1,5-Hexadiene, titanium

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

Allylic stannanes condense with nitro dienes in the presence of titanium tetrachloride to give alkenyl a-chlorooximes treatment of the a-chlorooximes with base then affords bicyclic isoxazolines.50 Thus, cyclization of the intermediates (107a-c), obtained from appropriate 1 -nitro- 1,5-hexadienes and (3-propenyl)trimethyltin, gave bicyclic isoxazolines (Scheme 31). Cyclization occurred exclusively on the double bond with the longer (three-atom vs. two-atom) intervening bridge, even when that double bond was trisubstituted. 

Polymer-supported titanium catalysts can be regenerated. A series of polymer-supported CpCp TiC (Cp = polymer substituted Gp rings) has been reduced by PPMgBr in situ, and used as catalysts for the hydrogenation of styrene, the isomerization of 1,5-cyclooctadiene and 1,5-hexadiene, and the reduction of carbonyl compounds. In some cases, the introduction of a polymer ligand on the Cp ring restricts the aggregation of active sites and the formation of inactive dimeric titanium species, and results in an activity increase. 

Cyclocodimerization of ethylene and 1,3-butadiene.1 This homogeneous titanium catalyst promotes the cyclocodimerization of ethylene and 1,3-butadiene to vinylcyclo-butane. The usual product of codimerization is 1,4-hexadiene. 

An important extension of Ziegler-Natta polymerization is the copolymerization of styrene, butadiene and a third component such as dicyclopentadiene or 1, 4-hexadiene to give synthetic rubbers. Vanadyl halides rather than titanium halides are then used as the metal catalyst. 

Ziegler-Natta Polymerization. NVK and other monomers, such as vinyl ethers, 1,5-hexadiene, dihydrofuran, and dihydropyrans can be polymerized using a titanium, hafnium or zirconium pentamethylcyclopentadienyl complex as an initiator in the presence of a borane co-initiator, e.g., ... 

The MBH adduct shown in Scheme 3.162, prepared from aromatic aldehydes and methyl acrylate, can directly undergo smooth dehydroxylation with concomitant olefin isomerization with low-valent titanium (LVT, prepared from TiCls-LAH-THF) reagent to afford the trisubstituted alkenes 364 with high ( )-selectivity. However, it is unsatisfactory in view of the low yield and the purity of products.More recently, Zhang et al. developed the samarium diiodide-promoted hydroxyl elimination of MBH adducts to form trisubstituted alkenes 364 with total ( )-stereoselectivity in good to excellent yields. This method also provided a new route to synthesizing a class of 1,5-hexadiene derivatives 365 by temperature tuning (Scheme 3.162). ... 

Ho, S. C. H. Wu, M. M. Xiong, Y. Novel cyclopolymerization polymers from nonconjugated dienes and 1-alkenes. PCT International Patent Application WO 95/06669 (Mobil Oil Corp.), March 9,1995. Hustad, P. D. Coates, G W. Insertion/isomerization polymerization of 1,5-hexadiene synthesis of functional propylene copolymers and block copolymers. J. Am. Chem. Soc. 2062,124, 11578-11579. Hustad, P. D. Tian, J. Coates, G. W. Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts an unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124,3614-3621. 

SCHEME 2.12 Polymerization of 1,5-hexadiene with bis(phenoxyimine) titanium catalyst system. 

Bis(phenoxyimine) titanium complexes have also been employed in the living copolymerization of ethylene/cyclic olefins and ethylene/l,5-hexadiene. Utilizing 31/MAO and varying ethylene pressure, a series of poly(E-co-CP)s with different cyclopentene contents were prepared (Fujita and Coates, 2002). When ethylene pressure was low (<1 psi), an almost perfectly alternating copolymer was formed (Mn = 21000 g/mol, Mw/Mn = 1.34, T = 10.1 C). However, the use of higher ethylene pressures (3 psi) resulted in the formation of a random copolymer containing 36 mol% cyclopentene (M = 133000g/mol,Mw/Mn = 1.24, T =—A.S°C). Tri- and multiblock copolymers were synthesized in which the constituent blocks differed in their cyclopentene content. 

Finally, Coates and coworkers found that bis(phenoxyimine) titanium complex 31 was also capable of living 1,5-hexadiene polymerization and 1,5-hexadiene/propylene copolymerization (Hustad and Coates, 2002). Homopolymerization of 1,5-hexadiene with 31/MAO at 0°C produced a high molecular weight polymer with a narrow PDI (M = 268 000 g/mol, Mw/Mn = 1.27). The polymer showed the presence of two distinct units - the expected MCP units as well as 3-vinyl tetramethylene (3-VTM) units. As shown in Scheme 9.4, the MCP units are proposed to arise from 1,2-insertion of 1,5-hexadiene followed by a 1,2-cyclization. However, an initial 2,1-insertion of 1,5-hexadiene followed by a 1,2-cyclization forms a... 

In addition to cyclic olefins, Hustad and Coates found that bis(phenox3dmine) titanium complex 50 (Figure 15) was also capable of living 1,5-hexadiene polymerization and... 

One of the first cyclizations was carried out with the titanium(III)hydride 94. Owing to antf-Markovnikov addition of the titanium hydride to the double bond, this type of reaction is suitable for the cyclization of 1,5-dienes such 1,5-hexadiene and 1,2-divinylcyclohexadiene to give methyl-enecyclopentanes. A typical example is cyclization of cis 1,2-divinylcyclohexane into a mixture of the cis 95c and trans products 95t (Scheme 39) [47,48]. 

There are a few examples of monocyclopentadienyl compounds that catalyze non-conjugated dienes. The cationic titanium compound [Cp TiMe2] [172] and mono(pentamethylcyclopentadienyl)yttrium alkoxides [53] catalyzed cyclopolymerization of 1,5-hexadiene, whereas neither of them gave highly stere-oregulated polymers. 

Adam, W. and Staab, E., Regio-controUed functionalization of 2,5-dimethyl-2,4-hexadiene into epoxy alcohols by photooxygenation in the presence of titanium(lV) or vanadium(V), Tetrahedron Lett., 29, 531, 1988. 

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