Functional groups, directed metalation

To illustrate to the reader the principles introduced in this section in the most direct and practical way, we present some case studies that compare the bonding properties of different ligands to the same metal center, and that compare the bonding properties of different metal functional groups to the same ligand. It is hoped that these experimental results will give the reader an appreciation of the types of detailed electronic structure information available from photoelectron spectroscopy. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

There are many transition metal ion oxidants used in organic chemistry for the interconversion of functional groups. Those which have been used for the preparation of sulphones from sulphoxides will be discussed below. It is very interesting to note that this type of oxidant often reacts more rapidly with sulphoxides than with sulphides and so sulphoxides may be selectively oxidized with transition metal ion oxidants in the presence of sulphides. This is in direct contrast to the oxidation of sulphides and sulphoxides with peracids and periodate, for example, where the rate of reaction of the sulphide is more than 100 times that for the corresponding sulphoxide. 

Many pharmacologically active compounds have been synthesized using 5-bromoisoquinoline or 5-bromo-8-nitroisoquinoline as building blocks.6 7 8 9 10 11 The haloaromatics participate in transition-metal couplings 81012 and Grignard reactions. The readily reduced nitro group of 5-bromo-8-nitroisoquinoline provides access to an aromatic amine, one of the most versatile functional groups. In addition to N-alkylation, TV-acylation and diazotiation, the amine may be utilized to direct electrophiles into the orthoposition. 

Many enzymes contain small nonprotein molecules and metal ions that participate directly in substrate binding or catalysis. Termed prosthetic groups, cofactors, and coenzymes, these extend the repertoire of catalytic capabilities beyond those afforded by the limited number of functional groups present on the aminoacyl side chains of peptides. 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

This is a very rare metal in cross-coupling reactions. Direct comparison of similar methylating reagents derived from Al, Ga, and In showed that the Ga derivative is the least reactive.165 Vinylgallium dichlorides underwent cross-coupling with aryl iodides in the presence of Pd catalysts with P(o-tol)3 the reaction is moderately tolerant to acidic functional groups.166... 

Examples of the operation of both types of effect have been documented. Nevertheless, while these effects are useful concepts, as mentioned previously, very often the role of the metal ion in a given in situ reaction may be quite complex and, for instance, involve aspects of both effects. As well, the metal may play less obvious roles in such processes. For example, it may mask or activate individual functional groups or influence the reaction in other ways not directly related to the more readily defined steric influences inherent in both template effects. 

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