Carbocations, stereochemistry with alkenes

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Addition reactions with alkenes to form cyclopropanes are the best-studied reactions of carbene intermediates, in terms of understanding carbene mechanisms and synthetic applications. Doering, in 1954, first reported the formation of cyclopropanes by the 1,2-addition of carbenes to alkenes. Singlet and triplet carbenes exhibit some important differences. Because it has an empty p orbital (like a carbocation) and a nonbonded pair of electrons (Hke a carbanion), the singlet carbene exhibits both carbocation and carbanion character. However, the triplet carbene behaves more as a diradical. These characteristics influence the types and stereochemistries of carbene reactions. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

Although the stereochemistry of bromination of alkenes could result from a freely rotating carbocation and a cyclic bromonium ion competing with each... 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

Evidence for the existence of the bromonium ion is provided from the observation that bromine adds to cyclic alkenes (e.g. cyclopentene) in an anti-stereochemistry (Following fig.). Thus, each bromine adds to opposite faces of the alkene to produce only the trans isomer. None of the ds isomer is formed. If the intermediate was a carbocation, a mixture of cis and trans isomers would be expected as the second bromine could add form either side. With a bromonium ion, the second bromine must approach from the opposite side. 

Treatment of an alkene with a strong acid, such as sulfuric acid, that has a relatively nonnucleophilic conjugate base results in the addition of the elements of water (H and OH) to the double bond. This reaction has many similarities to the addition of the halogen acids described in Section 11.2. First H+ adds to produce a carbocation and then water acts as the nucleophile. The reaction follows Markovnikov s rule and the stereochemistry is that expected for a reaction that involves a carbocation—loss of stereochemistry. Some examples are provided in the following equations. Note that the mechanism is the exact reverse of the El mechanism for acid-catalyzed dehydration of alcohols described in Section 10.13. 

Why does alkene hydroboration take place with non-Markovnikov regiochemistry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene addition reactions in that it occurs in a single step without a carbocation intermediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 p. 244). Since both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

HCl, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H give.s a carbocation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products ha%dng anti stereochemistry. If water is present during halogen addition reactions, a halohydrin is formed. 

Excellent yields of ene adducts can be obtained from BF3-Et20- or SnCU- catalyzed addition of formaldehyde to alkenes that can give a tertiary carbocation. Limonene reacts selectively at the 1,1-disub-stituted double bond to give ene adduct (9) in 80% yield (equation 6). SnCU- catalyzed addition of formaldehyde to 2,6-dimethyl-2,3-heptadiene gives lavandulol (10) in 35% yield (equation 7). BF3-Et20-catalyzed ene reaction of formaldehyde with (11) occurs exclusively from the less-hindered a-face to give ene adduct (12) with the correct stereochemistry at C-20 and functionality suitable for construction of the vitamin D side chain (equation 8). ... 

Oxymercutation-demercutation occurs with Markovnikov regiochemistry and results in hydration of alkenes without complication from carbocation rearrangement. It is often the preferred choice over acid-catalyzed hydration for Markovnikov addition. The overall stereochemistry of addition in acid-catalyzed hydration and oxymercuration-demercuration is not controlled—they both result in a mixture of cis and trans addition products. 

Dehydration (loss of H2O) of an alcohol is promoted by treatment with a strong, non-nucleophilic acid (such as H2SO4 or H3PO4) and heat. Alcohol dehydration involves an El mechanism with a carbocation intermediate. Since carbocations can rearrange, the double bond can end up anywhere on the carbon chain, and the most stable, most highly substituted alkene is expected as the major product (follows Zaitsev s rule, with possible rearrangement of the carbon skeleton). The planar carbocation intermediate also results in a loss of stereochemistry so the more stable stereoisomer will be produced as the major product (trans or E isomer). The dehydration reaction is most useful when only a single product is possible otherwise, a mixture of alkene products is likely to be obtained. 

With substituents on the alkyne that can stabilize a non-bridging carbocation, mixed stereochemistry is found in the products. The reaction of 1-phenylpropyne with Br2 in acetic acid gives the products shown in Eq. 10.23. The reaction obeys a kinetic expression such as Eq. 10.21 ([alkene] replaced by [alkyne]), and a p value of-5.17 for para substituents on the phenyl ring was found. The data support the intermediacy of a vinyl cation (see margin) that can combine with both bromide or solvent to form either E or Z products. 

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