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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

Summary of MPWB1K results comparing the originally intended retro-Diels−Alder reaction (dashed curve) with the Singleton rearrangement (solid black curve). The principal and secondary mechanisms for formation of acetylmethylketene are shown by the red and blue pathways, respectively.
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fig3: Summary of MPWB1K results comparing the originally intended retro-Diels−Alder reaction (dashed curve) with the Singleton rearrangement (solid black curve). The principal and secondary mechanisms for formation of acetylmethylketene are shown by the red and blue pathways, respectively.

Mentions: If taken literally, the curved arrows in Scheme 6 (which were not proposed by Singleton) would imply that the first step was a poorly precedented σ2s + σ2s + π2s pericyclic reaction. Furthermore, the orbital alignment of the breaking C−O and C−C bonds would seem to be very unfavorable. Nevertheless, preliminary DFT calculations carried out by Singleton suggested a barrier for the rearrangement that was far lower that the one for the intended retro-Diels−Alder reaction of the precursor. We confirmed that conclusion with calculations at the MPWB1K/6-31+G(d,p) level. The computed enthalpies at 298 K are shown in Figure 3.


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

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

Summary of MPWB1K results comparing the originally intended retro-Diels−Alder reaction (dashed curve) with the Singleton rearrangement (solid black curve). The principal and secondary mechanisms for formation of acetylmethylketene are shown by the red and blue pathways, respectively.
© Copyright Policy
Related In: Results  -  Collection

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

fig3: Summary of MPWB1K results comparing the originally intended retro-Diels−Alder reaction (dashed curve) with the Singleton rearrangement (solid black curve). The principal and secondary mechanisms for formation of acetylmethylketene are shown by the red and blue pathways, respectively.
Mentions: If taken literally, the curved arrows in Scheme 6 (which were not proposed by Singleton) would imply that the first step was a poorly precedented σ2s + σ2s + π2s pericyclic reaction. Furthermore, the orbital alignment of the breaking C−O and C−C bonds would seem to be very unfavorable. Nevertheless, preliminary DFT calculations carried out by Singleton suggested a barrier for the rearrangement that was far lower that the one for the intended retro-Diels−Alder reaction of the precursor. We confirmed that conclusion with calculations at the MPWB1K/6-31+G(d,p) level. The computed enthalpies at 298 K are shown in Figure 3.

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