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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

Experimental constraints on the relationship between the rate-constant ratios kb/ka and kc/ka. The dashed lines show the values required for these two ratios if the Wolff rearrangement of carbene 3 were to occur as expected by statistical kinetic models (i.e., with f = 0.5). The horizontal and vertical red lines, respectively, show the independently assessed values of kb/ka and kc/ka from coupled-cluster calculations.
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fig5: Experimental constraints on the relationship between the rate-constant ratios kb/ka and kc/ka. The dashed lines show the values required for these two ratios if the Wolff rearrangement of carbene 3 were to occur as expected by statistical kinetic models (i.e., with f = 0.5). The horizontal and vertical red lines, respectively, show the independently assessed values of kb/ka and kc/ka from coupled-cluster calculations.

Mentions: Simultaneous application of the steady-state approximation to the concentrations of carbene 13by and intermediate 17 allows one to write eq 1:1The results from the two experiments were simultaneously best fit by the conditions [4bx]/([4az]+[4by]) = 5.0 and [4az]/[4by] = 2.6. These constraints and the steady-state result lead to eqs 2 and 3,23which are depicted graphically in Figures 5 and 6. These graphs show that the ratio of mechanistic rate constants kc/ka has an upper bound of 0.2 (above which the ratio kb/ka becomes negative) and a lower bound of ∼0.056 (below which f becomes negative).


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

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

Experimental constraints on the relationship between the rate-constant ratios kb/ka and kc/ka. The dashed lines show the values required for these two ratios if the Wolff rearrangement of carbene 3 were to occur as expected by statistical kinetic models (i.e., with f = 0.5). The horizontal and vertical red lines, respectively, show the independently assessed values of kb/ka and kc/ka from coupled-cluster calculations.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Experimental constraints on the relationship between the rate-constant ratios kb/ka and kc/ka. The dashed lines show the values required for these two ratios if the Wolff rearrangement of carbene 3 were to occur as expected by statistical kinetic models (i.e., with f = 0.5). The horizontal and vertical red lines, respectively, show the independently assessed values of kb/ka and kc/ka from coupled-cluster calculations.
Mentions: Simultaneous application of the steady-state approximation to the concentrations of carbene 13by and intermediate 17 allows one to write eq 1:1The results from the two experiments were simultaneously best fit by the conditions [4bx]/([4az]+[4by]) = 5.0 and [4az]/[4by] = 2.6. These constraints and the steady-state result lead to eqs 2 and 3,23which are depicted graphically in Figures 5 and 6. These graphs show that the ratio of mechanistic rate constants kc/ka has an upper bound of 0.2 (above which the ratio kb/ka becomes negative) and a lower bound of ∼0.056 (below which f becomes negative).

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