Conjugated dienes addition reactions

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. 

Coniine, molecular model of. 28 structure of, 294 Conjugate acid, 49 Conjugate base, 49 Conjugate carbonyl addition reaction, 725-729 amines and, 727 enamines and, 897-898 Gilman reagents and, 728-729 mechanism of, 725-726 Michael reactions and, 894-895 water and. 727 Conjugated diene, 482... 

A general method for preparing allylic halides is by addition of hydrogen halides to conjugated dienes. This reaction usually produces a mixture of 1,2- and 1,4-addition products (see Sectioii 13-2) ... 

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. 

In the Pd(0)-catalyzed transformation of conjugated dienes the reaction is initiated by an oxidative addition to the Pd(0) catalyst. Typically, an organohalide RX or a pronucleophile HX reacts with Pd(0) to give RPdX or HPdX, respectively. These species will react with the conjugated diene throngh an insertion of one of the double bonds of the diene into the R—Pd or H—Pd bond, respectively, which results in the formation of a tt-allylpalladium complex (Scheme 1). 

FIGURE 12.84 Two typical reactions of conjugated dienes addition to give an allyl cation and the Diels—Alder reaction. 

Perfluorotetramethylcyclopentadienone (219), a highly volatile yellow solid, is a diene par excellence. An immediate colour change occurs upon addition of a dienophile (7t-complex formation) which fades as reaction proceeds. Unlike cyclo-pentadienone itself, (219) still functions as the diene component in [4 -i- 2] reactions with conjugated dienes. Slow reaction occurs with naphthalene, whereas in the case... 

Diels-Alder reaction is the 1,4-addition of an alkene or alkyne (dienophile) across a conjugated diene. An example is the addition of pro-penal to buta-l,3-diene to give A -tetrahy-... 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

A particular kind of conjugate addition reaction earned the Nobel Prize in chemistry for Otto Diels and Kurt Alder of the University of Kiel (Germany) m 1950 The Diels-Alder reaction is the conjugate addition of an alkene to a diene Using 1 3 buta diene as a typical diene the Diels-Alder reaction may be represented by the general equation... 

Section 10 12 Conjugate addition of an alkene (the dienophile) to a conjugated diene gives a cyclohexene derivative in a process called the Diels-Alder reaction It is concerted and stereospecific substituents that are cis to each other on the dienophile remain cis m the product... 

Addition (Section 6 1) Reaction in which a reagent X—Y adds to a multiple bond so that X becomes attached to one of the carbons of the multiple bond and Y to the other 1 2 Addition (Section 10 10) Addition of reagents of the type X—Y to conjugated dienes in which X and Y add to adja cent doubly bonded carbons... 

Conjugate acid (Section 1 13) The species formed from a Brpnsted base after it has accepted a proton Conjugate addition (Sections 1010 and 1812) Addition reaction in which the reagent adds to the termini of the con jugated system with migration of the double bond synony mous with 1 4 addition The most common examples include conjugate addition to 1 3 dienes and to a 3 unsaturated car bonyl compounds... 

Diels-Alder reaction (Section 10 12) Conjugate addition of an alkene to a conjugated diene to give a cyclohexene denva tive Diels-Alder reactions are extremely useful in synthesis... 

Reactions of trifluoroacetonitnle oxide with conjugated dienes also lead to regiospecific additions [36] (equation 36) Its addition to the strain-activated double... 

---