Alkenes catalytic isomerization

Isoforming A process for increasing the octane rating of thermally cracked gasolines by catalytic isomerization over silica/alumina. Terminal alkenes are thus converted to nonterminal alkenes. Developed by Standard Oil Company of Indiana in the 1940s. 

Comparison of the results for catalytic isomerization of pent-l-ene to trans-pent-2-ene with the basic and one-electron donating properties of the catalysts led to the conclusion that two different reaction mechanisms operate in double bond isomerization reactions (a) an ionic mechanism which involves proton abstraction from the alkene molecule by the super base site (pAia = 37 for pentenes) and (b) a free radical mechanism which involves the abstraction of a hydrogen atom from the alkene by the one-electron donor center (Scheme 39). 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

Isomerization of terminal olefins was achieved with the system Ln(C5H5)3/NaH (Ln = Y, Er, Lu). The reactions were carried out at 45 °C in THF and afforded cis- and frans-2-alkenes in very good yields. The catalytic isomerization may occur via organolanthanide hydride intermediates. The proposed mechanism is depicted in Scheme 9 [73]. 

In addition to the most important 1,2-difunctionalization assisted or catalyzed by palladium(II) complexes, a catalytic 1,1-arylamination process of alkenes, applied to the construction of nitrogen heterocycles from 4-pentenylamides, was realized29,30. The mechanism involves the formation of arylpalladium chloride from alkyl(aryl)stannanes, the addition to the alkene, the isomerization of the adduct to the more stable benzylic palladium complex, and the displacement of palladium by an internal nitrogen nucleophile. In the presence of a substituent, mixtures of diastereomers were generally obtained. 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

The supply of branched alkenes from cracking processes is limited and there is a need for catalytic isomerization processes to increase the supply. One method is to isomerize butane and then to dehydrogenate the isobutane (ref. 5). 

In conclusion, partially dehydroxylated oxide surfaces exhibit a large inventory of surface OH groups and water molecules together with Lewis acidic and Lewis basic sites with coordinative unsaturation (structures II and III of Scheme 1). The hydroxyl population is the souree of protons that cause enhanced surface electrical conductivity and catalytic activity. It is significant that the increase in the conductivity value is paralleled by increases in either the amount of weakly bound protons or their mobility [48]. Almost all metal oxides are active in catalytic isomerization of alkenes, which is one of the least demanding reactions in terms of the requirements for the acid strength of active sites [34]. Studies on several oxide systems show that the activity is lost after extensive dehydration and is partially restored by... 

The Chalk-Harrod type mechanism is proposed to be operative in the reaction of ethene with HSiMes, HSiEts and HSiPhs catalyzed by an early-late heterobimetallic complex, Cp2Ta(CH2)2lr(CO)2 . The proposed catalytic cycle is very similar to the one shown in Scheme 5, wherein [M] is Cp2Ta(CH2)2lr(CO), except for the dissociation of CO at the oxidative addition of HSiR3 and the coordination of CO at the reductive elimination of EtSiRs. This catalyst, however, only promotes the hydrosilylation of ethene, and higher alkenes are isomerized ". ... 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

Allylic sulfones and a, /5-unsaturated sulfones are known to be in equilibrium314-319. Allylic sulfones, such as 242, isomerize to a, /5-unsaturated sulfones 243 upon treatment with a catalytic amount of potassium t-butoxide in dry THF. The a, /5-unsaturated sulfones can be converted to the corresponding olefins upon desulfonation with sodium amalgam320 or aluminium amalgam294,321. Since treatment of allylic sulfones with potassium-graphite gives 2-alkenes, alkylation of allylic sulfones and subsequent desulfonation is a useful process for the synthesis of olefins, as shown in Scheme 6. 

The addition of a spillover proton to an adsorbed alkene to yield a secondary carbonium ion followed by abstraction of a proton from the C3 carbon would yield both isomers of 2-butene. The estimated faradaic efficiencies show that each electromigrated proton causes up to 28 molecules of butene to undergo isomerization. This catalytic step is for intermediate potentials much faster than the consumption of the proton by the electrochemical reduction of butene to butane. However, the reduction of butene to butane becomes significant at lower potentials, i.e., less than 0.1V, with a concomitant inhibition of the isomerization process, as manifest in Fig. 9.31 by the appearance of the maxima of the cis- and tram-butene formation rates. 

Complex a is readily converted into a Fe-y-H agnostic complex b within an early picosecond timescale and then the 7i-allyl hydride complex c is generated by hydride abstraction. The energy level of the 2-alkene isomer d, which is calculated by DPT experiments, is similar to that of the 1-alkene complex b. In the next step, Fe (CO)3(t -l-alkene)(ri -2-alkene) f, which is generated via intramolecular isomerization of the coordinated 1-alkene to 2-alkene and the coordination of another 1-alkene, is a thermodynamically favored product rather than formation of a Fe(CO)3(ri -l-alkene)2 e. Subsequently, release of the 2-aIkene from f regenerates the active species b to complete the catalytic cycle. 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

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