Tertiary butyl protons

A potassium bromide pellet prepared from the major component indicated the formation of an ester linkage, the presence of a phosphate, and an A -t-butoxycarbonyl group. A UV spectrum with a maximum absorption at 257.5 nm identical to that of ATP eliminated the possibility that aminoacylation may have taken place at 6-amino group of nucleotide. Nuclear magnetic resonance spectroscopy demonstrated the presence of tertiary butyl protons. On the basis of the above evidence, structure (II) has been assigned to this product. 

When tertiary butyl hydrogen peroxide (TBHP) was used alone as the radical initiator, no grafting of methylmethacrylate (MMA) onto wool was observed. However, TBHP in conjunction with mineral acids, such as H2SO4, HNO3, or HCIO4 afforded good results [26]. Protonation of TBHP by the acid aided in the dissociation of TBHP to yield free radicals, which initiated grafting reaction. 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

No fluorescence is observed at room temperature from TIN in non-polar solvents such as cyclohexane. In these solvents only the intramolecularly hydrogen-bonded form, which can undergo rapid ESIPT upon excitation, is present. The t-Bu-STIN derivative (see Table II) is very weakly fluorescent in all of the solvents examined. This is attributable to the protection of the intramolecular hydrogen bond from the solvent by the tertiary butyl group which is adjacent to the labile proton. 

It will be noted from the above examples that the tertiary butyl carbonium ions required for the reaction are constantly being replenished to establish a chain reaction. It is assumed that the reaction is initiated by olefin molecules accepting protons from the catalyst to form carbonium ions which react with isobutane to produce the necessary active tertiary butyl carbonium ions. 

Fig. 2. Isomerization of protonated tertiary butanol (1) to a hydrogen bonded complex between the tertiary butyl cation and water (2) is provided by the successive rotations of two methyl groups. Results of MP2/6-31G calculations...

Chain Initiotion. The theory pxKtulated by a number of investigators (Cupit etal., 1961, Schmerling, 1955) is that carbonium ions are generated by addition of a proton (H+) to an olefin molecule in the presence of HF. Albright and Li, 1970, and Hofmann and Schriesheim, 1962, indicate that initiation steps with H2SO4 catalyst may involve red oil hydrocarbons. However, only the tertiary butyl carbonium ion performs the chain carrying function in isobutane alkylation. Reactions follow ... 

The reaction of a carbocation with a neutral nucleophile such as water gives a protonated alcohol. Tertiary butyl carbocation, for example, reacts with water (neutral nucleophile) to give protonated tert-butyl alcohol, which eliminates a proton to give tert-butyl alcohol (Scheme 2.5). 

The highly unstable primary carbenium ion rearranges rapidly to give the relatively stable tertiary butyl cation, which gives isobutylene by proton abstraction ... 

The interaction of triphenylmethylenephosphoranes (108) with tertiary butyl lithium has been studied by proton and lithium-7 NMR. It was found that treatment of simple ylides such as (108 a) and (108b) with one equivalent of the butyl lithium resulted in ortho-metallation of one of the phenyl rings... 

Just as anions of allyl derivatives can be homoenolate equivalents (chapter 13) so anions of vinyl derivatives can be acyl anion equivalents. Vinyl (or enol) ethers can be lithiated reasonably easily, especially when there is no possibility of forming an allyl derivative, as with the simplest compound 81. The most acidic proton is the one marked and the vinyl-lithium derivative 82 reacts with electrophiles to give the enol ether of the product17 84. However, tertiary butyl lithium is needed and compounds with y-CHs usually end up as the chelated allyl-lithium 85. These vinyl-lithium compounds add directly to conjugated systems but the cuprates will do conjugate addition.18... 

Reeve, Chambers, and Puckett propose a two-step reaction, the first step involving addition of a positive chlofineJbo the center of high electron density of the olefin. Then the chloro tertiary butyl carbonium ion loses a proton to form the unsaturated methallyl chloride. 

A number of sterically hindered cyclopentadienones with tertiary butyl substituents has been prepared [270], The H-n.m.r. spectrum of the 2,4-di-t-butyl derivative has signals for the ring protons at higher field than those expected for protons attached to normal unsaturated ketones and it has been suggested that this is associated with a paramagnetic ring current due to a 4-ir-electron system [271]. 

Tertiary, allylic, and benzylic ethers are particularly susceptible to deavage by acid, often under quite mild conditions. Tertiary butyl ethers, for example, are cleaved by aqueous HCl at room temperature. Proton transfer from the add to the oxygen atom of the ether produces an oxonium ion, which then deaves to produce a particularly stable 3°, allylic, or benzylic carbocation. Reaction of the carbocation with CP completes the reaction. 

Cerfontain and co-workers suggested that the last step of the reaction, namely proton removal from the complex, was not rate-contributory, but this argument may not be valid since a more recent publication reported a higher value of the kinetic isotope effect (kn kxi) of 1.9-2.5. In sulfonylation using alkylsulfonyl chlorides, there is competing alkylation the latter is enhanced by increasing stability of the alkyl cation. Thus reaction of isopropyl, tertiary butyl and benzylsulfonyl chlorides caused facile alkylation of aromatic substrates in the presence of aluminium chloride in nitromethane at 25 °C. ... 

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