This rearrangement was originally encountered in the photolysis of barrelene to give semibullvalene.[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.
Notable examplesedit
Rearrangement of Mariano's diene.
One example was the photolysis of Mariano's compound, 3,3‑dimethyl-1,1,5,5‑tetraphenyl-1,4‑pentadiene. In this symmetric diene, the active π bonds are conjugated to arenes, which does not inhibit the reaction.[4][5][6]
Pratt's diene has two possibilities for rearrangement: a and b. It prefers a, because the intermediate diradical is conjugated to the phenyl substituents.
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.[7]
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 di‑π-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 probeedit
The dependence of the di-π-methane rearrangement on the multiplicity of the excited state arises from the free-rotor effect.[8] 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.
^Zimmerman, Howard E.; Armesto, Diego (1996). "Synthetic Aspects of the Di-π-methane Rearrangement". Chemical Reviews. 96 (8): 3065–3112. doi:10.1021/cr910109c. PMID 11848853.
^Zimmerman, H. E.; Grunewald, G. L. (1966). "The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene". J. Am. Chem. Soc. 88 (1): 183–184. doi:10.1021/ja00953a045.
^Zimmerman, Howard E.; Binkley, Roger W.; Givens, Richard S.; Sherwin, Maynard A. (1967). "Mechanistic organic photochemistry. XXIV. The mechanism of the conversion of barrelene to semibullvalene. A general photochemical process". Journal of the American Chemical Society. 89 (15): 3932–3933. doi:10.1021/ja00991a064. ISSN 0002-7863.
^Zimmerman, H. E.; Mariano, P. S (1969). "The Di-π-methane Rearrangement. Interaction of Electronically Excited Vinyl Chromophores". J. Am. Chem. Soc. 91: 1718–1727. doi:10.1021/ja01035a021.
^Hixson, Stephen S.; Mariano, Patrick S.; Zimmerman, Howard E. (1973). "The Di-π-methane and Oxa-di-π-methane rearrangements". Chemical Reviews. 73 (5): 531. doi:10.1021/cr60285a005.
^Zimmerman, H. E.; Pratt, A. C (1970). "Unsymmetrical Substitution and the Direction of the Di-π-methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry. LVI". J. Am. Chem. Soc. 92: 6259–6267. doi:10.1021/ja00724a026.
^Zimmerman, H. E.; Schissel, D. N (1986). "Di-π-methane Rearrangement of Highly Sterically Congested Molecules: Inhibition of Free Rotor Energy Dissipation. Mechanistic and Exploratory Organic Photochemistry". J. Org. Chem. 51: 196–207. doi:10.1021/jo00352a013.
December 15, 2023
methane, rearrangement, organic, chemistry, methane, rearrangement, photochemical, rearrangement, molecule, that, contains, systems, separated, saturated, carbon, atom, aliphatic, case, this, molecules, diene, aromatic, case, allyl, substituted, arene, reactio. In organic chemistry the di p methane rearrangement is the photochemical rearrangement of a molecule that contains two p 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 2 Di p methane rearrangementContents 1 Discovery 2 Notable examples 3 As excited state probe 4 ReferencesDiscovery editThis rearrangement was originally encountered in the photolysis of barrelene to give semibullvalene 3 Once the mechanism was recognized as general by Howard Zimmerman in 1967 it was clear that the structural requirement was two p groups attached to an sp3 hybridized carbon and then a variety of further examples was obtained Notable examples edit nbsp Rearrangement of Mariano s diene One example was the photolysis of Mariano s compound 3 3 dimethyl 1 1 5 5 tetraphenyl 1 4 pentadiene In this symmetric diene the active p bonds are conjugated to arenes which does not inhibit the reaction 4 5 6 nbsp Pratt s diene has two possibilities for rearrangement a and b It prefers a because the intermediate diradical is conjugated to the phenyl substituents Another was the asymmetric Pratt diene Pratt s diene demonstrates that the reaction preferentially cyclopropanates aryl substituents because the reaction pathway preserves the resonant stabil ization of a benzhydrylic radical inter mediate 7 nbsp The barrelene to semibullvalene transformation ISC is an intersystem crossing 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 p bonds and any two can undergo the di p methane rearrangement Moreover unlike the acyclic Mariano and Pratt dienes the barrelene reaction requires a triplet excited state Thus acetone is used in the barrel ene 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 s bond As excited state probe editThe dependence of the di p methane re arrange ment on the multiplicity of the excited state arises from the free rotor effect 8 Triplet 1 4 dienes freely undergo cis trans inter conversion of diene double bonds i e free rotation In acyclic dienes this free rotation leads to diradical reconnection short circuiting the di p methane process Singlet excited states do not rotate and may thus undergo the di p 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 edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 di p methane rearrangement doi 10 1351 goldbook D01745 Zimmerman Howard E Armesto Diego 1996 Synthetic Aspects of the Di p methane Rearrangement Chemical Reviews 96 8 3065 3112 doi 10 1021 cr910109c PMID 11848853 Zimmerman H E Grunewald G L 1966 The Chemistry of Barrelene III A Unique Photoisomerization to Semibullvalene J Am Chem Soc 88 1 183 184 doi 10 1021 ja00953a045 Zimmerman Howard E Binkley Roger W Givens Richard S Sherwin Maynard A 1967 Mechanistic organic photochemistry XXIV The mechanism of the conversion of barrelene to semibullvalene A general photochemical process Journal of the American Chemical Society 89 15 3932 3933 doi 10 1021 ja00991a064 ISSN 0002 7863 Zimmerman H E Mariano P S 1969 The Di p methane Rearrangement Interaction of Electronically Excited Vinyl Chromophores J Am Chem Soc 91 1718 1727 doi 10 1021 ja01035a021 Hixson Stephen S Mariano Patrick S Zimmerman Howard E 1973 The Di p methane and Oxa di p methane rearrangements Chemical Reviews 73 5 531 doi 10 1021 cr60285a005 Zimmerman H E Pratt A C 1970 Unsymmetrical Substitution and the Direction of the Di p methane Rearrangement Mechanistic and Exploratory Organic Photochemistry LVI J Am Chem Soc 92 6259 6267 doi 10 1021 ja00724a026 Zimmerman H E Schissel D N 1986 Di p methane Rearrangement of Highly Sterically Congested Molecules Inhibition of Free Rotor Energy Dissipation Mechanistic and Exploratory Organic Photochemistry J Org Chem 51 196 207 doi 10 1021 jo00352a013 Retrieved from https en wikipedia org w index php title Di p methane rearrangement amp oldid 1168734971, wikipedia, wiki, book, books, library,
