Conjugated diene, 1,2-addition reaction with

Cyclopentadiene contains conjugated double bonds and an active methylene group and can thus undergo a Diels-Alder diene addition reaction with almost any unsaturated compound, eg, olefins, acetylene, maleic anhydride, etc. The number of its derivatives is extensive only the reactions and derivatives considered most important are discussed. 

Perhaps the most striking difference between conjugated and nonconjugated dienes is that conjugated dienes undergo an addition reaction with alkenes to yield substituted cyclohexene products. For example, 1,3-butadiene and 3-buten-2-one give 3-cycIohexenyl methyl ketone. 

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. 

Cyclic carbodisilanes undergo addition reactions with conjugated dienes with splitting of their Si—Si bond. l,l,2,2-Tetramethyl-l,2-disila-cyclopentane reacted with butadiene by the catalysis of PdCI2(PPhs)2 at 100°C to give l,l,5,5-tetramethyl-l,5-disilacyclotrideca-7,ll-diene (70) in... 

The relative rates of reaction of the singlet TMM derivative 14b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels-Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 14b, on the other hand, follow the reactivity order of their addition reactions with monoradicals. 

The composition of the product mixture formed upon electrophilic addition to conjugated dienes often changes with the temperature at which the reaction is conducted. For example, when HBr is added to 1,3-butadiene at — 80°C, the major product is formed by 1,2-addition. In contrast, when the reaction is run at 45°C, the major product is the one resulting from 1,4-addition ... 

Because of the stability of iron tricarbonyl diene complexes, conjugated dienals are protected from polymerization when complexed, while other reactions can be carried out at the aldehyde functionaUty. A number of synthetically attractive nucleophilic transformations of the aldehyde can be performed on these complexes. These include, aldol reactions, Michael additions, reactions with organozinc, -silicon, -boron, and -tin... 

Acyl isocyanates are more reactive than alkyl or aryl isocyanates. However, the presence of an additional rr-bond conjugated to the C>i-N bond of the isocyanate opens the possibility for [4 + 2] cycloadditions to compete with normal [2 + 2] additions. Reactions with alkyl and aryl substituted alkenes are rather slow. Propene, tranj-2-butene, styrene and conjugated dienes give only 3-lactams, albeit in moderate yields (Scheme 25). The strained double bond of norbomene, a reactive dienophile, adds across the conjugated 4iT-system of trichloroacetyl isocyanate (equation 51). 

Conjugated dienes undergo hydroperfluoroalkylation with perfluoroalkyl iodides and zinc in the presence of titanium catalysts 02 Yields are moderate to good (52-74%) but stereoselectivity is low. From allylic halides functionalized on the a-vinylic position (TMS, carboxylate, phosphonate), additions to terminal alkynes provide functional dienes, used in further synthetic reactions leading to lactones and carboxylic or phosphonic esters.228... 

Just as conjugated dienes undergo addition reactions with HX or X2, conjugated carbonyl derivatives also react. The presence of the carbonyl makes the reaction follow a somewhat different path from the dienes, however. A simple conjugated ketone—but-3-en-2-one (methyl vinyl ketone, 10)— illustrates the difference. When an a,[3-unsaturated ketone such as 10 reacts with HCl, the most basic unit in the molecule is the oxygen, not the C=C unit. Therefore, the carbonyl oxygen reacts with H+ to generate 37A. 

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. 

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