Hydrocarbons, hydride abstraction reactions

The hydride abstraction reaction of NO+ has been employed in a modified Ritter-type reaction51 1 (Scheme 5.50, routea) as well as in ionic fluorination512 of bridgehead hydrocarbons (Scheme 5.50, route b). 

Reaction of a carbonium ion formed from step 1 or 2 with another hydrocarbon by abstraction of a hydride ion... 

The next step in complexity are systems in which alkylation competes with hydride transfer to give dimeric alkyl cations which, when hydride abstraction occurs, yield dimeric saturated hydrocarbons (equation 14). This reaction path for cyclic aliphatic alcohols and olefins is often accompanied by some rearrangement (Deno etal., 1964 Pittman, 1964). 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition... 

The unique hydride abstraction property has been gainfully employed in developing novel synthetic reactions.530 Reactive hydrocarbons such as triphenylmethane, adamantane, and diamantane are readily fluorinated in the presence of nitrosonium ion in HF-pyridine media.537 In the presence of a suitable oxygen donor such as dimethyl sulfoxide, the nitrosonium ion can act as a nitrating agent538 [Eq. (4.152)]. The initially formed nitrito onium ion 223 transfer nitrates aromatics rather readily.245 The NO+-induced reactions are further reviewed in Chapter 5. 

A systematic study showed that the main ionizing reactions of molecules containing heteroatoms occurred through acid-base reactions with C2H5+ and C3H5+. If, however, the sample is a saturated hydrocarbon RH, the ionization reaction will be a hydride abstraction ... 

The first reaction involves interaction of a hydrocarbon with the catalyst surface. Hydride abstraction occurs to form a carbonium ion. Abstraction can be of any suitable hydrogen atom but if this results in a primary ion as shown, this will rapidly isomerise by hydrogen shift to the more thermodynamically stable secondary ion. This may be further isomerised by carbon shift to a tertiary ion. This contrasts with free radicals and although isomerisation occurs it is relatively slower. The carbonium ions can also undergo inter-molecular transfer (not shown) when a carbonium ion meets another hydrocarbon molecule. 

Althou the evidence presented in support of such reaction is not entirely convincing, in its favour is the existence of a lower oxidation state of antimony which can be readily reached by smooth reduction. In the case of other Lewis acids a lower oxidation state is either unavailable or less easily obtained and the likelyhood of hydride abstraction seems therefore more remote, at least from saturated hydrocarbons. Kennedy s scheme implies however the removal of the allylic hydride ion from an olefin. Although plausible in certain cases (but never proved), this mechanism is obviously impossible with styrene and 1,1 -diphenylethylene With 3-phenylindene it would yield an aryl-substituted allylic carbenium ion which would not be expected to be in equilibrium with its precursor yet, this equilibrium was observed With 2,3-dimethylindene in the same conditions initiation did not take place yet Kennedy s mechanism shouldhave operated without impediments. Finally, with 1,1-diphenylpropene hydride abstraction would have produced an allylic ion incapable of giving back the precursor by reacting with methanol yet Bywater and Worsfold showed that this reversible reaction takes place. 

The superacid media can induce hydride abstraction, H-D exchange and other reactions even with saturated hydrocarbons.25 Carbonium ions are formed, some of which, notably the trimethylcarbonium ion, are quite stable Me3CH - Me3C+ <- CH3CH2CH2CH3... 

Aliphatic alkylation is widely used to produce high-octane gasolines and other hydrocarbon products. Conventional paraffin (alkane)-olefin (alkene) alkylation is an acid-catalyzed reaction it involves the addition of a tertiary alkyl cation, generated from an isoalkane (via hydride abstraction) to an olefin. An example of such a reaction is the isobutane-ethylene alkylation, yielding 2,3-dimethylbutane. 

Similarly to the oxidative addition reactions, for a long time the only intramolecular reactions known followed a multicentre pathway (a and P hydride abstractions at d metal centers are classical examples). Reactions 19 and 20 constitute the first examples of such intermolecular reactions with saturated hydrocarbons. 

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

The analysis of published data on reactions of ozone with low molecular hydrocarbons shows that double bonds react with ozone more quickly than saturated bonds (12). Ozone reacts with saturated hydrocarbons in reactions in which hydrogen abstraction s followed by re-hydridization of the carbon atom form sp to sp state (43,44) ... 

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