Alkenes trans-substituted

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

Trans -substituted diarylalkenes undergo oxidative cleavage upon treatment by potassium permanganate in the presence of moist alumina as a solid support (equation 32)158. Under the same conditions, cyclic alkenes, with medium-sized rings, give acyclic dialdehydes (equation 33). 

The use of prochiral alkenes such as propene, 2-butene, tran -dideutereoethylene, dialkyl fumarates, and trani-dimethoxyethylenes, have allowed detailed structural and mechanistic studies of alkene complexes. The diastereomers produced upon binding of prochiral alkenes to CpM(CO)X centers provided the key complexes to prove that interconversions occurred by rotations about the metal-(C=C) axis. Thus observations that neither the chirality at the metal nor at the alkene is changed in the rearrangement of a trans-substituted alkene provided proof that the nature of the dynamic process was a propeller rotation. Note that in equation (9) the equilibrium between (20) and (21) averages enviromnents b and d separately from a and c. A key feature is that olefin rotation does not alter the chirahty at the olefin because of the olefin-metal bond. 

The enantioselectivities obtained in the first catalytic AD process (Table 2) reveal that the best results are obtained with trans-substituted alkenes with at least one aromatic substituent (entries 2 and 3). If the aromatic substituent is moved further away from the alkene, the ee drops sharply (entry 6). Furthermore, terminal alkenes give moderate ee provided a directly bond aryl group is present (entry 1). 1,1-Disubstituted (entry 4) and frans-dialkyl-substituted alkenes... 

Bromine addition in [BMIM][PF6] and [BMIM][BF4] is a stereospecific anti-addition process with dialkyl substituted alkenes, alkyl substituted alkynes and trans-stilbenes, whereas ds-stilbenes and aryl alkynes give mixtures of syn- and anti-addition products, although in the case of dx-diaryl substituted olefins the anti-stereoselectivity is generally higher than in chlorinated solvents. In the case of diaryl substituted olefins, such as stilbenes, it has been shown that stereoselectivity in molecular solvents depends primarily on two factors (i) the nature of the intermediates and (ii) the lifetime of the ionic intermediates [53]. Bridged bromiranium... 

The ate complexes that result may be converted to vinyl halides or a,p-unsaturated carboxylic acids by procedures analogous to those employed in the previous examples. Organoaluminum reagents possess the characteristic of versatility, in that an alkyne may be converted stereospecifically to a cis- or trans-substituted alkene by appropriate choice of reagents. In the case of a,)8-unsaturated acids, the flexibility is even more pronounced, in that it is simply the order of addition of reagents that determines the stereochemistry of the product. 

Two types of effects can be suggested a) different stability of isomeric cis- and trans-substituted alkenes and b) different reactivity of two identical groups positioned cis and trans to a o-acceptor or a o-donor group (Figure 7.3). The first phenomenon is the basis of the well-known ds-effect discussed in the next section. The second phenomenon is surprisingly scarcely documented and will not be discussed in this book. 

Nucleophilic attack on the 18-electron Fp(alkene) cations cannot proceed by coordination of the nucleophile to the metal unless displacement of a ligand also occurs (as with iodide). Generally, however, nucleophiles add to the alkene, yielding substituted alkyl derivatives (Fig. 7.9). Such addition always takes place stereospecifically trans to the metal. The Fp group can be removed from Fp—R in... 

Typically, the thermodynamically most stable alkene or alkene mixture results from unimolecular dehydration in the presence of acid. Thus, whenever possible, the most highly substituted system is generated if there is a choice, trans-substituted alkenes predominate over the cis isomers. For example, acid-catalyzed dehydration of 2-butanol furnishes the equilibrium mixture of butenes, consisting of 74% trans-2- sa sae, 23% of the cis isomer, and only 3% 1-butene. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

Part B of Table 1.5 gives heats of formation for the C4, C5, and some of the Cg alkenes. A general relationship is also observed for the alkenes. The more highly substituted the double bond, the more stable is the compound. There are also other factors that enter into alkene stability. trans-Alkenes are usually more stable than cis-alkenes, probably largely because of increased nonbonded repulsion in the cis isomer. ... 

Cis-trans isomerism is not limited to d/substituted alkenes. It can occur whenever both double-bond carbons are attached to two different groups. If one of the double-bond carbons is attached to two ick-ntical groups, however, then cis-trans isomerism is not possible (Figure 6.4). 

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