Cations f-butyl

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

The most stable of all alkyl cations is the t-butyl cation. Even the relatively stable f-pentyl and f-hexyl cations fragment at higher temperatures to produce the t-butyl cation, as do all other alkyl cations with four or more carbons so far studied.21 Methane,22 ethane, and propane, treated with super acid, also yield r-butyl cations as the main product (see 2-18). Even paraffin wax and polyethylene give f-butyl cation. Solid salts of t-butyl and f-pentyl cations, e.g., Me3C SbF4, have been prepared from super-acid solutions and are stable below -20°CP... 

Simple acyl cations RCO+ have been prepared45 in solution and the solid state.40 The acetyl cation CH3CO is about as stable as the f-butyl cation (see, for example, Table 5.1). The 2,4,6-trimethylbenzoyl and 2,3,4,5,6-pentamethylbenzoyl cations are especially stable (for steric reasons) and are easily formed in 96% H2S04.47 These ions are stabilized by a canonical form containing a triple bond (G), though the positive charge is principally located on the carbon,48 so that F contributes more than G. 

There is direct evidence, from ir and nmr spectra, that the f-butyl cation is quantitatively formed when f-butyl chloride reacts with A1CI3 in anhydrous liquid HCI.246 In the case of olefins, Markovnikov s rule (p. 750) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride247 and 1-chloroadamantane248 alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole but not benzene.249 It was noted on p. 337 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates.250... 

A second type of anomalous cryoscopic behaviour is exhibited by f-butyl esters ". The benzoate and acetate both give varying / -values between 3 and 4, and by extrapolating the value for f-butyl benzoate to zero time Kuhn1" obtained a value close to 3.0. This he interpreted as evidence for alkyl-oxygen cleavage, and the formation of the f-butyl cation, viz-... 

This reaction proceeds as a conjugate process (see also Eq. 2, pathway B). It starts with ionization of 57 by action of super acid (HF/SbF5) to give f-butyl cation. Its further attack on TFE with formation of another cation 59, which is stabilized by adding fluoride anion, from either a counter anion or HF ... 

First the double bond is protonated to give a f-butyl cation, which then reacts with methanol, a nucleophile, to give an oxonium ion. Loss of a proton gives f-butyl methyl ether. Notice that the acid is a catalyst it is needed for a reaction to occur, but it is not consumed. 

Extra stabilization comes to the planar structure from weak donation of a bond electrons into the empty p orbital of the cation. Three of these donations occur at anyone time in the f-butyl cation. It... 

So far we have added heteroatoms only—bromine, nitrogen, or sulfur. Adding carbon electrophiles requires reactive carbon electrophiles and that means carbocations. In Chapter 17 you learned that any nucleophile, however weak, will react with a carbocation in the S>jl reaction and even benzene rings will do this. The classic S>jl electrophile is the f-butyl cation generated from t-butanol with acid. 

Like Cbz, the f-Boc group is a carbamate protecting group. But, unlike Cbz, it can be removed simply with dilute aqueous acid. Just 3M HCl will hydrolyse it, again by protonation, loss of f-butyl cation, and decarboxylation. 

In Chapter 17 we showed you that it is possible to run the NMR spectra of carbocations by using a polar but nonnucleophilic solvent such as liquid SO2 or SOC1F. Treating an alkyl halide RX with the powerful Lewis acid SbFs under these Conditions gives a solution of carbocation the carbocation reacts neither with solvent nor the SbF5X counterion because neither is nucleophilic. We know, for example, that the chemical shifts in both the l3C and NMR spectra of the f-butyl cation are very large, particularly the 13C shift at the positively charged centre. 

Primary cations can never be observed by NMR—they are too unstable. But secondary cations can, provided the temperature is kept low enough, sec-Butyl chloride in SO2CIF at -78 °C gives a stable, observable cation. But, as the cation is warmed up, it rearranges to the f-butyl cation. Now this rearrangement truly is a carbocation rearrangement the starting material is an observable car-bocation, and so is the product, and we should just look at the mechanism in a little more detail. 

Isobutane loses an electron upon El and yields the corresponding radical cation, which will fragment mainly through the loss of a hydrogen radical to yield a f-butyl cation, and to a... 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the f-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

This crystalline complex was found to initiate the polymerisation of isobutene at —78°C more effectively than an equivalent amount of AIQ3 alone or with f-butyl chloride added to the mixture. This was taken as a proof that the activity of the complex pointed to its structure approaching that of a carbenium salt. This work was never continued with other monomers to demonstrate that the f-butyl cation was the initiating species. 

A more detailed description of such as investigation was published two years later by Sparapany . This author demonstrated the cationic nature of the process and showed that one could initiate the polymerisation of isobutene also in the gas phase and in various solvents. Yields were low but proportional to the duration of irradiation and to its intensity. Very high molecular weights were obtained. The following reactions were proposed to account for the formation of the f-butyl cation ... 

Product B must arise from a Friedel-Crafts alkylation with the f-butyl cation as intermediate This comes from the loss of carbon monoxide from the acylium ion. Such a reaction happens oniv when the simple carbocation is stable. 

If we dissolve f-butanol in a nitrile as solvent and add strong acid, a reaction does take place. The acid does not protonate the nitrile, but does protonate the alcohol to produce the f-butyl cation in the usual way. This cation is reactive enough to combine with even such a weak nucleophile as the nitrile. 

In other cases substituents compete by directing to different positions. For example, in the synthesis of the food preservative BHT (p. 000) from 4-methylphenol (p-cresol) by a Friedel-Crafts alkylation, the methyl and OH groups each direct ortho to themselves. The -OH group is much more powerfully directing than the methyl group because it provides an extra pair of electrons, so it wins and directs the electrophile (a t-butyl cation) ortho to itself. The f-butyl cation can be made from the alkene or f-butanol with protic acid or from t-butyl chloride with AICI3. 

Initiation (or olefin protonation) In this step, a f-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a f-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C and a t-butyl cation. 

Oligomerization After the primary reaction forms a Cg+ carbocation, a second olefin reacts to form a higher molecular weight hydrocarbon (e.g., C12) and another f-butyl cation. Further reactions can result in even larger products (e.g., CifiS, etc.). 

CF3COOH, PhSH, 20°C, 1 h, 100% yield. Thiophenol is used to scavenge the liberated f-butyl cations, thus preventing alkylation of methionine or tryptophan. Other scavengers such as anisole, thioanisole, thiocresol, cresol, and... 

Organic compounds with covalent bonds to electronegative elements may dissociate to form carbocations, especially if the cation is stabilized by the inductive effect, as in the f-butyl cation, or by resonance, as in the cumyl (2-phenylprop-2-yl) cation. This can happen slowly in polar solvents such as water. The SN1 reaction of t-butyl halides is an example (reaction 5.6). The slow and rate-determining heterolysis of the halide is followed by a rapid reaction of the f-butyl cation with water to give the alcohol product. -Cl HiO... 

The t-BuCl in the system presumably acts as a chain transfer agent producing a transitory tertiary chloride and the f-butyl cation ... 

But we want to use alcohols in nucleophilic substitution reactions because they are easily made. The simplest answer is to protonate the OH group with strong acid. This will work only if the nucleophile is compatible with strong acid, but many are. The preparation of f-BuCl from t-BuOH simply by shaking it with concentrated HCl is a good example. This is obviously an Sjsjl reaction with the f-butyl cation as intermediate. 

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