1,2-and 1,4-Electrophilic addition reactions

Figure 11.9 Energy Profile for 1,2- and 1,4-Electrophilic Addition Reactions... 

Fluorination with XeF2 or C H IF2 gives both the 1,2- and 1,4-difluoro products. This reaction proceeds via the initial electrophilic addition of F to the diene (53). 

Adducts of type 13, arising from the rearrangement of the allylic intermediate, have never been observed. The product distribution in methanol depends, however, on the reaction conditions. When the addition of XeF2 is carried out in the presence of boron trifluoride as a catalyst, the formation of the complex b has been suggested. This complex would react with 2,3-dimethylbutadiene as a positive oxygen electrophile to give, besides 1,2- and 1,4-difluoro derivatives, 1,4- and 1,2-fluoromethoxy products with a predominance of the anti-Markovnikov adduct (equation 26). 

Formation of both 1,2- and 1,4-addition products occurs not only with halogens, but also with other electrophiles such as the hydrogen halides. The mechanistic course of the reaction of 1,3-butadiene with hydrogen chloride is shown in Equation 13-1. The first step, as with alkenes (Section 10-3A), is formation of a carbocation. However, with 1,3-butadiene, if the proton is added to C1 (but not C2), the resulting cation has a substantial delocalization energy, with the charge distributed over two carbons (review Sections 6-5 and... 

Iodine electrophiles including r-BuOI/BF3, AcOI, IC1 and IBr undergo 1,2- and 1,4-addition reactions with 1,3-butadiene (equation 5). Reactions of J-BuOBr and J-BuOCl... 

Because of the increased steric hindrance at the carbonyl carbon, similar reactions involving a,/3-unsaturated ketones often result in a mixture of 1,2- and 1,4-addition. The exact amount of each product depends on the relative amounts of steric hindrance at the two electrophilic carbons and may be difficult to predict in advance. An example is provided in the following equation ... 

The amount of 1,2- and 1,4-addition products formed in the electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions. 

In Summary Conjugated dienes are electron rich and are attacked by electrophiles to give intermediate allylic cations on the way to 1,2- and 1,4-addition products. These reactions may be subject to kinetic control at relatively low temperatures. At relatively higher temperatures, the kinetic product ratios may change to thermodynamic product ratios, when such product formation is reversible. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

The reaction of the solvent-separated Lewis-acidic ion pair C with cyclohexenone is the only multiple-step addition in Figure 10.32. It does not involve the cyclohexenone itself, but its (solv)3Li complex E, which, of course, is the better electrophile. The carbanion R R2R3C may now choose between both the 1,2- and the 1,4-addition. The former proceeds via transition state F, the latter via transition state G. These transition states are about equal in energy, since there is a close (solv), I Tl /(/ contact in both cases. This is why the reaction of organolithium compounds of type C and tt,/i-unsaturated ketones typically leads to a mixture of 1,2- and 1,4-addition products. 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

We have talked a lot about regioselectivity, without calling it that, in the last two chapters. In Chapter 21 you learned how to predict and explain which productfs) you get from electrophilic aromatic substitution reactions. The functional group is the aromatic ring where it reacts is the reaction s regioselectivity. in Chapter 22 you saw that nucleophilic addition to an unsaturated ketone can take place in a 1,2- or 1,4-fashion—the question of which happens (where the unsaturated ketone reacts) is a question of regioselectivity. We wUl address regioselectivity in much more detail in the next chapter. 

---