Reactions of Hydride Ion

Potassium hydride (KH) is a source of the strongly basic hydride ion ( H ) Using curved arrows to track electron movement write an equation for the reaction of hydride ion with water What is the conjugate acid of hydride lon ... 

For paraffins and naphthenes, the important reaction of hydride ion exchange (2) is postulated, which is in turn initiated by carbonium ions derived from small amounts of thermally produced olefins in the cracking system. 

The mechanism of the reaction shows why the product of the reaction is an amine. Take a minute to note the similarities between the mechanisms for the reaction of hydride ion with an N-substituted amide and with a carboxylic acid. 

This terminology can also be applied to the faces of trigonal planar groups such as carbonyl. Thus the two faces of acetophenone are enantiotopic as seen by the fact that reaction of hydride ion at one as opposed to the other gives enantiomeric products (14) and (15) (section 1.3). The faces of the carbonyl group in (55) are diastereotopic because reduction on one or other gives diastereomers (56) and (57) (section 1.11). 

Makar and Kruger (1993) considered the key reactions leading to the diagram in Fig. 2.1 originated by Perrault (1974, 1978). This diagram corresponds to the Mg-H20 system in the presence of H2 molecules at 25°C and considers the reactions of hydride-ion and hydride-hydroxide formation ... 

C. D. Ritchie and H. F. King, Theoretical studies of proton-transfer reactions. I. Reactions of hydride ion with hydrogen fluoride and hydrogen molecules, J. Amer. Chem. Soc. 90 825 (1968). 

The labile intermediate (96) has been isolated from the low-temperature reaction of hydride ion (tri- ec-butylborohydride) with the complex [Cr(f/" -l,3-butadiene)(CO)3 P(OMe)3 ], and its structure confirmed from an X-ray structural analysis of the tetraethylammonium salt. NMR studies show that this unstable (Z)-allyl structure, with open face of the allyl group pointing toward the P(OMe)3 ligand, is retained at low temperature (— 5 C) in solution. However, at room temperature (96) isomerizes to the thermodynamically more stable E isomer (97) see Eq. 24. 

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

Addition of hydride ion from the catalyst gives the adsorbed dianion (15). The reaction is completed and product stereochemistry determined by protonation of these species from the solution prior to or concurrent with desorption. With the heteroannular enolate, (13a), both cis and trans adsorption can occur with nearly equal facility. When an angular methyl group is present trans adsorption (14b) predominates. Protonation of the latter species from the solution gives the cis product. Since the heteroannular enolate is formed by the reaction of A" -3-keto steroids with strong base " this mechanism satisfactorily accounts for the almost exclusive formation of the isomer on hydrogenation of these steroids in basic media. The optimum concentration of hydroxide ion in this reaction is about two to three times that of the substrate. 

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Alteration of the relative reactivity of the ring-positions of quinoline is expected and observed when cyclic transition states can intervene. Quinoline plus phenylmagnesium bromide (Et20,150°, 3 hr) produces the 2-phenyl derivative (66% yield) phenyllithium gives predominantly the same product along with a little of the 4-phenylation product. Reaction of butyllithium (Et 0, —35°, 15 min) forms 2-butylquinoline directly in 94% yield. 2-Aryl- or 6-methoxy-quinolines give addition at the 2-position with aryllithium re-agents, and reaction there is so favored that appreciable substitution (35%) takes place at the 2-position even in the 4-chloroquinoline 414. Hydride reduction at the 2-position of quinoline predominates. Reaction of amide ion at the 2-position via a cyclic... 

On treatment with a strong base such as sodium hydride or sodium amide, dimethyl sulfoxide yields a proton to form the methylsulfinyl carbanion (dimsyl ion), a strongly basic reagent. Reaction of dimsyl ion with triphenylalkylphosphonium halides provides a convenient route to ylides (see Chapter 11, Section III), and with triphenylmethane the reagent affords a high concentration of triphenylmethyl carbanion. Of immediate interest, however, is the nucleophilic reaction of dimsyl ion with aldehydes, ketones, and particularly esters (//). The reaction of dimsyl ion with nonenolizable ketones and... 

