Metal Ions as Electron Sinks

The role of metal ions as electrophilic catalysts is often underestimated. The formation of good ion-pair stabilization in the transition state in an anionic nucleophilic attack lowers the energy of the transition state. Even a powerful hydride nucleophile such as LiAlH4 will not add to a carbonyl if the lithium ion is kept from complexing with the carbonyl. 

A good example of a metal ion as an electrophile is the oxymercuration reaction. The nucleophile is often solvent, usually water or an alcohol. The charge is distributed, so little rearrangement occurs. The mercury is removed in a subsequent reduction step. 

Chromium trioxide and chromic acid, H2C1O4, are commonly used to oxidize alcohols. The alcohol adds to the chromium species to give a chromate ester, which undergoes elimination by an E2 process. 

If water and a littie acid are present, the initially formed aldehyde adds water to form a hydrate, which is further oxidized to the carboxyUc acid. 

Step 1 is the oxidative addition of hydrogen the metal loses two electrons to form a bond to each of the two hydrogen atoms. Step 2 is the complexation of the alkene to the metal, activating the pi bond. Complexation usually occurs from the least hindered face of the pi bond. Step 3 is the insertion of the metal hydride bond into the pi bond, forming a metal-carbon bond. Step 4 is the reductive elimination (the reverse of oxidative addition), which returns the metal to its original oxidation state and releases the alkane. The reductive elimination is usually very fast. Both hydrogens are added to the same face of the pi bond, a syn addition. 

---

Transition metal ions such as Mn04" and Cr04 can serve as electron acceptors in oxidation reactions, for they can have many available oxidation states. Metal ions as electron sinks will be covered in more detail in Section 8.10... 

---