Ethylene cyclodimerization

The effect of various ligands on the yield of DMCDeT is illustrated in Table X and should be compared with the cyclodimerization of butadiene and the co-oligomerization of butadiene with ethylene (Tables III and IX). 

Tetrabenzyltitanium or two-component titanium catalysts of the Ziegler type have been observed to catalyze the cyclodimerization of ethylene and 1,3-butadiene to vinylcyclobutane [Eq. (16)] (128). While propylene... 

The reaction of bis(77 -ethylene)(77 -toluene)iron 117 with/-butylphosphaethyne, at a temperature no more than room temperature, produced, in addition to cyclodimerization of the ethyne, a mixture of a coordinated 1,3-phosphete in (77 -2,4-di-/-butyl-l,3-diphosphete)(77 -toluene)iron 118 and penta-/-butyl-(l,2,4-triphospholyl)(l,3-diphospholyl)iron... 

Contrary to the parent compound, double bond disubstituted methylenecyclopropanes (70) react with Ni(0) or Pd(0) catalysts by cleavage of the C(2)-C(3) bond. In the presence of olefins such as ethylene, styrene or norbornene a relatively mild cyclodimerization into methylenecyclopentanes (71-73) is achieved in 75% or better yields (equation 47) . 

The last observations automatically lead to the conclusion that non-activated alkenes also could undergo these reactions. Indeed it was found that ethylene, norbornene, norbomadiene198) and allene 199) react with methylenecyclopropane to give cycloadducts (Scheme 7). The reason for the limitation to these alkenes lies in the ability of methylenecyclopropane to compete successfully with alkenes in it-complexation to the metal. Thus cyclodimerization of methylenecyclopropane is much faster than codimerization with other alkenes, which give less stable ic-com-plexes with Pd(0). 

By far the largest outlet for benzene (approx. 60%) is styrene (phenyl-ethene), produced by the reaction of benzene with ethylene a variety of liquid and gas phase processes, with mineral or Lewis acid catalysts, are used. The ethylbenzene is then dehydrogenated to styrene at 600-650°C over iron or other metal oxide catalysts in over 90% selectivity. Co-production with propylene oxide (section 12.8.2) also requires ethylbenzene, but a route involving the cyclodimerization of 1,3-butadiene to 4-vinyl-(ethenyl-) cyclohexene, for (oxidative) dehydrogenation to styrene, is being developed by both DSM (in Holland) and Dow. 60-70% of all styrene is used for homopolymers, the remainder for co-polymer resins. Other major uses of benzene are cumene (20%, see phenol), cyclohexane (13%) and nitrobenzene (5%). Major outlets for toluene (over 2 5 Mt per annum) are for solvent use and conversion to dinitrotoluene. 

The application of OGAMS to the paradigmatic cyclodimerization of ethylene was dealt with in detail in the primary publication on the method [9]. 

The discussion in this chapter has ranged well outside the main theme of the book. In addition to the writer s early involvement with secondary isotope effects, which can serve as partial extenuation, the mechanism of [2-f2]-cycloaddition has sufficient intrinsic interest to justify the digression. Orbital symmetry conservation plays but a small part in its mechanistic analysis, but it is a crucial one. Fig. 6.2 applies strictly only to the cyclodimerization of ethylene, or to an olefin symmetrically tetrasubstituted by substituents that do not add to the essential number of electrons involved in the reaction. Nevertheless, the principal conclusion drawn from it, that the initial plane-rectangular interaction of the two tt systems leads to formation of a bond between diagonally situated atoms, is remarkably robust. It can be applied to a variety of reactions with different electronic and steric requirements, provided that the specifics of each reacting system are kept firmly in mind. The wealth of diverse, superficially contradictory, experimental results cannot be fit into a consistent logical framework without it. 

In order to further illustrate how bond "ionicity" controls the Cl correction, we now consider the transition states of two different 2ir + 2tt cyclodimerizations of ethylene to form cyclobutane One involving s + s and the other s + a union of the two reactants. In bond diagrammatic terms these two transition states can be symbolized as follows a. s + s... 

Free-radical Reactions.—Publications have appeared dealing with the bidirectional addition of trifluoroiodomethane and hydrogen bromide across the C=C bond in the olefin CF3 CH CHMe, peroxide-initiated addition of 1,2-dibromotetrafluoroethane to ethylene, propene, and isobutene, the addition of pentafluoroiodoethane to 3,3,4,4-tetrafluorohexa-l,5-diene (see p. 29), peroxide-initiated cyclodimerization of 3,3,4,4-tetrafluoro-4-iodobut-l-ene (see p. 29), and telomers from tribromofluoromethane or tetrabromomethane and bromotrifluoroethylene as high-density fluids for gyroscope flotation. The telomerization of chloromethanes with tetra-fluoroethylene provides a measure of the relative reactivity for both chlorine and hydrogen abstraction by the CF2 CF2 radical. The chain-transfer... 

