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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

Schematic representation of a bifurcated potential energy surface that could in principle allow preferential reaction via the less-stable carbene.
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fig2: Schematic representation of a bifurcated potential energy surface that could in principle allow preferential reaction via the less-stable carbene.

Mentions: One conceivable explanation could have been that following the rate-determining rDA transition state a bifurcation in the reaction path occurred,15 yielding branches leading monotonically down in PE to the two carbenes, as illustrated schematically in Figure 2. Under such circumstances, the yields of the carbenes need not be determined by their relative energies,16 although it had to be admitted that outside of an ill-defined dynamical effect, no obvious reason for preferential formation of carbene 13 could be offered.


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

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

Schematic representation of a bifurcated potential energy surface that could in principle allow preferential reaction via the less-stable carbene.
© Copyright Policy
Related In: Results  -  Collection

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

fig2: Schematic representation of a bifurcated potential energy surface that could in principle allow preferential reaction via the less-stable carbene.
Mentions: One conceivable explanation could have been that following the rate-determining rDA transition state a bifurcation in the reaction path occurred,15 yielding branches leading monotonically down in PE to the two carbenes, as illustrated schematically in Figure 2. Under such circumstances, the yields of the carbenes need not be determined by their relative energies,16 although it had to be admitted that outside of an ill-defined dynamical effect, no obvious reason for preferential formation of carbene 13 could be offered.

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