Organometallic compounds carboxyl carbon

Carboxylic acid groups can also be installed in molecules using the reaction of an organometallic compound with carbon dioxide. This is a reductive method since the carbon dioxide is reduced to a carboxylic acid by formation of a new carbon-carbon bond. Both Grignard reagents and organolithium compounds work well in this reaction. 

A further interesting, and synthetically useful, reaction of carbanions— and of organometallic compounds acting as sources of negative carbon—is addition to the very weak electrophile C02, to form the corresponding carboxylate anion (36)—carbonation ... 

Decarboxylation reactions of metal carboxylates [Eq. (1)], are of increasing value in the synthesis of organometallic compounds (1-5). The reverse reaction, e.g., carbonation of Grignard and organolithium reagents (6,7), is a well-known source of carboxylic acids. Early reviews of the... 

When a nucleophile containing a heteroatom reacts at a carboxyl carbon SN, reactions occur that convert carboxylic acid derivatives into other carboxylic acid derivatives, or they convert carbonic acid derivatives into other carbonic acid derivatives. When an organometallic compound is used as the nucleophile, SN reactions at the carboxyl carbon make it possible to synthesize aldehydes (from derivatives of formic acid), ketones (from derivatives of higher carboxylic acids), or—starting from carbonic acid derivatives—carboxylic acid derivatives. Similarly, when using a hydride transfer agent as the nucleophile, SN reactions at a carboxyl carbon allow the conversion of carboxylic acid derivatives into aldehydes. 

Most SN reactions of hydride donors, organometallic compounds, and heteroatom-stabi-lized carbanions at the carboxyl carbon follow the mechanism shown in Figure 6.2. Thus, the substitution products, i.e., the aldehydes and ketones C, form in the presence of the nucleophiles. Thus, when the nucleophile and the acylating agent are used in a 2 1 ratio, alcohols F are always produced. 

Fig. 6.40. On the chemo-selectivity of the reactions of hydride donors, organometallic compounds, and heteroatom-stabilized "carbanions with acylating agents (kM t refers to the rate constant of the addition of the nucleophile to the carboxyl carbon, and kadd2 refers to the rate constant of the addition of the nucleophile to the carbonyl carbon).

Figure 12.13 shows that the iso-A enols of the /3-diketones A react with an a,/3-unsaturated carboxonium ion C that acts as a C electrophile. This oxocarbenium ion is formed by reversible protonation of the oc,/3-unsaturated methyl vinyl ketone in acetic acid. However, the oxocarbenium ion C in this figure does not react with the iso-A enols at its carbonyl carbon atom—as the protonated acetone in Figure 12.12 does with the enol of acetone—but at the center C-/3 of the conjugated C=C double bond. Accordingly, an addition reaction takes place whose regioselectivity resembles that of a 1,4-addition of an organometallic compound to an 0C,/3-unsaturated carbonyl compound (see Section 10.6). 1,4-additions of enols (like in this case) or enolates (as in Section 13.6) to a,/3-unsaturated carbonyl and carboxyl compounds are referred to as Michael additions. 

Telomerization is defined as an oligomerization of dienes accompanied by addition of a heteroatom or carbon nucleophilic reagent10. It is catalyzed by various organometallic compounds of transition metals, especially palladium compounds. The nucleophiles, such as water, alcohols, amines or carboxylic acids, as well as enamines, nitroalkanes and stabilized carban-ions, are mainly introduced in the terminal position of the dimeric molecule in excellent yield10. It is also possible to direct the reaction towards an internal product functionalization. Telo-merizations with heteronucleophiles are regarded as heterocarborative addition reactions and are described in Section 1.5.8.4. 

Molecules containing carboxylic acid functionalities are not confined to organic systems. For example, the C=C double bond in fumaric acid can interact with a low oxidation state metal centre (see Chapter 23) to form organometallic compounds such as Fe(C0)4(r -H02CCHCHC02H) the T -prefix (see Box 18.1) indicates that the two carbon atoms of the C=C bond of the fumaric acid residue are linked to the Fe centre. Hydrogen bonding can occur between adjacent pairs of molecules as is depicted below, and such interactions extend through the solid state lattice to produce an extensive, three-dimensional array. 

Many transition metal complexes contain anionic oxygen donor ligands, and many of these complexes display structures and reactivity that resemble that of more conventional organometallic compounds containing metal-carbon bonds. In some cases the alkoxo ligand is the site of reaction, and in other cases the alkoxide is an ancillary ligand. This section will focus on three main classes of compounds alkoxides (including aryloxides and the parent hydroxides), carboxylates, and p-diketonates. 

PO can be made degradable by means of additives. The types of additives include aromatic ketones (benzo-phenone and substituted benzophenones [47], qui-none), aromatic amines (trisphenylamine), polycyclic aromatic hydrocarbons (anthracene, certain dyes such as xanthene dyes), or transition metal organic compounds. The transition metal compounds of Fe, Co, Ni, Cr, Mn are widely used. Organo-soluble acetyl acetonates of many transition metals are photooxidants and transition metal carboxylates are also thermal pro-oxidants. Co acetylacetonate appears to be an effective catalyst for chemical degradation of PP in the marine environment. The preferred photoactivator system is ferric dibutyldithiocarbamate with a concentration range of 0.01. 1%. Scott has patented the use of organometallic compounds hke iron (ferric) dibutyldithiocarbamate or Ni-dibutyl-dithiocarbamate [48]. Cerium carboxylate [49] and carbon black are also used in such materials [50]. 

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