Chemistry

3D visualization of organic reactions with CAVOC

3D visualization of organic reactions with CAVOC


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Literature example [1] by Vedejs et al. - More reactions

Wharton and Johnson (Wharton, P. W. (1978): . In: J. Chem.. 38, 4117) have shown that the equilibrium between compounds IV and V, due to the ring strain in the cyclodecadiene, is clearly on the side of compound IV (Keq= 1,3 · 10-6). That trans-1,2-divinylcyclohexane is thermodynamically favored compared to itsCope rearrangement product: at 40 ° C is the difference in free energies ΔG 8.5 kcal mol-1; under standard conditions ΔG° equal to 12 kcalmol-1and, as already mentioned, this difference is essentially due to the hoop stress energy of connection V.

Vedjes and Cammers-Goodwin have carried out experimental and theoretical investigations in order to find out which changes are necessary with regard to the choice of substituents in order to shift the equilibrium to the side of the rearrangement product and thus the so-called Cope ring expansion - as a variant of a [3,3] - sigmatropic rearrangement - to make synthetically usable for the representation of 10-membered rings.

The Cope rearrangement of 1,2,6-heptatriene to 3-methylene-hexa-1,5-diene is exothermic: This is explained by the conversion of a cumulative double bond (starting material) into a conjugated double bond (product). The energy released during this functional conversion from an allenyl to a 1,3-dienyl unit is estimated by the authors to be about -13 kcal mol-1estimated; see example [1.4]. This estimate is based on the values ​​for the standard enthalpies of formation HB.° for 3-methylbutadiene (HB.°= 30.9 kcal mol-1) compared to isoprene (HB.° = 18.0 kcal mol-1); see example [1.5].

The functional conversion of an allenyl unit into a 1,3-dienyl unit is referred to by the authors as decumulation (from English decumulation). Since the hoop stress energies of cyclodeca-1,5-diene derivatives under standard conditions are 8.5-15.5 kcal mol-1estimated (for compound V: 12 kcal mol-1) and the examples [1.4] and [1.5] show that the so-called decumulation effect releases energies of this magnitude, the authors make the following working hypotheses:

With suitable use of the decumulation effect, it is possible

  1. to compensate for the hoop stress energies in cyclodeca-1,5-diene derivatives and thus
  2. to shift the equilibrium between the constitutional isomers in the direction of theCope ring expansion product and thus
  3. to establish the Cope ring expansion for the preparation of cyclodeca-1,5-diene derivatives as a synthetic method.


Video: STRUCTURAL REPRESENTATION OF ORGANIC COMPOUNDS (May 2022).