Cycloadditions with Allylic Cations

In the presence of zinc chloride, [4 + 3] cycloadducts between the allylic cation formed from 29 and furan are obtained, Eq. 18. However, the major product of the reaction arises from electrophilic addition to furan [46]. 

Harmata has reported the formation of intramolecular cycloadducts derived from trimethylsilylmethyl allylic sulfones 30, Eq. 19 [47]. Optimized reaction conditions involved the use of trimethylaluminum as the Lewis acid. 

Furthermore, 30 also afforded the same cycloadducts under photolytic conditions, Eq. 20. While the reaction is not synthetically useful in its present form, 

West has recently reported intramolecular [4 + 4] cycloadditions of furan and 2-pyrone under photolysis in aqueous solution [49]. The exo and endo adducts are obtained in varying ratios when 31 bearing a variety of substituents is photolyzed in an aqueous solution of LiCl, Eq. 21. 

In substrate 32, modest diastereoselectivity was observed as a result of the preexisting stereocenter in the tether, Eq. 22. 

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There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

A review of intramolecular 4 + 3-cycloadditions of allyl cations has been presented.277 The 4 + 3-cycloaddition reaction of C(2)-substituted furans with 1,3-dimethyloxyallyl cations show high endo diastereoselectivity and a cis dia-stereospecificity.278 The tandem Peterson olefination/[4 + 3]-cycloaddition of tertiary alcohols (149) in the presence of filran and Lewis acids (TiCLt) furnishes cycloheptanes (150) in modest yields (Scheme 57).279 (Trimethylsilyl)methyl allylic sulfones (151) were used to investigate the scope and limitations of intramolecular 4 + 3-cycloadditions of allylic sulfones (Scheme 58).280 Lewis acid-catalysed 4 + 3-... 

Similar to the cycloaddition of allyl cations, allenyl cations have been found to undergo cycloadditions with alkenes to afford bicyclic compounds. The allenyl cations were generated from propargyl chlorides by treatment with Lewis acids. This reaction sequence proceeds via the cyclization 34... 

Cycloaddition of allyl cations to conjugated dienes provides a route to seven-membered carbocycles. One of several methods can be used to generate the allyl cation, such as from an allyl halide and silver trifluoroacetate, or from an allyl alcohol by way of its trifluoroacetate or sulfonate. Cycloaddition of the allyl cation proceeds best with a cyclic diene, particularly for intermolecular reactions. Thus, cyclohexadiene and methylallyl cation gave the bicyclo[3.2.2]nonadiene 187 (3.125). Many intramolecular examples are known, such as the formation of the... 

Cycloaddition Reactions with Allyl Cations and Allyl Anions... 

Allyl vinyl ethers (66) undergo [l,3]rearrangements to give cyclopentanones (67) when heated with Pd° catalysts,in vivid contrast to the uncatalysed thermal rearrangement of compounds such as (66), which results in the formation of cycloheptenones. The recently developed oxyanionic Cope rearrangement of cyclononatrienols has been used by Paquette and Crouse as the key step in a brief synthesis of ( )-multifidene (68)/ Highly substituted alkylcyclopentenes may be prepared by [3 + 2]cycloadditions between allyl cations and olefins/ ... 

The [4+3] cycloaddition reaction is an efficient and convergent way to assemble seven-membered rings, which are important stmctural subunits in natural products and pharmaceutically important molecules. There have been many examples of [4+3] cycloadditions of dienes since the first report of this reaction in 1962 [1], of which the largest subclass are those with allyl cations having oxygen as an anchoring substituent, that is, 2-oxyallyl cations (Scheme 18.1) [2]. 

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 ... 

Intramolecular cycloaddition reactions of allylic cations with participation and/ or formation of heterocycles, mainly [4+3]-cycloaddition to furan system 97T6235. 

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

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). 

The synthesis of fenozan BO-7 4 involves two key steps, the first of which employs a 4 + 2 cycloaddition of singlet oxygen to the diene 83a122-20 123. This provides the endoper-oxide 83b that can be transformed into the target cis-fused 1,2,4-trioxane by treatment with the Lewis acid, TMSOTf, in the presence of a carbonyl compound. The reaction proceeds by Lewis acid promoted heterolysis of the C—O bond to give an intermediate peroxy allyl cation 83c that is captured by the carbonyl compound (in this case, cyclopen-tanone) to give the product (Scheme 30). A number of different carbonyls have been used in this reaction along with a number of different endoperoxide templates and detailed SAR have been developed (Scheme 30). 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

The suprafacial thermal addition of an allylic cation to a diene (a 3 + 4 cycloaddition) is allowed by the Woodward-Hoffmann rules (this reaction would be expected to follow the same rules as the Diels-Alder reaction1095). Such cycloadditions can be carried out1096 by treatment of a diene with an allylic halide in the presence of a suitable silver salt, e.g,1097... 

We have already seen in the reactions of an allyl cation with a diene, that cycloadditions may take place stepwise, with one bond forming in a separate step from the second, in which case, although they are still cycloadditions, they are no longer pericyclic. It is not absolutely certain that all the cycloadditions illustrated above are pericyclic. All that is required for a stepwise reaction to be plausible is that the intermediate produced when one bond forms ahead of the other should have substituents powerful enough to stabilize the charges. 

Draw the frontier orbital interactions for the all-suprafacial cycloaddition of an allyl anion to an alkene and for an allyl cation to a diene showing that they match, and show that the alternatives, allyl cation with alkene and allyl anion with diene are symmetry-forbidden. 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

Allylcations (c/., 10, 461-462) cyclopentenes. Allyl chlorides are converted into allyl cations by ZnCl2 complexed with ether. These cations undergo [3 + 2] cycloaddition to C=C bonds to form cyclopentenes. Unsymmetrical cations are preferentially attacked at the less substituted terminus unsymmetrical alkenes react in the Markovnikow sense. The most thermodynamically stable cyclopenlene is formed.a... 

The allyl cation has three atoms but only two electrons so it can take part in cycloadditions with dienes—the total number of electrons is the required six. This is one of the few reactions that works only to produce a seven -me mbered ring. 

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. 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 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

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