Addition to a Conjugated Diene

Step 1 Protonation of one of the double bonds forms a resonance-stabilized allylic cation. 

Step 2 A nucleophile attacks at either electrophilic carbon atom. 

The key to formation of these two products is the presence of a double bond in position to form a stabilized allylic cation. Molecules having such double bonds are likely to react via resonance-stabilized intermediates. 

Treatment of an alkyl halide with alcoholic AgN03 often promotes ionization. 

When 3-chloro-l-methylcyclopentene reacts with AgN03 in ethanol, two isomeric ethers are formed. Suggest structures, and propose a mechanism for their formation. 

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Electrophilic addition to a conjugated diene at or below Toom temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct. When the same reaction is carried out at higher temperatures, though, the product ratio often changes and the 1,4 adduct predominates. For example, addition of HBr to 1,3-butadiene at 0°C yields a 71 29 mixture of 1,2 and 1,4 adducts, but the same reaction carried out at 40 °C yields a 15 85 mixture. Furthermore, when the product mixture formed at 0 °C is heated to 40 °C in the presence of HBr, the ratio of adducts slowly changes from 71 29 to 15 85. Why ... 

Acetoacetic Ester Synthesis Section 20.4 Addition of an Alcohol to an Aldehyde or a Ketone Section 18.9 Figure 18.5 Addition of a Carbene to an Alkene Section 11.8 Addition to a Conjugated Diene Section 11.13 Addition of a Grignard Reagent to an Aldehyde or a Ketone Section 18.6... 

Although 2 1 telomerization reactions can be considered as a special case of 1,4-addition to a conjugated diene by a carbon nucleophile and a second nucleophile [Eq.(14)], these reactions will not be covered by this chapter. The reader is advised to consult Refs. [8] and... 

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

This reaction is a double bond (dienophile) 1,4-addition to a conjugated diene (4+2 cycloaddition), and the product is always a six-membered ring. Competing reactions are polymerization of the diene or dienophile or both and 1,2-cycloaddition. However, yields are usually quite high. No catalyst is needed but this reaction was known for several years to proceed through catalysts with Lewis acidic properties (1-5), usually for reactions in which there is a C=0 or C=N bond in one of the reactants. 

The reactions of a conjugate diene reflect the fact that a conjugated diene should be viewed as a functional group in its own right, rather than as two separate alkenes. Electrophilic addition to a conjugated diene results in a mixture of two possible products arising from 1,2-addition and 1,4 addition ... 

The addition of SiMej and SiMeOMe to 2,3-dimethylbutadiene studied by Weyenberg and Atwell was actually the first reported silylene addition to a conjugated diene system. In the vapor phase, the yields of silacyclopentenes were generally high and over 60%. Experimental support for the mechanism of such addition to include an initial step of 1,2-addition followed by isomerization was recently provided by Ishikawa, Ohi, and Kumada, who studied the reaction as shown in equation (109) with SiMePh. They succeeded in intercepting the silacyclopropane intermediate with CH3OH and obtained 2,3-dimethyl-4-(methylphenylmethoxysilyl)-2-butene (9), as an identifiable product. 

Predicting the Product of Electrophilic Addition to a Conjugated Diene... 

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