Trans geometry alkenes

The cis-trans geometry of the alkene is influenced by the nature of the substituents, solvent and dissolved salts. Polar protic or aprotic solvents favor the cis isomer. 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

Because the epoxidation takes place in one step, there is no opportunity for the alkene molecule to rotate and change its cis or trans geometry. The epoxide retains whatever stereochemistry is present in the alkene. 

The sodium donates an electron to the LUMO of the triple bond (one of the two orthogonal 7t orbitals). The resulting radical anion can pick up a proton from the ammonia solution to give a vinyl radical. A second electron, supplied again by the sodium, gives an anion that adopts the more stable trans geometry. A final proton quench by a second molecule of ammonia or by an added proton source (t-butanol is often used, as in the Birch reduction) forms the E-alkene. 

Similarly, with the frans-alkene, two products are formed and both retain the trans geometry of the dienophile. But once again a nearly 50 50 mixture of endo and exo products is formed. 

The substituent R prefers an equatorial position as the molecule reacts and R retains this position in the product. The new alkene bond is shown in black and the substituents in green. Notice that the trans geometry of the alkene in the product is already R R... 

For alkene-diene or diene-diene reactions, [2 + 2] additions are often in competition with Diels-Alder reactions the balance is extremely system dependent. Only [2 + 2] products will be formed when the diene is restricted to the (S)-trans geometry, as in the reactions of dienes (82)" and (84)" with TONE to give [2 + 2] adducts (83) and (85). Other conformationally mobile dienes give [2 -t- 2] rather than [2 -t-4] adducts when reacting intramolecularly because the Diels-Alder alternative would lead to relatively unstable products, as in the isomerizations of (86) to (87)" and of (88) to (89)." ... 

Acyclic 1,2-disubstituted alkenes from sulfoxide pyrolyses are obtained with the trans geometry because of increased torsional interaction in the transition state leading to the cis isomer (e.g. equation 22). The stereochemistry of tri- and tetra-substituted alkenes is determined by the stereochemistry of the precursor sulfoxides and is predictable on the basis of the syn elimination mechanism. In practice the stereospeciflc synthesis of such alkenes is limited by the availability of isomerically pure starting materials. 

Control of cis-trans-geometry is well illustrated by catalytic hydrogenation or alkali metal reduction of alkynes, as discussed in Sections 4.8.B and 4.9.C, respectively. The Lindlar catalyst (sec. 4.8.B) allows selective reduction of alkynes to the cis-alkene, as in the conversion of 23 to 24 in Kaiser s synthesis of niphatoxin B. This contrasts sharply with treatment of an alkyne with alkali metals (sec. 4.9.C) to give the... 

Methods were developed to make the 8,9-alkene stereoselectively with either cis-or trans-geometry, prior to glucosylation using tetra-O-benzoyl-a-D-glucopyranosyl bromide and AgOTf7 ... 

This process leads to both cis- [or (Z)] and trans- [or ( )] alkenes and a preference for the final geometry [(Z)/( )j does not seem to be expressed ... 

Seven-membered ring trani-alkenes have been accessed diastereoselectively from dienes and aldehydes by the silver-catalysed transfer of silylene (Scheme 13). The trans geometry of the double bond has been assigned from cleavage of the silicon-oxygen bond (path (a)). The high strain of these cyclic trawi-alkenes can... 

Finally, since the phenyl group is on a wedge and the propyl is on a dash, the resulting alkene must have trans geometry. This is because formation of the alkene occurs simultaneously with the breakdown of the four-membered ring, so the substituents do not have a chance to rotate thus the relative trans configuration on the ring translates to a trans alkene in compound B. 

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