Metal hydrides carboxylic acids

Ethyl acetate has sometimes been used to destroy lithium tetrahydrogen aluminate (the reaction is similar to the one that results from the effect of a carboxyl acid on this metal hydride described on p.321 the acid formed destroys the metal hydride). Such an attempt had been made for this purpose. It led to a very violent detonation. 

The mechanism of such reactions using unsaturated carboxylic acids and Ru(BINAP)(02CCH3)2 is consistent with the idea that coordination of the carboxy group establishes the geometry at the metal ion.26 The configuration of the new stereocenter is then established by the hydride transfer. In this particular mechanism, the second hydrogen is introduced by protonolysis, but in other cases a second hydride transfer step occurs. 

A less powerful complex metal hydride is Na BH4 which will reduce aldehydes and ketones only, and does not attack carboxylic acid derivatives nor does it—as Li AlH4 does—attack NO2 or C=N present in the same compound. It has the great advantage of being usable in hydroxylic solvents. A wide variety of other reagents of the MH4 , MH3OR , MHjfORlj type have been developed their relative effectiveness is related to both the nucleophilicity and size of MH4 , etc. 

The reduction of carboxylic acids or esters requires very powerful reducing agents such as lithium aluminum hydride (LiAlH,) or sodium (Na) metal. Aldehydes and ketones are easier to reduce, so they can use sodium borohy-dride (NaBH,j). Examples of these reductions are shown in Figure 3-13. 

However, the most important methods for preparing alcohols are catalytic hydrogenation (H2/Pd-C) or metal hydride (NaBH4 or LiAlH4) reduction of aldehydes, ketones, carboxylic acids, acid chlorides and esters (see Sections 5.7.15 and 5.7.16), and nucleophilic addition of organometalhc reagents (RLi and RMgX) to aldehydes, ketones, acid chlorides and esters (see Sections 5.3.2 and 5.5.5). 

Acid chlorides are easy to reduce than carboxylic acids and other carboxylic acid derivatives. They are reduced conveniently all the way to 1° alcohols by metal hydride reagents (NaBH4 or LiAlH4), as well as by catalytic hydrogenation (H2/Pd—C). 

Amides, azides and nitriles are reduced to amines by catalytic hydrogenation (H2/Pd—C or H2/Pt—C) as well as metal hydride reduction (LiAlH4). They are less reactive towards the metal hydride reduction, and cannot be reduced by NaBITj. Unlike the LiAlIU reduction of all other carboxylic acid derivatives, which affords 1° alcohols, the LiAlIU reduction of amides, azides and nitriles yields amines. Acid is not used in the work-up step, since amines are basic. Thus, hydrolytic work-up is employed to afford amines. When the nitrile group is reduced, an NH2 and an extra CH2 are introduced into the molecule. 

Concerning the mechanism of metal-catalyzed carboxylations, two basic pathways—the metal hydride and the metal carboxylate mechanisms—gained general acceptance.14 16 118 119122151 The actual mechanism depends on the catalyst used and the reaction conditions particularly in the presence of additives (acids, bases). It seems reasonable to assume that both mechanisms may be operative under appropriate conditions. 

The involvement of zinc in nicotinamide-based hydride-transfer reactions has led to numerous studies of Group IIB complexes of pyridine carboxylic acid derivatives. Cadmium complexes of 2-pyridinecarboxylic acid, 5 3-pyridinecarboxylic add497 and 3-pyridinecarboxamide498 have been reported. The crystal structure of [Cd(HC02)2L2(H20)2] (L = 3-pyridinecarboxamide) has also been described the metal is in an octahedral environment in which the amide acts as a monodentate N donor.498... 

Metal hydrides, such as lithium aluminum hydride, also can be used to reduce derivatives of carboxylic acids (such as amides and nitriles see Table 16-6) to aldehydes. An example follows ... 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

The carbonylchloroiridium(III) porphyrins can be transformed into a variety of other carbonyl complexes by chloride exchange with acids or salts (path e). Concentrated sodium hydroxide in ethanol appears to destroy the carbonyl ligand in these compounds (path — d, a) in a manner similar to the alkoxide addition to RhCl(TPP) CO (path f) here, this should give a carboxylic acid RhCOOH(P) which is decarboxylated to a hydride RhH(P) according to the typical base reaction of metal carbonyls. The hydride may then be autoxidized to the hydroxide. 

The following compounds with H—C and II—M bonds undergo oxidative addition to form metal hydrides. This is examplified by the reaction of 6, which is often called ortho-metallation, and occurs on the aromatic C—H bond at the ortho position of such donar atoms as N, S, 0 and P. Reactions of terminal alkynes and aldehydes are known to start by the oxidative addition of their C—H bonds. Some reactions of carboxylic acids and active methylene compounds are explained as starting with oxidative addition of their O—H and C—H bonds. 

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