Inorganic chemistry, thedi-π-methane rearrangement is thephotochemicalrearrangement of amolecule that contains twoπ-systems separated by a saturatedcarbon atom. In thealiphatic case, this molecules is a1,4-diene; in thearomatic case, anallyl-substitutedarene. The reaction forms (respectively) an ene- or aryl-substitutedcyclopropane. Formally, it amounts to a1,2 shift of one ene group (in thediene) or the aryl group (in the allyl-aromatic analog), followed by bond formation between the lateral carbons of the non-migrating moiety:[1][2]
This rearrangement was originally encountered in thephotolysis ofbarrelene to givesemibullvalene.[3] Once the mechanism was recognized as general byHoward Zimmerman in 1967, it was clear that the structural requirement was two π groups attached to ansp3-hybridized carbon, and then a variety of further examples was obtained.
One example was the photolysis of Mariano's compound, 3,3‑dimethyl-1,1,5,5‑tetraphenyl-1,4‑pentadiene. In thissymmetric diene, the active π bonds areconjugated to arenes, which does not inhibit the reaction.[4][5][6]
Another was the asymmetric Pratt diene. Pratt's diene demonstrates that the reaction preferentially cyclopropanates aryl substituents, because the reaction pathway preserves theresonant stabilization of abenzhydrylic radical intermediate.[7]
The barrelene rearrangement is more complex than the Mariano and Pratt examples since there are twosp3-hybridized carbons. Each bridgehead carbon has three (ethylenic) π bonds, and any two can undergo the di‑π-methane rearrangement. Moreover, unlike the acyclic Mariano and Pratt dienes, the barrelene reaction requires atriplet excited state. Thusacetone is used in the barrelene reaction; acetone captures the light and then delivers triplet excitation to the barrelene reactant. In the final step of the rearrangement there is aspin flip, to provide paired electrons and a newσ bond.
The dependence of the di-π-methane rearrangement on themultiplicity of the excited state arises from thefree-rotor effect.[8] Triplet 1,4-dienes freely undergocis-trans interconversion of diene double bonds (i.e. free rotation). In acyclic dienes, this free rotation leads to diradical reconnection,short-circuiting the di-π-methane process. Singlet excited states do not rotate and may thus undergo the di-π-methane mechanism. 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.