Isoprene, selective

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

Lutetium complexes of chelating NPN ligands (40) were shown to polymerize isoprene selectively to its 3,4-derivative in a high syndiotactic stereochemistry. Interestingly, replacing P by S leads to an isotactic polymer. ... 

The reaction of isoprcnc with Et NH in the presence of triethylammonium iodide (10 mol%) gives the 1 1 adducts 51 and 52 with high selectivity(53]. The reaction of isoprene with ammonia or diethylamine affords the tail-to-tail dimer 53 when Pdfacac) and (BuO)jP are used as the catalyst. The head-to-head dimer 54 is obtained with Pd(acac)2, BF,. and tricyclohexylphosphine(54]. 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Synthesis. Because of the limited availabiUty of by-product isoprene much effort has been devoted to synthesis of isoprene. Most routes tend to have marginal selectivity and require large amounts of energy. The choice of which route is preferable depends on availabiUty and cost of raw materials and cost of energy. Several synthetic routes have been practiced commercially (103—108). 

The ethynylation reaction takes place at 10—40°C and 2 MPa (20 atm) and hquid ammonia is the solvent. The methylbutynol is converted into methylbutenol by selective hydrogenation and then is dehydrated over alumina at 250—300°C. Polymerization-grade isoprene is obtained. 

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 ... 

Dimethylfuran, in a sensitized reaction, gave 1,3-dimethylcyclopropene (the main product), isoprene, cis- and rran.s-l,3-pentadiene, 2-pentyne, and 1-methylcyclopropenyl methyl ketone (Scheme 7) (70JPC574) the ring contraction showed a high selectivity. 

The interest in chiral titanium(IV) complexes as catalysts for reactions of carbonyl compounds has, e.g., been the application of BINOL-titanium(IV) complexes for ene reactions [8, 19]. In the field of catalytic enantioselective cycloaddition reactions, methyl glyoxylate 4b reacts with isoprene 5b catalyzed by BINOL-TiX2 20 to give the cycloaddition product 6c and the ene product 7b in 1 4 ratio enantio-selectivity is excellent - 97% ee for the cycloaddition product (Scheme 4.19) [28]. 

In the reaction between isoprene (IP) and methyl vinyl ketone (MVK), the selectivities between the two isomers produced in this reaction can be improved from 4 1 to 20 1 by the addition of a mild Eewis acid such as 2inc(II) iodide (5 mol %) to the ionic liquid [BMIM][PE(3] (Scheme 5.1-18). One of the key benefits of this is that the... 

There are several different routes for producing isoprene. The choice of one process over the other depends on the availahility of the raw materials and the economics of the selected process. 

Highly uMtr-diastereofacial selective cycloaddition of isoprene (2) with 4-isopropyl-2-cyclohexenone allowed a short regiocontrolled and stereocon-trolled synthesis [13] of jS-cadinene and (y2-cadinene, Scheme 3.3). High anti-diastereofacial selectivity also occurs in the Diels-Alder reaction of optically active cyclohexenones 6-9 (Figure 3.2), readily available from the chiral pool, with open chain dienes [14-16]. Their cycloadducts are valuable intermediates in the synthesis of optically active sesquiterpenes in view of the easy conversion of the gem-dimethylcyclopropane and gem-dimethylcyclobutane in a variety of substituents. 

Similarly a marked increase of regioselectivity has been shown in the catalyzed Diels-Alder reactions of the chiral bicyclic lactame 24 (Scheme 3.9) with a variety of dienes [27] (isoprene, mircene, (E,E)-L4-dimethylbutadiene, 2,3-di-methylbutadiene, 2-siloxybutadiene). The catalyzed reactions were more regio-selective and totally enJo-antz-diastereoselective anti with respect to the bridgehead methyl group). The results of the cycloadditions with isoprene and mircene are reported in Scheme 3.9. The cycloadducts have then been used to provide interesting fused carbocycles [28] with high enantiomeric purity as shown in Scheme 3.10. 

The use of Lewis acids (ZnU, BF3 Et20) in ionic liquids, tested in the cycloaddition of but-3-en-2-one with isoprene, increases both the rate and selectivity of the reaction. The ionic liquid remains catalytically active after the work-up and can be reused. 

The Diels Alder reactions of maleic anhydride with 1,3-cyclohexadiene, as well the parallel reaction network in which maleic anhydride competes to react simultaneously with isoprene and 1,3-cyclohexadiene [84], were also investigated in subcritical propane under the above reaction conditions (80 °C and 90-152 bar). The reaction selectivities of the parallel Diels-Alder reaction network diverged from those of the independent reactions as the reaction pressure decreased. In contrast, the same selectivities were obtained in both parallel and independent reactions carried out in conventional solvents (hexane, ethyl acetate, chloroform) [84]. 

