Dearomatization

A dearomatization is an organic chemical reaction in which an aromatic system is reduced. Metal-organic catalysts are used to overcome the high resonance energy of the transition state . The reverse reaction is aromatization .

Examples

Dearomatization of benzene to cyclohexane.

Cyclohexane can be obtained by adding hydrogen to benzene . This is catalyzed by Raney nickel .

Dearomatization is also an integral part of daily life: Nicotinamide adenine dinucleotide (abbreviation: NAD ⁺ ) is used in biochemical processes to transfer hydride ions in the human body. An oxidation / reduction of the aromatic pyridine system takes place as aromatization / dearomatization.

An example of a method frequently used in organic synthesis is the Birch reduction , in which an aromatic is reduced regioselectively in the 1,4-position by reacting with sodium , potassium or lithium in liquid ammonia .

In a new process in the research group of Dean Harman, a stereoselective tandem reaction of reduction and umpolung of phenol is now possible. Phenol is usually found in solution as an enol , but can be stabilized in its keto form by a tungsten complex . This can then be converted stereoselectively with electrophiles and nucleophiles to form cyclohexenone derivatives.

The keto form phenol can be stabilized by complex formation with a tungsten complex [W]. This can be used to stereoselectively implement this complex with an electrophile E ⁺ and a nucleophile Nu ^- . [W] = TpW (NO) (PCH ₃ )

Tungsten complex TpW (NO) (PCH ₃ ) (5,6-η ² -phenol), Tp: (Hydriotris (pyrazolyl)) borate with.

Individual evidence

1. ^ Graham, PM. et al. (2003): Dearomatization of Benzene, Deamidization of N, N-Dimethylformamide, and a Versatile New Tungsten π Base . In: Organometallics 22 (22); ; 4364-4366; doi : 10.1021 / om030560h
2. ↑ Todd, MA. et al. (2008): Stereoselective Umpolung Tandem Addition of Heteroatoms to Phenol . In: J. Am. Chem. Soc. 130 (22); 6906-6907; doi : 10.1021 / ja801076z