Iodine cocatalyst

By using a transition metal chloride catalyst and an iodine modified cocatalyst, ring-opening polymerization of C5 and C8 monocyclic olefins is controlled to prepare either cis polymers or trans products that are crystallizable. In copolymerization, the cis/trans units in the copolymers are regulated by adjusting the C5/C8 olefin monomer ratio. As the comonomer is increased, the copolymer becomes less crystalline and then completely amorphous at equal amounts of cis/trans units. Polymerization results are reported from WC16 and MoCl5 catalysts. 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Thus, the terms initiator and coinitiator, as well as catalyst and cocatalyst, must be clearly distinguished. As proposed earlier [68], an initiator is consumed in the initiation process whereas a catalyst remains unchanged during the polymerization. In the polymerization of alkenes initiated directly by Lewis acids (e.g., iodine initiated polymerization of vinyl ethers) [69], the Lewis acid plays both roles. Nevertheless, Lewis acids usually act only as catalysts rather than as initiators, with protonogenic compounds such as adventitious moisture being the initiator. 

The main differences are concerned with the role of iodide cocatalyst, not yet well understood, and the DPU formation mechanism. It is suggested that the iodide promotes the formation in situ of molecular iodine (reaction 7) which then reacts with the intermediate carbamoyl complex coming from reaction (8) to afford iodoformamide (reaction 9). The in situ reaction of the last one with aniline gives DPU (reaction 10). 

In 2005 Jorgensen and coworkers successfully exploited 33 for the first asymmetric a-bromination of aldehydes in the presence of benzoic acid as the cocatalyst and using 4,4-dibromo-2,6-di-f-butylcyclohexa-2,5-dienone as the brominating agent (Scheme 11.30). The analogous a-iodination reaction... 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

Hypervalent iodine compounds, in stoichiometric amounts, are also known to oxidize alcohols and the use of iodosyl benzene or a polymer-supported iodine(III) reagent, in combination with KBr as a cocatalyst, for the oxidation of alcohols in water has been described. More recently a related catalytic system, consisting of PhI02 (2 mol%), Br2 (2 mol%) and NaN02 (1 mol%), for the aerobic oxidation of alcohols in water at 55°C has been described. ... 

Iodine electrophile [(CH3)3Si I, HI, etc.] and a zeolite cocatalyst Iodine electrophile [(CH3)3Si I, HI, etc.] and a zeolite cocatalyst HI + cadmium halide BF3, Et20, or other Fredel-Crafts catalyst (CH3)3Si CF3SO3 in presence of Me2S... 

With the help of a bifiinctional thiourea catalyst 55, Mukherjee and coworkers contemporaneously developed a related catalytic asymmetric iodoetherification of oximes (Scheme 2.31). A variety of p.y-unsaturated ketoximes were cyclized using commercially available A-iodosuccinimide (NIS) as the iodine source and iodine as the cocatalyst to furnish A -isoxazolines containing a quaternary stereogenic center in high yields and good to excellent enantioselectivities [47]. Besides, the oxime iodoetherification products could be subjected to a number of synthetically useful transformations to produce other important organic compounds. 

Iodine compounds are essential cocatalysts, and the reaction is believed to proceed via methyl iodide, which alkylates the transition metal. At elevated pressures and temperatures both acetic acid and the iodine compounds are highly corrosive. Thus, the development of corrosion-resistant alloys such as Hastelloy C was a prerequisite to commercializa-... 

An enrichment of monodisperse OPEs from oligodisperse mixtures by GPC is laborious therefore Meier and coworkers used selective reactions with different reactive sites and different protecting groups for the preparation of the higher OPEs Ic-i. The iodine in compound 38 (Scheme 11.17) is much more reactive than the bromine it couples even without Cul cocatalyst. Thus, Br is a dormant substituent which can be used in a later reaction step under more severe conditions. The trimethylsilyl group in 39 can be cleaved by the action of bases like... 

The polymerization of VAc was also controlled by an orga-nometallic system composed of ethyl-2-iodoisobutyrate (or the HI adduct with VAc CH3CH(0C(0)CH3)I), [Fe(Cp)(CO)2]2 as catalyst, and Al(Oi-Pr)3 as cocatalyst in anisole at 60 C (Scheme 18c).Since the initiator is an iodinated alkyl iodide, ITP can occur competitively with metal-catalyzed radical polymerization, the latter being expected to be the dominant mechanism in this system. 

In another study, Koumura et cd have shown that alternated copolymers of methyl acrylate and 1-hexene could be formed by Mn2(CO)io-induced polymerization under weak visible light at 40 °C in a protic fluoroalcohol solvent such as (Cp3)2CHOH in the presence of ethyl-2-iodoisobutyrate as an initiator and molecular iodine I2 as a cocatalyst (Scheme 23b). 

However, there is another approach for the rare direct asymmetric a-iodination of aldehydes (Scheme 4.23). In this reaction, catalyst 46b and benzoic acid as cocatalyst were used to promote the a-iodination of several aldehydes 17 using A-iodo succinimide (95c) as a iodinating agent, affording the corresponding chiral a-iodoaldehydes 102 in moderated isolated yields (30-80%) and with excellent enantioselectivities (90-99% ee) [132]. 

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