Isomers of the Alkenes

As in the alkanes, it is possible for carbon atoms to align themselves in different orders to form isomers. Not only is it possible for the carbon atoms to form branches which produce isomers, but it is also possible for the double bond to be situated between different carbon atoms in different compounds. This different position of the double bond also results in different structural formulas, which, of course, are isomers. Just as in the alkanes, isomers of the alkenes have different properties. The unsaturated hydrocarbons and their derivatives are more active chemically than the saturated hydrocarbons and their derivatives. 

Treatment of an a-halosulfone 1 with base leads to extrusion of sulfur dioxide and formation of an alkene 2. This reaction is referred to as the Ramberg-Bdcklund reaction it usually yields a mixture of E- and Z-isomers of the alkene. 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Given a good diastereoisomeric excess in the initial formation of the /3-hydroxysilane, either geometrical isomer of the alkene can be obtained. 

Write the structural formulas for and name all the isomers (including geometrical isomers) of the alkenes (a) C4H8 (b) C5H10. 

The triplet state of the alkene is most stable when the p orbitals, which make up the normal tt system of the double bond, are not parallel to one another (Figure 6-17). Therefore, if the energy-transfer process leads initially to a planar triplet, this is converted rapidly to the more stable nonplanar form. The excitation of either the cis or the trans isomer of the alkene appears to lead to a common triplet state, as shown in Figure 28-4. 

The intermediates that form react further to give E- and Z-isomers of the alkene ... 

The preference for anti elimination results in the (l/ ,2/ )-diastereomer of the bromide producing only the (Z)-isomer of the alkene, and the (lS j-bromide producing only the ( >alkene. 

Finally, the reduction of the carbon-carbon triple bond of an alkyne can also be accomplished by using sodium or lithium in liquid ammonia. This reaction is especially useful because it produces the (ZT)-isomer of the alkene product. (Recall that the (Z)-isomer can be prepared by catalytic hydrogenation of the alkyne see Section 11.12.)... 

Like the Sn2 reaction (Section 6-12), the E2 is stereospecific Different stereoisomers of the reactant give different stereoisomers of the product. The E2 is stereospecific because it normally goes through an and and coplanar transition state. The products are alkenes, and different diastereomers of starting materials commonly give different diastereomers of alkenes. In Problem 6-38, you showed why the E2 elimination of one diastereomer of l-bromo-l,2-diphenylpropane gives only the trans isomer of the alkene product. 

Y hen we discussed the Peterson reaction in Chapter 31, we explained that each diastereoisomer of a.P-silyl alcohol can eliminate, depending on the reaction conditions, to give either geometrical isomer of the alkene but we did not explain how these diastereoisomers could be made. This is how they are made. Elimination in base is a Wittig-style syn process but an anti elimination occurs in acid. Here are the reactions on one of the diastereoisomers we have just made. 

Wittig-based reactions have been used in both the late stages of multi-step synthetic sequences and also to produce large quantities of key synthetic intermediates (see Ref. 2 for a discussion and some examples). A real advantage of this chemistry is that both the (Z) and (E) isomers of the alkene are generally available by careful choice of conditions (compare Protocols 11 and 12). The products of Protocols 11 and 12 have been conveniently converted into isomeric tetrahydrofurans which bear substituents at all four atoms of the ring. These ring systems form the basis of many complex natural products.25... 

As far as the cobalt complex is concerned, on simple stcric grounds, the preferred rr-alkene complex is that involving the terminal isomer of the alkene substrates, because, under rapid isomerization, it is always the ot- and jJ-aldehyde products which predominate. 

Isomerization does not take place on the products. No isomerization of alkenes occurs with the action of chloroplatinic acid alone, but recovered alkenes from a silane-alkene mixture often contain isomers of the alkene. D-H exchange is observed for the recovered hydrosilane from the hydrosilation mixture with DSiCb. These facts suggest the mechanism outlined in Scheme 2, which is known as the Chalk-Harrod mechanism. " The mechanism involves oxidative addition of hydrosilane to the active species of the catalyst, coordination of alkene, followed by <7 — tt conversion, and reductive elimination of the product. 

We saw earlier that for E2 //-elimination, an erythro isomer would result in the cis alkene isomer. For erythro-l-acetoxy-2-deuterio-l,2-diphenylethane, suggest which isomer of the alkene will be formed on pyrolysis. 

The trans isomer of the alkene stilbene is formed, as indicated by the presence of the deuterium atom in the product. In this case, elimination is via a cyclic transition state, and so the opposite stereochemistry is obtained than was achieved in the E2 mechanism. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. 

These two versions of the HWE are close to stereochemical control the formation of either isomer (E or Z) at will from (more or less) the same starting materials. The next two reactions achieve this aim. By purification of Wittig-type intermediates the stereospecific elimination gives a single isomer of the alkene. 

The Julia olefin synthesis is rather like the Wittig reaction with a sulfone instead of a phosphonium salt but with one other important difference the elimination step is stereoselective and both dia-stereoisomers of the intermediate can give the same isomer of the alkene. Treatment of the sulfone 147 with a strong base gives the anion 148 (or a metal derivative) that combines with an aldehyde to give a diastereomeric mixture of adducts 149. Elimination by various methods gives, in open chain compounds, mostly -150 but, in cyclic compounds, mostly the Z-alkene.29... 

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