Intramolecular cooperative catalysis

Fig. 13.4 General concept of rare earth bimetallic asymmetric catalysis (a) cooperative intramolecular catalysis, and (b) cooperative intermolecular catalysis.

In order to assess whether intramolecular cooperativity occurs, catalysis was performed with very low (dendritic) catalyst loading (0.027 mol% vs. 0.5 mol% for the monomeric catalyst). The dendritic Co complex effected complete kinetic resolution (98% ee, 50% conversion), while the unsubstituted analogue showed no measurable conversion. 

Another example of this cooperative catalysis has been presented by Konsler et al.101 in the course of their asymmetric ring-opening (ARO) study. They found that the ARO of mew-epoxides with TMS-N3, catalyzed by Cr salen compound 132, showed a second-order kinetic dependence on the catalyst.102 They then proposed that there might be cooperative, intramolecular bimetallic catalysis taking place, with one metal activating the substrate mew-epoxide and... 

Another important point in enzyme catalysis is that an enzyme is a multiple catalyst in vdiidi many catalytic groiqts cooperate intramolecularly (i). In other words, an enzyme is an intramolecular multiple catalyst or a multifunctional catalyst. On this aspect of ens me catalysis some investigations have been carried out through the cooperative functions of pendant catalytic groups of cyclic peptides. These enz mie... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

Acyl Transfer Blfunctlcfnal Catalysis- Many mechanisms for enzjrme cataly-sls postulate cooperative ("push-pull") catalysis by an acid-base pair in the active site. The analogous bifunctional intramolecular catalysis has been widely sought the evidence presented generally consists of a maximum rate at a pH sufficiently acidic for the acid-catalytic function to be pro-tonated yet sufficiently basic for the base-catalytic function to be free. The hydrolysis of hexachlorophene monosuccinate (14). for example, exhibits such a rate maximum at pH 6.8 (between pK 5.20 for Che carboxyl group and... 

Fig. 13.14 Postulated Lewis acid-Lewis acid intramolecular cooperative catalysis.

Intramolecular arylcyanation of simple alkenes proceeds smoothly by cooperative nickel/Lewis acid catalysis (Schemes 37, 38, and 39) [102-104], whereas palladium catalysts have been shown to be useful for intramolecular... 

Inspired by the success of intramolecular addition and tautomerization of aldehydes with a pendant alkyne through cooperative catalysis of a secondary amine and an An complex, in 2008, Yang et al. reported a cascade reaction with the combination of a copper complex and an achiral secondary amine catalyst for the synthesis of attractive carbocycles [48]. This chemistry merged a pyrrolidine-promoted Michael addition via iminium ion intermediates and a Cu-catalyzed cycloisomerization protocol (Scheme 9.54). Various ketones and alkyne-tethered active methylene compounds could be converted into densely functionalized cyclopentene derivatives. Although the asymmetric version was not given, the chemistry described here was amenable for the implementation of asymmetric synthesis of such functionalized molecules by a combination of chiral amines and suitable Au complexes. 

---