Palladium-catalyzed silylstannylation -cyclization of 1,6-diynes; axial chirality in (Z,Z)-1,3-dienes

The silylstannane-mediated cyclization of mono-substituted alpha,o-diynes is readily catalyzed by palladium in the presence of a weakly coordinating phosphine. The products of the reaction are highly functionalized carbo- or heterocycles. The reaction displays wide functional group compatibility, tolerating groups like carboxylic esters, amines and amides. The reaction is not sensitive to moisture or air, and the isolation of the products is operationally simple.;The 1,2-bis-alkylidenecycloalkane products display very interesting stereochemical features. First, the silyl and stannyl groups are pointing toward each other, despite the apparent steric hindrance thus generated. Second, this stereochemistry of the individual double-bonds can lead to axial chirality in the diene system. It has been shown that the 1,2-bis-alkylidenecycloalkanes are indeed axially chiral at low temperatures, but they display a fluxional behavior at room temperature. The coalescence temperatures have been determined for several compounds, ranging from -70°C to +20°C, giving insight into the exchange process and the influence of various substituents. Also, the energies of activation of the exchange process have been determined via NMR Line Shape Analysis for a selected set of compounds. These energies appear to depend not only on the groups on the silicon and the tin, but also on the cyclopentane ring substituents. Free energies of activations in the range of 52.3 to 56.9 kJ.mol-1 were observed.;The synthetic applications of these compounds have been the object of preliminary studies. Although many reactions did not yield the desired products, tin-lithium and tin-halogen exchange and epoxidation of the alkenes seem to offer some promise. Diels-Alder reactions of the destannylated products can be carried out in quantitative yield. The first example of a Stille coupling with a vinyl iodide is also described.