New light on the mechanism of the solid state [2+2] cycloaddition of alkenes: a database analysis

The mechanism of the solid state [2 + 2] cycloaddition of alkenes has been investigated using the structure correlation method based on geometrical data calculated from single crystal X-ray diffraction studies retrieved from the Cambridge Structural Database (CSD). Searches were carried out for non-bonded alkene...alkene reactant interactions, within a limiting C...C separation of the sum of van der Waals radii plus 20%, and for bonded cyclobutane product fragments. The results were visualised and interpreted using principal component analysis and symmetry deformation coordinates. The reaction pathway for [2 + 2] cycloaddition was established and it was shown that ( a) the alkene moieties are not required to be parallel for the reaction to occur and (b) a large twist angle of the reacting alkene fragments is permissible to form a puckered cyclobutane reaction product, as long as similar intra-annular valence angles are maintained around the four-membered ring.