Tetrasubstituted ethenes

There are enthalpy-of-formation data for only two tetrasubstituted ethenes (2,3-dimethyl-2-butene and 2,3-dimethyl-2-pentene). Although homologous, we will not... 

Similar rate decelerations in liquid-crystalline solvents have been observed for the thermal cis trans isomerization of a bulky tetrasubstituted ethene in cholesteric phases [728], On the other hand, the activation parameters for the thermal cis trans isomerization of less-dipolar substituted azobenzenes show no dependence on the solvent order. This indicates that the cis isomers and their corresponding activated complexes present a similar steric appearance to the solvent environment [729]. This result is more consistent with an isomerization mechanism which proceeds by inversion rather than by rotation cf. Eq. (5-40) in Section 5.3.2 and [527-529, 561]. The latter reaction represents a nice example of the use of liquid-crystalline solvents as mechanistic probes [729]. 

Figure 16.9 Dissociation energy for the central CC bond in tetrasubstituted ethenes R2C = CR2 at (U)TPSS-D3/def2-TZVP levels. Open circles denote data from single-point calculations without D3 dispersion corrections and illustrate the dramatic effects of...

Titanium (iv) chloride with zinc in pyridine has been found to couple ketones reductively to afford symmetrical tetrasubstituted ethenes. Unsymmetrical alkenes can be synthesized in useful yields by titanium-induced ketone coupling if the less reactive component is used in excess. Cycloalkenes (ring size 4—16) are prepared in good yield by intramolecular coupling of the corresponding alkanedione. ... 

More complex than ethene but amenable to a similar attack is the fascinating molecule orthogonene. This is so named because in this C14 molecule four C2 clamps hold the C(, tetrasubstituted double bond moiety twisted through ca. 90° ... 

Alkenes exhibit only weak fluorescence, which is thought to involve emission from the 7t,R(3s) state. On excitation at 185 nm, the emission maximum moves from 231 to 263 nm with increasing alkyl substitution, and the fluorescence quantum yield increases from 1 x 10 to 1.5 x 10". Ethene itself does not fluoresce. The excitation maximum increases with increasing wavelength, moving from 195 nm for disubstituted to 229 nm for tetrasubstituted alkenes. The fluorescence lifetime is estimated at 15 psec. 

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