As with the reduction of carbonyl compounds discussed in the previous section, we ll defer a detailed treatment of the mechanism of Grignard reactions until Chapter 19. For the moment, it s sufficient to note that Grignard reagents act as nucleophilic carbon anions, or carbanions ( R ), and that the addition of a Grignard reagent to a carbonyl compound is analogous to the addition of hydride ion. The intermediate is an alkoxide ion, which is protonated by addition of F O"1 in a second step. 

The reduction of carbonyl compounds by reaction with hydride reagents (H -) and the Grignard addition by reaction with organomagnesium halides (R - +MgBr) are examples of nucleophilic carbonyl addition reactions. What analogous product do you think might result from reaction of cyanide ion with a ketone ... 

Reduction Conversion of Nitriles into Amines Reduction of a nitrile with LiAIH4 gives a primary amine, RNH . The reaction occurs by nucleophilic addition of hydride ion to the polar C=N bond, yielding an imine anion, which still contains a C=N bond and therefore undergoes a second nucleophilic addition of hydride to give a dianion. Both monoanion and dianion intermediates are undoubtedly stabilized by Lewis acid-base complexafion to an aluminum species, facilitating the second addition that would otherwise be difficult Protonation of the dianion by addition of water in a subsequent step gives the amine. 

Still has also carried out mechanistic experiments9 3 from which he could deduce that the major reduction pathway is by attack of hydride ion at the sulphur atom. This conclusion was deduced from the fact that reduction with sodium borodeuteride-aluminium oxide gave a sulphoxide that had only incorporated about 25% mole equivalent of deuterium on to a methyl carbon atom bound to the sulphur atom. The mechanistic pathway for direct reduction is outlined in equation (38), whereas the pathway whereby deuterium could be incorporated is portrayed in equation (39). These reactions support the proposed mechanism for the hydride reduction of sulphones as outlined in Section III.A.l, namely that attack at sulphur by hydride ions may occur, but will be competitive with proton abstraction in cases when the attack at sulphur is not facilitated. 

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... 

Hydride Transfer In some reactions, a hydride ion is transferred to or from the substrate. The reduction of epoxides with LiAlH4 is an example (10-85). Another is the Cannizzaro reaction (19-60). Reactions in which a carbocation abstracts a hydride ion belong in this category ... 

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... 

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

B2H6 from BHg. The simplest reaction observed involves the abstraction of hydride ion from BHi by a boron halide to generate BH3 units which combine to form B2H6. Reaction (16) represents the general reaction observed. 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

Phenylcyclohexane also was dehydrogenated under base catalysis at 240°. This is presumably because of the formation of carbanion (III) by reaction with the catalyst, followed by the elimination of hydride ion to yield phenylcyclohexane, which can then react as before. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Both reactions are directed to C-2(6) and C-4 (attack at C-2 is shown below). Electrophilic reactions require the loss of a proton, whereas those for nucleophiles require the loss of hydride ion. 2(4)-Halogenopyridine A-oxides are good substrates for nucleophilic substitution, and the thus site of attack is strongly iniluenced by the position of the halogen atom. 

Free radical attack at the pyridine ring is noted for its low selectivity and substituents have little effect. Arylation takes place at all three positions, but halogen atoms preferentially attack the a-, and alkyl radicals the a- and y-positions. Metals such as sodium and zinc transfer a single electron to pyridine to form anion radicals. These can dimerize by reaction at the a- or y-position to yield dipyridyls by loss of hydride ion. Thus, reduction of pyridine by chemical and catalytic means is easier than reduction of benzene. 

This process has not been studied in detail. It has been shown that diphenylnitren-ium ion reacts with various hydrocarbons and metal hydrides to give diphenyl amine. An analysis of the rate constants for these processes showed that the reaction was most likely a hydride transfer, rather than a hydrogen atom transfer (Fig. 13.56). Novak and Kazerani found a similar process in their study of the decay reaction of heteroarylnitrenium ions. 

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