Di-π-methane rearrangement

[3] Once the mechanism was recognized as general by Howard Zimmerman in 1967, it was clear that the structural requirement was two π groups attached to an sp3-hybridized carbon, and then a variety of further examples was obtained.

Moreover, unlike the acyclic Mariano and Pratt dienes, the barrelene reaction requires a triplet excited state.

In the final step of the rearrangement there is a spin flip, to provide paired electrons and a new σ bond.

In acyclic dienes, this free rotation leads to diradical reconnection, short-circuiting the di-π-methane process.

For cyclic dienes, as in the barrelene example, the ring structure can prevent free-rotatory dissipation, and may in fact require bond rotation to complete the rearrangement.

Di-π-methane rearrangement
Di-π-methane rearrangement
(Ph2C=C)2CMe2 rearranges to form Ph2C=C(C-CPh2-CMe2-).
Rearrangement of Mariano's diene.
Irradiation of Ph2C=CCMe2C=CMe2 forms Ph2C•(C-CMe2-C-)C•Me2. The central atom can then attack the bond towards the isopropyl group (a) or the benzhydryl group (b). It chooses a, to form Ph2C•C(C•Me2)C=CMe2, which then closes to form a benzhydrilic cyclopropane ring.
Pratt's diene has two possibilities for rearrangement: a and b . It prefers a , because the intermediate diradical is conjugated to the phenyl substituents.
The barrelene to semibullvalene transformation. ISC is an intersystem crossing .