Limits...
Evidence for nonstatistical dynamics in the Wolff rearrangement of a carbene.

Litovitz AE, Keresztes I, Carpenter BK - J. Am. Chem. Soc. (2008)

Bottom Line: The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene.Preliminary molecular dynamics simulations are consistent with this conclusion.Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA.

ABSTRACT
Two (13)C-labeled isomers of the formal Diels-Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

No MeSH data available.


Related in: MedlinePlus

Decomposition of the Singleton rearrangement into three hypothetical electron-pair transfers. The np → σ* transfer identifies the reaction as a pseudopericyclic process.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC3585475&req=5

fig4: Decomposition of the Singleton rearrangement into three hypothetical electron-pair transfers. The np → σ* transfer identifies the reaction as a pseudopericyclic process.

Mentions: In hindsight, one can perhaps rationalize the unexpected facility of the rearrangement by identifying it as a pseudopericyclic17 rather than a truly pericyclic reaction. The hypothetical electron transfers in the pseudopericyclic process are shown schematically in Figure 4. The implication is that for the C−C bond, σ and σ* orbitals are involved in the reaction, while the C=C bond uses π and π* orbitals. However, the C−O bond uses a p-type lone-pair orbital (np) and the σ* orbital instead of the σ and σ* orbitals. The orthogonality between np and σ* leads to the pseudopericyclic classification17a and circumvents the orbital-alignment problems that the truly pericyclic process would seem to face. The implication of this analysis is that the analogous rearrangement would be much more difficult if the reactant had a cyclopropane in place of the oxirane ring. That prediction has not been pursued.


Evidence for nonstatistical dynamics in the Wolff rearrangement of a carbene.

Litovitz AE, Keresztes I, Carpenter BK - J. Am. Chem. Soc. (2008)

Decomposition of the Singleton rearrangement into three hypothetical electron-pair transfers. The np → σ* transfer identifies the reaction as a pseudopericyclic process.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3585475&req=5

fig4: Decomposition of the Singleton rearrangement into three hypothetical electron-pair transfers. The np → σ* transfer identifies the reaction as a pseudopericyclic process.
Mentions: In hindsight, one can perhaps rationalize the unexpected facility of the rearrangement by identifying it as a pseudopericyclic17 rather than a truly pericyclic reaction. The hypothetical electron transfers in the pseudopericyclic process are shown schematically in Figure 4. The implication is that for the C−C bond, σ and σ* orbitals are involved in the reaction, while the C=C bond uses π and π* orbitals. However, the C−O bond uses a p-type lone-pair orbital (np) and the σ* orbital instead of the σ and σ* orbitals. The orthogonality between np and σ* leads to the pseudopericyclic classification17a and circumvents the orbital-alignment problems that the truly pericyclic process would seem to face. The implication of this analysis is that the analogous rearrangement would be much more difficult if the reactant had a cyclopropane in place of the oxirane ring. That prediction has not been pursued.

Bottom Line: The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene.Preliminary molecular dynamics simulations are consistent with this conclusion.Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA.

ABSTRACT
Two (13)C-labeled isomers of the formal Diels-Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

No MeSH data available.


Related in: MedlinePlus