Olefins conjugated dienes

Isoprene is the second important conjugated diene for synthetic rubber production. The main source for isoprene is the dehydrogenation of C5 olefins (tertiary amylenes) obtained by the extraction of a C5 fraction from catalytic cracking units. It can also be produced through several synthetic routes using reactive chemicals such as isobutene, formaldehyde, and propene (Chapter 3). 

Grubbs et al. reported that the ruthenium-catalyzed RCM of a conjugated diene proceeds in such a way that the less hindered olefin moieties participate in the reaction. Consequently, RCM of 115 gives exo-methylene compound 116, and not exo-vinyl compound 117 (Scheme 24) [105]. This regioselectivity is complementary to that observed for enyne metathesis of 118, which gives exclusively 117 (Scheme 24) [106a]. 

Olefin metathesis of a conjugated diene has recently been exploited by Hiemstra et al. for the synthesis of bicyclic analogs of 116. Second-generation... 

Table 5 gives typical results of the wax cracking process to surfactant olefins. Compared with the pure a-olefins produced by the oligomerization reactions of ethylene the crack olefins are decreased in quality, especially due to the conjugated diene part (2-4%). Moreover, there are some problems in guaranteeing the wanted amounts of C20-C30 n-alkanes. Therefore in industrially de-... 

The copper-catalyzed additions of sulfonyl chlorides to conjugated dienes and trienes as well as to aryl-substituted cyclic olefins and substituted styrenes have been described for example, arenesulfonyl chlorides add to vinylarenes providing good to excellent yields of )S-chlorosulfones ... 

The second part will be devoted to alternating copolymerization of conjugated dienes with olefins. 

Recently, a metallocene/MAO system has been used for the polymerization of non-conjugated dienes [204, 205]. The cyclopolymerization of 1,5-hexadiene has been catalyzed by Zieger-Natta catalyst systems, but with low activity and incomplete cyclization in the formation 5-membered rings [206]. The cyclopolymerization of 1,5-hexadiene in the presence of ZrMe2Cp2/MAO afforded a polymer (Mw = 2.7 x 107, Mw/Mn = 2.2) whose NMR indicated that almost complete cyclization had taken place. One of the olefin units of 1,5-hexadiene is initially inserted into an M-C bond and then cyclization proceeds by further... 

Acyclic S-S dications 51 and 52 also give 1,2-disulfonium salts 90 in reactions with olefins.79 Formation of a mixture of diastereomers is observed in the case of 1,2-disubstituted alkenes. Reaction with conjugated dienes leads exclusively to the product of 1,4-addition 89 as mixtures of E- and Z-products whose ratio depends on steric factors (Scheme 34).79... 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

Agreement is somewhat poorer for substituted hexatrienes. Consider now the ( )-isomer of 2,5-dimethyl-l,3,5-hexatriene (81) for which Roth gives an enthalpy of formation of 95.8 kJ mol-1. Simple olefin additivity, as done above for the parent hexatriene, results in a value of 103 kJ mol-1. Modifying the above 5 to 3.5 kJ mol-1 as found for the relatedly branched conjugated diene (isoprene, 82) gives a new value of ca 100 kJmol-1 for the enthalpy of formation of 81. The discrepancy has shrunk to ca 4 kJmol-1. 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Doubts remain in the absolute identification of the individual olefinic carbon atoms of the (9Z,11Z) (38) and (9 ,11 ) (42), expect for the fact that the shifts of the inner (CIO and Cll) and outer (C9 and Cl2) olefinic carbon atoms of the conjugated diene system are distinguishable. 

Yasuda and coworkers51 reported a route to niobocene-allyl compounds by hydrometa-lation of conjugated dienes with niobium hydrido-olefin complexes, NbH(C5H5)2(olefin),... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

Allenes generally react with conjugated dienes to give [4 + 2] type of adducts in contrast with ketenes which generally react with dienes in a [2 + 2] kind of way. Some enhancement of reactivity is expected in comparison with olefins because of the significant amount of strain that allenes have. Semi-empirical calculations, however, have shown allene to be less reactive than ethylene due to higher deformation energies135. 

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. 

The reduction of conjugated dienes by dissolving metals is not extensively reported. This method appears to be nonselective, giving rise to a mixture of the expected olefins and polyolefins as by-products". 

Fe(CO)5 isomerizes simple olefins, nonconjugated dienes to conjugated dienes, and it can also catalyze the migration of both double bonds in a conjugated diene to give a new conjugated diene. An attractive mechanism for the isomerizations involves 7r-allyliron hydrides ... 

---