1 Supporting Information On the Mechanism of the Cycloaddition Reactions between Ketene and N-Silyl, N-Germyl, and N- Stannylimines: A Theoretical Investigation Pablo Campomanes, M. Isabel Menéndez, and Tomás L. Sordo* Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, C/ Julián Clavería, 8, 33006 Oviedo, Principado de Asturias, Spain