Dimerization of a-Methylstyrene

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]. 

We also obtained NMR spectra of the phenylindanyl cation 13 in the large-pore zeolite HY, and a small amount of cation 14 was formed on HZSM-5 by dimerization of a-methylstyrene. The dimethylphenyl carbenium ion 15 was not persistent on any zeolite we examined. This is not surprising if one reads the solution acid literature. 15 cannot be observed in 100% H2S04 stabilizing this cation requires 30% oleum (S03/H2S04) or other superacids (115). HZSM-5 is not a superacid. The observation of the much less stable styryl cation 11 was hailed as a triumph of superacid solution chemistry (116). If the styryl cation, with the phenyl group provid-... 

Harmer and co-workers996,997 tested various Nafion-based samples in the dimerization of a-methylstyrene to form isomeric pentenes and a cyclic dimer, which are of industrial interest 13% and 40% Nafion-silica nanocomposites exhibited near-complete conversion and gave the cyclic dimer 300 with high selectivity [Eq. (5.360)], whereas compound 299a was the main product over Nafion NR50. In sharp contrast, isomeric pentenes 299 could be obtained in a continuous process (86% selectivity at conversion >95%). Similar findings were reported with Nafion immobilized in MCM-41 mesoporous silica. In kinetic studies, 13% Nafion-silica and the catalyst with... 

The dimer of a-methylstyrene is useful in controlling molecular weight in polystyrene manufacture.467468 The control of molecular weight with... 

The activity of Nafion as a catalyst has been increased by applying it on silica by the sol-gel method using tetramethoxysilane20 or tetraethoxysilane and dimethyl-diethoxysilane.21 This increased its activity as a solid acid catalyst up to 100 times that of bulk Nafion. It performed better than Nafion on carbon in the dimerization of a methylstyrene. It was a much better catalyst (seven times on a weight basis) than Amberlyst 15, a typical sulfonic acid ion-exchange resin. It was more active in the benzyla-tion of benzene (6.6) than trifluoromethylsulfonic acid, a reaction in which Amberlyst 15 and p-toluenesulfonic acid... 

Nafion resins have been used not only for the opening of epoxides but also for their isomerization to aldehydes or ketones [137]. Various other rearrangements and isomerizations are catalyzed by this solid acid, in some cases with selectivities higher than those obtained with other solid catalysts [138-140]. Other reactions that have been studied include the Peterson methylenation of carbonyl compounds [141], hetero-Michael additions to unsaturated ketones [142], the Koch-type carbon-ylation of alcohols to form carboxylic acids [143], dimerization of a-methylstyrene [144], addition of carboxylic acids to olefins [145] and Diels-Alder reactions [146]. Notably, in most cases, reutilization of the catalyst is considered but only after an appropriate washing protocol to regenerate its acidity/activity. 

The activity of Nafion composites of greater surface area was investigated in different organic reactions, e. g. Friedel-Crafts alkylation and acylation, the Fries rearrangement, the dimerization of a-methylstyrene, esterification reactions, and isobutane alkylation. 

In Figure 10.29 another spectrum is reported, namely the MALDl-TOF spectrum of a copolymer containing units of a-methylstyrene (A) and units of methylmethacrylate (B). The sample is a reference material denoted SRM1487, and it was obtained by anionic polymerization using a bifunctional initiator, namely the dimer of a-methylstyrene (sodiirai salt). The most intense peaks are between... 

Carbon-based soUd acids with high surface areas have been also reported. SOjH-modified amorphous carbon/mesoporous sihca SBA-15 composites showed a distinct activity for a hydrophobic acid-catalyzed reaction, the dimerization of a-methylstyrene, whereas bulk carbon-based acids were completely inactive [72]. 

The dimerization of a-methylstyrene leads to the desired linear 2,4-diphenyl-4-methyl-l-pentene (1-PT) product as well as two other dimers, as shown in Scheme 8. Consequently, selectivity toward 1-PT is crucial for the catalyst. The use of HMIm BF4 at 60 °C led to selectivities... 

Styrene derivatives with a bulky substituent at the a or P position to the vinyl group cannot be made into high polymers because of the steric hindrance of the substituent. Consequently, these monomers give dimers exclusively at high temperatures irrespective of the kind of initiator few studies, however, deal with the dependence of the structure of these dimers on reaction conditions and on the nature of counteranions. This section briefly discusses cationic dimerization of a-methylstyrene ( MS) and anethol (P-methyl-p-methoxystyrene) as a- and P-substituted styrenes, respectively. 

Commercial poly(styrenesulfonic acid) ion exchange resins catalyze hydrolyses of carboxylic acid derivatives, acetal and ketal formation, esterification, condensations, alkylations and rearrangements. The catalyst is filtered from reaction mixtures. In examples such as the dimerization of a-methylstyrene in equation (20), higher yields and cleaner products have been obtained than with p-toluenesulfonic acid or sulfuric acid as the catalyst. 

