Metal and proton source

The ability of certain metals to donate electrons to electrophilic or unsaturated functional groups has proven useful in several reductive procedures. The facility with which these metals donate electrons is given by their standard reduction potentials. 

Carbonyl groups and conjugated Tr-electron systems are reduced by metals such as Li, Na and K, usually in liquid ammonia solution. Other reactive metals such as zinc and magnesium reduce aldehydes and ketones in the presence of a proton source. 

Clemmensen reduction Aldehydes and ketones are reduced by heating with a solution of zinc metal in mercury (zinc amalgam) and hydrochloric acid. 

functional substituents a to the carbonyl group such as hydroxyl, alkoxyl and halogens are reduced, and then the resulting unsubstituted aldehyde or ketone is reduced to the parent hydrocarbon. 

Reduction with alkali metals The solvents used for alkali metal reductions include hydrocarbons, ethers and, most commonly, liquid ammonia. Alcohols may also be used, but usually as co-solvents, since they react vigorously with these metals. Aldehydes are not usually reduced in this manner, because they react with ammonia to form unreactive imine condensation products. 

---

Nitrosobenzene and oximes are also reduced to the corresponding amine with metal and proton source. 

The metal and proton source system such as Zn and AcOH reduces 2-haloketones to ketones. 

In 1991,Inanaga et al. described the reduction at room temperature of some dis-ubstituted alkynes by the combination SmI2/proton source in the presence of some transition-metal catalysts (3% equiv) in THF [155]. By a good choice of catalyst (CoCl2,4 PPh3) and proton source (MeOH, z -PrOH or AcOH) it was possible to orientate the reaction towards the exclusive formation of the Z-alkene. When the same reaction was performed in the presence of HMPA then the E-alkene was produced. Iron(III) and Ni(II) catalysts were found to be less efficient. It was assumed that the reactive species were the corresponding transition-metal hydrides obtained by reduction of the initial complexes. 

The following is a brief survey on these chemical N2-reducing systems giving ammonia (and sometimes hydrazine). At first reactions of N2 promoted by the mixtures of transition-metal species with reducing agents and proton sources are illustrated, and then transformations of the N2 ligand in well-defined complexes to form ammonia are summarized. 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

The term Birch reduction was originally applied to the reduction of aromatic compounds by alkali metals and an alcohol in ammonia. In recent years many chemists have used the term to include all metal-ammonia reductions, whether an alcoholic proton source is present or not. The author prefers to use the term Birch reduction to designate any reduction carried out in ammonia with a metal and a proton donor as or more acidic than an alcohol, since Birch customarily used such a proton donor in his extensive pioneering work. The term metal-ammonia reduction is best reserved for reductions in which ammonia is the only proton donor present. This distinction in terminology emphasizes the importance of the acidity of the proton donor in the reduction process. 

Organolithium compounds are sometimes prepared in hydrocarbon solvents such as pentane and hexane, but nonnally diethyl ether is used. It is especially important that the solvent be anhydrous. Even trace amounts of water or alcohols react with lithium to form insoluble lithium hydroxide or lithium alkoxides that coat the surface of the metal and prevent it from reacting with the alkyl halide. Furthennore, organolithium reagents are strong bases and react rapidly with even weak proton sources to fonn hydrocarbons. We shall discuss this property of organolithium reagents in Section 14.5. 

Ta metal under proton irradiation Study of radiation effects of the Ta(p, n) W reaction in Ta foil, recording of emission spectra before and after annealing with metallic Ta absorber, and of absorption spectra before and after annealing W/W source... 

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

The organometallic side-product MAr is conveniently removed by selective reaction with a proton source such as Bu Cl or NH4C1. This reaction is extremely effective for certain symmetrical triarylphos-phines and for mixed aralkylphosphines. However, detailed investigations of P-C cleavage in functionalized and/or unsymmetrical triaryl-phosphines (17-19) indicate that such reactions are far from straightforward. The products obtained on treatment of triarylphos-phines of type IV or V with alkali metals depend both on the nature of the substituents X and on the alkali metal. 

Scheme 41.1 Reduction of anthracene and pyrene using electropositive metals in ionic liquids with HCI as the proton source.

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

At the outset of our studies of the reactivity of I and II, it was necessary to investigate claims that tertiary henzamides were inappropriate substrates for the Birch reduction. It had been reported that reduction of A,A-dimethylbenzamide with sodium in NH3 in the presence of tert-butyl alcohol gave benzaldehyde and a benzaldehyde-ammonia adduct. We formd that the competition between reduction of the amide group and the aromatic ring was strongly dependent on reaction variables, such as the alkali metal (type and quantity), the availability of a proton source more acidic than NH3, and reaction temperature. Reduction with potassium in NH3-THF solution at —78 °C in the presence of 1 equiv. of tert-butyl alcohol gave the cyclohexa-1,4-diene 2 in 92% isolated yield (Scheme 3). At the other extreme, reduction with lithium in NH3-THF at —33 °C in the absence of tert-butyl alcohol gave benzaldehyde and benzyl alcohol as major reaction products. ... 

Dihydroaromatics find diverse applications. The main way to prepare them is through Birch reduction of aromatic compounds (Birch 1944, Wooster and Godfrey 1937, Hueckel and Bretschneider 1939). Aromatic compounds are hydrogenated in diethyl ether or liquid ammonia, with alkali metals as reductants and alcohols as proton sources. 

This complex can also transfer hydride to another molecule of the carbonyl compound in a similar manner, and the process continues until all four hydrides have been delivered. Since all four hydrogens in the complex metal hydride are capable of being used in the reduction process, 1 mol of reducing agent reduces 4 mol of aldehyde or ketone. Finally, the last complex is decomposed by the addition of water as a proton source. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

---