Conjugated dienes resonance forms

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

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. 

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate. 

Electrophilic addition to simple alkenes takes place in such a way as to form the most stable intermediate carbonium ion. Addition to a,j3-unsaturatec arbonyl compounds, too, is consistent with this principle to see that this is so, however, we must look at the conjugated system as a whole. As in the case of conjugated dienes (Sec. 8.20), addition to an end of the conjugated system is preferr jd, since this yields (step 1) a resonance-stabilized carbonium ion. Addition to the carbonyl oxygen end would yield carbonium ion 1 addition to the j3-catbon end would yield carbonium ion II. 

The key step is hydride shift from a 2° carbocation to a 2° allylic, resonance-stabilized carbocation, which can subsequently lose a proton to form a conjugated diene. 

Here a surface Lewis acid (denoted by j) abstracts a hydride ion from the methylene group adjacent to the double bond. This mechanism is in accord with the essential Lewis acid nature of the silica-alumina surface and is consistent with the previously demonstrated ability of this surface to abstract hydride ions from tertiary hydrocarbons. Since an alkenyl carbonium ion is stabilized by resonance to a greater extent than is a saturated carbonium ion, it may well be the most stable species which could form in the chemisorption of an aliphatic olefin or its precursor. It seems reasonable, therefore, to presume that such species may be involved in heterogeneous acid catalysis to a greater extent than has been generally recognized. This chemisorption process does not, of course, exclude the more conventional acid addition to the double bond which may occur under suitable circumstances but rather, it introduces an alternate path which may well exert a considerable influence on the overall course of catalytic reactions. Thus, for example, since a substituted ally lie carbonium ion may be converted to a conjugated diene by loss of a proton, it may be an important intermediate in the formation... 

The carbocation formed by addition of an electrophile to a conjugated diene, in contrast, is stabilized by resonance. The positive charge is shared by two carbons, and as a result, both direct (1,2-) and conjugate (1,4-) addition occur. 

In the case of conjugated dienes the free radical is subject to resonance and can thus react through the last carbon atom of the chain or the antepenultimate one monomeric units of respectively 1,4- or 1,2- (or 3,4-, depending on the regiose-lectivity in the case of monosubstituted dienes) type are formed. The free radical polymerization of butadiene leads to approximately 80% of 1,4-type units. 

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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