Butadienes dimerisation

Several additions have been kinetically studied by more than one research group, but most of them were studied in solution and different workers used different solvents, so that the published data cannot be compared for reproducibility, Such a difficulty does not arise for reactions in the gas and in the pure liquid phases. Two popular Diels-Alder reactions, the dimerisation of cyclopentadiene to endo-dicyclopentadiene and that of butadiene to 4-vinyl-cyclohexene are suitable for a comparison of experimental results. Rate coefficients from 8 different sources for the former reaction in the pure liquid phase at various temperatures (in the range where comparison is possible) are given in Table 2. The coefficients are all extrapolated to zero time, as both secondary reactions and variation of the environment during the reaction cause a drift in the observed values of k. Rate coefficients for butadiene dimerisation in the gas phase, from different sources, are collected in Table 3. In this case also the temperature range has been limited to that where comparison is possible. In both Tables 2 and 3 activation parameters, as given by the authors (unless otherwise indicated) are also listed. 

EXPERIMENTAL RATE COEFFICIENTS (10 IN L.MO LE". S EC ) FOR BUTADIENE DIMERISATION IN CAS PHASE IN THE RANGE 282-418°C, TAKEN FROM DIFFERENT SOURCES... 

Thus the rate of both cyclopentadiene and butadiene dimerisations are remarkably little affected by a change of phase. 

However a typical Diels-Alder reaction, the butadiene dimerisation (Table 3, p. 94), and its reversal (case (g) in Table 10) have been recently considered by Benson " - as examples of two-step processes. The initial argument is that 1,5-cyclooctadiene, when pyrolysed in the gas phase, gives 4-vinylcyclohexene as the main product but also a small amount of butadiene, which cannot be produced from 4-vinylcyclohexene under the conditions of the experiments. A kinetic and thermochemical analysis shows that the ds-ds-l,7-octadiene-3,6-diyl biradical (singlet) could be the common precursor of 4-vinylcyclohexene... 

Butadiene dimerisation is very sensitive to catalysis by zero-valent nickel complexes, which can direct the reaction towards 1,2-cycloaddition or (4-1-4) cycloaddition, through a bis-7r-allyl intermediate, with small amounts of Diels-Alder product . Larger quantities of the latter were obtained in other cases . This and other examples of activation of olefins by transition metal complexes have been associated with excitation of the coordinated 7r-system . 

Fig. 2. 1,2-Cycloaddition versus 1,4-cycloaddition in the photosensitised butadiene dimerisation, using sensitisers of different triplet excitation energy .

Variations to the cis addition have been found in the transtition state in some cases and a mixture of products has been reported. Two possible stereochemical variations have been reported because of endo and exo addition. Thus in the dimerisation of butadiene Hoffmann and Woodward have shown that besides the primary orbital interactions between C, and C4 of the diene and Cj and C2 of the dienophile, there are also secondary interactions (shown by dotted lines and also called endo addition) between C-2 of the dieno and C-3 of the dienophile. Such orientations are only possible in endo orientation and this will stabilize the transition state. 

Having made extensive studies of the behaviour of models of the propagation center of 1,3-diphenyl butadiene this monomer would be the most appropriate one to investigate. Unfortunately, it seems that it cannot be isolated because of extremely facile dimerisation via the Diels Alder reaction. A cursory examination has, however, been made of the polymerisation of... 

Nickel-triarylphosphite complexes catalyze the dimerisation of butadiene to cyclooctadiene. Cyclododecatriene is an unwanted by-product, which results from trimerization catalyzed by the same catalyst. Table 3.2 shows the product yields using various ligand-metal complexes (the remainder in each case is a tarry polymeric material). 

Von besonderer praparativer und theoretischer Bedeutung sind die Er-gebnisse der photosensibilisierten Dimerisation von 1,3-Dienen. Hier konkurrieren C4-Cyclodimerisation und C6-Cyclodimerisation (Diels-Alder-Typ). So bildet sich aus Butadien bei der photosensibilisierten Dimerisation neben cis- und trans-l,2-Divinylcyclobutan (Formeln 16 tmd 17) auch 4-Vinylcyclohexen (Formel 18) (134, 139). Isopren liefert... 

Weniger systematisch wurde bisher die photosensibilisierte Dimerisation des Myrcens (187), der isomeren Piperylene (139) sowie des 2,3-Di-methyl-l,3-butadien (139) untersucht. Auch hier variiert die Zusammen-setzung des Reaktionsproduktes bei Variation des Sensibilisators. Bei... 

