1.4- Hexadiene, from butadiene and

Figure 9 Representation of the Rh-catalyzed formation of E 1,4-hexadiene from butadiene and ethylene (other ligands on Rh omitted for simplicity).

Fig. 24-B-3. Catalytic cycle for the formation of 1,4-hexadiene from ethylene and butadiene by use of nickel complexes. The numbers 4 or 5 after the electron configurations denote the coordination number of the species. [Reproduced by permission from a diagram provided by Dr. C. A. Tolman, Central Research Laboratory, E. I. du Pont de Nemours and Co., and published in J. Amer. Chem. Soc., 1970, 92, 6777].

The purpose of the ethylene-butadiene codimerization process is to produce rrmns-1,4-hexadiene from ethylene and butadiene, as shown in reaction (S.6S). The product is an important monomer in synthetic rubber production. 

Olefin insertion to 7c-allyl derivatives similarly may proceed through a (7-allyl intermediate. A good example is given by the formation of 1,4-hexadiene from ethylene and butadiene in the presence of rhodium chloride, equation (6-85). (See Zeigler-Natta catalyst in Chapter 7). 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

The above catalyst system is long-lived and like the Miller catalyst, it formed primarily 1,4-hexadiene, 3-methyl- 1,4-pentadiene, and 2,4-hex-adiene from ethylene and butadiene. A typical distribution of products formed by this catalyst and by the (Bu3P)2NiCl2/i-Bu2AlCl catalyst is shown in Table IX. The improved conversion and yield can be attributed to a better cocatalyst system, as shall be discussed later. Su and Collette s studies are summarized in the following discussions. 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

Selenosulfonylation of olefins in the presence of boron trifluoride etherate produces chiefly or exclusively M products arising from a stereospecific anti addition, from which vinyl sulfones can be obtained by stereospecific oxidation-elimination with m-chloroper-benzoic acid134. When the reaction is carried out on conjugated dienes, with the exception of isoprene, M 1,2-addition products are generally formed selectively from which, through the above-reported oxidation-elimination procedure, 2-(phenylsulfonyl)-l,3-dienes may be prepared (equation 123)135. Interestingly, the selenosulfonylation of butadiene gives quantitatively the 1,4-adduct at room temperature, but selectively 1,2-adducts at 0°C. Furthermore, while the addition to cyclic 1,3-dienes, such as cyclohexadiene and cycloheptadiene, is completely anti stereospecific, the addition to 2,4-hexadienes is nonstereospecific and affords mixtures of erythro and threo isomers. For both (E,E)- and ( ,Z)-2,4-hexadienes, the threo isomer prevails if the reaction is carried out at room temperature. 

Ethylene and propylene are produced primarily by the cracking of naphtha. They also are available from the fractionation of natural gas. Ethylidene norbornene is produced by reacting butadiene with cyclopentadiene. 1,4 Hexadiene is produced from butadiene and ethylene. Dicyclopentadiene is obtained as a by-product from the cracking of heavy feedstocks to produce ethylene. 

Figure 7.6 Industrial use of (from the top) propylene dimerization, butadiene dimerization, butadiene trimer-ization, and butadiene plus ethylene codimerization. In EPDM rubber, the terminal double bond of 1,4-hexadiene takes part in polymer formation. The internal double bond is used during curing.

Hexadiene in its E form is manufactured from butadiene and ethylene (Equation 20). The Z isomer must be kept to a minimum owing to its adverse effect on polymerization. The reaction, first reported in 1961, was commercialized by DuPont. The present world production is estimated around 2.5-3 kt/a. Various metal salts (Ni, Co, Fe) were tested, but the best results were obtained with RhCls.hydrate. 

Vulcanization is an industrial process applied to various polymers from the class of unsaturated polyhydrocarbons. The major practical use of vulcanized elastomers is the tire industry. Tires are made from various polymer blends, including natural rubber, typically between 20 and 50%. The other polymers used in various blends that can be vulcanized include copolymers such as poly(styrene-co-1,3-butadiene) or SBR, poly(acrylonitrile-co-1,3-butadiene-co-styrene) or ABS, poly(isobutylene-co-isoprene), poly(ethylene-co-propylene-co-1,4-hexadiene, etc. 

However, the appropriate symmetry is present if mirror symmetry is conserved. That is, the mirror symmetry for excited cyclobutene is a, symmetric n, symmetric 71, antisymmetric while for 1,3-butadiene the corresponding states are Tq, symmetric 7t2, antisymmetric ti i, symmetric. Thus, in this excited state cyclobutene has two symmetric and one antisymmetric mirror relationships as does 1,3-butadiene. The interconversion of these species in these states conserves symmetry. Since mirror symmetry requires that disrotatory processes occur, we can confidently predict that the photochemical process will generate cis,trans- or (ZT0-2,4-hexadiene from trans- or ( )-3,4-dime thy Icy clobutene and trans,trans- or ( , )-2,4-hexadiene from cis- or (Z)-3,4-dimethylcyclobutene with no cross over since only a disrotatory process will conserve orbital symmetry. 

Ethylidene norbornene is produced by reacting butadiene with cyclopentadiene. 1,4 Hexadiene is produced from butadiene and ethylene ... 

Cyclopentadiene (6) reacted with 105 at 0°C to give 108 (entry 1). At 100°C, butadiene (12) afforded 109 (entry 2). No [2 + 2] cycloadduct was formed in either reaction. Perfluoromethylenecyclopropane (105) failed to react with cis,cis- or cis,trans-2,4-hexadiene at 100 °C, although 110 was readily formed from trans,frans-2,4-hexadiene (106) under these conditions [29] (entry 3). Anthracene (107) added to 105 at 100 °C. The dienophilicity of 105 is exceptional when compared with the reactivity of simple fluoroolefins, such as perfluoro-isobutylene, which require 150 and 200 °C to undergo cycloaddition to cyclopentadiene [30] and anthracene, respectively. 

Irradiation of 2-cyanonaphthalene in the presence of 2,3-dimethyl-1,3-butadiene yielded (4 + 4) cycloadducts (70, 71) onto the naphthalene ring (Scheme 25). The main product from the reaction of 2-cyanonaphthalene with cyclohexa-1,3-diene is the (2 + 2) adduct (72), whereas a minor product is the (4 + 4) adduct (73). The reaction with 2,5-dimethyl-2,4-hexadiene leads again to a (2 + 2) cycloadduct (74) and aminoketone (75). 

Hegedus and Varaprath studied the reactions of various bromodienes with Ni(CO)4 and with bis(cyclooctadiene)nickel. l-Bromo-2,5-hexadiene and 2-bromomethyl-1,3-butadiene give the stable products 62 and 63, respectively, which resemble allyl nickel halides in their properties (217). Similar compounds had been prepared several years previously from geranyl halides (218). l-Bromo-2,4-pentadiene and l-bromo-2,4-hexadiene, however, formed intractable materials which could not be isolated and purified. In these cases the red color of the solution which was first produced faded and NiBr2 was deposited. The desired compounds, however, could be generated in situ at — 30° C and used in coupling reactions with aryl, alkenyl, and allyl halides (217). 

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

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