How tethers control the chemo- and regioselectivities of intramolecular cycloadditions between aryl-1-aza-2-azoniaallenes and alkenes.

Cationic 1-aza-2-azoniaallenes react intermolecularly with terminal alkenes to give 1,5-substituted (3 + 2)-cycloadducts, but intramolecular reactions lead to either 1,5- or 1,4-substituted (3 + 2)-cycloadducts or (4 + 2)-cycloadducts, depending on the tether length. DFT calculations and distortion/interaction analyses show that the (CH2)3 tether prevents the reacting partners from aligning efficiently to give 1,5-substituted (3 + 2)-cycloadducts, and the 1,4-regioselectivity dominates. With the (CH2)2 tether, the (3 + 2) cycloaddition is disfavored due to the forming four-membered ring in the transition state, and the (4 + 2) cycloaddition prevails.