Carbocation incipient

This order compares well with that of the decreasing susceptibility to elearophilic substitution of the three positions of the ring and of benzene. This comparison is justified by the analogy of the transition states of both reactions. The observed order agrees also with that of the calculated v net charge on the site of fixation of the incipient carbocation (133) ... 

Electronegative, nonconjugating groups (which interact with an incipient carbocation only by an inductive or field effect) discourage attack at C. This is due to destabilization of transition state (49) by the juxtaposition of positive charge. 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

An ipso attack on the fluorine carbon position of 4-fIuorophenol at -40 °C affords 4-fluoro-4-nitrocyclohexa-2 5-dienone in addtion to 2-nitrophenol The cyclodienone slowly isomenzes to the 2-nitrophenol Although ipso nitration on 4-fluorophenyl acetate furnishes the same cyclodienone the major by-product is 4 fluoro-2,6-dinitrophenol [25] Under similar conditions, 4-fluoroanisole pnmar ily yields the 2-nitro isomer and 6% of the cyclodienone The isolated 2 nitro isomer IS postulated to form by attack of the nitromum ion ipso to the fluorine with concomitant capture of the incipient carbocation by acetic acid Loss of the elements of methyl acetate follows The nitrodienone, being the keto tautomer of the nitrophenol, aromatizes to the isolated product [26] (equation 20) Intramolecular capture of the intermediate carbocation occurs in nitration of 2-(4-fluorophenoxy)-2-methyIpropanoic acid at low temperature to give the spiro products 3 3-di-methyl-8 fluoro 8 nitro-1,4 dioxaspiro[4 5]deca 6,9 dien 2 one and the 10-nitro isomer [2d] (equation 21)... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

The actual proportions of products obtained in many cases are not necessarily found to reflect the relative stabilities of the incipient carbocations, unrearranged and rearranged, however. This follows from the fact that their relative rates of reaction with the aromatic species almost certainly do not follow the order of their relative stabilities, and may well be diametrically opposed to it. Attack on the aromatic species by the first formed polarised complex may be faster than its rearrangement. The study of these rearrangements is also complicated by the fact that Lewis acids are found to be capable of rearranging both the original halides, and the final, alkylated end-products, e.g. ... 

This corresponds to an isotope effect of approximately 3.5% per deuterium. In comparison, the secondary /3-deuterium KIEs in SN1 reactions are all normal and range from 5% to 15% per deuterium. Because the normal KIEs in SN1 reactions result from the weakening of the C,—L bond by a hyperconjugative interaction with the incipient carbocation in the transition state, the authors concluded that hyperconjugative interactions are present also in the transition state for the insertion reaction. The normal secondary /3-deuterium KIE observed for the insertion reaction is consistent with the dipolar three-centre transition state structure [15] proposed by Seyferth et al. (1970a,b) because the partial positive charge on the a-carbon is stabilized by hyperconjugation. 

The tendency to react according to (6) or (7) depends on the stability of the (incipient) carbocation [16, 17] and on the oxidizing power (redox potential) of the nitro compound [12, 18]. It also depends on solvent, more polar solvents favoring the ionic path (Eq. 7) [18]. 

Stabilization of the (incipient) C 6) carbocation developing in the C-O heterolysis. The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidine-6-yl radicals. On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition [27]. 

Intramolecular interception of an Af-alkoxy-AT-acylnitrenium ion by an aromatic ring results in a spirocyclic species [203], There follows a C-migration (instead of N-migration) to restore aromaticity. This migration is assisted by the stabilization of the incipient carbocation by the resident donor nitrogen atom N-migration would not profit from this favorable electronic interaction. 

Other mechanisms with the involvement of an incipient methyl cation were also proposed.418,460,462 An attack of methyl cation released from extensively polarized methoxy groups on the carbon-hydrogen bond forms pentacoordinated carbocation intermediates that, in turn, yields ethyl methyl ether after the loss of a proton ... 

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

Fluorination has a particularly profound effect on the additions of nucleophiles to per-fluorinated alkenes where the intermediate is anionic. Such processes are dramatically assisted by the strongly stabilizing influence of perfluoroalkyl groups substituted at the incipient anionic site.66 Similar to carbocations (see Section 1.4.), the effect of fluorination in such systems is often ambiguous when monofluorination is involved. a-Halogens generally stabilize anions in the order bromine > chlorine > fluorine, which is the exact opposite to the inductive electron-withdrawing order of the substituents. This effect reflects the importance of l7t-repulsion.67... 

In understanding these reactions, it is helpful to view the metal-alkene tt complex as an incipient carbocation (just as tt complexes of halogens are incipient carbocations). Alkyl and hydride shifts then bear analogy to carbocation rearrangements. This may be an oversimplification but it makes the chemistry easier to follow. 

Electron demand at the incipient carbocation is also important in determining whether or not homoallylic T aHicipation Takes place. Gassman and Fentiman have plotted the logs of the rates of solvolysis of both 57 and 58 in dioxane-water vs. the Hammett cr+ constants of the X substituents. They... 

Double bonds further removed from the incipient carbocation than the homoallylic position can also assist, in jonizatien-if the..geometry of the system. allows Tt. For example -nitrobenzoate (59) solvolyzes 1500 times faster than the corresponding saturated compound in aqueous acetone. The product is trans,trans-1 -decalol (60). If after 12 half-lives the product is iso-... 

As is apparent in the last step, isobutane is not alkylated but transfers a hydride to the Cg+ carbocation before being used up in the middle step as the electrophilic reagent (tert-butyl cation 4). The direct alkylation of isobutane by an incipient tert-butyl cation would yield 2,2,3,3-tetramethylbutane,142 which indeed was observed in small amounts in the reaction of ferf-butyl cation with isobutane under stable ion conditions at low temperatures (vide infra). 

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