Disproportionation of alkenes

Although the noncatalytic thermal disproportionation of alkenes is symmetry-for-bidden, the formation of metal complex 1 with the four-centered cyclic intermediate makes the reaction symmetry-allowed.61... 

The metal-catalysed disproportionation of alkenes has been postulated to involve an adsorbed cyclobutane intermediate. However, since cyclobutanes are not products of the reactions and cyclobutanes do not react readily with the catalytic systems the cyclobutane model has been criticized. Instead it has been proposed that the reaction proceeds via a reversible rearrangement of two co-ordinated alkenes into a multi-three-centred complex which involves four -hybridized CHa units (Scheme 1). An important feature of this... 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Di-f-butyl sulfone is different from the other dialkyl sulfones in that RH is mainly alkene and not alkane [G(isobutene) = 3.2 and G(isobutane) = 1.2]. The preference for isobutene over isobutane means that the formation of the alkene cannot be due to disproportionation of two t-butyl radicals but is due to a hydrogen atom expulsion as suggested by Bowmer and O Donnell70... 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

The lower alkenes and the heavy alkenes must be "disproportionated" to give the full range of alkenes. In the second part of the process the higher and... 

Introduction of mesityl groups at the porphyrin ring can prevent the formation of the dimeric products and the reaction with dioxygen now leads to ruthenium(VI)-dioxo complexes of TMP (tetramesitylporphyrin) [35], The tram-Ru(VI)02-TM P species can catalyse the epoxidation of alkenes as well as whole range of other oxidation reactions. After transfer of one oxygen atom to an organic substrate Ru(IV)0-TMP is formed, which disproportionates to an equilibrium of Ru02 and llu ). 

The kinetics of the reaction of [Ru (0)2(poi with various alkenes have been investigated either in CH2Cl2/MeOH or CH2Cl2/Hpz. In the absence of MeOH or Hpz, clean kinetics are not always observed, presumably because dimerization or disproportionation of the intermediate Ru =0 species occurs at comparable rates as epoxidation. The reactions have the following rate law rate = k[Ru (0)2(por)][alkenel. The rate constants for the oxidation of styrene are in the range 6 x 10 —4.8 x 10 s. The oxidation of norbornene occurs at... 

Some typical epoxidations are listed in Table 3.1. The first Ru-catalysed epoxida-tion was reported in 1983 by James et al., in which RuBrlPPh XOEPl/PhlO/CHjClj was used to epoxidise styrene, norbomene and c/x-stilbene in low yields trans-stilbene was not oxidised [588]. It was later noted that tranx-RulOl lTMPl/Oj/C H aerobically oxidised cyclic alkenes, and a catalytic cycle involving Ru 0(TMP) was proposed, in which there is disproportionation of Ru(0)(TMP) to Ru(TMP) and fran -Ru(0)2(TMP), the latter epoxidising the alkene and the former being oxidised back to the latter by (Fig. 1.26) [46, 583]. Stilbene, tranx-styrene and norbomene were efficiently epoxidised by trani-RulOl lTMPl/lCl pyNOl/CgH [589], as was epoxidation of exo-norbomenes catalysed by trani-RulOl lTMPl/Oj/ CgHg [590]. 

The disproportionations of both ally lie and alkenic groups are described. 

Basic alcohol solutions have been found to be capable of reducing ir-allylpalladium complexes to the corresponding alkene (equations 90 and 91 ).278-283 In similar fashion, aqueous NaOH media has been reported to effect a disproportionation of the allylpalladium chloride dimer (equation 92).284... 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

Depending on the type of iron catalyst, the reaction seems to take different mechanistic pathways. According to Johannsen and Jorgensen s results, the catalytic cycle starts with the formation of nitrosobenzene 32 either by disproportionation of hydroxylamine 29a to 32 and aniline in the presence of oxo iron(IV) phthalocyanine (PcFe4+=0) or by oxidation of 29a [131]. The second step, a hetero-ene reaction between the alkene 1 and nitrosobenzene 32, yields the allylic hydroxylamine 33, which is subsequently reduced by iron(II) phthalocyanine to afford the desired allylic amine 30 with regeneration of oxo iron(IV) phthalocyanine (Scheme 3.36). That means the nitrogen transfer proceeds as an off-metal reaction. The other byproduct, azoxybenzene, is probably formed by reaction of 29a with nitrosobenzene 32. 

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

Ivin et al. doubt the general possibility of alkene insertion into the transition metal—carbon bond [303]. But insertion into the metal—H bond is regarded as established. They noted the similarity of disproportionation [303] and ZN catalysts, and of the respective reactions. They postulated the following mechanism of homogeneous and heterogeneous alkene polymerization. 

Intramolecular disproportionation of [1] could conceivably result in the formation of unsaturated aldehyde or ester. Both paths are well known to occur with cyclic alkanones (la). Abstraction of H by the acyl radical portion of [1] would produce alkenal [3] while abstraction of by the alkyl radical portion of [1] would produce ketene [4] which may be efficiently trapped in nucleophilic alcoholic solvents to yield ester [5]. The intramolecular nature of alkenal formation is supported by deuterium labeling experiments (5), while the intramolecular nature of ketene formation is supported by (a) deuterium labeling experiments (6,7), and (b) the observed decrease in ketene formation with decreasing ring size (8). 

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