Carbonium Oxocarbonium

The reaction between carbonium ions and carbon monoxide affording oxocarbonium ions (acyl cations) is a key step in the well-known Koch reaction for making carboxylic acids from alkenes, carbon monoxide and water ... 

In Sections II and III it was shown that secondary and tertiary alkyl cations can be formed by decarbonylation of the corresponding oxo-carbonium ions. This has been found impossible in the case of primary alkyl cations (Hogeveen and Roobeek, 1970) the oxocarbonium ions 13 and 14 were unchanged after one hour at 100°C k < 1-3 x 10 sec ), whereas ion 15 is fragmented by a j3-fission under these circumstances ... 

Y is a strong interacting pi electron acceptor. In addition to carbonium and oxocarbonium ion centers, examples of Y groups from Table XIII include Nj and several electrophilic substitution transition states (cf. sets 10,21 and 23) of the type... 

The alternative role for direct involvement of aspartic acid-52 is electrostatic stabilization of an oxocarbonium ion, a reasonable expectation in view of the steric situation in the active site, the poor internal stabilization of a glycosyl carbonium ion, and the apparent need for some kind of stabilization if general acid catalysis by glutamic acid-35 is to occur. Nevertheless, this assignment is questionable since electrostatic stabilization has not been demonstrable with o-methoxymethoxyisophthalic acid (Dunn and Bruice, 1970) and benzaldehyde disalicyl acetals (Anderson and Fife, 1973). In the former example, such stabilization should be favourable because the carbonium ion intermediate is unstable. 

In the reverse direction, protonation of the phosphate of glucose-1-phosphate destabilizes the glycosidic bond and promotes formation of a glucosyl oxocarbonium ion-phosphate anion pair. In the subsequent step, the phosphate anion becomes essential for promotion of the nucleophilie attack of a terminal glucosyl residue on the carbonium ion. This sequence of reactions brings about a-1,4-glycosidic bond formation and primer elongation. 

Stabilisation of the positively charged oxocarbonium ion could result keeping the C2 carbonium planar. This is achieved by multiple interactions between the functional groups of the intermediate and the active site residues. Only a such strong binding is possible when the C2 atom is in a planar conformation. 

There is no direct evidence on the form of the glycosyl-enzyme intermediate. Nonetheless, stabilization of a noncovalent oxocarbonium for the period between product fructose release and acceptor binding may not be realistic for slow reactions, given the extremely short lifetime of a glycosyl oxocarbonium ion (9). Levansucrase and particularly GTase-S are quite slow enzymes, with sucrose hydrolysis Heat of 48 sec 122) and 9.1 see" 213), respectively. Thus, the carbonium ion may well collapse to a more stable covalent complex or develop an equilibrium between the two forms. Nucleophilic catalysis is consistent with... 

The main effect of the oxygen atom next to a reaction centre in chemical terms is to stabilise the buildup of positive charge on the central atoms the lone pairs of electrons on the oxygen atom release electrons into the vacant p orbital of an adjacent carbonium ion centre. This means that, compared with nucleophilic substitutions at purely carbon centres, the C—X bond can break to a much greater extent before there is much C—Y bond formation. Indeed, in the limit, for most processes in water the C—X bond completely breaks first and the oxocarbonium ion is a discrete intermediate. 

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