Nonconjugated diene

We saw m Section 9 10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes In the pres ence of an alcohol this same combination reduces arenes to nonconjugated dienes Thus treatment of benzene with sodium and methanol or ethanol m liquid ammonia converts It to 1 4 cyclohexadiene... 

Section 1111 An example of a reaction m which the ring itself reacts is the Birch reduction The ring of an arene is reduced to a nonconjugated diene by treatment with a Group I metal (usually sodium) m liquid ammonia m the presence of an alcohol... 

Ethylene-propylene-diene rubber is polymerized from 60 parts ethylene, 40 parts propylene, and a small amount of nonconjugated diene. The nonconjugated diene permits sulfur vulcanization of the polymer instead of using peroxide. 

Nonconjugated diene residual unsatuiation from the diene is in the pendent group. 

An important difference between conjugated and nonconjugated dienes is that the former compounds can react with reagents such as chlorine, yielding 1,2- and 1,4-addition products. Eor example, the reaction between chlorine and 1,3-butadiene produces a mixture of 1,4-dichloro-2-butene and 3,4-dichloro- 1-butene ... 

Because a monosubstituted alkene has a AT/Ohyc rog of approximately -126 kj/mol, we might expect that a compound with two monosubstituted double bonds would have a Af/0hyjrog approximately twice that value, or -252 kj/mol. Nonconjugated dienes, such as 1,4-pentadiene (AH°hydrog = —253 kj/mol), meet this expectation, but the conjugated diene 1,3-butadiene (AT/°hydr0g = -236 kj/mol) does not. 1,3-Butadiene is approximately 16 kj/mol (3.8 kcal/mol) more stable than expected. 

Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability. 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

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. 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

Expected AfThydr0g for allene is -252 kj/moi. Allene is less stable than a nonconjugated diene, which is less stable than a conjugated diene. 

The polymerization of nonconjugated diene monomers might be expected to afford polymer chains with pendant unsaturation and ultimately, on further reaction of these groups, crosslinked insoluble polymer networks. Thus, the finding by Butler et a .,, 03, n5 that polymerizations of diallylammonium salts, of general structure 8 [e.g. diallyldimethylammonium chloride (9)] gave linear saturated polymers, was initially considered surprising. 

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

Dicyanopyridazine 4-145 has been used for the synthesis of carbo- and het-ero-cage systems employing nonconjugated dienes such as cyclooctadiene 4-146, or 4-148 or 4-150 to give 4-147, 4-149 and 4-151, respectively (Scheme 4.31) [50]. In a similar way, dihydrofurans, dihydropyrans, pyrrolines, and enol ethers have also been used [51],... 

Di-tert-butyl- 1,3-butadiene behaves like a nonconjugated dienes because the bulky tert-butyl groups twist the structure and prevent the double bonds from lying in the same plane => the p orbitals at C2 and C3 do not overlap and delocalization (and therefore resonance) is prevented. 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

Rotational isomerism normally complicates the study of gaseous nonconjugated dienes and polyenes because many conformers appear simultaneously, and hence only few structures of free molecules in this category have been studied. 

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

Compared with conjugated dienes, the electrochemistry of nonconjugated dienes is classified into two types, A and B11,12. In type A, the double bond of the diene behaves essentially the same as the double bond of a monoolefin in the anodic oxidation. A typical... 

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

Reactions with alkenes and nonconjugated dienes have been described in many publications (101, 103, 106-111). Various alkenes, such as cycloalkenes as well as acyclic alkenes, up to tetrasubstituted derivatives, can react with nitroalkenes (42) (110). Only one double bond is involved in the reactions of heterodienes (42) with nonconjugated dienes (111), whereas the second double bond can be used in subsequent transformations of target nitronates (35). The reactions of heterodienes (42) with inactivated alkenes require the presence of LA as catalyst. 

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

As for olefins different from propene, molecular modeling studies have also been able to rationalize the dependence on metallocene symmetry of E-Z selectivity for 2-butene copolymerization as well as the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings, for cyclopolymerization of nonconjugated dienes. 

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