Allyl carbonium ions, stability

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Substituents at the 3-position, if capable of stabilizing an adjacent positive charge by resonance and/or inductive effects, direct strongly to the adjacent (i.e. 2) position (equation 3). Stabilization of the allylic carbonium ion intermediate is obviously involved. This effect can be quite pronounced for R = Ph in equation (3) the ratio of 2- to 5-bromo derivatives for electrophilic bromination is ca. 660 (68JOC2902). Even more striking is the exclusive formation of the 2,2 -dibrominated product (19) from 3,3 -dithienyl (equation 4) (69JOC343). As expected, substituents not capable of stabilizing the cationic intermediate direct substitution to the 5-position. 

The reactivity of the organosilanes65 and organostannanes66 towards electrophiles is dependent on the characteristics of the organic ligands. Typically, the alkylsilanes and alkylstannanes are unreactive, which is a consequence of the weakly polarized carbon-silicon and carbon-tin cr-bonds (C8-—Ms+). However, allylsilanes67 and allylstannanes are highly reactive to electrophiles because of extensive ct-tt (C—Si or C—Sn) conjugation in the ally metals and the 0-carbonium ion stabilization effect of the metal center. Consequently, electrophiles add exclusively with allylic transposition. 

A benzyl or allyl carbonium ion is formed with particular ease, provided that an electron withdrawing group is not conjugated with the system. These carbonium ions are stabilized by the resonance forms VIII, IX, and X ... 

Enzymes catalyze the formation of carbon-carbon bonds between allylic and homoallylic pyrophosphate species by mechanisms that are very different from those for carbonyl compounds. Here, carbonium ions, stabilized as ion pairs and generated from allylic pyrophosphates, are likely to be the intermediates that add to the TT-electron density of carbon-carbon double bonds to form new carbon-carbon single bonds. Reaction patterns are consistent with model systems and the mechanisms are based on analogies with the models, stereochemical information (which is subject to interpretation), and the structural requirements for inhibitors. Detailed kinetic studies, including isotope effects, which provide probes in the aldolase and Claisen enzymes discussed in Section II, have not yet been performed in these systems. The possibility for surprising discoveries remains and further work is needed to confirm the proposed mechanisms and to generalize them. 

Although steric effects and substituent effects leading to carbonium ion stabilization are of greatest importance in governing the mechanism and relative rate of nucleophilic substitution processes, there are other substituent effects that are recognized and of importance. We have mentioned earlier in this chapter that arylmethyl and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in such systems than in simple alkyl systems. It has also been observed that nucleophilic substitutions of the direct displacement type also take place more readily, but the reason for this is not apparent. Allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone, and benzyl chloride is 93... 

In a different approach, a styryl terminating group was also found to favor five-membered ring formation by virtue of the relative stability of the allylic carbonium ion formed. 

It is of interest to note that artemisia alcohol (18) produced in the hydrolysis of 16-OPy I" is essentially completely racemic (>98%) (57). Apparently (18) is formed by nucleophilic capture of the acyclic allylic carbonium ion (29) rather than direct attack in the 3 position of the chrysanthemyl carbonium ion (28). Nucleophilic substitution upon cyclopropylcarbinyl cations to give homoallyl products occurs with inversion of configuration 74—75). In the case of (28), however, position 3 is highly hindered by the adjacent gem dimethyl groups thus collapse to the allylicly stabilized (29) is faster than direct substitution. The formation of a small amount of cw-chrysanthemol (27, 0.25%) is taken to indicate that allylic ion (29) recyclizes, at least in part, back to (28) and its cis isomer (30). 

The results of this work are not limited to just S-b-MM and S-b-tBM, but may be extended to include styrene derivatives such as p-methylstyrene and p-t-butylstyrene 1). In addition to t-butyl methacrylate, other alkyl esters capable of stabilizing a carbonium ion, such as benzyl methacrylate and allyl methacrylate, should exhibit similar reactivity toward acidic hydrolysis and TMSI. In contrasting the hydrolysis of tBM blocks with TsOH and their reaction with TMSI, it should be noted that the hydrolysis is reportedly catalytic in nature (7-10), whereas the reaction with TMSI is stoichimetric. Therefore the latter approach may allow one to more easily "dial in" a desired level of methacrylic acid or metal methacrylate. 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

For alkenes, the reactivity is based on the stability of the carbonium ion formed and they follow the order tertiary > secondary > primary. Thus olefins react as follows (CH3)2C=CH2 = (CH3)2C=CHCH3 > CH3CH=CH2 > CH2=CH2. Allylic and benzylic carbonium [19,20] ions are also favored where appropriate. 

That this mechanistic pathway is followed is of some interest since the 1-tri-quinacenyl cation (420) has been shown on the basis of semiempirical calculations to be a twofold allyl-stabilized bridgehead ion.389 This cation, as well as 421 and 422, could be generated by allowing the respective chlorides to react with antimony pentafluoride in S02C1F at —78 °C.388 The and 13C NMR spectra have been recorded and suggest that almost planar divinyl carbonium ion units are present in... 

Most examples of quinone dehydrogenations adjacoit to have been earned out on steroidal ketones and are essentially limited to readily enolizable species. Reactions on esters and amides (Table 8) are far less common and, because of their relatively low ease of enolization, require hanh conditions. Thus, unless stabilization of the intermediate carbonium ion is possible, - elevated temperatures and prolonged reaction times are required (Table 8), which increases the incidence of unwanted side reactions. Frequent by-products are those arising as a result of Diels-Alder reactions or Michael addition to the quinone." Allylic alcohols may be rapidly oxidized to aldehydes or ketones under these conditions and requite prior protection. 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

This subject has already arisen in connection with the stabilities of olefinic bonds at various positions in the steroid nucleus (p. 14) and as a side-reaction in certain hydrogenation processes. Rearrangements of allylic systems will be covered in Chapter 9, and photolytic reactions in Chapter ii. We are concerned here with the double-bond migrations which involve carbonium ions generated by protonation of simple olehns. 

Unimolecular solvolyses of allylic secondary halides and similar derivatives are often favoured by the capacity of the double bond to stabilise the intermediate carbonium ion as the mesomeric "allylic cation 4). This extra stability of the intermediate is reflected in the energy of the transition state for its formation whenever the path of departure of the anion... 

The activating effects of unsaturated a-substituents are probably due to tt-orbital overlap with the allylic system in the transition state for carbonium ion formation. This hypothesis is supported by the fact that a- and /3-methyl substituents which sterically hinder attainment of coplanarity of the allylic system and the unsaturated substituent produce abnormally small rate accelerations, and may even decrease isomerization rate. The activating effects of alkyl substituents probably also arise because of their stabilizing effect on the transition state for carbonium ion formation. Selected data (mostly from Braude s publications) which illustrate substituent effects on reactivity are collected in Table 1. 

To carry this analogy a little further, allylic and benzyUc halides fragment faster than the saturated counterparts. The carbonium ions formed in both types of reaction are stabilized by resonance with neighboring jt-electron systems (Equation 2.25). Thus, the carbon-halogen bonds in the unsaturated compounds are more susceptible to both solvolytic and El-induced cleavage. 

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