Coordinating functional groups

The stereochemical outcome of all these reactions was rationalized in terms of a chelate complex formed between the reactants, from which the R -group is then transferred to the alkene as drawn in (45). Nevertheless, in the case of substrates containing additional stereogenic centers and other coordinating functional groups, a reliable prediction of the product configuration is not always possible.30... 

In recent years the understanding of colloid surfaces and soils constituents has increased tremendously. Surface coordinating functional groups on particulate inorganic and humic materials are viewed as complexant ligands (Stumm, 1992). 

Activation of vinyl C-H bonds with RuH2(CO)(PPh3)3 catalyst has allowed the formal insertion of a,/l-unsaturated ketones or esters into the C-H bond of vinylsilanes and led to a regioselective C-C coupling at the -position [9] (Eq. 6). Activation of the sp2 C-H bond occurred with the aid of chelation of a coordinating functional group and provided vinylruthenium hydride 14. Insertion of olefin afforded the tetrasubstituted alkene 13. The ruthenium activation of a variety of inert C-H bonds has now been performed by Murai [10]. 

This is an important observation, since functionalised carbenes usually possess a sidechain with a coordinating functional group. Thus, functionahsation normally introduces added stability to the catalyst. 

Despite the above description, some substrates bearing a coordinating functional group near to the allylic hydroxy group show unusual behavior (substandard enantioselectivity and/or slow reaction). For example, the allyhc alco-... 

A chelation-assisted strategy is another useful method for C—C bond cleavage. In this method, the substrates containing a coordinating functional group will first coordinate to a metal and form a stable metallacycle. A representative example of this strategy is the activation of the a-C—C bond to the carbonyl group in 8-quinolinyl alkyl... 

Although unclear, these effects impose certain requirements on the substrate and the chiral ligands to be used. Thus, the well-known positive effects of having additional basic and coordinating functional groups present in the substrate are believed to stem from bi- or polydentate coordination of the substrate to the metal center. This renders the intermediate complexes more rigid and stereochemically uniform, and consequently the number of possible complex geometries is reduced. 

Substitution of the coordinating functional groups is synthetically facile so that the reactivity of the complex can be optimized. 

Many reports of aerobic alcohol oxidation involve homogeneous Pd- [7] and Ru-based [8] catalysts that are effective with benzylic, allylic, and ahphatic primary and secondary alcohols. These catalysts are often inhibited by coordinating functional groups such as heterocycles, amines, and oxygen- or sulfur-containing moieties and are capable of oxidizing alkenes. Efforts to develop scalable applications of Pd-based catalysts raised concerns about large-scale prospects for these... 

The attachment of metal complexes to the polymer most often occurs via coordinating functional groups (ligands) boimd to the polymer in a covalent fashion as outlined in the earlier chapters of this book, but various types of noncovalent attachment are also well documented. The latter can be achieved, for instance, by means of electrostatic interactions, physisorption by amphiphilic polymer micelles (either as common association micelles or as unimolecular micelles), by hydrogen bonding, or by specific interactions of proteins with a molecule (Figure 14). 

P(0Pr )3] favor elimination of jS-H, larger ligands [P(0-o-tol)3] favor elimination in p position. The turnover numbers are low. The site of nucleophilic attack on palladium 7r-allyl intermediates can be directed by coordinating functional groups of the substrate. While normally anti stereochemistry would be expected, a syn pathway of attack has now been observed for the first time in phosphinated substrates (Scheme 15). The reaction of terminal alkynes with CO2 and secondary... 

Indeed, upon reaction of 32 with catalyst 13 at ambient temperature, the desired dimer 33 was formed hy cross-metathesis in 58 % yield. Although the material return is modest, this result is still remarkable because there are so many seemingly deleterious coordinating functional groups near the terminal olefin, including two free hydroxy groups and a ketone. 

Fig. 49. Terminal, tri- and tetrasubstituted olefins and olefins with poorly coordinating functional groups.

Scheme 12.11 Trans effect by tethered coordinating functional groups [24].

Interesting attributes of this new reaction are (i) the use of only Cu, and not Pd, to catalyze the desulfltative carbon-carbon bond formation, (ii) the strict requirement for a Cu-coordinating functional group on the sulfur-pendant, and (iii) the very mild (room temperature, neutral pH) reaction conditions. S -acylthiosalicylamides proved to be particularly effective substrates for this chemistry. A study of peptidic S -acylthiosalicylamides was used to demonstrate broad functional group compatibility and high stereoretention (Scheme 23.6) [46]. 

---