Catalytically competent intermediates

Zirconocene metallacycles are invoked as key intermediates in the catalytic cyclomagnesiation of dienes. To probe for the intermediacy of metallacycles, zirconacyclopentane derivative 658 was synthesized from the reaction of Cp2ZrCl2 and 9,9-diallylfluorene in the presence of 2equiv. of BunLi482 (Scheme 154). This structurally characterized zirconacyclopentane is a catalytically competent intermediate for the cyclization of dienes. For example, cyclomagnesiation of 1,7-octadiene occurs smoothly in the presence of 10mol% of 658. 

The enzyme 2-C-methyl-D-erythritol-4-phosphate synthetase appears to catalyse a Bilik reaction (Figure 6.10) the substrate l-deoxyxylulose-5-phosphate is converted to the title compound via an intermediate aldehyde, whose carbonyl derives from C3 of the substrate. The first step is thus a Bilik reaction and the aldehyde is subsequently reduced by the enzyme using NADPH as reductant, The X-ray crystal structure of the Escherichia coli enzyme in complex with the promising antimalarial Fosmidomycin (a hydroxamic acid) reveals a bound Mn " coordinated to oxygens equivalent to the substrate carbonyl and 03. The stereochemistry and regiochemistry follow the normal Bilik course, although the crystallographers favour an alkyl shift rather than a reverse aldol-aldol mechanism. The intermediate aldehyde has been shown to be a catalytically competent intermediate. 

The second finding was that the pentacyclic intermediates formed by the trimeric E. coli enzyme and the pentameric M. jannaschii riboflavin synthase are diastereomers, as shown in Figure 19. These pentacyclic diastereomers are formed by the condensation of two molecules of 6,7-dimethyl-8-ribityllumazine with the same regiochemistry in both enzymes. " This difference clearly results from the fact that these are different enzymes with different active sites. Each of the diastereomers is a catalytically competent intermediate for its respective enzyme but does not serve as a substrate for the other enzyme. ... 

Scheme 4.8 Synthesis and characterization of catalytically competent intermediates in bis(imino)pyridine iron-catalyzed hydrogenation and hydrosilylation reactions.

Suarez J, Ranguelova K, Jarzecki AA et al (2009) An oxyferrous heme/protein-based radical intermediate is catalytically competent in the catalase reaction of Mycobacterium tuberculosis catalase-peroxidase (KatG). J Biol Chem 284 7017-7029... 

The electronic spectra of the cobalt-substituted derivative are characteristically different when different anions are bound to the metal (Figure 2.37). A catalytically competent ternary complex intermediate displays the electronic absorption pattern typical of anion adducts. ... 

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