1.3- Butadiene resonance forms

The oxidative coupling of two molecules of butadiene with Pd(0) forms the bis-TT-allylpalladium complex 31, which is the resonance form of 2,5-divinyb palladacyclopentane (30) formed by oxidative cyclization. 

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. 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to form the more stable carbocation => a resonance stabilized allylic cation, i) Addition to one of the inner carbon atoms would have produced a much less 1 ° cation, one that could not be stabilized by resonance. 

From the advent of organic chemistry, dienes (and polyenes) have played a very important role in both the theoretical and synthetic aspects. For example, 1,4-addition of bromine to 1,3-butadiene to form l,4-dibromo-2-butene rather than 3,4-dibromo-l-butene as the major product was a challenging problem for theoretical chemists, who interpreted the phenomenon in terms of resonance or delocalization of jr-electrons1. 

One of the polymerization routes involves polymerization of one or the other of the double bounds in the usual manner. The other route involves the two double bonds acting in a unique and concerted manner. Thus addition of an initiating radical to a 1,3-diene such as 1,3-butadiene yields an allylic radical with the two equivalent resonance forms LI and LII... 

Exercise 28-24 Why must the resonance forms 20a, b, c, etc. correspond to a singlet state Formulate the hybrid structure of a triplet state of butadiene in terms of appropriate contributing resonance structures. 

The above structure is the accepted structure for 1,3-butaddiene. This structure, however, is not altogether correct in that 1,3-butadiene has several resonance forms which originate from the interactions of the 7r-cloud electrons. 

Insertion of dienes into M-H bond or M-alkyl bond affords r -allylic complexes or its )7 -alken-J7 -yl resonance form. The allylic complex may further undergo insertion of other unsaturated compounds such as alkene or diene into the unsubstituted or substituted terminal of the allyhc ligand. If successive butadiene insertion takes place, polymers with internal unsaturated bonds are produced as will be described later. A nickel-catalyzed reaction of butadiene with 2 mol of HCN affords adiponitrile, an important feedstock in polymer synthesis (Eq. 1.15). 

Such is not the case with 1,3-butadiene. Protonation of the diene is still regiospe-cific for the end carbon, but the two resonance forms of the resulting allylic carbocation are not equivalent. 

In contrast, the rotation barrier increases to 29kcal/mol when an electron is removed from 1,3-butadiene to form the respective radical cation (Figure 3.12). This significant increase in the energy cost is consistent with the much stronger isovalent resonance stabilization in the one-electron oxidized species. ... 

The more bonds in a resonance form, the more important a contributor the form is. For example, 1,3-butadiene can be Avritten as a resonance hybrid of four structures (Fig. 1.31). 

ABC FIGURE 1.31 Four resonance forms contributing to 1,3-butadiene. Form A is by far the best. 

FIGURE 1.35 In 1,3-butadiene, all electrons are paired, and all resonance forms contributing to the structure must also have all electrons paired. 

The identical arrows indicate that the spins of the two electrons are the same this is not a resonance form of 1,3-butadiene... 

First, let s look at 1,3-butadiene from a resonance point of view. 1,3-Butadiene does have resonance forms that contribute to the overall electronic structure of the molecule (Fig. 12.15). 

FIGURE 12.15 Resonance forms contributing to the stmcture of 1,3-butadiene. 

The best (most stable) resonance form is the best representation for 1,3-butadiene... 

The r-bond order of the CN bond in aniline is therefore low. This means that its resonance integral will be small and this in turn implies an increase in the associated NBMO coefficient [cf equation (3.36)] and so in the Tc-electron density at nitrogen. In other words, aniline is much more like a classical structure with a localized CN bond than is the benzyl anion. The same effect is seen in other cases, too. Thus the n energy of union of ammonia and butadiene to form pyrrole is much less than that for the corresponding carbanion. 

Cyclobutadiene, a An tt system (n = 1), is an air-sensitive and extremely reactive molecule in comparison to its analogs 1,3-butadiene and cyclobutene. Not only does the molecule have none of the attributes of an aromatic molecule like benzene, it is actually destabilized through tt overlap by more than 35 kcal moF (146 kJ moF ) and therefore is antiaromatic. As a consequence, its structure is rectangular, and the two diene forms represent isomers, equilibrating through a symmetrical transition state, rather than resonance forms. 

The addition of an initiator radical to a 1,3-diene leads to a radical which can have two equivalent resonance forms as in the case of 1,3-butadiene... 

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