Hydride abstracting agents

The nitrosonium ion does not react toward aromatics except in activated systems. It forms a Jt-complex with aromatics with deep color.533,534 However, it is a powerful hydride-abstracting agent in the case of activated benzylic or allylic positions. Olah and Friedman535 have demonstrated that isopropylbenzenes undergo hydride abstraction to cumyl cations 222 [Eq. (4.151)] which further reacts to give various condensation products. The reaction has been employed to prepare a variety of stable carbocations.536... 

The nitrosonium ion (NO ), the electrophilic species formed in nitrous acid media, is also a particularly effective hydride abstracting agent. Cumene reacts with NO+ to give various condensation products that involve intermediate formation of the cumyl cation. °The formation of the cumyl cation in a nonlinear hydride transfer reaction involves a pentacoordinate carbocation [Eq. (6.45)]. 

Nitrosonium ion (NO ) is an excellent hydride abstracting agent. Cumene undergoes hydride abstraction to provide cumyl cation, which further reacts to give condensation products The hydride abstracting ability of NO has been exploited in many organic transformations such as the Ritter reaction, ionic fluorination and so on (equation 59). ... 

Summary Treatment of the sterically hindered silanes TsiSiRR H (where RR = H2 or PhMe and Tsi = (MesSilaC) with the hydride abstraction agent Ph3CB(C6Fs)4 affords cationic species TsiSiRR" which are in equilibrium with bridged cationic species. Bridged cations in which Ph or H bridge between 1,3-silicon atoms have been isolated and characterized by X-ray crystallography. 

All nltrosonlum ions are in principle hydride-abstracting agents, and nltrosylsulfurlc acid has occasionally been used as a dehydrogenating agent (eq 13). ... 

Other organic mediators act as hydride atom-abstracting agents. This is true, for example, with 2,2-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and the oxoammonium ion which is anodically accessible from 2,2,6,6-tetramethylpiperidyl oxide (TEMPO). DDQ has been electrochemically regenerated either externally or internally The in situ electrochemical oxidation, of TEMPO to the active oxoammonium ion is performed in lutidine-containing acetonitrile. Thus, primary alcohols can be oxidized to the aldehydes, while secondary ones are stable Primary amines are transformed to nitriles. If water is present, the amines are cleaved via the Schiff bases to the corresponding carbonyl compounds... 

The Friedel-Crafts acylation of alkanes requires hydride abstraction, which can be induced by the acylium ion itself, to form the corresponding carbocation. This may undergo carbocationic rearrangements prior to a proton loss to form an alkene, which then reacts with the acylating agent. Similar to the acylation of alkenes, the product is an unsaturated ketone. The reaction is limited to alkanes that are prone to undergo hydride transfer. 

To avoid the limitation of one-electron redox catalysts due to the necessity to use compounds with relatively high oxidation potentials, other types of mediator systems with very low oxidation potentials would be interesting to obtain high chemoselectivities. However, mediators with much lower oxidation potentials cannot react via electron transfer but must act as hydride ion abstracting agents. Mediators of his type are ortho- and / ra-quinones, or ortho- and pora-diamines [107]. The reactivity of such systems has been rationalized as shown in Fig. 17. 

Figure 18. Indirect electrochemical NAD(P)" regeneration using mediators acting as hydride ion-abstracting agents.

The nitronium ion is also capable of acting as an oxidizing agent effecting hydride abstraction from a variety of functionalized alkanes. The oxidation of diarylmethyl methyl ethers is best illustrated involving pentacoordinate carbocations [Eq. (6.44)]. 

Other hydrogen abstracting agents also have proved useful in hydroperoxide preparations. The excited uranyl ion, Uaq02, for example, reacts rapidly with (NH3)4(H20)RhH and other rhodium(III) hydrides (82) to generate Rh(EI) species, Eq. (5), followed by O2 capture of Eq. (4). Finally, the UaqOj produced in Eq. (5) reduces (NH3)4(H20)Rh00 to hydroperoxide and regenerates UaqO, Eq. (6). 

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