Radical anions from butadiene

Fig. 12.P8. Spectrum of the butadiene radical anion. [From D. H. Levy and R. J. Myers, J. Chem. Phys. 41, 1062 (1964).]...

Compared with the anodic oxidation of a 1,3-diene, the cathodic reduction of a 1,3-diene may be less interesting since the resulting simple transformation to monoolefin and alkane is more conveniently achieved by a chemical method than by the electrochemical method. So far, only few reactions which are synthetically interesting have been studied15. The typical pattern of the reaction is the formation of an anion radical from 1,3-diene followed by its reaction with two molecules of electrophile as exemplified by the formation of the dicarboxylic acid from butadiene (equation 22)16. 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

Several other compounds which are easily reduced in a comparably explicable manner are exemplified by butadiene 7.173,159 already discussed on p. 108, diphenylethene 7.174,1076 naphthalene 7.175,1077 anthracene 7.1761078 and phenanthrene 7.177.1079 In reductions in liquid ammonia without alcohol present, two electrons must be added to the LUMO of the starting material before it is basic enough to abstract a proton from ammonia. The addition of the second electron to the radical anion is sometimes so slow that dimerisation occurs to some extent, as we can see in the case of diphenylethene and have already seen in earlier reactions. 

Reaction 10 represents coupling of the radical anions, and Reaction 11 represents a second electron-transfer step. Reaction 11 does occur in the case of sodium and butadiene (10). This result is not surprising because an electron transfer step such as Reaction 9 can only occur at the metal surface and cannot give rise to a high concentration of radical anions in the homogeneous phase (as in the case of the naphthalene complex). Hence, these radical anions are quite likely to remove a second electron from the sodium when they are in the vicinity of the metal surface. The above reactions of butadiene are all shown as 1,4 for the sake of simplicity, although a substantial proportion of 1,2 additions could occur as well. These additions depend on the nature of the medium. 

Mechanism (3) is the direct transfer of an electron from a donor to the monomer to form a radical anion. This can be accomplished by means of an alkali metal, and Na or K can initiate the polymerization of butadiene and methacrylonitrile the latter reaction is carried out in liquid ammonia at 198 K. 

The polymerization of 2-chloro-l,3-butadiene(chloroprene), which is made from acetylene or 1,3-butadiene [284-287] is strongly exothermic (75kJ/mol). It can be initiated radically, anionically, cationically, or with Ziegler catalysts [288]. Only the free-radical process, which is usually run as an emulsion polymerization, is of technical importance [289-294]. Compared with polybutadiene and polyisoprene, polychloroprene features improved gasoline and aging resistance, low-temperature flexibility, and is less combustable [295-297]. 

ROMP to provide polybutenamer, a product with the structure of a fully 1,4-polybutadiene. While this microstructure differs from polybutadiene prepared by radical, anionic or Ziegler-Natta-type polymerizations, no compelling performance advantage has been reported. Due to the added relative monomer cost of COD and CDDT with respect to butadiene, there has been no motivation to develop a polybutenamer product. 

PBD is the rough designation for a family of polymers derived from butadiene. The polymerization of butadiene can be effected using radical and ionic species (preferentially anionic) and results in products of varying microstructure, depending on temperature, solvent, initiator, catalysts, etc.. Typical microstructures are depicted in Fig. 1. 

This reaction takes place in liquid ammonia and in this solvent the active centres are present in the form of free ions. On the other hand, styrene initiated by butyl lithium in benzene polymerizes by addition to ion-pairs. The second type of initiation that can occur is by the transfer of an electron from an electron donor to a monomer molecule to form a radical anion. A simple example of this is the initiation of the polymerization of butadiene by sodium metal... 

Detection indirectly from the nature of the final large-scale electrolysis products [8-10] or by the use of free-radical acceptors, such as unsaturated compounds (styrene, 1,3-butadiene, acrylonitrile, etc.) for example, which readily enter into reaction with the radicals [7] this also applies to the observation by Bezuglyi and Ponomarev that radical anions formed during electrolysis of acrylic and methacrylic acids can act as initiators of polymerization of the original monomeric depolarizers, which was discovered from the suppression of polarographic maxima [11] ... 

Hexacyanobutadiene [5104-27-4] (4), 1,3-butadiene-1,1,2,3,4,4-hexacarbonitrile, is prepared in good yield by a two-step process from the disodium salt of tetracyanoethane (30). It is like TCNE in forming colored TT-complexes and an anion radical. 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

Both theory and experiment point to an almost perpendicular orientation of the two butadiene H2C=C(t-Bu) moieties (see Scheme 3.53). On passing from the neutral molecule to its anion-radical, this orthogonal orientation should flatten because the LUMO of 1,3-butadiene is bonding between C-2 and C-3. Therefore, C2-C3 bond should be considerably strengthened after the anion-radical formation. The anion-radical will acquire the cisoidal conformation. This conformation places two bulky tert-butyl substituents on one side of the molecule, so that the alkali metal counterion (M+) can approach the anion-radical from the other side. In this case, the cation will detain spin density in the localized part of the molecular skeleton. A direct transfer of the spin population from the SOMO of the anion-radical into the alkali cation has been proven (Gerson et al. 1998). 

An alternative rationale for the unusual RLi (hydrocarbon) copolymerization of butadiene and styrene has been presented by O Driscoll and Kuntz (71). Rather than invoking selective solvation, these workers stated that classical copolymerization kinetics is sufficient to explain this copolymerization. They adapted the copolymer-composition equation, originally derived from steady-state assumptions for free-radical copolymerizations, to the anionic copolymerization of butadiene and styrene. Equation (20) describes the relationship between the instantaneous copolymer composition c/[M,]/rf[M2] with the concentrations of the two monomers in the feed, M, and M2, and the reactivity ratios, rt, r2, of the monomers. The rx and r2 values are measures of the preference of the growing chain ends for like or unlike monomers. 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

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