Dimerization and aromatization

Several single and binary oxides have a capacity to oxidize propene to dimerization products. The first compound formed is 1,5-hexadiene, which may undergo further dehydrogenation and cyclization leading to benzene. 

Many authors assume that the initial reaction step in the dimerization is identical with that in the acrolein production, namely hydrogen abstraction and formation of an allylic intermediate. Dimerization is then supposed to occur because the ability to oxidize the allyl radical to acrolein is absent. 

The best known dimerization catalysts are Bi203, bismuth salts and binary oxide mixtures containing Bi203. A very effective catalyst is Bi203— Sn02, in particular for the production of benzene. 

Bismuth phosphates and various other bismuth salts (e.g. arsenate, basic sulfate, and titanate) are capable of producing benzene, as reported by Seiyama et al. [283]. A selectivity of 49% is reached with a combination 2Bi203 P205 at 500°C. Sakamoto etal. [271] varied the Bi/P ratio and stated that a 2/1 ratio gives the maximum selectivity. 

The Bi203—Sn02 combination was studied by Solymosi and Bozso [299] and by Seiyama et al. [284,285]. The former carried out pulse experiments in the absence of oxygen and report that even small amounts of Sn02 added to Bi203 have a promoting effect and shift the product spectrum from hexadiene to benzene. The best combination is a mechanical mixture of the two oxides in a 1/1 ratio. With this catalyst, a selectivity of 80% (benzene) is reached at a 40% conversion level (at 500° C), 

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Dimerization and aromatization have been reported for isobutene and n-butenes, analogous to propene, over catalysts like Bi203. Isobutene radicals dimerize more easily than n-butene radicals, which are less stable and rapidly form butadiene. 

The dimerization and aromatization of olefins occurs in consecutive reaction steps. Isobutene, for example, reacts as follows... 

Noncarbonyl transition metal complexes catalyze dimerization and aromatic cyclo-trimerization of ethynylcyclopropane. The product composition depends on the catalyst and the reaction conditions. Thus, Co(acac)2 in the presence of phosphines and AIEt2Cl afforded either the dimer 1,3-dicyclopropyl-1 -butyn-3-ene or a mixture of 1,2,4- and 1,3,5-tris(cyclopropyl)benzenes, whereas Pd(0 Ac)2 gave the same dimer in the presence of PPh3 but only a tris(cyclopropyl)fulvene in the absence of phosphines (equation 176)245. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

On heating, the dichlorooxazolo[3,4- ]azepine 26, for which dimerization is prevented by the chloro groups, undergoes ring contraction and aromatization, involving a [l,2]-chlorine shift, to 5,8-dichloro-l,4-dihydro-2//-3,l-benzoxazin-2-one (27).11,153... 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

N-Substituted amides can be prepared by direct attack of isocyanates on aromatic rings.The R group may be alkyl or aryl, but if the latter, dimers and trimers are also obtained. Isothiocyanates similarly give thioamides. The reaction has been carried out intramolecularly both with aralkyl isothiocyanates and acyl isothiocyanates.In the latter case, the product is easily hydrolyzable to a dicarboxylic acid this is a way of putting a carboxyl group on a ring ortho to one already there (34 is... 

Although the above profusion of in vivo studies evidence their health potentialities, the problem of the bioavailabihty of proanthocyanidins supplied by dietary supplementation has still not been completely resolved since unequivocal evidence for absorption is missing so far [11]. However, studies carried out using radio-labelled procyanidins revealed that dimers and trimers may be absorbed by intestinal cells, whereas a recent study demonstrated that procyanidin oligomers are readily adsorbed in rats [55], while it has been shown that colon microflora may be able to degrade proanthocyanidins to low-molecular-weight aromatic compounds [56]. 

Another method for reductive dimerization has been developed in hy-drosilylation. NiCl2-SEt2 is an effective catalyst in silylative dimerization of aromatic aldehydes with a hydrosilane (Scheme 12) [40]. A catalytic thiolate-bridged diruthenium complex [Cp RuCl(/ 2-SPrI)2RuCp ][OTf] also induces the conversion to 1,2-diaryl-1,2-disiloxyethane [41]. A dinuclear (siloxyben-zyl)ruthenium complex is considered to be formed, and the homolytic Ru - C bond fission leads to the siloxybenzyl radicals, which couple to the coupling product 14. 

Fatty acids have also been converted to difunctional monomers for polyanhydride synthesis by dimerizing the unsaturated erucic or oleic acid to form branched monomers. These monomers are collectively referred to as fatty acid dimers and the polymers are referred to as poly(fatty acid dimer) (PFAD). PFAD (erucic acid dimer) was synthesized by Domb and Maniar (1993) via melt polycondensation and was a liquid at room temperature. Desiring to increase the hydrophobicity of aliphatic polyanhydrides such as PSA without adding aromaticity to the monomers (and thereby increasing the melting point), Teomim and Domb (1999) and Krasko et al. (2002) have synthesized fatty acid terminated PSA. Octanoic, lauric, myristic, stearic, ricinoleic, oleic, linoleic, and lithocholic acid acetate anhydrides were added to the melt polycondensation reactions to obtain the desired terminations. As desired, a dramatic reduction in the erosion rate was obtained (Krasko et al., 2002 Teomim and Domb, 1999). 

Many selenoketones18 20 and telluroketones21 23 can be obtained in the solid state as stable 1,3-diselenetanes and 1,3-ditelluretanes and their formation can be assumed as proof of the existence of unstable selenoketone and telluroke-tone. In solution dimers of aromatic selones equilibrate with monomeric species, according to Scheme 5, where equilibrium is shifted by dilution towards the monomer.18,24... 

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