Trans-Disubstituted alkenes

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

As regards the regioselectivity of monosubstituted, cis and trans disubstituted alkenes, bromination most frequently exhibits anti-Markovnikov behaviour. Only in the case of propene and 1-butene, i.e. when the double bond bears only one linear substituent, bromination in methanol is predominantly but not completely Markovnikov. Steric effects obviously play an important role in determining the... 

The process is stereoselective. When applicable, a trans disubstituted alkene will be favored over a cis disubstituted alkene. 

Regioselectivity with cis and trans disubstituted alkenes. The large-group non-bonded effect... 

The most characteristic vibrational modes of alkenes are the out-of-plane C—H bending vibrations between 1000 and 650 cm-1. These bands are usually the strongest in the spectra of alkenes. The most reliable bands are those of the vinyl group, the vinylidene group, and the trans-disubstituted alkene. Alkene absorption is summarized in Appendix Tables D-l and D-2. 

An 11 kJ/mol (2.7 kcal/mol) stability difference is typical between a monosubstituted alkene (but-l-ene) and a trans-disubstituted alkene (frans-but-2-ene). In the following equations, we compare the monosubstituted double bond of 3-methylbut-l-ene with the trisubstituted double bond of 2-methylbut-2-ene. The trisubstituted alkene is more stable by 14 kJ/mol (3.4 kcal/mol). 

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans disubstituted, the dipole moment is zero. For example, n. v-but-2-ene has a nonzero dipole moment, but iran.v-but-2-ene has no measurable dipole moment. 

The Sharpless asymmetric dihydroxylation works best for trans disubstituted alkenes, while the Jacobsen epoxidation works best for cis disubstituted alkenes. Even In this small area, there is a need for better and more general methods. Organic chemistry has a long way to go. 

Semireduction of internal alkynes in the presence of a transition metal catalyst (e.g., Ni2B, Pd/C) provides disubstituted cw-alkenes. On the other hand, dissolving metal reduction of alkynes or reduction of propargylic alcohols with LiAlH4 or with Red-Al [sodium bis(2-methoxyethoxy)aluminum hydride] furnishes tran -disubstituted alkenes. ... 

Enolisable aldehydes such as 101 or 103 do not give quite such good yields but the ees are still good and the diastereoselectivity in favour of the trans epoxides 102 and 104 is excellent. The secret of this method is the simple preparation of the reagent 96. In the next chapters you will see that superior catalytic methods are available for asymmetric epoxidation of allylic alcohols and of m-alkenes but they are less good for the trans disubstituted alkenes that would give 97, 102, or 104. You will also see catalytic versions of sulfur ylid epoxidation. 

The first examples of the enantioselective Simmons-Smith cyclopropanations mediated by a chiral catalyst are very recent. Scheme 6.33 shows three catalysts for the cyclopropanation of rrans-cinnamyl alcohol. The most selective appears to be Charette s dioxaborolane (Scheme 6.33c, [120-122], which also affords the highest yield of product, although this procedure is only suitable for small scale.With other olefins, such as cis and trans disubstituted alkenes and P,P-trisubstituted alkenes, the yields are nearly as good and the enantioselectivities are 96-97%. An important finding in this study [120] was that, in addition to the Lewis acid (boron) that binds the alcohol, a second atom to chelate the zinc is also necessary. In the... 

Asymmetric osmylation. Chiral ligands of /V,/V -dialkyl bispiperazines linked by two carbons (I, n = 2) can effect highly enantioselective dihydroxylation of trans-disubstituted alkenes. 

The process thus provides a stereoselective route to trans-disubstituted alkenes. If methallyl chloride, CH2 =C(CH3)CH2C1, is used in place of allyl chloride, trisubstituted olefins can be prepared, usually as a mixture of E- and Z-isomers (6) and (7). 

Both 1 and 2 react with monosubstituted and tran -disubstituted alkenes to form complexes that give Bic-diamines on reductive cleavage (equation III). The reaction involves stereospecific cw-addition of the reagent. ... 

An 11 kJ/mol (2.7 kcal/mol) stability difference is typical between a monosubstituted alkene (1-butene) and a trans-disubstituted alkene (rrans-2-butene). In the following... 

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole... 

The chemical shifts of sp -hybridized carbon atoms in the vicinity of double bonds can be estimated using the additivity rule given on page 78. The conformational correction factors, K, for y-substituents of cis- vs. trans-disubstituted alkenes differ by 6 ppm because the relative position of these substituents is fixed by the double bond. 

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