In organic chemistry, the di-π-methane rearrangement is the photochemical rearrangement of a molecule that contains two π-systems separated by a saturated carbon atom. In the aliphatic case, this molecules is a 1,4-diene; in the aromatic case, an allyl-substituted arene. The reaction forms (respectively) an ene- or aryl-substituted cyclopropane. Formally, it amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog), followed by bond formation between the lateral carbons of the non-migrating moiety:[1]
This rearrangement was originally encountered in the photolysis of barrelene to give semibullvalene.[2] 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.
One example was the photolysis of Mariano's compound, 3,3dimethyl-1,1,5,5tetraphenyl-1,4pentadiene. In this symmetric diene, the active π bonds are conjugated to arenes, which does not inhibit the reaction.[3] [4] [5]
Another was the asymmetric Pratt diene. Pratt's diene demonstrates that the reaction preferentially cyclopropanates aryl substituents, because the reaction pathway preserves the resonant stabilization of a benzhydrylic radical intermediate.[6]
The barrelene rearrangement is more complex than the Mariano and Pratt examples since there are two sp3-hybridized carbons. Each bridgehead carbon has three (ethylenic) π bonds, and any two can undergo the diπ-methane rearrangement. Moreover, unlike the acyclic Mariano and Pratt dienes, the barrelene reaction requires a triplet excited state. Thus acetone 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 a spin flip, to provide paired electrons and a new σ bond.
The dependence of the di-π-methane rearrangement on the multiplicity of the excited state arises from the free-rotor effect.[7] Triplet 1,4-dienes freely undergo cis-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.