The next criterion was the selection of materials that would yield the desired mechanical properties. Butyl elastomer (HR), a copolymer of isobutylene with a small amount of isoprene, has outstanding low-temperature properties and very high damping, but has very high creep without cross-linking." It was theorized that a blend of SIBS TPE and butyl elastomer, filled to achieve the required minimum density of 2.4 g cm, would be a promising composite for less-lethal ammu-... 

Many of these cobalt complexes will catalyze the reduction of organic compounds by borohydride, hydrazine, thiols, etc. Cobalt cyanide complexes will catalyze the reduction of a,j8-unsaturated acids by borohydride (105) DMG complexes the reduction of butadiene and isoprene by borohydride, but not by H2 (124) Co(II) salen, the reduction of CHCI3 and CH3CCI3 to the dichloro compounds by borohydride (116) and cyanocobalamin, the selective reduction of -CCI2- by borohydride to -CHCl- in compounds such as aldrin, isodrin, dieldrin, and endrin without... 

Figure 3.41 Conversion of isoprene and selectivity for citraconic anhydride as a function of temperature and catalyst used [27],...

Table 5 summarizes the reactions of isoprene with aromatic aldehydes and unsaturated aldehydes. Salicylaldehyde provides the expected product as a cyclic boric ester derivative and shows apparently lower stereoselectivity, giving a mixture of 1,3-anti and 1,3-syn isomers in a ratio of 6 1 (run 1, Table 5). 2-Furfural reacts as usual and provides a 1,3-anti isomer as a single diastereomer in good yield (run 2). Unsaturated aldehydes, irrespective of their substitution patterns, undergo homoallylation selectively with excellent 1,3-anti selectivity, the geometry of the double bond of the starting aldehydes remaining intact (runs 3-5). 1,2-Addition to unsaturated aldehyde takes place selectively and no 1,4-addition is observed.

The present homoallylation with isoprene under Ni-Et3B catalysis shows marginal success for the reaction with aliphatic aldehydes. Results are summarized in Table 6. Primary alkyl aldehydes (bearing no a-substituents) and sterically less-hindered secondary alkyl aldehydes undergo the homoallylation successfully to provide the expected products in good yields with excellent stereoselectivity (runs 1-5). The results in runs 3-5 indicate that the present reaction shows almost no diastereofacial selectivity with respect to the a-stereo centers of secondary alkyl aldehydes. Sterically demanding aldehydes, such as cyclohexanecarbaldehye and pivalaldehyde, provide the... 

The reaction can be performed in one flask with great operational ease a mixture of an aldehyde and p-anisidine is stirred in THF for 5-10 h at 50 °C. Then, without removing the water produced, Ni(acac)2, isoprene, and Et2Zn are added in this order at room temperature. The mixture is stirred at the same temperature for the period of time indicated (Table 8). The products 57 and 58 are isolated as a mixture by column chromatograph after the usual work-up. Table 8 demonstrates the scope regarding the kind of aldehyde that encompasses not only aromatic aldehydes but also aliphatic aldehydes and even the parent formaldehyde. Despite the diminished electrophilic reactivity of aldimines, the reaction is complete at room temperature within a reasonable reaction time. The reaction of aldimines proceeds in an opposite sense of stereoselectivity to that of aldehydes and selectively provides 1,3-syn isomers 57. 

The change in selectivity is not credited to the catalyst alone In general, the bulkier the alkyl residue of the diazoacetate is, the more of the m-permethric acid ester results 77). Alternatively, cyclopropanation of 2,5-dimethyl-2,4-hexadiene instead of l,l-dichloro-4-methyl-l,3-pentadiene leads to a preference for the thermodynamically favored trans-chrysanthemic add ester for most eatalyst/alkyl diazoacetate combinations77 . The reasons for these discrepandes are not yet clear, the interplay between steric, electronic and lipophilic factors is considered to determine the stereochemical outcome of an individual reaction77 . This seems to be true also for the cyclopropanation of isoprene with different combinations of alkyl diazoacetates and rhodium catalysts77 . 

The arene organolanthanide system, Nd(C6H6)(AlCl4)3/AliBu3 (Al/Nd = 30), also induces the catalytic polymerization of isoprene to give cis-1,4-polymers having 92-93% selectivity at low conversion (17-36%). Neither the... 

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