Stereoselective polycyclisations of allyl and enyne silanes: evidence for a bicyclo[3.2.0]hept-1(7)ene structure

The intramolecular copper(I)-catalyzed [2+2]-photocycloaddition of diphenyldiallylsilane (1) (or tetraallylsilane (4)) led to sila-3-bicyclo[3.2.0]heptane (3) (or to spiro analogous 6) in the cis (or cis–cis) configuration whereas the α,ω-diiodide (2) obtained by cyclozirconation of 1 (or from the homologous tetraiodide 5) followed by addition of n-BuLi, produced the sila-3-bicyclo[3.2.0]heptane (3) (or to the spiro analogous 6) in the trans (or trans–trans configuration). The same cyclozirconation reaction, starting from the hetero enyne 7, selectively led to the highly strained silacyclobutene moiety 15 which represents the first stable hetero bicyclo[3.2.0]hept-1(7)ene skeleton, hypothetical intermediate in metathesis reactions.