1,3-Butadiene cation

Ethene Cation Radical s-frans-1,3-Butadiene Cation Radical... 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

Scheme 57. The conrotatory, nonelectrocyclic path from the cyclobutene cation radical to 1,3-butadiene cation radical.

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

Ding, Y.-Q., Fang, D.-C. Theoretical Studies on Cycloaddition Reactions between the 2-Aza-1,3-butadiene Cation and Olefins. J. Org. Chem. 2003, 68, 4382-4387. 

Reactions of the 1,3-butadiene cation with various isomeric pen-tenes 164,207 show for each pentene a unique complex (C9H16) which decomposes in a specific manner without the randomization observed in the case of small hydrocarbons. It is noteworthy that the isomeric cyclopentane does not react with C4H6. 

Fig. 7. Comparison of the experimental and calculated optical absorption of the monomeric (M) and dimeric (D) butadiene cation radicals. Absorption curve was recorded by Shida and Hamill, Ref. 1, the thick vertical lines represent the positions and optical density of the absorption maxima recorded by Badger and Brocklehurst, Ref. Lower part of the figure contains the results of the CNDO calculations (Carsky, P., Zahradnfk, R.. Ref. 11)...

An ESR spectrum was observed by Shida et al. for 1,3-butadiene y-irradiated in butyl chloride glass at —196 °C. It was attributed to the butadiene cation-radical which is responsible for the cationic polymerization mechanism. The successive addition of butadiene monomers to the cationic end produced a polymer of butadiene, with an allylic radical at its other end. The allylic radical is considered too stable to induce radical polymerization of butadiene at low temperature. Thus, y-ray polymerization of butadiene should take place through ionic propagating species. 

Loss of an electron from ypt results in a cation in which the positive charge at the terminal position is clearly greater than at a 2-position. The same result is obtained from arguments based on resonance structures. For butadiene cation, we write the six important structures Vla-f. 

The 3-alkyi-1,3-butadiene-2-carboxylate (2-vinylacrylate) 850 is obtained in a high yield by the carbonylation of the 2-alkyl-2,3-butadienyl carbonate 849 under mild conditions (room temperature, atm)[522]. The corresponding acids are obtained in moderate yields by the carbonylation of 2,3-alkadienyl alcohols under severe conditions (100 °C, 20 atm) using a cationic Pd catalyst and p-TsOH[523],... 

Refer to the molecular orbital diagrams of allyl cation (Figure 10 13) and those presented earlier in this chapter for ethylene and 1 3 butadiene (Figures 10 9 and 10 10) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Floffmann rules... 

Styrene—butadiene block copolymers are made with anionic chain carriers, and low molecular weight PS is made by a cationic mechanism (110). Analytical standards are available for PS prepared by all four mechanisms (see Initiators). 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

Monomers for manufacture of butyl mbber are 2-methylpropene [115-11-7] (isobutylene) and 2-methyl-l.3-butadiene [78-79-5] (isoprene) (see Olefins). Polybutenes are copolymers of isobutylene and / -butenes from mixed-C olefin-containing streams. For the production of high mol wt butyl mbber, isobutylene must be of >99.5 wt % purity, and isoprene of >98 wt % purity is used. Water and oxygenated organic compounds iaterfere with the cationic polymerization mechanism, and are minimized by feed purification systems. 

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