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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

The MPWB1K/6-31+G(d,p) PE profile for retro-Diels−Alder fragmentation of an analogue of precursor 1a. A local minimum was found for the oxirene, but the barrier to its ring opening to the carbene was so low that no transition structure could be located.
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fig1: The MPWB1K/6-31+G(d,p) PE profile for retro-Diels−Alder fragmentation of an analogue of precursor 1a. A local minimum was found for the oxirene, but the barrier to its ring opening to the carbene was so low that no transition structure could be located.

Mentions: Precursor 1a was selected because the work of Lewars and Morrison12 provided good precedent for its synthesis and fragmentation and because MPWB1K/6-31+G(d,p) DFT calculations on a model for compound 1a lacking the carbomethoxy substituents suggested a PE profile (Figure 1) for the reaction sequence in Scheme 1 that might promote nonstatistical dynamics.13 In particular, the ∼16 kcal/mol drop in PE following the rate-determining step and the very small barriers for the ensuing unimolecular rearrangements suggested that the reactive intermediates involved could be generated with substantial selective excitation of certain vibrational modes and that formation of the final products might occur before energy relaxation by way of intramolecular vibrational redistribution (IVR) could reach completion.


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

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

The MPWB1K/6-31+G(d,p) PE profile for retro-Diels−Alder fragmentation of an analogue of precursor 1a. A local minimum was found for the oxirene, but the barrier to its ring opening to the carbene was so low that no transition structure could be located.
© Copyright Policy
Related In: Results  -  Collection

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

fig1: The MPWB1K/6-31+G(d,p) PE profile for retro-Diels−Alder fragmentation of an analogue of precursor 1a. A local minimum was found for the oxirene, but the barrier to its ring opening to the carbene was so low that no transition structure could be located.
Mentions: Precursor 1a was selected because the work of Lewars and Morrison12 provided good precedent for its synthesis and fragmentation and because MPWB1K/6-31+G(d,p) DFT calculations on a model for compound 1a lacking the carbomethoxy substituents suggested a PE profile (Figure 1) for the reaction sequence in Scheme 1 that might promote nonstatistical dynamics.13 In particular, the ∼16 kcal/mol drop in PE following the rate-determining step and the very small barriers for the ensuing unimolecular rearrangements suggested that the reactive intermediates involved could be generated with substantial selective excitation of certain vibrational modes and that formation of the final products might occur before energy relaxation by way of intramolecular vibrational redistribution (IVR) could reach completion.

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