Hydride ion abstraction

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Alternative high-yield syntheses of these various boranes via hydride-ion abstraction from borane anions by BBra and other Lewis acids have recently been devised l (see p. 162). 

We have previously shown (8) that the chemical ionization spectra using methane as reactant are generated by the combination of dissociative proton transfer from CH5 + and hydride ion abstraction and alkyl ion... 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

For straight chain and cycloalkanes, RoCek et al. prefer a mechanism involving hydride ion abstraction to give a partly-developed carbonium ion which suffers further reaction with the Cr(IV) portion before it can become free to give acetate or olefin... 

A few results have been reported on the oxidation of cyclohexanol by acidic permanganate In the absence of added fluoride ions the reaction is first-order in both alcohol and oxidant , the apparent first-order rate coefficient (for excess alcohol) at 25 °C following an acidity dependence k = 3.5-1-16.0 [H30 ]sec fcg/A , depends on acidity (3.2 in dilute acid, 2.4 in 1 M acid) and D2o/ H20 is f-74. Addition of fluoride permitted observation of the reaction for longer periods (before precipitation) and under these conditions methanol is attacked at about the same rates as di-isopropyl ether, although dioxan is oxidised over twenty times more slowly. The lack of specificity and the isotope effect indicates that a hydride-ion abstraction mechanism operates under these conditions. (The reactivity of di-isopropyl ether towards two-equivalent oxidants is illustrated by its reaction with Hg(II).) Similar results were obtained with buffered permanganate. 

Two studies have been performed by Littler on the oxidation of cyclohexanol by Hg(II), the second leading to more detailed and reliable data. The reaction is first-order in both oxidant and substrate but the rate is independent of acidity. E is 24.8 kcal.mole AS is 1 eu, Ath/Acd is 3.0 and ko ol HzO 1-30-At 50 °C di-isopropyl ether is attacked at about one-half the rate of isopropanol, which implies that hydride ion abstraction is occurring in both cases. This is supported in the case of cyclohexanol by the isotope effects. 

Oxidation of isopropyl alcohol (H2R) by chromic acid has been studied in det ai by Westheimer and Novick , and it was found that acetone (R) is formed nearly quantitatively. The reaction proved to be first order with respect to hydrogen chromate and second order with respect to hydrogen ions. Measurements using 2-deutero-2-propanol under identical conditions as those for the oxidation of ordinary isopropyl alcohol showed the rate of reaction to be of that with the hydrogen compound. This fact is considered to prove that the secondary hydrogen atom is removed in the rate-controlling step and that the assumption of hydride-ion abstraction can be excluded. The data are consistent with the following mechanism... 

A related dienyl species may be obtained by reaction of the diene complex with trityl ion, when hydride ion abstraction occurs from the diene to yield a dienyl group coordinated to one metal center, [(C6H7)Os3(CO)10]+ (153). 

Towards, or after, the end of the reaction leading to the formation of the SD ions, the spectra of the solutions slowly developed a rather broad peak around 450 m X (Figure 1). We attribute this peak to allylic carbonium ions formed by hydride ion abstraction from an unsaturated polymer molecule by an SD ion X+ ... 

The initiation with some carbocations, especially trityl, does not involve direct addition to monomer. The carbocation abstracts a hydride ion from the a-carbon of monomer and the newly formed carbocation initiates polymerization [Afsar-Taromi et al., 1978 Kuntz, 1967]. This hydride ion abstraction is so facile with 1,3-dioxolane that it is used to preform stable l,3-dioxolan-2-ylium salts (XII) that can be used subsequently as initiators [Jedlinski et al., 1985],... 

These Cl [M + H]+ ions (quasimolecular ions) are often prominent. Chemical ionization spectra sometimes have prominent [M — H]+ ions because of hydride ion abstraction from the M,+ ion by CH5+. Since the [M + H]+ ions are chemically produced, they do not have the great excess of energy associated with ionization by electron impact, and they undergo less fragmentation. For example, the El spectrum of 3,4-dimethox-yacetophenone shows, in addition to the molecular ion at m/z 180, 49 fragment peaks in the range of mJz 40-167 these include the base peak at m/z 165 and prominent peaks at m/z 137 and m/z 77. The CH4 induced Cl spectrum shows the quasimolecular ion (M + H+, m/z 181) as the base peak (100%), and virtually the only other peaks, each of just a few percent intensity, are the... 

