π4s + π2s cycloadditions: Allyl anions plus ethylene

Abstract

Ab initio calculations failed to locate the expected transition structure with Cs symmetry for the concerted π4s + π2s cycloaddition of allyl anion with ethylene. Unsymmetrical structures were found that satisfied the normal criteria for first order saddle points, i.e. one imaginary frequency. Similarly, cycloadditions between ethylene with 2-fluoroallyl anion, 2-azaallyl anion CH 2NCH 2(−), and the lithium-complexed 2-azaallyl system CH 2NCH 2Li failed to generate first order transition structures with Cs symmetry. Calculations at the MP2(FC)/6-311++G(d) level show that allyl anion prefers a structure with C2 symmetry as opposed to the classical (planar) C2v structure.