Oxocarbenium

An oxocarbenium ion is a canonical form of a carbocation, real or hypothetical, which places a formal positive charge on an oxygen atom. They are common reactive intermediates in the hydrolysis of glycosidic bonds, and are a commonly used strategy for chemical glycosylation. These ions have since been proposed as reactive intermediates in a wide range of chemical transformations, and have been utilized in the total synthesis of several natural products. In addition, they commonly appear in mechanisms of enzyme-catalyzed biosynthesis and hydrolysis of carbohydrates in nature.

The general structure of an oxocarbenium ion contains an oxygen–carbon double bond, with the oxygen atom attached to an additional group. The ion has a formal positive charge, which is placed on the oxygen atom. The oxocarbenium cation is stabilized by resonance, which places the positive charge on the carbon. Compared to a ketone, the oxocarbenium ion is more polar, and more reactive with nucleophiles. A common example of this is the activation of a ketone by the addition of a Lewis Acid to the oxygen. An average dipole moment for a general ketone R₂CO is δ = 0.51. With the addition of an acidic hydrogen to the oxygen atom to produce [R₂COH]⁺, the dipole moment increases to δ = 0.61, making the oxocarbenium ion more susceptible to nucleophilic addition than the ketone.

Formation of oxocarbenium ions can proceed through several different pathways. Most commonly, the oxygen of a ketone will bind to a Lewis Acid, which activates the ketone, making it a more effective electrophile. The Lewis acid can be a wide range of molecules, from a simple hydrogen atom to metal complexes. The remainder of this article will focus on alkyl oxocarbenium ions, however, where the atom added to the oxygen is a carbon. One way that this sort of ion will form is the elimination of a leaving group. In carbohydrate chemistry, this leaving group is often an ether or ester. An alternative to elimination is direct deprotonation of the molecule to form the ion, however, this can be difficult and require strong bases to achieve.

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