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Concept
Insertion reaction
Summary
An insertion reaction is a chemical reaction where one chemical entity (a molecule or molecular fragment) interposes itself into an existing bond of typically a second chemical entity e.g.:
The term only refers to the result of the reaction and does not suggest a mechanism. Insertion reactions are observed in organic, inorganic, and organometallic chemistry. In cases where a metal-ligand bond in a coordination complex is involved, these reactions are typically organometallic in nature and involve a bond between a transition metal and a carbon or hydrogen. It is usually reserved for the case where the coordination number and oxidation state of the metal remain unchanged. When these reactions are reversible, the removal of the small molecule from the metal-ligand bond is called extrusion or elimination.
There are two common insertion geometries— 1,1 and 1,2 (pictured above). Additionally, the inserting molecule can act either as a nucleophile or as an electrophile to the metal complex. These behaviors will be discussed in more detail for CO, nucleophilic behavior, and SO2, electrophilic behavior.
Homologation reactions like the Kowalski ester homologation provide simple examples of insertion process in organic synthesis. In the Arndt-Eistert reaction, a methylene unit is inserted into the carboxyl-carbon bond of carboxylic acid to form the next acid in the homologous series. Organic Syntheses provides the example of t-BOC protected (S)-phenylalanine (2-amino-3-phenylpropanoic acid) being reacted sequentially with triethylamine, ethyl chloroformate, and diazomethane to produce the α-diazoketone, which is then reacted with silver trifluoroacetate / triethylamine in aqueous solution to generate the t-BOC protected form of (S)-3-amino-4-phenylbutanoic acid.
Mechanistically, the α-diazoketone undergoes a Wolff rearrangement to form a ketene in a 1,2-rearrangement. Consequently, the methylene group α- to the carboxyl group in the product is the methylene group from the diazomethane reagent.
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