The linear dimerisation of butadiene with palladium(II) catalyst precursors has been investigated in [C4Ciim]+ with a variety of different anions.[24] Observed turnover frequencies, which range from 37-49 mol mol h, are affected only slightly by the nature of the ionic liquid or catalyst precursor. Best activities were obtained with four equivalents of triphenylphosphine per palladium at a reaction temperature of 70°C. Contrary to the reaction in THF, no formation of metallic palladium was observed and reuse of the catalyst solution was possible. Pressurising the reaction mixture with 5-10 bar of carbon dioxide led to a decrease in reaction rates, which was explained by decreased substrate solubility in the C02-expanded ionic liquid. 

In the classical oxidative alkyne-dimerisation reaction large quantities of solvent are often required which makes alternative procedures attractive. With 20 mol% CuCl and 20 mol% TMEDA, the dimerisation product of phenylacetylene, viz. 1,4-diphenyl-1,3-butadiene, was obtained at ambient temperature in [C4Ciim][PF6] in 95% y ield.1 2 Products were isolated by extraction of the ionic liquid phase with toluene and reuse of the catalyst was possible, but addition of further catalyst was necessary every two runs to keep reaction rates more or less constant. 

Dithienothiophenes give cation polymeric radicals capable of further copolymer addition" while polystryene with a narrow polydispersity has been prepared through the use of an end-capped photoactive anthryl group. ° Large differences in radical termination rates have been found to be responsible for the marked variations in molecular weights of polymer from the UV flash polymerisation of 1,3-butadiene. tra 5-l,2-bis(5-Phenyl-2-oxazolyl)ethene has been found to exhibit low laser conversion efficiency due to preferential dimerisation while thermally activated patterns can be formed on the surface of poly(methyl methacrylate) by coating with photodimerisable 9-anthraldehyde. " ... 

Vinylidene and carbyne complexes also offer a rich chemistry following electron transfer, and a comprehensive review on that topic is available in the literature.As an example, oxidation of vinylidene complexes generates highly reactive radical cations. These may undergo a host of different follow-up reactions and they are summarised in Scheme 6.10. Possible follow reactions include dimerisation by direct Cp—Cp homocoupling to dinuclear butanediyli-dyne complexes M " =C—CRH—CRH—deprotonation to 17 valence-electron alkynyl radicals M —C=CR, which subsequently dimerise to the corresponding bis(vinylidenes) (= 1,3-butadiene-1,4-diylidene derivatives) M = C = CR-CR = C = C Mf and CH-bond homolysis. The latter... 

The synthesis of such eight-membered rings by the nickel-catalysed dimerisation of butadienes, is used here with 2-methylbutadiene. The regio-selectivity and stereo-specificity of the hydro-boration of symmetrical 124 are as expected for a trisubstituted alkene. Conversion of the alcohol 125 into a good leaving group 126 made it reactive enough to carry out electrophilic addition on the other alkene. In aqueous solution, the final nucleophile was water and the product a mixture of diastereoisomers of the tertiary alcohol 127. 

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. 

Butadiene-1-carboxylic acid and styrene, Y. A. Titov and A. I. Kuznetsova, Izvest. Akad. Nauk SSSR, Otdel. Khim. Nauk, 1960, 1810 and 1815 Chem. Abs., 1961, 55, 15408c and 15408f) H. R. Snyder and G. I. Poos, J. Am. Chem. Soc., 1949, 71, 1057 2-cyanolbutadiene dimerising (which is also a Z-substituted dienophile), C. S. Marvel and... 

For the dimerisation of butadiene, some kinetic results for the reaction in the pure liquid phase are available. Rate coefficients can be calculated from experimental conversion data at low total conversion to a good approximation, and so compared with coefficients for the gas-phase reaction, calculated from kinetic results at higher temperatures , by using the appropriate Arrhenius equation the k values (I.mole. sec" ) are gas phase 10 /t = 2.6 (60°C) 24 (82°C) pure liquid phase 10 = 3.4(60°C) 21 (82°C). 

Metal complexes can occasionally give good results as catalysts in Diels-Alder reactions, as for butadiene and isoprene dimerisations with Fe(CO)2(NO)2 and butadiene addition to 2-butyne in the presence of Fe (cyclooctatetraenela . However, these catalysts usually promote formation of several products in non-specific manner, through a two-step mechan-ism - , not dissimilar to most photochemical processes (see following section). 

Butadiene gives a few percent of /ra/j5-1,2-divinyl-cyclobutane when dimerised without added catalysts at 150°C and 100 atm , while the dimerisa-tion of 2-chlorobutadiene at 35-40°C yields in nearly equal amounts a cyclohexene and a cyclobutane derivatives ... 

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