Multidentate nucleophiles

Application of multidentate nucleophiles in this procedure provides access to a wide range of heterocyclic structures and is illustrated with recently published examples in Scheme 29 [90-92]. The 1990s literature on this subject has been reviewed [6,8,93]. 

Reactions of alkynyliodonium salts with multidentate nucleophiles can be employed for the synthesis of heterocyclic compounds. Recent examples include preparations of thiazoles, selenazoles, and 2-mercaptothiazoles by the treatment of alkynyliodonium mesylates or tosylates with thioamides, selenoamides, and ammonium dithiocarbamate (Scheme 62) [169-171]. A novel hetero-Claisen rearrangement of tricovalent iodine(III) intermediates was proposed to account for the 2,4-disubstitution pattern of the thiazoles [169]. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

The antimony analogue of diars, o-phenylenebis(dimethylstibine), has been prepared, as has its phenyl analogue (equation 43).162 Attempts to prepare multidentate stibines have been unsuccessful, probably due to the weaknes of the carbon-antimony bond. For example, in the reaction of BunLi with o-C6H4(SbPh2)Br the nucleophile attacks the C—Sb as well as the C—Br bond.142... 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

In line with these results, multidentate podands such as 7 and 8 can operate as catalysts under solid-liquid phase-transfer conditions. A series of solid-liquid PTC nucleophilic displacements was performed in the presence of simple polyglymes 6 with a sufficiently long chain (n = 7-9)... 

Two issues seem to be ubiquitous. First, the redox chemistry needs to be finely balanced oxidation of RPt(II) has to be fast to compete with protonolytic cleavage and selective, so that the Pt(ll) species that activates the C-H bond is not itself oxidized. Second, the RPt(IV) species has to be able to undergo facile dissociation, so that a five-coordinate intermediate needed to facilitate nucleophilic C-X bond formation is readily accessible for complexes of multidentate ligands, this criterion will probably require the ability to isomerize easily, as in Scheme 29 above. And, of course, all of this must be achieved within the context of maintaining a Pt(ll) center capable of activating the C-H bond. 

Carbonyls. The title compound is widely used as an oxophilic Lewis acid, efficiently increasing the reactivity of carbonyl groups toward the additions of many nucleophiles. a-Dicarbonyls are particularly effective substrates for this kind of activation. A catalytic amount of In(OTf)3 combined with a multidentate chiral ligand allows nucleophiles, such as allylstannanes (eq 23), electron-rich... 

Schiff base enzymes, reflects essentially the sensitivity of carbonyl functions toward acid or base catalysis. Whereas the protein side chains provide adequate nucleophiles and bases for catalytic activity, the superiority of metal ions as acidic catalysts in comparison with protons is amply demonstrated by metallo-enzymes. A further reason for the occurrence of numerous metalloenzymes might be sought in the multidentate nature of metal complexes. The precise stereochemical positioning of several reaction components in the same complex provides an easy optimization of proximity and orientation effects. 

Elimination of the multidentate interaction of a counterion with the siloxane chain is crucial. Otherwise, as mentioned before, the equilibration reactions would make the precision polymerization impossible. Specific initiator-solvent systems used for this purpose may be divided into three groups (1) basic solvent and a hard counterion, which interacts with solvent stronger than with siloxane, for example, lithium/THF (2) bulky and soft counterions, for example, Me4N BtuP, and phosphazenium cations, which weakly interact with nucleophiles (3) basic promoters strongly interacting with counterions, such as HMPT, DMSO, DMF, cryptands, and crown ethers. ... 

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