Carbenium ions stabilities

Song, W., Nicholas, J.B. and Haw, J.F. (2001). Acid-base chemistry of a carbenium ion in a zeolite under equilibrium conditions verification of a theoretical explanation of carbenium ion stability. J. Am. Chem. Soc. 123, 121-129... 

Apeloig, Y. Biton, R. Abu-Freih, A. Importance of electronic geminal interactions in solvolysis reactions. Application to the determination of the a-silyl effect on carbenium ion stability./. Am. Chem. Soc. 1993, 115, 2522-2523. 

Apeloig, Y. Stanger, A. Are carbenium ions stabilized or destabilized by a-silyl substitution The solvolysis of 2-(trimethylsilyl)-2-adamantyl p-nitrobenzoate. J. Am. Chem. Soc. 1985, 107, 2806-2807. 

Starting with the knowledge of carbenium ion stability and the understanding that it is necessary to decrease this stability enabling the reaction of these ions with nitriles102 (Section III.B.l), it should be assumed that acetals 209, oc-haloethers 211, enols 217 and vinyl ethers 218 are ineffective precursors since they are the sources of the highly stable hydroxy- (223) and alkoxycarbenium ions 224. In contrast, the carbonyl compound derivatives, which can produce destabilized ions (in comparison to ions 223 and 224) are the most interesting for reactions with nitriles. This includes acyloxycarbocations 225 (see Section III.B.3), halocarbocations 226, A-acyliminium ions 228 and vinyl cations 229. 

Carbenium ions are usually unstable, very reactive species which readily rearrange or decompose by /3-proton elimination. The propensity for such reactions are decreased by stabilization through inductive effects, through resonance and/or by hyperconjugation. Alkyl substituents are electron donating and stabilize carbenium ions inductively. The carbenium ion stability increases with increasing substitution, and tertiary carbenium ions are much more stable than secondary, which are more stable than primary ions all are more stable than the unsubstituted methylium ion. 

In addition, the endo double bonds activate neighboring hydrogen atoms or alkyl groups to hydride and alkyl anion abstraction, respectively, to produce tertiary carbenium ions stabilized by both the double bond and the a-substituent [Eq. (91)]. 

Polyindanes have been obtained from a variety of monomers which are capable of forming benzyl carbenium ions stabilized with dialkyl groups upon the appropriate initiation. Linear polymers containing indane units in the main chain have been obtained by the BClj-initiated polymerization of 1,4-diisopropenylbenzene [Eq. (6)] [8]. Polyindanes with structural unit IS have also been obtained when difunctional monomers capable of forming a,a-dimethylbenzylic carbenium ions upon treatment with an initiator [8-12] (Fig. 4). 

The rearrangement, as postulated by Yamashita, seems probable, because of the formation of the carbenium ion stabilized by adjacent oxygen atom. 

On the other hand, cyclobutyl compounds are not obtained from cyclopropylmethyl derivatives with carbenium ion stabilizing substituents on the carbinyl carbon. For example, reactions of methylcyclopropyl methanol (101) with acidic reagents led to unrearranged or on further heating to 3-penten-l-yl derivatives (equation 70) . 

Cyclopropylmethyl systems containing a carbenium ion stabilizing group at the 1-position of the cyclopropane ring lead to the exclusive formation of cyclobutanols.Thus, 1-methylcy-clobutanol (2 a) was obtained in 90% yield by hydrochloric acid catalyzed ring expansion of (l-methylcyclopropyl)methanol (la). Hydrolysis of 1-phenyl-l-(tosyloxymethyl)cyclo-propane (lb) in 90% acetone/water or dimethyl sulfoxide afforded a quantitative yield of 1-phenylcyclobutanol (2b). 

An alternative and excellent route to oxaspiropentanes 83 and thus of cyclobutanones 84 proceeds through the condensation of cyclopropyldiphenylsulfonium tetrafluoroborate (82) with aldehydes and ketones. Table 2 presents some illustrative examples. With cyclopropyl methyl ketone and benzophenone as substrates, the oxaspiropentanes cannot be isolated but rapidly rearrange to the corresponding cyclobutanones." Here ring expansion is facilitated by the presence of excellent carbenium ion stabilizing groups. 

The effects of substituents at the 1-, 2-, 3-, and 7-positions of 7-methylcyclohepta-triene on the cycloheptatriene-norcaradiene equilibrimn have been examined. Where the C-7 substituent is a carbenium ion stabilized by heteroatoms, e.g. the bis(methylamino) substituted cation (255), the equilibrium is towards cycloheptatriene ... 

Table 5. Effect of Carbenium Ion Stability on Copolymerization Reactivity Ratios...

Staley RH, Wieting RD, Beauchamp JL. Carbenium ion stabilities in the gas phase and solution. An ion cyclotron resonance study of bromide transfer reactions involving alkali ions, alkyl carbenium ions, acyl cations, and cyclic halonium ions. J Am Chem Soc. 1977 99 5964-72. 

Heteroatoms are not the only groups that will facilitate an SnI reaction when in proximity to the cationic center. Resonance effects in allyl or benzyl carbenium ions stabilize the cationic center and hence facilitate the substitution reaction. Yet, n systems further away from the cationic center can also get involved if their geometry is such that they are oriented toward the carbenium ion s empty p orbital. Consider the solvolysis of cholesterol tosylate in Figure 11.7. Two products are formed, and the solvolysis rate is approximately 100 times... 

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

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