Hydride ion removal

In any of these cases, an analogy of the initiatory mechanism to that encountered in olefin cracking is clear thus, association with a proton, rather than hydride ion removal (as required for paraffins and naphthenes), normally constitutes the first step in the cracking of both aromatics and olefins. 

In mitochondria, hydride ions removed from substrates by NAD-linked dehydrogenases donate electrons to the respiratory (electron-transfer) chain, which transfers the electrons to molecular 02, reducing it to H20. 

Dehydrogenation of alkylbenzene over the platinum component yields phenylalkenes. Protonation of the phenylalkene over the acid component forms a carbonium ion. (A phenylallyl cation may be produced by proton addition to phenylbutadiene or by hydride ion removal from a phenylalkene.) Attack of this carbonium ion on the aromatic ring closes either a five- or six-membered ring. Stabilization of the product occurs by proton elimination or hydride abstraction. This step may be followed by dehydrogenation to the thermodynamically most-stable species (e.g., to an alkylnaphthalene in the case of six-membered ring closure). (See p. 308.)... 

O The strongly basic hydride ion removes the acidic proton from the carboxylic acid. 

Q Like the reaction shown in Figure 19.8, the mechanism begins with the basic hydride ion removing an acidic proton. Here, it is the proton on the nitrogen. 

To examine the feasibility of cation formation through hydride ion abstraction, we obtained an estimate for the barrier for hydride ion removal. Due to complications associated with the gas-phase reaction, we modeled the abstraction by considering the 1-aminoethyl cation abstracting a hydride ion from the neutral aminopropyl radical. Although hydride abstraction might have been expected to be a high-energy process, the calculated barrier for this model reaction is only 13... 

A hydride ion removes the acidic hydrogen from the nitrogen of the amide, and the electrons left behind are delocalized onto oxygen. 

In addition to the effects of a cyclic transition state, of lone-pair repulsions, and of rate of removal of hydride ion mentioned above, the position of nucleophilic substitution can be altered by a) hydrogen... 

Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion.

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 similar mechanism was invoked by Ohshima and Kawabata (2) to account for their results in the nitrosation of tertiary amines and amine oxides. In applying these concepts to the nitrosative dealkylation of tetraalkyltetrazenes, Michejda al. 5) introduced an interesting variant by suggesting that immonium ions could be formed in two successive one-electron oxidation steps (for example by ferric ion oxidation of tertiary amine to the radical cation followed by radical abstraction of a hydrogen atom from the alpha position), rather than exclusively through the one-step removal of a hydride ion as nitroxyl. The resulting immonium ion was again considered to react directly with nitrite to produce the N-nitroso derivative. These reactions are summarized in Fig. 2b. 

We have met the hydride ion in the guise of NaBH4 and L1AIH4 twice already (frames 41-45 and 76 - 79) - If we now use hydride ion Id remove the carbonyl oxygen atom altogether we shall obviously get a hydrocarbon ... 

Both coenzymes undergo reversible reduction of the nicotinamide ring (Fig. 13-15). As a substrate molecule undergoes oxidation (dehydrogenation), giving up two hydrogen atoms, the oxidized form of the nucleotide (NAD+ or NADP+) accepts a hydride ion ( H, the equivalent of a proton and two electrons) and is transformed into the reduced form (NADH or NADPH). The second proton removed from the substrate is released to the aqueous solvent. The half-reaction for each type of nucleotide is therefore... 

NAD-linked dehydrogenases remove two hydrogen atoms from their substrates. One of these is transferred as a hydride ion ( II ) to NAD+ the other is released as H+ in the medium (see Fig. 13-15). NADH and NADPH are water-soluble electron carriers that associate reversibly with dehydrogenases. NADH carries electrons from catabolic reactions to their point of entry into the respiratory chain, the NADH dehydrogenase complex described below. NADPH generally supplies electrons to anabolic reactions. Cells maintain separate pools of NADPH and NADH, with different redox potentials. This is accomplished by holding the ratios of [reduced form]/[oxidized form] relatively high for NADPH and relatively low for NADH. Neither NADH nor NADPH can cross the inner mitochondrial membrane, but the electrons they carry can be shuttled across indirectly, as we shall see. 

The reaction of 5-azaindolizines with dimethyl acetylenedicarboxylate gave the compounds 23 and 24. Attempts to remove a hydride ion from the dihydro-4a-azacycl[3,2,2]azines (24) failed.27... 

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