Cyclopentadiene allyl cation

A similar transformation results when trimethylsilyloxy-substituted allylic halides react with silver perchlorate in nitromethane. The resulting allylic cation gives cycloaddition reactions with dienes such as cyclopentadiene. The isolated products result from desilyla-tion of the initial adducts ... 

In the presence of water the tropylium ion (50) has been found to undergo polar cycloaddition of the 4 + 2 type. Ito and Itoh found that cyclopentadiene reacted with the tropylium ion to afford a 90% yield of a complex mixture of alcohols, all of which could be derived from the allylic cation (51). 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

Silver(I) compounds are known to promote different kinds of cycloaddition. Reactions of 2-alkoxyallyl halides with 1,3-dienes in the presence of silver(I) compounds provide a beneficial route to cycloheptanones [2,3]. When a mixture of 2-(trimethyl-siloxy)allyl chloride 1 and cyclopentadiene (2) is treated with 2 equiv. AgC104 in THF-ether (1 2) at 0 °C, bicyclo[3.2.1]oct-6-en-3-one 3 is produced in 91 % yield [3] (Sch. 1). The 2-(trimethylsiloxy)allyl cation 4 is believed to be involved as a reactive species in the reaction. 

There is some evidence157 that the cycloaddition of the allyl cation (183) to cyclopentadiene (184) takes place with a transition state like 186 rather than 185. This is clearly in agreement with a frontier orbital analysis, provided that we look at the HOMO of the allyl cation and the LUMO of the diene. 

Treatment of p-methallyl iodide (4a X = I, Y = Me) with a silver salt gives a methallyl cation (5a Y = Me) which reacts with 1,3-dienes such as 1,3-butadiene, 1,3-cyclopentadiene or 1,3-cyclohexadiene to give the corresponding seven-membered ring compounds (20) and (21) and their isomers. However, cycloaddition of the parent allyl cation (5a) with 1,3-dienes generally produces satisfactory results. In fact, (5a Y = H) derived from allyl iodide (4a X = I, Y = H) and silver trifluoroacetate reacts with 1,3-cyclopentadiene to yield bicyclo[3.2.1]octa-2,6-diene (21 Y = H, n = 1) though in poor yield, along with... 

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions, although they are by far the most numerous and the most important. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.27, derived from its iodide 6.26, with cyclopentadiene giving a... 

The cycloaddition of a,a -dibromo (or dichloro) ketones with furan (or cyclopen-tadiene) gave very good yields when the reaction was conducted in pure water with iron powder. Furthermore, in the presence of triethylamine as the base, monobro-mo (or chloro) ketones react to furan (or cyclopentadiene) in water to afford the corresponding cycloadducts in near-quantitative yields (Eq. 5). In both cases, 2-oxy-allyl cation, the formation of which is favored in water, was considered as the reactive intermediate [54]. 

Another way to make allylic chlorides is by treating dienes with HCl. Electrophiles attack conjugated dienes more readily than they do isolated alkenes. There was some discussion of this in Chapter 19, establishing the main point that the terminal carbon atoms are the most nucleophilic and that the initial attack produces an allylic cation. A simple example is the addition of HCl to cyclopentadiene. 

In contrast, the less strained four-7r-electron cyclopentadienyl cation is very unstable. Its p r+ has been estimated as --40, using an electrochemical cycle. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. " Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is doubly allylic. When the bromide and antimony pentafluoride react at -78°C, the EPR spectrum observed indicates that the cyclopentadienyl cation is a triplet. Similar studies indicate that pentachlorocyclopentadienyl cation is also a triplet, but the ground state of the pentaphenyl derivative is a singlet. 

An improved procedure has been reported for performing the homo-Diels-Alder reaction between allyl cations and cyclopentadiene. In the new method the allyl cations are generated by reaction of aa -dibromoketones with triethylborate and zinc. A typical example is illustrated by the conversion of (233) into bicyclo[3,2,l]oct-6-en-3-ones (234a—c), and the tentative reaction pathway is outlined (Scheme 1). 

So far we have considered only reactions in which the pericyclic ring contains an even number of atoms. Reactions of this kind are, however, known in which an odd-numbered ring is involved. A simple example is the Diels-Alder-like addition of 2-methylallyl cation (148) to cyclopentadiene (149) to form the methylbicyclooctyl cation (150). The transition state for this reaction is easily seen to be of Hiickel type (151) and so isoconjugate with tropylium. Since the allyl cation contains only two n electrons, we are dealing here with a six-electron system isoconjugate with the tropylium cation (147) and hence aromatic. In reactions of this kind, both the reactants and the transition state are odd. The reactions are therefore of 001 type. Since, moreover, the aromaticity or antiaromaticity of the transition state is again unrelated to the structures of the reactants or products, the reactions are of anti-BEP type and are consequently classed as 00 J. 

Several allyl cations can serve as dienophiles and allyl anions and pentadienyl cations as dienes in Diels-Alder cycloadditions for example, cycloaddition of 2-methyl allyl cation 88 with cyclopentadiene. 

Trimethylsilyloxy-substituted allyl halides in the presence of silver perchlorate in nitromethane generate allyl cations 89, which react efficiently with cyclopentadiene. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

A related rhodium catalyzed enantioselective reductive coupling of acetylene to N arylsulfonyl imines leads to the formation of (Z) dienyl allylic amines (Scheme 1.28) [105]. The scope of the reaction is comparable to that demonstrated for the analogous iridium catalyzed process. The reaction between the acetylene and rhodium leads to the oxidative dimerization of acetylene to form a cationic rhoda cyclopentadiene that then reacts with the imine to generate the product after the protolytic cleavage and reductive elimination. 

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