Dimerization and Polymerization of Alkenes

FIGURE 9.39 Dimeric products formed from 2-methylpropene. 

FIGURE 9.40 Many different acids add to 2-methylpropene (and other alkenes). Here we see HsO and the tert- mX cation adding to 2-methylpropene to give tertiary carbocations. In each case, addition occurs in the Markovnikov sense, the more stable carbocation is formed preferentially. 

Now we have a structure that contains eight carbons, as do the two new products. It looks as though we are on the right track. The only remaining trick is to see how to lose the extra proton and form the new alkenes. Losing H as shown in 

The proton can t just leave, however. It must be removed by a base. In concentrated sulfuric add, strong bases do not abound. There are some bases present, however, and the carbocation is a high-energy spedes, and thus rather easily deprotonated. Under these conditions, both water and bisulfate ion (HS04 ) are capable of assisting the deprotonation. 

Note the exact correspondence of the steps involved in alkene protonation and deprotonation. The two reactions are just the two sides of an equilibrium (Fig. 9.42). 

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The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

Simple a,3-unsaturated aldehydes, ketones, and esters (R = C02Me H > alkyl, aryl OR equation l)i, 60 preferentially participate in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, and selected simple alkene and alkyne dienophiles, - although the thermal reaction conditions required are relatively harsh (150-250 C) and the reactions are characterized by the competitive dimerization and polymerization of the 1-oxa-1,3-butadiene. Typical dienophiles have included enol ethers, thioenol ethers, alkynyl ethers, ketene acetals, enamines, ynamines, ketene aminals, and selected simple alkenes representative examples are detailed in Table 2. - The most extensively studied reaction in the series is the [4 + 2] cycloaddition reaction of a,3-unsaturated ketones with enol ethers and E)esimoni,... 

The insertion of alkenes into M-H bonds has been examined in Chap. 4. This reaction is very important because, it leads to the dimerization, oligomerization and polymerization of alkenes. It is broad and concerns not only transition metals, but also main-group metals (group 13 Lewis acids), lanthanides and actinides. For instance, AlEt3 is an excellent initiator of olefin polymerization. This reaction can also be considered as the hydrometallation or the hydroelementation of an olefin, and stoichiometric examples have been shown. If the element E does not have the property of a Lewis acid allowing olefin pre-coordination onto a vacant site and thus facilitating insertion, the insertion reaction is not possible without a catalyst. 

Acid-catalyzed dimerization and oligomerization of 1,2,4-trioxolanes will be covered in Section 4.16.5.2.1. In general, ozonides are not prone to spontaneous polymerization. Polymeric products can be obtained from the ozonolysis of alkenes but most likely arise from reaction of the primary ozonide. Bicyclic 1,2,4-trioxolanes such as 2,5-dimethylfuran endoperoxide can dimerize on warming in CCI4 (Section 4.16.5.1.1). 1,2,4-Trithiolane tends to polymerize at room temperature especially if left open to air, whilst more highly substituted ring systems are stable. 

Polymerization of alkene monomers, with or without functional groups, are very important industrial processes. Until recently the use of homogeneous catalysts was restricted to relatively small-volume production of specialty dimers and oligomers. The manufacture of the two largest-tonnage plastics— polyethylene and polypropylene—has so far been based on heterogeneous catalytic processes. 

However, the equilibrium monomer concentrations of disubstituted alkenes is measurable. The equilibrium constants for dimerization, tri-merization, and polymerization of a-methylstyrene have been determined as a function of temperature under anionic conditions [12] similar values should be obtained under cationic conditions. Unfortunately, the equilibrium position can t be determined directly under cationic conditions due to the irreversible side reactions of isomerization and indan and spirobiindan formation (Section II. A). The equilibrium monomer concentrations of isobutene and isopropenyl vinyl ethers should also be relatively high, albeit lower than those of a-methylstyrenes. However, the true equilibrium can t be reached with these monomers due to irreversible side reactions, and reliable data are therefore not available. Nevertheless, the ceiling temperature of isobutene polymerization is apparently between 50 and 150° C. 

First of all we compared the behaviour of these catalysts in the benchmark reaction of ethyl diazoacetate (1) with styrene (2) (Scheme 1) using equimol amounts of both reagents or even a twofold excess of diazoacetate (Table 1). Under these conditions the selectivity with regard to diazoacetate was low and did not depend on the catalyst. This result was not unexpected because this reagent has a great tendency to dimerize and polymerize so a large excess of alkene is generally used. 

Diels-Alder reaction of the highly strained alkene (48) with benzene is one of the strikingly unusual reactions. The driving force of the unusual cycloaddition would be a favorable electron transfer from benzene to the low-lying LUMO of the highly electron-deficient double bond of 48 [15]. In contrast, the corresponding hydrocarbon (50) dimerizes and polymerizes rapidly at ambient temperature (Scheme 1.43) [16]. 

The polymerization and oligomerization of alkenes has been one of the most successful applications of organometallic chemistry to the synthesis of organic products on a large scale. As noted in the introduction to this chapter, organometallic complexes are involved in the synthesis of close to, or in excess of, fifty to one hundred million metric tons of polyolefins and a-olefins per year. In most cases, these products are formed by a series of alkene insertions into metal alkyl complexes in competition with p-hydrogen elimination processes. In other cases, selective dimerization or trimerization of alkenes occurs by the intermediacy of metallacyclic intermediates. 

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