Carbenium center

In the initial step one hydroxy group is protonated, and thus converted into a good leaving group—i.e. water. Subsequent loss of water from the molecule proceeds in such a way that the more stable carbenium ion species 2 is formed. The next step is a 1,2-shift of a group R to the tertiary carbenium center to give a hydroxycarbenium ion species 4 ... 

The 5,6-dihydro-4//-l,3-oxazinium salts are in essence cyclic alkoxycar-benium ions bearing two heteroatoms at the carbenium center. They are similar to other heterocarbenium ions and have an ambident character which has been described in detail in reviews (64AG400 72CRV357). 

Deprotonation of the a-position of a carbenium center is one of the most typical properties of carbocations. All the alkyl-substituted heterocyclic ions possess an appreciable CH-acidity [82AHC(S)1] the 3-azapyrylium salts are no exceptions to this rule. The formation of anhydro bases, i.e., methylene-1,3-oxazines (e.g., 69), from methyl-3-azapyrylium salts is well known (72S333). 

A carbocation in which a group or moiety bridges two or more potential carbenium centers (such that there are alternative Lewis structures having different carbenium centers). 

CARBENIUM CENTER CARBENIUM ION CARBONIUM ION CARBENE Carbenoid,... 

In dication 188, the NBO charge at the carbenium ion center is +0.69 and at the acyl carbon is +1.09. The tert-butyl cation has been found to have NBO charge at its carbenium center of +0.67, suggesting a modest superelectrophilic activation of the carbenium ion center in 188, compared with the tert -butyl cation. When charges on the methyl groups are also considered, structure 188 is similar to the protosolvated tert -butyl cation 196. 

The 13C NMR spectrum shows a characteristic absorption of < 13C 331.3 for the carbenium centers. 

Water elimination in the superacidic solution is a highly exothermic step, but nevertheless the 2,6-adamantadiyl dication 34 is not formed. This observation suggests that structures like 34 can be distonic superelectrophiles. As in the case of other 1,4- and 1,5-carbodications, the 2,6-adamantadiyl dications are stabilized and persistent when the carbenium centers bear an aryl substituted (vide infra). 

Due to coulombic repulsion, there is also less charge delocalization into the phenyl group. The diminished neighboring group participation (stabilization) is one of the characteristics of superelectrophilic activation.16 In the case of dication 45, the aryl group is less capable of donating electron density to either carbenium center. 

Although formally considered a 1,5-dication, 50 possesses a structure in which the carbenium centers are constrained at a distance of separation of 3.11 A. NMR studies show the carbenium ion centers at 513C 207.7, consistent with the carbocationic structure 50. In cyclic voltamographic analysis, the compound 51 shows an especially high oxidation potential (two-electron oxidation peak at 1.10 V), when compared to analogous dications and triarylmethyl monocations.19 It has also been shown that... 

Another mechanistic possibility involves a single electron transfer within the ion pair 58 with the formation of the corresponding radical pair 59 (equation 48), since the GeCl3 substituent in 58 must a priori destabilize the carbenium center due to the negative I-effect. 

Two reactions were explored in continuation of these attempts hydride transfer from silanes to a carbenium center and halide transfer from silanes to a Lewis acid. Media of low nucleophilicity and high ionization power were employed. Stable silylenium ions A and B were postulated to be formed by H- transfer from corresponding silyl hydride to triphenyl-methylium perchlorate in CH2C12 (71,72). However, soon after, and in view of results of additional experiments, this evidence was shown to be insufficient (19,73). 

On the one hand, the acyloxycarbocations 163 differ from carboxonium ions 105 by the presence of a carbonyl group which considerably increases the electron deficiency on the carbenium center of 163 due to electron withdrawal. On the other hand, ions 163 are the heteroanalogs of Af-acyliminium ions 164 and, as will be shown below, they possess many similarities to the latter. 

Interaction of the carbenium center with one of the phenyl rings, as shown in formula 4 (Scheme 19) has been suggested as an alternative explanation for the observed stereoselectivities [91]. 

Although an interplay of enthalpic and entropic effects has been shown to be responsible for the reactivity order of the methylenecyclo-alkanes, one can summarize that generally the entropy effects caused by substituent variation at the developing carbenium center are small compared with the enthalpic effects. 

In organic syntheses allylsilanes and allylstannanes have been used extensively as allyl anion equivalents during the last two decades [187-190]. The regioselective attack of electrophiles, which finally yields products with allylic inversion (Scheme 43), has been explained by the hyperconju-gative stabilization of carbenium centers by the carbon-silicon or carbon-tin bond in the j3-position [191-196], which has initially been derived from solvolytic experiments [197-199]. 

The comparison of propene, allyltrimethylsilane, and isobutene indicates, that introduction of a trimethyl silyl group in /3-position of the developing carbenium center activates more than a methyl group in a-position. Both series of triphenyl element compounds (left and right column Scheme 44) show the reactivity pattern Si < Ge < Sn, but variation of the substituents at silicon and tin was found to largely affect the reactivity of the double bond. While in the allyl series (right column), the trialkylsilanes and -stannanes are 2 to 3 orders of magnitude more reactive than the... 

Desilylation reactions are based on the well-known stabilization of a,/3-carbenium centers by silicon. The conversion of allylsilanes into alkenes by use of the BF3. (AcOH)2 complex has been studied in considerable detail. The reaction occurs by protonation, followed by nucleophile-induced desilylation (Eq. 61) [107]. The stereochemical implications have also been considered in detail, and in the deuteration-desilyation sequence shown in Eq. (62) the structure shown is the major product [108]. Protonation-desilation of alkynes to give allenes in yields ranging from 70 to 97 % can evidently be considered to involve a /3-vinyl cation (Eq. 63) [109]. 

An ESCA study by Olah and coworkers " succeeded in observing the ESCA spectrum of the 2-norbornyl cation (126) and compared it with those of the related 2-methyl-2-norbomyl cation (142) and other trivalent carbenium ions such as the cyclopentyl (75), 1-methyl-1-cyclopentyl (141), and tert-butyl (50) cations. The Is electron spectrum of the 2-norbomyl cation shows no high binding energy carbenium center and a maximum separation of <1.5 eV is observed between the two equivalent cyclopropyl -type carbons... 

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