Surprising exocyclic regioselectivity in electrophilic additions to alkylidenecyclobutenes

The addition of peracid, bromine, TCNE and singlet oxygen to alkylidenecyclobutenes shows a strong regioselectivity - in some cases regiospecificity - for the exocyclic double bond. Depending on the reagent, the endocyclic double bond will only undergo reaction upon the addition of a second equivalent of reagent. In the case of the first three reagents, this exocyclic regioselectivity is rationalized by invoking the formation of the more stabilized carbocation intermediate or transition state. Regarding ¹O₂, where polar mechanisms are rare, we attribute this exocyclic regioselectivity to the improper alignment and, hence, lack of reactivity of the endocyclic allylic hydrogen.