Isoprene formation

Unseparated Catalyst. Unaged Catalysts. Our previous work (13) with the triisobutylaluminum-titanium tetrachloride catalysts preformed at 20° C. had shown that the optimum Al/Ti ratio for c is-1,4 poly isoprene formation is about 0.9 to 0.1. The yields of solid polymer fell off rapidly on either side of this ratio. Lower ratios led to mixed leathery-rubbery products of high gel, whereas higher ratios (above 1.3) produced increasing amounts of low molecular weight oily materials, so-called extractables. 

Wagner W.P., M. Nemeeek-Marshall and R. Fall Three distinct phases of isoprene formation during growth and sporulation of Baeillus subtilis, J. Bacteriol. 181 (1999) 4700-4703. 

Kreuzwieser, J., Graus, M., Wisthaler, A., Hanse, A., Rennenberg,H.,Schnitzler, J.P. (2002) Xylem-transported glucose as an additional carbon source for leaf isoprene formation in Quercus robur. The New Phytologist, 156, 171-178. 

FIGURE 523 The leaf isoprene formation at subcellular compartment level based on Reference [11].This figure illustrates the role of the cytosolic mevalonate and chloroplastic deoxyxylulose-5-phosphate (DOXP) pathways in the formation of dimethylallyl diphosphate (DMAPP), the precursor of isoprene by the isoprene synthase reaction. IPP, isopentenyl diphosphate HMG-CoA, 3-hydroxy-3-methylglutaryl-coen-zyme A MEP, 2-C-methyl-D-erythritol 4-phosphate PEP, phosphoenolpyruvate. (Adapted and reprinted with kind permission from Springer Science + Business Media [16], figure 1.)... 

The capability of PTR-MS to detect stable isotopes is potentially a very useful tool to study the biological pathways in relation to plant volatiles. A well-studied example is the formation of C-isoprene (CsHg) during C02 fumigation, demonstrating the linkage between photosynthesis and isoprene emission in intact leaves from oak and cottonwood [16]. Despite the fact that the biochemistry of isoprene formation is known, the role of isoprene biosynthesis in plants is stiU not clear, especially in its relation to photosynthetic precursors in leaf chloroplasts. 

Santos, L.S., Dalmazio, I., EberMn, M.N., Claeys, M., and Augusti, R. (2006) Mimicking the atmospheric OH-radical-mediated photooxidation of isoprene formation of cloud-condensation nuclei polyols monitored by electrospray ionization mass spectrometry. Rapid Common. Mi s Spectrom., 20, 2104-2108. 

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

Section 26 9 Carbon-carbon bond formation between isoprene units can be understood on the basis of nucleophilic attack of the tt electrons of a double bond on a carbocation or an allyhc carbon that bears a pyrophosphate leaving group... 

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

Isoprene is highly reactive both as a diene and through its allyhc hydrogens, and its reactions are similar to those of butadiene (qv) (8). Apart from polymerisation, the most widely investigated isoprene reactions are the formation of six-membered rings by the Diels-Alder reaction ... 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

Dehydrogenation of Tertiary Amylenes, The staiting material here is a fiaction which is cut from catal57tic clacking of petroleum. Two of the tertiary amylene isomers, 2-methyl-l-butene and 2-methyl-2-butene, are recovered in high purity by formation of methyl tertiary butyl ether and cracking of this to produce primarily 2-methyl-2-butene. The amylenes are mixed with steam and dehydrogenated over a catalyst. The cmde isoprene can be purified by conventional or extractive distillation. 

A second route based on olefin disproportionation was developed by Phillips Petroleum (131). Here isobutylene reacts with propylene to form isoamylenes, which are dehydrogenated to isoprene. 2-Butene can be used in place of propylene since it also yields isoamylene and the coproduct propylene can be recycled. Use of mixed butylenes causes the formation of pentenes, giving piperjlene, which contaminates isoprene. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

Assuming selective formation of the most stable carbocation, which product(s) would be obtained from HCl addition to isoprene Would this outcome be different from the one predicted on the basis of thermodynamic control ... 

Another interesting applieation of MDGC is in the rapid determination of isoprene (the most reaetive hydroearbon speeies) and dimethyl sulfide (DMS) (the major souree of sulfur in the marine troposphere and a preeursor to eloud formation) in the atmosphere (16). The deteetion limits were 5 and 25 ng 1 respeetively. 

Unlike polyethylene and other simple aikene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

Methane, ethene, ethane, propene, acetaldehyde, methyl formate, butene, acetone, furan, dimethyl sulfide, isoprene, isobu-tyraldehyde, diacetyl, methylfuran, and isovaleraldehyde... 

Ionol is a commercial antioxidant, 2,6-di-/cr/-butyl- -cresol, manufactured by Shell Chemical Corp. Inhibitors appear to minimize formation of polymeric side products, although with isoprene the effect is often small. 

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