Catalysis group transfer

Full details on the phosphorylation of water and alcohols by 4-nitrophenyl dihydrogen phosphate and the NfC H ) - and N(CH3) -salts of its mono- and dianion have been published 146>. Phosphoryl group transfer from the monoanion and dianion is thought to proceed via the monomeric POf ion. Addition of the sterically unhindered amine quinuclidine to an acetonitrile solution containing the phosphate monoanion and tert-butanol produces t-butyl phosphate at a faster rate than does the addition of the more hindered diisopropylethylamine. This nucleophilic catalysis of the phosphorylation reaction is also explained by the intermediacy of the POf ion. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

Baldini, L., Bracchini, C., Cacciapaglia, R., Casnati, A., Mandolini, L. and Ungaro, R. (2000) Catalysis of acyl group transfer by a double-disiplacement mechanism the cleavage of aryl esters catalyzed by calixcrown-Ba complexes. Chem.-Eur. J., 6, 1322. 

Additional catalytic mechanisms employed by enzymes include general acid-base catalysis, covalent catalysis, and metal ion catalysis. Catalysis often involves transient covalent interactions between the substrate and the enzyme, or group transfers to and from the enzyme, so as to provide a new, lower-energy reaction path. 

Diazoamides of type 300 rapidly cyclize to form aziridines 302 (342) (Scheme 8.73). It is conceivable that this reaction proceeds through a 1,2,3-triazoline intermediate 301, which is the consequence of a LUMO(dipole)— HOMO(dipolarophile) controlled intramolecular [3 + 2] cycloaddition. Some remarkable steric effects were encountered for this cyclization. While the piperidine derivative [300, R R2 = (CH2)4] readily cyclized by diazo group transfer at 0 °C in 88% yield, the pyrrolidine analogue [300, R, R2 = (CH2)3] had to be heated for 1-2 days in polar solvents. The corresponding acyclic diazoamide (300, R1 = R2 = H) possessed a half-life of >10 days at ambient temperature. The intramolecular aziridination reaction, however, could be readily achieved under catalysis using Rh2(OAc)4. 

Aldol reactions of both (E)- and (Z)-ketene acetals are highly susceptible to KOBuc catalysis. In the presence of 5 mol% of KOBuc, aldol reactions proceeded to completion within minutes at -78 °C < 1994JA7026>. A double-label crossover experiment, devised to probe the nature of the silicon group transfer in the alkoxide-catalyzed aldol reaction, suggested that free metal enolates are the true reactive species adding to the aldehydes. 

Stabilization of enolate anions generated from abstraction of a proton a to a carboxylate Hydrolysis, phosphoryl group transfer via hydrolytic nucleophilic substitution Stabilization of diverse oxyanion intermediates via metal-assisted catalysis Schiff base dependent formation of an electron sink ... 

The side chains of amino acids present a number of nucleophilic groups for catalysis these include RCOO-, R—NH2, aromatic—OH, histidyl, R—OH, and RS. These groups attack electrophilic (electron-deficient) parts of substrates to form a covalent bond between the substrate and the enzyme, thus forming a reaction intermediate. This type of process is particularly evident in the group-transfer enzymes (EC Class 2 see Table 8.1). In the formation of a covalently bonded intermediate, attack by the enzyme nudeophile (Enz-X in Example 8.10) on the substrate can result in acylation, phosphorylation, or glycosylation of the nucleophile. 

Coordination polymerizations are becoming an inspiration source for further methodical development in addition polymerizations. The nearest aim could possibly be the insertion of polar monomers (as indicated by group transfer polymerization) and a deepening of our understanding of catalysis. Research in this field should lead to partial or even total replacement of catalysts by other means. I shall try to indicate one of the possibilities. 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

The catalytic mechanism of the pyrimidine-5 methylation in nucleic acids is more complex as it involves covalent catalysis. The mechanism is common for numerous DNA/RNA cytosine and uracil MTases as well as for thymidylate synthase (although the latter uses tetrahydrofolic acid as the methyl donor) and has been studied in detail in several systems (50). Here, the cytosine-5 methylation in DNA is presented as an example (see Fig. 4a). The C5-position of cytosine, which is part of an aromatic ring, does not carry sufficient nucleophilicity for a direct methyl group transfer. The continuity of the aromatic system is disrupted by a nucleophilic attack of thiolate (from a conserved... 

Gencic S, LeClerc GM, Gorlatova N, Peariso K, Penner-Hahn JE, Grahame DA. Zinc-thiolate intermediate in catalysis of methyl group transfer in Methanosarcina barker . Biochemistry 2001 40 13068-13078. 

Finally, group-transfer chain catalysis involves the transfer of a group in the cross-propagation step a few examples are known with transfers of groups such as allyl [59], cyclopentadienyl [60], and rhodium octaethylporphyrin [57]. 

A.R. Hopkins et al., Electrophilic Catalysis of Sulphate (-SOj ) Group Transfer Hydrolysis of Salicyl Sulphate Assisted by Intramolecular Hydrogen Bonding, J. Chem. Soc., Perkin Trans. 2, 1983, 1279. 

A similar conjugate addition- silyl group transfer process was reported later by Danishefsky and co-workers for the synthesis of PGF2a (Scheme 52) (103). In this case, the silyl ketene acetal adds, under Hgl2 promotion, cis to the OTBS group in the optically pure enone 52.1 to provide silyl enol ether 52.2 as the exclusive product. The indicated aldol products are obtained from 52.2 in subsequent reactions with ( )- and (Z)-octenal using TiCl4 catalysis. 

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