Linear olefin epoxidation

The results confirm the previously reported low reactivity of cyclohexene when the catalyst is TS-1 and indicate that Ti-beta is active for the oxidation of cyclohexene and other bulky olefins. However, for cyclohexene and the linear olefins, the major reaction products formed in the presence of Ti-beta are glycols and glycol ethers, whereas in the presence of TS-I, epoxides are predominantly formed. Also in this case, the epoxides initially formed in the presence of Ti-beta undergo secondary reactions catalyzed by the acidic centers associated with the aluminum in the material, as previously seen for allyl alcohol and for the epoxidation of 1-butene on aluminum-containing TS-1 (Bellussi et al., 1991a). A different product composition was observed for cyclododecene,... 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

It is clear from the preceding discussion that several systems are now available for epoxidations with aqueous H202. The key features are compared in Table 4.5 for epoxidation of cyclohexene and a terminal olefin. They all have their limitations regarding substrate scope, e.g. TS-1 is restricted to linear olefins and... 

The number of metal zeolites and their application to the epoxidation of olefins rose in parallel from the late 1980s. TS-2, Ti,Al-P, Ti-P, Ti-MWW and, rarely, Ti-MOR are catalysts that have been studied in some detail [7-9, 35, 77-84]. TS-2 behaves, according to the few studies published, similarly to TS-1. The greater spaciousness of pores in Ti-Beta zeolites and of external cups in Ti-MWW allows the epoxidation, under mild conditions, of olefins unable to diffuse in TS-1 and TS-2, such as methylcyclohexenes, cyclododecene, norbornene, camphene and methyl oleate [80-83]. Steric constraints still prevail over electronic factors, however, as in medium pore Ti-zeolites, even in the epoxidation of linear olefins (Table 18.9). It is generally believed that active sites and epoxidation mechanisms are not significantly different from those of TS-1. 

An added advantage of the TS-1 catalyst, which could have commercial benefits, is the possibility for accomplishing shape-selective epoxidations. Owing to the limited dimensions (5.6 A X 5.4 A) of its micropores, linear olefins are epox-idized much faster than branched or cyclic olefins, e.g., 1-hexene is smoothly epoxidized while cyclohexene is virtually unreactive [45]. This reactivity is completely the opposite to that observed with the metal catalyst-alkyl hydroperoxide reagents (see earlier). It could be utilized in, for example, the selective epoxidation of linear olefins in mixtures of linear and branched or cyclic olefins. 

The first part concludes with a discussion of the similarity between the mechanisms of initiation and chain transfer, the appreciation of which led to the inifer concept, which in turn yielded new telechelics, networks, sequential copolymers, etc. The second part of this presentation focuses on practical consequences of understanding details of the mechanism of initiation. The synthesis of a new family of telechelic linear and tri-arm star polyisobutylenes will be described. Among the new prepolymers are telechelic olefins, epoxides, aldehydes, alcohols, and amines. The preparation of new ionomers and polyisobutylene-based polyurethanes will be outlined and some fundamental properties of these new materials will be discussed. 

Among such oxidations, note that liquid-phase oxidations of solid paraffins in the presence of heterogeneous and colloidal forms of manganese are accompanied by a substantial increase (compared with homogeneous catalysis) in acid yield [3]. The effectiveness of n-paraffin oxidations by Co(III) macrocomplexes is high, but the selectivity is low the ratio between fatty acids, esters, ketones and alcohols is 3 3 3 1. Liquid-phase oxidations of paraffins proceed in the presence of Cu(II) and Mn(II) complexes boimd with copolymers of vinyl ether, P-pinene and maleic anhydride (Amberlite IRS-50) [130]. Oxidations of both linear and cyclic olefins have been studied more intensively. Oxidations of linear olefins proceed by a free-radical mechanism the accumulation of epoxides, ROOH, RCHO, ketones and RCOOH in the course of the reaction testifies to the chain character of these reactions. The main requirement for these processes is selectivity non-catalytic oxidation of propylene (at 423 K) results in the formation of more than 20 products. Acrylic acid is obtained by oxidation of propylene (in water at 338 K) in the presence of catalyst by two steps at first to acrolein, then to the acid with a selectivity up to 91%. Oxidation of ethylene by oxygen at 383 K in acetic acid in... 

Longer chain olefins are similarly epoxidized, with yields in die range of 80-98% (13). The rate of reaction strongly depends on structural features of the olefin including chain length, presence of substituents, and position and steric configuration of the double bond (13). As a result, a different order of reactivity is shown by TS-1 as compared to other epoxidation catalysts a-olefin > internal olefin, linear olefin > branched and cycloolefin, linear-C > linear-C +,. C -2-butene reacts 16 times faster than the trans-isomer (J3). 

Olefins Oxidative Cleavage Photoirradiation (>280 nm) to an acetonitrile solution containing various cyclic and linear olefins with mesoporous silica containing isolated Ti-oxide species produces the corresponding epoxide with high selectivity (>98 %) [2]. 

