations. The originally proposed reaction mechanism (Houk) involves a highly peri-, regio-, and stereoselective [6F + 4T] cycloaddition of tropone [4π] to dimethylfulvene [6π], followed by a [1,5] hydrogen shift, and, finally, a second [6 + 4] cycloaddition of tropone [6π] to the cyclopentadiene moiety [4π]. Paddon-Row and Warrener proposed an alternative mechanism: the initial cycloaddition involves a different [6T + 4F] cycloaddition in which fulvene acts as the 4π component, and a subsequent Cope rearrangement produces the formal [6F + 4T] adduct. Computations now demonstrate that the initial cycloaddition proceeds via an ambimodal transition state that can lead to both of the proposed [6 + 4] adducts. These adducts can interconvert through a [3,3] sigmatropic shift (Cope rearrangement). Molecular dynamics simulations reveal the initial distribution of products and provide insights into the time-resolved mechanism of this ambimodal cycloaddition. Competing [4 + 2] cycloadditions and various sigmatropic shifts are also explored.