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

Gumylphenol. -Cumylphenol (PGP) or 4-(1-methyl-l-phenylethyl)phenol is produced by the alkylation of phenol with a-methylstyrene under acid catalysis. a-Methylstyrene is a by-product from the production of phenol via the cumene oxidation process. The principal by-products from the production of 4-cumylphenol result from the dimerization and intramolecular alkylation of a-methylstyrene to yield substituted indanes. 4-Cumylphenol [599-64-4] is purified by either fractional distillation or crystallization from a suitable solvent. Purification by crystallization results in the easy separation of the substituted indanes from the product and yields a soHd material which is packaged in plastic or paper bags (20 kg net weight). Purification of 4-cumylphenol by fractional distillation yields a product which is almost totally free of any dicumylphenol. The molten product resulting from purification by distillation can be flaked to yield a soHd form however, the soHd form of 4-cumylphenol sinters severely over time. PGP is best stored and transported as a molten material. 

Arylalkenes can undergo various reactions when treated with sodium, since compounds such as a-methylstyrene are both olefins with allylic hydrogens and styrenes, both of which are reactive. The reaction of a-methylstyrene with sodium has been reported by Bergmann et al. (6i) to yield tetramers. More recent work by Kolobielski and Pines (56) has shown that dimers and products derived from dimers are formed when this compound is heated with a sodium-benzyl-sodium catalyst. Some of the major products were cumene (VII), p-terphenyl (VIII), and 1-methyl-1,3-diphenyl-cyclopentane (IX). 

Acid-treated clays Alcylation reactions (e.g. of benzene with benzyl chloride) Dimerization reactions (e.g. of a-methylstyrene) Etherification reactions (e.g. of ferf-butanol with methanol) Condensation reactions (e.g. of cyclohexanone) Separation of close boiling aromatic amines Separation of isomers of xylene... 

Prolonged contact of a-methylstyrene with potassium or sodium-potassium alloy in tetrahydrofuran yields a living, tail-to-tail dimer, i.e. 

It will be shown later (see section 6) that the addition of a-methylstyrene to both ends of the dimer proceeds much faster than further growth of the resulting tetramer. Consequently, a living tetramer, denoted by t2, may be prepared by the addition of the required amount of a-methylstyrene to the living a-methylstyrene dimer (K+, -aa, K+). The structure of T2 follows from the method of preparation, namely,... 

Note that zt gives the concentration of a-methylstyrene units added to -aa-, whether they form the trimer or the tetramer. It is implicitely assumed that the added unit has the same chance to dissociate whatever the state of the other end of the dimer. 

The monomer units in I are linked tail to tail and not in the normal head-to-tail manner. Therefore I is not comparable with a normal dimer. In substance II a dimer lies to the right of the tail-to-tail linkage. In the copolymerization of a-methylstyrene with, for example, methyl methacrylate, the addition of one molecule of a-methylstyrene corresponds to substance I,... 

Chloro-2,4-diphenyl-4-methylpentane (adduct of a-methylstyrene dimer and hydrogen chloride) Difurylmethane... 

In principle, dimerization is reversible and the equilibrium is rapidly established. The equilibrium concentration of radical ions derived from vinyl monomers is so small that it cannot be detected by ESR (for monomer concentrations 10-1 mol dm-3 it is < 10-7 mol dm-3). The rate constant of dissociation of the dimeric dianion of a-methylstyrene (aMeS)... 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

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. 

The exo double bond is formed first in polymerizations of a-methylstyrene, but is later isomerized by protonic acid to the more stable endo isomer [14]. Carbocationic polymerizations initiated by protonic acids with extremely basic counteranions, as in triflic acid-initiated polymerizations of isobutene, produce predominantly the unsaturated dimer [285]. The exo dimer forms first and then isomerizes to the more stable endo isomer [Eq. (90)]. 

Because the concentration of carbocations in a real polymerization is very low, model NMR studies have been used to obtain a deeper insight into the nature of the growing species. These experiments are restricted to sufficiently stable carbocations, such as those derived from vinyl ethers. Styrene derivatives are not stable enough and participate in Friedel-Crafts alkylation. For example, derivatives of a-methylstyrene easily deproto-nate, dimerize and then form intramolecularly indan derivatives. 

Figure 24.4 Addition-fragmentation mechanism for the chain-transfer activity of a-methylstyrene dimer...

It is also possible to form the dimer directly by adding a-methylstyrene to a sodium emulsion in tetrahydrofuran (5). Its structure was examined and as predicted (19,20, 2i), it was shown (5) to be C (Ph) (CH3).CH2.CH2.C-(Ph)-(CH3). Now, the addition of two equivalents of a-methylstyrene to this living dimer produces a living tetramer. However, the equilibrium concentration of the monomer in contact with such a tetramer is at least 10 times greater than that observed in the system containing the directly formed tetramer 26). It is obvious, therefore, that the living tetramer formed from the living dimer must have a different structure from that of the directly formed tetramer. The mode of preparation of the former implies that this is... 

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