The kinetics of the ADMET reaction is not amenable to study by many traditional means, as these polymerizations are mostly conducted in bulk. The most effective way to measure the kinetics of the polymerization is to monitor the volume of evolved ethylene. This technique has been used to probe the difference in activity between [Mo] 2 and [Ru]l for ADMET polymerization of 1,9-decadiene [37]. At 26 °C in bulk monomer, [Mo] 2 promotes ADMET polymerization of 1,9-decadiene at a rate approximately 24 times that of [Ru]l. Additionally, [Mo] 2 polymerizes 1,5-hexadiene 1.7 times faster than 1,9-decadiene, while [Ru]l only cyclodimerizes 1,5-hexadiene to 1,5-cyclooctadiene. Monomers with coordinating functionality, specifically ethers and sulfides, were also investigated. Predictably, these monomers did not undergo polymerization as rapidly as hydrocarbon monomers however, this difference was dramatically more pronounced with [Ru]l than with [Mo]2. In fact, the dialkenyl sulfide monomers that were studied completely shut down the polymerization with [Ru]l, whereas the catalytic activity of [Mo]2 was only slightly lowered. This reduction in polymerization rate is most likely due to coordination of the heteroatom to the vacant coordination site of [Ru] 1, following phosphine dissociation. This reversible coordination of heteroatoms to the ruthenium complex likely occurs both intramolecularly and intermolecularly. Conversely, the steric bulk of the ligands in [Mo] 2 makes it less likely to intramolecularly form a coordinate complex, despite molybdenum being far more electrophilic than ruthenium. 

The trouble with this approach is that very few organic molecules have any symmetry and even those that do need not retain it during a reaction. Consider, for example, a classic example of this kind of argument, the cis-n-cyclodimerization of ethylene. 

The scheme considered is fully valid also in the case of cation-radical [2 + l]-cycloadditions. These reactions, like the corresponding thermal reactions of the [2 + 2]-cycloaddition of neutral molecules, are forbidden by the orbital symmetry conservation rules. The same calculations [95] have shown that the addition of the cation-radical of ethylene to a neutral ethylene molecule proceeds in an unconcerted and nonsynchronous fashion. Unlike the [2 + 2]-cyclodimerization of ethylene (Sect. 10.1.1), the [2 + l]-cycloaddition involves the formation of an intermediate LIII with the energy barrier calculated for this highly exothermal step being extremely low (1.3 kcal/mol by the MNDO method). A barrier lower still (1.0 kcal/mol) is expected for the step of transformation of LIII into the cation-radical of cyclobutane LIV in which the... 

Dodecamethylcyclohexasilane can now be prepared in very good yield by the one step reduction of MeaSiCla using lithium in excess. With a deficiency of lithium, however, larger cyclopolysilanes resulted (MeaSi) (x = 7, 8, or 9). A series of silyl and phenyl substituted cyclopolysilanes have been synthesized, and while ethylene oxide inserts into Li(SiPha)5Li, dimethylsilylene, generated from (MeaSi)6, will additively cyclodimerize adamantanone and norbornone through a mechanism thought to involve the formation of an oxasilacyclopropane intermediate. ... 

Ethylene oxide 3 may be cyclodimerized to 1,4-dioxan 4 in the presence of superacids, like chlorosulfonic or perchloric acid (Equation 1). The reaction was accelerated by aprotic dipolar solvents, but was retarded by less polar solvents. Chlorosulfonic acid also catalysed ring-opening polymerization of the saturated oxygen heterocycle oxepine 5 to the polymer 6 (Equation 2). ... 

Further experimental results (see also ref. 164) on the stereospecificity of [2 -1- 2] cycloadditions are available. Tetrafluoroethylene adds to both cis- and tra s-2-butene and to ethylene in a non-stereospecific manner, interpreted as evidence of biradical intermediates. Some stereospecificity is observed in allene-keten [2 + 2] cycloadditions and a one-step [,2, + J cycloaddition is indicated. Further deuterium-isotope studies indicate that the cyclodimerization of allenes by a [2 q- 2] addition is non-concerted, but their reactions in [3 + 2] and [4 + 2] cycloadditions are concerted. ... 

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