Computational analysis of the selective cyclopropanation mode for the PtCl2-catalyzed cycloisomerization of a polyunsaturated precursor

PtCl2-catalyzed cycloisomerization of polyunsaturated precursors derived from hep-1-en-6-yne skeleton, such as 1, and 3, as well as the O-(bromomethyl)dimethylsilyl (2, 4) derivatives, gave mixtures of skeletal rearrangement products and/or polycyclic molecules incorporating a cyclopropyl ring system. The introduction of additional unsaturated moieties on the precursor, such as O-propenyl derivative (15), may give rise to further cycloisomerization processes. However, some critical features on the precursor seem to control and drive the catalytic process to specific products with a high regio- and stereoselectivity. In this paper, we have carried out a computational DFT study for the PtCl2-cycloisomerization of the polyunsaturated O-propenyl precursor in order to propose a mechanism and identify these key properties. Different cyclopropanation modes have been evaluated and the results have revealed that the most favorable pathway involves a 5-exo carbocyclopropanation mode, in agreement with the experimental observations. Electronic and structural factors have been shown to be responsible for the observed selectivity. (c) 2006 Elsevier B.V. All rights reserved.