Anionic catalytic hydrogenation

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

Alkylation of the protected azetidinyl bromide 61 with the anion from m-trifluormethyl-phenol gives ether 62. Removal of the N-(alpha-methylbenzyl)- protecting group by catalytic hydrogenation gives the secondary amine 63. Reaction of that compound with methyl isocyanate gives the anticonvulsant urea fluzinamide (64) [14]. 

Strategy Compare the product with the starting material, and catalog the differences. In this case, we need to add three carbons to the chain and reduce the triple bond. Since the starling material is a terminal alkyne that can be alkylated, we might first prepare the acetylide anion ol 1-pentyne, let it react with 1-bromopropane, and then reduce the product using catalytic hydrogenation. 

Amination of aromatic nitro compounds is a very important process in both industry and laboratory. A simple synthesis of 4-aminodiphenyl amine (4-ADPA) has been achieved by utilizing a nucleophilic aromatic substitution. 4-ADPA is a key intermediate in the rubber chemical family of antioxidants. By means of a nucleophibc attack of the anilide anion on a nitrobenzene, a o-complex is formed first, which is then converted into 4-nitrosodiphenylamine and 4-nitrodiphenylamine by intra- and intermolecular oxidation. Catalytic hydrogenation finally affords 4-ADPA. Azobenzene, which is formed as a by-product, can be hydrogenated to aniline and thus recycled into the process. Switching this new atom-economy route allows for a dramatic reduction of chemical waste (Scheme 9.9).73 The United States Environmental Protection Agency gave the Green Chemistry Award for this process in 1998.74... 

Anionic and cationic species are particularly easy to study in the gas phase using mass spectrometric techniques. Studies of organometallic reactions involving neutral species are becoming more prevalent. The elegant study of the catalytic hydrogenation of ethylene by photochemical... 

Scheme 8.4. Catalytic hydrogenation of imines using cationic iridium catalysts with the C02-philic BARF anion...

Tetracyclic keto ester 467, prepared earlier (253), was treated with the anion of diethyl methoxycarbonylmethylphosphonate in dimethylformamide. The reaction supplied the unsaturated ester 492, which was catalytically hydrogenated to diester 493. Dieckmann condensation of 493 yielded two nonenolizable keto esters (494 and 495), which could be separated by fractional crystallization. Sodium borohydride reduction of 18a-methoxyyohimbinone (494) gave two alcohols (496 and 497) in a ratio of about 10 1 at the same time, reduction of 180-methoxyyohimbinone (495) furnished another two stereoisomeric alcohols (498 and 499) in approximately equal amounts. Demethylation of the four stereoisomers (496-499) resulted in the corresponding 18-hydroxyyohimbines (500-503)... 

The general concept of phase transfer catalysis applies to the transfer of any species from one phase to another (not just anions as illustrated above), provided a suitable catalyst can be chosen, and provided suitable phase compositions and reaction conditions are used. Most published work using PTC deals only with the transfer of anionic reactants using either quaternary ammonium or phosphonium salts, or with crown ethers in liquid-liquid or liquid-solid systems. Examples of the transfer and reaction of other chemical species have been reported(24) but clearly some of the most innovative work in this area has been done by Alper and his co-workers, as described in Chapter 2. He illustrates that gas-liquid-liquid transfers with complex catalyst systems provide methods for catalytic hydrogenations with gaseous hydrogen. 

Bipyridine is catalytically hydrogenated to 3,3 -bipiperidine, and the reduction may also be accomplished electrochemically. Erythro and threo forms of 3,3 -bipiperidine are produced by the latter method. 3,3 -Bipyridine is reduced with difficulty by means of tin and hydrochloric acid or sodium and alcohol. Some l,2,3,4,5,6-hexahydro-3,3 -bipyridine (also known as nicotidine) is produced, but several other partly reduced 3,3 -bipyridines are obtained as well. 3,3 -Bipyridine is reported to give a radical anion in tetrahydrofuran on reduction by sodium. Although there is some doubt about the authenticity of the starting material (see Section 3,4 -bipyridine is reported to be fully reduced to 3,4 -bipiperidine by catalytic hydrogenation and to l, 2, 3, 4, 5, 6 -hexahydro-3,4 -bipyridine by tin and hydrochloric acid. ... 

Bipyridine is likewise reduced to 4,4 -bipiperidine by sodium and amyl alcohol and by catalytic hydrogenation. " 2,2 -Dimethyl-4,4 -bipyridine is reduced to 2,2 -dimethyl-4,4 -bipiperidine. Electrochemical reduction of 4,4 -bipyridine affords 4,4 -bipiperidine and some partly reduced 4,4 -bipyridines. Further work on the electroreduction of 4,4 -bipyridine has been reported. Reduction of 4,4 -bipyridine by tin and hydrochloric acid " or by controlled catalytic hydrogenation - gives l,2,3,4,5,6-hexahydro-4,4 -bipyridine. 4,4 -Bipyridine is reduced to its 1,4-dihydro derivative by bisdihydropyridyl metal complexes and to its radical anion by alkali metals and related processes. " " The ionization constant of the radical anion has been determined. ... 

The percent ring substitution (% RS) of the polymer with active sites affects catalytic activity. Polystyrenes with < 25 % RS with lipophilic quarternary onium ions are swollen in triphase mixtures almost entirely by the organic phase. Water reduces the activity of anions by hydrogen bonding. In most triphase nucleophilic displacement reactions onium ion catalysts with <25% RS are highly active, and those with >40% RS, such as most commercial ion exchange resins, are much less active. However, low % RS is not critical for the reactions of hydroxide ion with active methylene compounds, as commericial ion exchange resins work well in alkylation of active nitriles. 

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