Of allyl cations

Refer to the molecular orbital diagrams of allyl cation (Figure 10 13) and those presented earlier in this chapter for ethylene and 1 3 butadiene (Figures 10 9 and 10 10) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Floffmann rules... 

FIGURE 10 13 Their molecular orbitals of allyl cation The allyl cation has two IT electrons and they are in the orbital marked it. 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

Rotational Earner of Allyl Cation Reaction Path Following... 

Draw Lewis structures for allyl cation. Where is the positive charge Examine atomic charges as well as the electrostatic potential map for localized and delocalized forms of allyl cation. Which carbon (s) carries the charge in each ... 

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

Addition can be attributed directly to the stability and delocalized nature of allylic cation. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

The positive charge of allyl cations essentially distributes itself among C-1 and C-3 of the allyl skeleton. Terminal alkylation concentrates the positive charge at the alkylated carbon, as shown for the 1-methylcyclopentenyl cation [494],... 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

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-... 

The formation of allyl cations from halocyclopropanes via ring opening of the unstable cyclopropyl cations also has been investigated159,161,162 [Eq. (3.35)]. 

The same answer is obtained if the HOMO of allyl cation and the LUMO of ethylene are examined. 

This reaction is allowed by the Woodward-Hoffmann rules. Both interactions are bonding. The same prediction would be arrived at if the HOMO of allyl cation and LUMO of 1,3-buta-diene were the orbitals considered. 

Propargyl cations (alkynylcarbonium ions) (172) may be viewed as the unsaturated analogues of allyl cations (173). The close relationship with vinyl cations is shown by the vinyl cation-type structure 172a... 

Nucleophilic selenenylation (RSe ) of allyl cation equivalents (ally halides, sulfonates, epoxides). 

Concerning the possible causes of the premature cessation of polymerisation in most systems characterised by direct initiation, we believe that the accumulation of anions derived from the Lewis acid is the most important sin e factor responsible. We have already discussed the possible reactions giving rise to an increasing concentration of these anions in the case of direct initiation by the neutral Lewis acid. To these must be added the formation of carbenium ions which cannot propa te either because they are sterical-ly hindered, as in the case of isobutene unsaturated dimers and trimers , or because they are too stable to attack the monomer, as in the case of termination by formation of allylic cations. The accumulation of these species is inevitably accompanied an equivalent rise in anion concentration. Whatever the origin(s) of this rise, a higher anion concentration can determine various detrimental effects on the survivd of active species ... 

Gassman first observed the intermediacy of allyl cations as dienophiles in connection with efforts to extend the aminium cation-radical catalyzed Diels-Alder reaction to intramolecular cases. It was found that the cyclization of tetraene (26) performed in the presence of 0.2 equiv. of tris(p-bromophe-nyl)aminium hexachloroantimonate [ArsN+ CleSb ] was in fact catalyzed by protic acid. - A better means of catalyzing the cyclization of (26) involves use of 4 mol % trifluoromethanesulfonic acid at -23 C for 6 min, which provides the trans-fused cycloadduct in 88% yield (98% isomeric purity). Similar treatment of (27), however, provided a mixture of cis- and trans-fused products. A subsequent report revealed that allylic alcohols and allylic ethers are useful precursors to the allyl cation dienophiles with... 

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