Di-π-methane rearrangement

Discovery

This rearrangement was originally encountered in the photolysis of barrelene to give semibullvalene. 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.

Notable examples

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.

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.

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.

As excited-state probe

The dependence of the di-π-methane re­arrange­ment on the multiplicity of the excited state arises from the free-rotor effect. 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.

References

95. "The Di-pi-Methane Rearrangement of Naphthobarrelenes. Mechanistic and Exploratory Organic Photochemistry. L." Zimmerman, H. E.; Bender, C. O. J. Am. Chem. Soc., 1970, 92, 4366-4376.
96. "Electron Delocalization as a Controlling Factor in the Di-pi-Methane Rearrangement. Exploratory and Mechanistic Organic Photochemistry. LXII, Zimmerman, H. E.; Baum, A. A. J. Am. Chem. Soc., 1971, 93, 3646-3653.
97. "Competitive Photochemical Pathways in the Di-pi-Methane Rearrangement. Exploratory and Mechanistic Organic Photochemistry. LXIII," Zimmerman, H. E.; Hackett, P.; Juers, D. F.; McCall, J. M.; Schröder, B. J. Am. Chem. Soc., 1971, 93, 3653-3662.
98. "The Stereochemistry of the Di-pi-Methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry. LXIX," Zimmerman, H. E.; Baeckstrom, P.; Johnson, T.; Kurtz, D. W. J. Am. Chem. Soc., 1972, 94, 5504-5505.
99. "An Unexpected Sigma Plus Pi Rearrangement of a Di-pi-Methane Reactant. Inhibition of the Di-p-Methane Rearrangement. Mechanistic and Exploratory Organic Photochemistry. LXX," Zimmerman, H. E.; Pincock, J. A. J. Am. Chem. Soc., 1972, 94, 6208-6209.
100. "The Di-p-Methane and Oxa-di-pi-Methane Rearrangements," Hixson, S. S.; Mariano, P. S.; Zimmerman, H. E. Chem. Revs, 1973, 73, 531-551.
101. "Five Decades of Mechanistic and Exploratory Organic Photochemistry", Zimmerman, H. E., Pure and Appl. Chem., 2006, 2193-2203. Porter Award Address. --

References

1. "di-π-methane rearrangement".
2. (1996). "Synthetic Aspects of the Di-{{pi}}-methane Rearrangement". Chemical Reviews.
3. (1966). "The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene". J. Am. Chem. Soc..
4. (1967). "Mechanistic organic photochemistry. XXIV. The mechanism of the conversion of barrelene to semibullvalene. A general photochemical process". Journal of the American Chemical Society.
5. (1969). "The Di-{{pi}}-methane Rearrangement. Interaction of Electronically Excited Vinyl Chromophores". J. Am. Chem. Soc..
6. (1973). "The Di-{{pi}}-methane and Oxa-di-{{pi}}-methane rearrangements". Chemical Reviews.
7. (1970). "Unsymmetrical Substitution and the Direction of the Di-{{pi}}-methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry. LVI". J. Am. Chem. Soc..
8. (1986). "Di-{{pi}}-methane Rearrangement of Highly Sterically Congested Molecules: Inhibition of Free Rotor Energy Dissipation. Mechanistic and Exploratory Organic Photochemistry". J. Org. Chem..