Relative thermodynamic stabilities of isomeric 1,2-dialkoxyethenes

Abstract

The relative thermodynamic stabilities of the geometrical isomers of a number of 1,2-dialkoxyethenes have been determined by chemical equilibration in the neat liquid and in 1,4-dioxane solution with mercuric acetate as catalyst. From the variation of the value of the equilibrium constant with temperature, the thermodynamic parameters AG ~ AH ~ , and AS e of the E ~ Z isomerization were evaluated. In all cases the Z isomer proved to be thermodynamically the more stable species, its favor increasing with the bulkiness of the alkoxy groups. The thermodynamic data obtained for the E -, Z isomerization of 1,2-dimethoxyethene differ significantly from those reported in the literature. An attempt to correlate the experimental thermodynamic data with MM2 calculations for the title compounds as well as for 1,2-dimethoxypropenes is presented.

Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme

“…The latter is apparently a (kinetically) more effective alkylating agent, allowing the intermediate carbene complex 15 to be observed. Similar transformations have been reported for CpFe(CO) 2 (C(O)CH 2 OMe) ,, as well as unstabilized iron and rhenium carbenes. − Apparently hydride migration is stereoselective, as only trans olefins are produced, even though the cis isomers are thermodynamically preferred; if trace acid is present, some isomerization to the more stable cis species is observed. , …”

17 O, 13 C and 1 H NMR spectra of 1,2‐dialkoxyethenes

“…In the Z-form of the latter ethers, the higher shielding of the O atom, suggesting a weaker strength of conjugation, is reasonable in view of a proposed slightly non-planar gauche conformation of the CwOwCxC moiety.11 On the other hand, the origin of the reversed order of the relative d(17O) values of 1E and 1Z is less obvious : the strength of pÈn interaction appears to be stronger in the more crowded (and probably non-planar) Z-isomer, in line with its higher thermodynamic stability. 5 In ROCHxCHOMe (1È6), the di †erence in d(RO) between the Z-and E-isomers increases, whereas that in d(MeO) decreases with enhancing bulkiness of the RO group. Moreover, the chemical shift of the MeO group of the Z-form is essentially constant, whereas that of the E-form increases slightly in this sequence.…”

“…The relative thermodynamic stabilities of the geometrical isomers of the title compounds have been studied previously. 5 According to an IR and Raman spectral study,6 the E-and Z-forms of 1,2-dimethoxyethene (R1xR2xMe) are mixtures of at least two conformers about the OÈ C(sp2) bonds. One of these conformers was concluded to have a planar heavy atom skeleton, at least in the case of the E-isomer [see the s-cisÈs-cis structure shown in Fig.…”

Methoxyethenes: Structures and Conformations

“…We report structure determinations by GED and ab initio calculations for 1,1-dimethoxyethene (1,1-DME), ( Z )-1,2-dimethoxyethene ( Z -1,2-DME), and tetramethoxyethene (TME). The conformational properties of 1,1-DME and Z -1,2-DME have been studied previously by various theoretical methods. − Molecular mechanics and ab initio methods (STO 3G and HF/3-21G) result in different predictions for the most stable conformers of these two compounds. Vibrational spectra for the Z -1,2-DME were interpreted in terms of a mixture of two conformers, one of which possesses a planar structure .…”