Dimerization of a-olefins

A head-to-head dimerization of a-olefin catalyzed by a bis(imino)pyridine iron complex has been reported by Small and Marcucci [126]. This reaction delivers linear internal olefins (up to 80% linearity) from a-oleftns. The linearity of products, however, depends on the catalyst structure and the reaction conditions. 

Linear (head-to-head) dimerization of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefrn mixture can be promoted when less sterically hindered examples of 1 (e.g., Ic) are employed in combination with MAO (Scheme 5.7) [48]. The mechanism for dimerization is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene, and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerization, albeit with lower activities [62]. Oligomerization of the a-olefrns propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems Ih and 2h yielding highly linear dimers [39]. 

Scheme 5.7 Use of Ic/MAO to mediate the head-to-head dimerization of a-olefins.

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

Constrained geometry chromium alkyls catalyzed the polymerization of ethylene however, the reaction was relatively slow, and elevated pressures (PC2H4 = 500 psi) were required to generate significant amounts of polymer. Not surprisingly then, no homopolymoization or copolymerization of a-olefins was observed. Instead, catalytic isomerization and dimerization of the alkyl-substituted olefins was found. 

The catalytic transformation of olefins by transition metal complexes has received a great deal of attention during the past two decades. These catalytic reactions are important, especially industrially, because they represent some of the most economical ways to synthesize olefinic monomers or polymers. The more common types of these transformation reactions are (a) dimerization or polymerization of a-olefins (b) dimerization, oligomerization, cyclooligomerization, or polymerization of con-... 

Methyldichlorosilane was by far the most reactive in hydrosilation of 1,1-disubstituted olefins. Trialkylsilanes did not add at all, even at 120°C. Trichlorosilane gave complicated results involving isomerization of olefins and dimerization of a-methylstyrene, and products were not optically active. 2-Methylbutene-2 and trichlorosilane gave two adducts, 2-meth-ylbutyltrichlorosilane and 3-methylbutyltrichlorosilane. The latter required isomerization of the olefin. 2,3-Dimethylbutene-l gave one adduct in 70% yield, and it was optically slightly active [0.8% (R) isomer]. 

Figure 14.1. (a) Orbital correlation diagram for nls + n2s dimerization of two olefins to form a cyclobutane. (b) Orbital correlation diagram for n4s + n2s cycloaddition of a diene and an olefin (the Diels-Alder reaction). 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

Dimerization of activated olefins or ketones as a means of ring closure was mentioned in Part I reports of more examples have been published.132 A special kind of coupling and ring closure occurs in the reduction of 6-phenyl-2,3-dihydrodiazepinium salt (76) in DMF to the diphenylpyrrolo-diazepine derivative (77). A plausible explanation of the formation of 77 involves dimerization of an initially formed radical, followed by intramolecular displacement of ethylenediamine.133-135... 

The dimeric hydride acts as a single-component, regiospedfic polymerization catalyst. It catalyzes the polymerization of a-olefins RCH = CH2 (R = Me, Et, nPr) to polyolefins with M = 4000-6000 and MJMn = 1.7-2.1. As in the case of [( 5Me5)Y(ORX/i-H)]2 the polymerizations are slow as compared to single-component zirconium catalysts [59]. The proposed mechanism is illustrated in Scheme 5 [52, 60, 61]. 

Dimeric homochirotopic [rac.-Me2Si(Me3Si, t-BuCp)2YH]2 is the first singlecomponent isospecific catalyst it is suitable to study the subtle steric factors that govern the remarkably high stereospecificities exhibited in the polymerisation of a-olefins by this and related two-component class III catalysts of C2 symmetry based on group 4 metallocenes. 

For instance, out of many studies carried out with crystalline olefins that possess slightly different interbond distances and orientations one would only answer Yes or No to questions of reactivity and selectivity. An oversimplified and pictorial representation of this analysis is illustrated in Fig. 2 with a reaction coordinate for the excited state dimerization of a hypothetical set of crystalline alkenes. The reaction coordinate in this case would be given primarily by the displacement between the two Tr-systems which, for simplicity, may be assumed to have the required parallel arrangement. In the figure, different crystals can be though of as positioning the prospective reactants at different distances. Each crystal represents a point along the reaction coordinate [50]. Some molecules may... 

Oxidation and Dehydrogenation.—Some novel oxidations of A -olefinic steroids are reported. Silver oxide, in refluxing benzene or toluene, converted cholesterol into the 6,6 -dimeric compound (148), probably through a one-electron process. The dimer decomposed above its m.p. to give an equimolar... 

An important aspect of photocycloaddition consists of its regio- and stereoselectivity. Thus, dimerization of substituted olefins generally yields head-to-head adducts 61a and 61b in a regiospecific reaction, while head-to-tail dimers 63 are obtained from 9-substituted anthracenes (62) in polar solutions (Applequist et al., 1959 cf. Kaupp and Teufel, 1980). 

Signal-time behavior of the ESR response following a current pulse has been calculated by Goldberg and Bard [367] for a number of mechanisms, including first-order decomposition, radical ion dimerization, and radical ion-substrate coupling, and working curves from which rate constants can be calculated were presented. Application of this approach, which is very similar to that taken, for example, in transmission spectroelec-trochemistry, was demonstrated for the reductive dimerization of a series of activated olefins (Fig. 58), a reaction that has been studied by a number of different electrochemical... 

Figure 14.1. a) Orbital correlation diagram for dimerization of two olefins to form a... 

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