Additioaal uses for higher olefias iaclude the productioa of epoxides for subsequeat coaversioa iato surface-active ageats, alkylatioa of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomeriza tion to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from a-olefias either by direct hydroalumination or by transalkylation. In addition, a number of heavy olefin streams and olefin or paraffin streams have been sulfated or sulfonated and used in the leather (qv) iadustry. 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

The results of the olefin oxidation catalyzed by 19, 57, and 59-62 are summarized in Tables VI-VIII. Table VI shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbomene) are selectively oxidized to epoxides. Cyclopentene shows exceptional behavior, it is oxidized exclusively to cyclopentanone without any production of epoxypentane. This exception would be brought about by the more restrained and planar pen-tene ring, compared with other larger cyclic nonplanar olefins in Table VI, but the exact reason is not yet known. Linear inner olefin, 2-octene, is oxidized to both 2- and 3-octanones. 2-Methyl-2-butene is oxidized to 3-methyl-2-butanone, while ethyl vinyl ether is oxidized to acetaldehyde and ethyl alcohol. These products were identified by NMR, but could not be quantitatively determined because of the existence of overlapping small peaks in the GC chart. The last reaction corresponds to oxidative hydrolysis of ethyl vinyl ether. Those olefins having bulky (a-methylstyrene, j8-methylstyrene, and allylbenzene) or electon-withdrawing substituents (1-bromo-l-propene, 1-chloro-l-pro-pene, fumalonitrile, acrylonitrile, and methylacrylate) are not oxidized. 

Chiral ketone catalysts of the Yang-type (5a and 5b, see above) and of the Shi-type (10, Scheme 10.2) have been successfully used for kinetic resolution of several racemic olefins, in particular allylic ethers (Scheme 10.4) [28, 29]. Remarkable and synthetically quite useful S values of up to 100 (ketone 5b) and above 100 (ketone 10) were achieved. Epoxidation of the substrates shown in Scheme 10.4 proceeds with good diastereoselectivity. For the cyclic substrates investigated with ketone 10 the trans-epoxides are formed predominantly and cis/trans-ratios were usually better than 20 1 [29]. For the linear substrates shown in Scheme 10.4 epoxidation catalyzed by ketone 5b resulted in the predominant formation of the erythro-epoxides (erythro/threo-ratio usually better than 49 1) [28]. 

The results of the olefin oxidation catalyzed by 1 to 6 are summarized in Tables 1-3. Table 1 shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbornene) are selectively oxidized to epoxides. Cyclopentene shows an exceptional behavior it is oxidized exclusively to cyclopentanone without any produc-... 

A third difference concerns Ti-MWW only. The siting of Ti in different porous environments, that is in external pockets, in internal supercages and in sinusoidal 10-MR channels, leads to active species associated with different diffusional and steric constraints [79]. Thus, the epoxidation of bulky olefins can occur exclusively in external pockets, whereas the linear ones are not subject to site limitations. Ti-MWW is also an unusual catalyst in the epoxidation of stereoisomers. At odds with TS-1 and Ti-Beta zeolites, trons-olefins are epoxidized faster than their as analogues [85]. Though the mechanism is still unclear, a better fitting of the trans configuration to the tortuous nature of 10-MR channels could be an explanation. 

Geometric constraints and related factors including active site accessibility, steiic effects of transition states and diffusion limitation of reactants and products play a crucial role in liquid phase catalyzed reactions [240]. Several examples are presented hereafter for illustration (i) TS-1 is more active in the oxidation of linear vs. branched alcohols [241], and in the epoxidation of linear vs. cyclic olefins [153] (ii) in the presence of TBHP, TS-1 has no activity [242], while Ti-6 is less active than Ti-MCM-41 [243] (iii) large TS-1 particles are less effective catalysts than smaller ones [244]. 

Hydrocarboxylation of the Ce-Cs a-olefins with cobaltcarbonyl/pyridine catalysts at 200 °C and 20 MPa gives predominantly the linear carboxylic acids. The acids and their esters are used as additives for lubricants. The Ce-Cio a-olefins are hydroformylated to odd-numbered linear primary alcohols, which are converted to polyvinylchloride (PVC) plasticizers with phthalic anhydride. Oligomerization of (preferably) 1 -decene, applying BF3 catalysts, gives oligomers used as synthetic lubricants known as poly-a-olefins (PAO) or synthetic hydrocarbons (SHC) [11, 12]. The C10-C12 a-olefins can be epoxidized by peracids this opens up a route to bifunctional derivatives or ethoxylates as nonionic surfactants [13]. 

Epoxy resins based on glycidylation of bisphenols, cresol and phenol novolacs, polycarboxylic acids, polyols, amines, and aminophenols have been long known. Epoxidized linear and cyclic olefins have also been used as specialty epoxy resins. More recently, glycidylated heterocycles have been introduced, initially as specialty resins promising improved resistance to weathering. One heterocycle in particular, the hydantoin ring, has become of particular interest as an epoxy substrate (J ). 

Quantitative epoxidation of olefin-telechelic polyisobutylenes yielded interesting new epoxy-telechelic products. Isomerization of the linear epoxy-telechelic polyisobutylene quantitatively yielded aldehyde-telechelic chains ... 

In many examples the reactivity has been discussed in terms of strain release. Whereas it is apparent that some strain is released in addition reactions, other factors have to be considered. Thus, a linear correlation between the rate of epoxidation and the ionization potential of bridgehead olefins has been observed (227). Other factors such as the polarizability and hyperpolarizability, which are associated with the outer region of the electronic structure of molecules as well as with intermolecular forces and chemical reactivity have not yet been considered (277). 

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