Kinetic control conjugated dienes

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

We showed (14) that formation of the isomerization products is kinetically controlled and that it depends on the catalyst system employed, the principal conjugated diene isomer being either the trans-2-trans-4-hexadiene, cis-2-trans-4-hexadiene, or 1,3-hexadiene. 

Burgess and coworkers investigated the hydrogenation of the conjugated diene 87 (Scheme 30.1) [48]. Kinetic studies showed that the reaction occurred mostly stepwise via 2,3-diphenyl-l-butene, while only a small part of the diene was converted directly to 2,3-diphenylbutane, without dissociation of the catalyst from the intermediate mono-alkene. The first hydrogenation step was found to proceed with low enantioselectivity, whereas the second step was characterized by strong catalyst and strong substrate control. 

Generally, the addition of sulfenyl halides to conjugated dienes occurs, under kinetic control, at either double bond with anti stereospecificity to give 1,2-adducts with either... 

These dications react with alkenes to give 1,2-disulfonium salts, and with conjugated dienes to afford 1,4-adducts. Furthermore, while 1,4-disubstituted linear dienes yield complex mixtures of unidentified substances, 1,3-cyclohexadiene (96) produces a moderately stable salt 102 (equation 106). The formation of the kinetically controlled 1,2-addition product has never been observed. 

The seven-membered metallacycles 89 (formed by ketone addition to [(butadiene)ZrCp2] cleanly add a nitrile molecule at elevated temperatures to yield the respective nine-membered metallacyclic products 96. Their hydrolysis then yields the 6-hydroxy-substituted non-conjugated unsaturated imines 97 under kinetic control. Within a few hours at room temperature these rearrange to the thermodynamically favored primary dienamine products 98.107 In this case the thermochemical diene conjugational energy makes the primary dienamines more stable than their conjugated imine tautomers (Scheme 32). 

Enamines (e.g. 377) have been shown to react with conjugated nitroolefins 378 to give mainly dihydro-l,2-oxazine A -oxide derivatives 379 as products of kinetic control (sometimes a cyclobutane ring is formed in these reactions see Section II.B). The stability of these heterocycles is largely dependent on the parent enamine and the type of substituent used on the nitro olefin as has been extensively studied by Valentin and coworkers " . Usually they open into the corresponding nitroalkylated enamines 380 (equation 82), in particular in a solution of methanol or deuteriated chloro-and often an equilibrium between the two forms is established. Stable 1,2-oxazine A -oxides have been obtained in the reaction of 2-nitro-l,3-dienes with cyclic enamines . 

The regiochemistry of the final product is determined at the last protonation step—the anion itself is of course delocalized and could react at either end to give a conjugated diene, which would be more stable. Why then does it choose to pick up a proton in the middle and give a less stable isomer Kinetically controlled reactions of pentadienyl anions with electrophiles typically take place at this central carbon as a result of orbital interactions. For more information see the further reading section at the end of the chapter. 

With electrophilic addition to standard alkenes such as propene, the product predicted by Markovnikov s rule is also more stable. For reactions under thermodynamic (equilibrium) control, the distribution of products is determined by the relative stability of each. Thus, kineticaUy controlled and thermodynamically controlled electrophilic additions of H—to standard alkenes results in the same dominant product. This is the case with many reactions the product formed fastest is also most stable. Yet, many other reactions do not behave this way. Below we will see that the addition of HBr to conjugated dienes exemplifies reactions in which kinetic and thermodynamic control produce different dominant products. 

To complete our discussion of electrophilic addition to conjugated dienes and of kinetic versus thermodynamic control, we need to ask the following questions. 

Conjugated dienes that undergo addition at low temperature are said to be under kinetic control. 

An important fundamental concept in this chapter involves the difference between thermodynamic and kinetic control of a reaction. Conjugated dienes will react with HX or X2 to give 1,2-addition products when kinetic conditions are used. The use of thermodynamic conditions will favor formation of the most stable product, usually the 1,4-addition product. 

ELECTROPHILIC ATTACK ON CONJUGATED DIENES KINETIC AND THERMODYNAMIC CONTROL... 

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. 

Electrophilic addition to conjugated dienes proceeds via allyl cations. Under conditions of kinetic control, the outcome is determined by the relative stabilities of the resonance forms of the allyl cation. At high temperature, under conditions of thermodynamic control, the most stable alkene is favored. 

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