Stability allyl cation

In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to form the more stable carbocation => a resonance stabilized allylic cation, i) Addition to one of the inner carbon atoms would have produced a much less 1 ° cation, one that could not be stabilized by resonance. 

Some of these modules also show fairly low (17.7 to 21.9 kcal/mol) barriers to rotation about the N=C bond. This can be ascribed to the stabilized allylic cation appearing in the transition state. Although no further splitting occurs above... 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

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... 

Fragmentation in the mass spectrometer gives resonance-stabilized cations whenever possible. The most common fragmentation of alkenes is cleavage of an allylic bond to give a resonance-stabilized allylic cation. 

The key to formation of these two products is the presence of a double bond in position to form a stabilized allylic cation. Molecules having such double bonds are likely to react via resonance-stabilized intermediates. 

The [4+3] cycloaddition between furan and amino-stabilized allyl cations has not been as actively studied. An intramolecular cycloaddition between a furan and a 2-aminoallyl cation, generated from methyleneaziridine under Lewis acid-promoted conditions, is shown in Equation (50) <2004AGE6517>. An AgBF4-promoted asymmetric intermolecular [4+3] cycloaddition of 2-aminoallyl cations, derived from chiral a-chloroimines, with furan to give cycloadducts of up to 60% ee was also reported <1997TL3353>. 

Acid-catalyzed hydrolysis of isobutylene oxide (8) is >750 times faster than that of ethylene oxide (6), and > 99% of the glycol product is from addition of solvent at the tertiary carbon.23 These results are consistent with a mechanism in which there is significant positive charge on the tertiary carbon at the transition state, as discussed in the previous section. Butadiene monoepoxide (10) is slightly less reactive than isobutylene oxide,36 and its acid-catalyzed hydrolysis can potentially proceed via a resonance-stabilized allyl cation (Scheme 6). However, the acid-catalyzed hydrolysis of 10 yields 96% of 3-buten-l,2-diol (15) and only 4% of 2-butene-1,4-diol (16),36 and the acid-catalyzed methanolysis of 10 is reported to yield only 2-methoxy-3-buten-l-ol.37 An A-2 mechanism proceeding via transition state 17 may account for the observation that 1,2-diol 15 is the predominant product from acid-catalyzed hydrolysis of 10. The minor yield of the 1,4-diol 16 may be formed from reaction of... 

The most effective tricyclization process reported was separated into a sequence of mono- and bicyclization steps85. After inducing cyclization with a dimethylcyclopentenyl cation the first ring closure leads to a stabilized allylic cation which induces a concerted bicyclization in the second step. An anti-trans-anti-trans skeleton is formed stereoselectivelv in good yield and the acetoxy substituent is found exclusively in the a-position85. 

Since pyrophosphate is a good leaving group, it leaves to form the resonance stabilized allylic cation... 

In their qualitative chemistry, conjugated dienes behave very much like alkenes They readily react with electrophiles in addition reactions. Just as in the case of alkenes. this addition proceeds to give the most stable intermediate. For conjugated dienes, this normally turns out to be a re.sonance stabilized allylic cation ... 

Diels-Alder reactions. Compounds that give an oxygen-stabilized allyl cation are super-dienophiles in the presence of LiClQ,.OEt2. The method obviates the difficulties experienced with the low reactivities of 2-cyclohexenones. An intramolecular variant serves to construct tricyclic ketones expediently. 

Formation of a random copolymer via incorporation of small amounts of isoprene in the PIB would thus occur as in eq 12, via addition of isoprene to an isobutyl cationic center to form a stabilized allylic cation. Although attack by a molecule of isobutene... 

Hyperconjugation, which is at least partially responsible for the tertiary > secondary > primary stabilization order, is more important for cations than for radicals. The effect is large enough for tertiary cations to exceed resonance-stabilized allylic cations in stability (the reverse of the order for radicals). 

Step 1 Protonation of one of the double bonds forms a resonance-stabilized allylic cation. 

Alkenes ionize and frequendy undergo fragmentations that yield resonance-stabilized allylic cations ... 

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