A coordinated organic carbonyl is also electrophilic.
Figure MI4.1. Activation of an organic carbonyl by a metal ion.
Figure MI4.2. Coordination of an organic carbonyl to a metal ion makes the carbonyl more reactive towards nucleophiles.
A hydride attached to the metal can donate to the carbonyl.
Figure MI4.3. The intermolecular reaction of a coordinated organic carbonyl with a coordinated nucleophile.
Unlike migratory insertion, the nucleophile does not move to the atom attached to the metal.
The nucleophile moves to the second atom away from the metal: the position.
- In this case, the insertion can be reversible.
- The reverse of an insertion is called a 1,2-elimination or a beta-elimination.
Figure MI4.4. The reverse of a 1,2-addition is a 1,2-elimination.
The Greek lettering refers to the number of atoms away from the metal. The first atom attached to the metal is called the alpha position. A hydrogen on that atom is called an alpha hydrogen. The next atom along the chain is called the beta position. The third atom along the chain is the gamma position. A hydrogen attached to the beta position can undergo 1,2-elimination or beta-elimination.
- elimination leads to formation of a double bond.
- The double bond forms between the alpha and the beta position.
This nomenclature can be confusing because a carbonyl compound already has an alpha position and a beta position. The position is the carbonyl carbon and the position is the carbon next to the carbonyl. This Greek lettering system is a general way of designating positions and it is used in a number of different contexts; you need to be able to decide which context fits. If elimination is occurring, then the term, position, may mean one thing. If enolate formation is occurring, then the term, position, means something else.
In the following structures, identify any alpha, beta or gamma positions on the groups attached to the metal. In each case, show the potential products of 1,2-elimination of hydrogen.
Hydride transfer reduction is a method of converting aldehydes or ketones to alcohols. This catalytic reaction involves the use of a sacrificial alcohol as the solvent. As the aldehyde or ketone substrate is converted into alcohol, the alcohol solvent is converted into ketone. The mechanism is quite complicated, but at a simple level it could be imagined as taking place through a series of binding steps, eliminations and insertions.
Provide mechanisms based on the following descriptions for the reduction of benzaldehyde to benzyl alcohol.
- The reaction involves binding of isopropanol or 2-propanol to a metal ion, such as Ru(II) or Ru2+. A deprotonation ensues, followed by elimination, and 2-propanone dissociates from the metal ion.
- The ruthenium hydride formed in step (a) binds to the substrate, benzaldehyde. An insertion occurs, forming a metal alkoxide.
- The metal alkoxide is protonated, and the resulting alcohol dissociates from the metal.