Water participates in carbocation formation59,60 by generating the strong acid H[Al(OH)Hlg3],6,59 which forms a carbocation through an alkyl halide intermediate60 or via direct hydride ion abstraction.6 Experiments with heavy water by Pines further proved its cocatalytic effect indicating that HC1 (HBr) formed is not the real activator.60,61... 

Initiation by both types of salt is relatively inefficient and the simple fast quantitative alkylation proposed by Yamashita (119) is quite wrong. Inevitably significant induction periods precede an accelerating propagation reaction and for the case of Et30+ BF4 as initiator, both ethane and ethyl fluoride are detectable products. Presumably hydride ion abstraction from the monomer occurs, together... 

Cycloheptalnene complexes can be oxidized (hydride ion abstraction) to form cydoheptatrienyl (sometunes called tropylium) complexes 120... 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

Carbonium ions and isoparaffins are formed by hydride ion abstraction and hydride ion transfer reactions. This mechanism has been described for HF.SbFg (5). Isomerization of n-paraffins over monofunctional acidic catalysts has also been claimed for mordenite (6, 7), for sieve Y (8), and for the base of the catalyst of undisclosed composition applied in the isomerization process using a noble metal on an acidic zeolite base (3). 

Bawn et al. (19), and Kuntz (43) suggested that after hydride ion abstraction the resulting species reacts with another molecule of THF to form a growing polymer molecule which has an acetal end group ... 

Initiation by p-ClC6H4N3 PFjj also seemed to occur via a hydride ion abstraction. Dreyfuss and Dreyfuss (25) showed that the expected product of hydride ion abstraction, chlorobenzene, is formed in the decomposition of p-ClC6H4N PF3 in 2-MeTHF and the product of thermal decomposition, p-chlorofluorobenzene, was absent. In this case... 

The implication is that where hydride ion abstraction is indicated, the probable true initiator is the add HSbCl6 or HPFS. Dreyfuss et al. further point out that one cannot make the apriori assumption that the reactivity of the dialkyl oxonium ion (THF HSbCle) is comparable to the reactivity of the trialkyl oxonium ion (i.e. the propagating species). In fact, it may be slower (46). Thus any assumptions about the rate of initiation, or about the number of active centers formed, or any attempts to correlate the degree of polymerization of the resulting polymer with the amount of carbonium ion salt or aryl diazonium salt initiators charged should be made with extreme caution. Again, for theoretical studies of the polymerization, the use of preformed trialkyloxonium salts is to be preferred. 

Bawn, Bell, Fitzsimmons, and Ledwith (20) have suggested that transfer reaction in the bulk polymerization of THF must involve either hydride ion abstraction from the alpha methylene of THF or of tetra-methyleneoxy units in the polymer, or degradative oxonium ion formation with the ethereal oxygen atoms of polymers of the type discussed in Section IIID4. 

Hydride ion abstraction from 177-2-benzothiopyrans by triphenylcarbenium fluoroborate produces the highly colored 2-benzothiopyrylium salts 303 (Equation 43) <1998JOC4626>. 

The mass spectrometric evidence on the hydride-ion transfer from C3H8 to methyl, ethyl and vinyl ions has been discussed vide supra). A careful quantitative study (Bone e al., 1967) showed that the hydride ion abstraction (29) represents indeed the predominant reaction of propynyl ions in propane, being four to ten times faster than any competing process. The hydride-ion transfer (33), which causes the formation of C3H7T, is observed in the mass spectrometer (Derwish et al., 1964b Bone et al., 1967 Munson et al., 1964), as well as the transfer from CsHs to the C2HI and 03 ions, that represents the source of ethane and propylene, according to Eqs. 30 and 32. 

In the case of C2—C4 paraffins, the competition between reactions (48) and (49) is biased in favour of the hydride-ion abstraction, which determines a large yield of HT and relatively low yields of tritiated... 

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