Elimination of hydride

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. 

Alkaline solutions of Ru(CO)t2 (KOH in aqueous ethoxyethanol) have also been found to catalytically decompose formic acid (5 7,5S). Presumably this occurs by way of anionic ruthenium hydride derivatives [e.g., HRu3(CO)7,] reacting with HCOOH to provide a ruthenium formate derivative and H2. Subsequent / -elimination of hydride from the ruthenium formate led to regenerating the anionic ruthenium hydride species and carbon dioxide. We have recently synthesized and fully characterized a possible ruthenium formato intermediate for this process, Ru3(CO),0-(02CH) (9) (59). Indeed this species in part extrudes C02 in the presence of CO with concomitant production of Ru3(CO),, H. ... 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

Thus, spontaneous elimination of hydride ion commonly demands a high temperature and is characteristic for anionic aH-complexes only. 

If the elimination of hydride ion is rate-determining and it is assumed that the addition step is rapidly reversible, the results might be explained as follows (Scheme IV), taking 3-picoline as an example The aromatization step in Scheme IV probably involves the abstraction by the lithium cation of the hydrogen atom with its bonding pair of electrons, so that an electron-repelling ortAo-methyl should lower the activation energy of such a process more than a para-methyl... 

Molecular orbital calculations of the w-electron distribution in pyridine predict that more 4- than 2-aminopyridine should be formed in the Tschitschibabin reaction.4 The fact that no 4-aminopyridine can be detected when the two positions are allowed to compete for a deficiency of sodamide (see, e.g., Abramovitch et al 268) has led to the suggestion that the observed orientation in this reaction depends on the relative ease of elimination of a hydride ion from C-2 and C-4 and not upon the initial mode of addition (which, by implication, must take place predominantly at C-4 as predicted by the molecular orbital calculations).4 This hypothesis necessitates that the addition step be rapidly reversible and that the second stage, the elimination of hydride ion, be the rate-determining one (Scheme VII). Although it seems reasonable to assume that the hydride ion eliminations are the slow steps in this reaction, the fact that no deuterium isotope effect was observed in the reaction of 3-picoline-2d and of pyridine-2d with sodamide implies that the first stage must be virtually irreversible,268 as was found also in the case of the addition of phenyllithium to pyridine.229 The addition stage must, therefore, be the product-... 

The anionic cr-adducts were rapidly formed in liquid ammonia as shown by spectroscopic methods. The elimination of hydride is frequently facilitated by the addition of an oxidant. When the diazine was dissolved in a mixture of liquid ammonia and potassium amide, the addition of potassium permanganate resulted in oxidation to an aminoheterocycle. Treating 155,156, and 157 in this way led to the corresponding aminoheterocycles 158-160 in good yield (Scheme 59). This is a general oxidation method which has been applied to other substituted diazines (82JHC1285). Amination of 3-phenylpyridazine (161) gave a mixture of all three possible amino derivatives (Scheme 60). 

The reaction may involve the addition of an amide anion to the pyridine ring, followed by elimination of hydrogen and the formation of 2-aminopyridine upon hydrolysis (Scheme 1). More plausibly, the reaction may proceed via the addition of an amide anion to the pyridine ring, followed by the elimination of hydride, which then deprotonates at the amino group, as shown in Scheme 2. 

It might be interesting to note that the proponents of the carbene mechanism (mentioned earlier), point out that this is also consistent with their mechanism [254, 255], The reaction can consist of (a) an insertion of a metal into an a-CH bond of a metal alkyl to form a metal-carbene hydride complex. This is followed by (b) reaction of the metal-carbene unit with an alkene to form a metal-cyclobutane-hydride intermediate. The final step (c), is a reductive elimination of hydride and alkyl groups to produce a chain-lengthened metal alkyls. This assures that a chiral metal environment is maintained [254]. It is generally believed [258], however, that stereospecific propagation comes from concerted, multicentered reactions, as was shown in the Cossee-Arlman mechanism. The initiator is coordinated... 

In conclusion, Oppenauer oxidation may also be regarded as an elimination of hydride (H ) ion. The reverse reaction is therefore hydride addition to carbonyl, as in the Meerwein-Ponndorf-Verley reduction (Section 4.15.3) ... 

This conventional beginning is followed by a rather unconventional conclusion, as aromaticity is regained through elimination of hydride (Fig. 14.104).This step is unconventional because loss of hydride from an organic compound is rare. Aminopyridine and potassium hydride are the initially formed products. These react rapidly and irreversibly to form hydrogen gas and a nicely resonance-stabilized amide ion. A final hydrolysis step regenerates aminopyridine. It is presumably the irreversible nature of hydrogen gas formation that helps drive this reaction to completion. 

Square-redox scheme for the CO-induced reductive elimination of hydride from an iron-hydride complex... 

Anthracene and Related Compounds. The formation of anthracene by elimination of hydride from 9,10-dihydroanthracene has been reported in the preceding section. An improved high-yield synthesis of 7,12-dimethylbenz-[ajanthracene (336), a compound of biological interest, utilizes the reaction of 7,12-benz[a]anthraquinone with MeLi. Subsequent reaction with dry HCl in ethyl acetate produces 7-(chloromethyl)-12-methylbenz[a]anthracene, reduction of which gives (336). 

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