Merging of 4+2 and 2+4 cycloaddition paths in the regiospecific dimerization of methacrolein. A case of concerted crypto-diradical cycloaddition

The regiospecific dimerization of methacrolein takes place through a very asynchronous and symmetrical transition structure, which shows a merging of the 4+2 and 2+4 cycloaddition paths. The geometrical features of the transition structures correspond well to a case of diradical formation. A cage of secondary orbital interactions restricts the flexibility of the diradical transition structure and stabilizes it.