Stetter Reaction - Variations

Variations

Several variations of the Stetter reaction have been developed since its discovery in 1973. In 2001, Murry et al reported a Stetter reaction of aromatic aldehydes onto acylimine derivatives to give α-amido ketone products. The acylimine acceptors were generated in situ from α-tosylamide substrates, which underwent elimination in the presence of base. Good to excellent yields (75-90%) were observed. Mechanistic investigations showed that the corresponding benzoins were not adequate substrates, contrary to traditional Stetter reactions. From this, the authors conclude the Stetter reaction of acylimines is under kinetic control, rather than thermodynamic control.

Another variation of the Stetter reaction involves the use of 1,2-dicarbonyls as precursors to the acyl anion intermediate. In 2005, Scheidt and coworkers reported the use of sodium pyruvate, which loses CO₂ to form the Breslow intermediate. Similarly, in 2011 Bortolini and coworkers demonstrated the use of α-diketones to generate an acyl anion. Under the conditions they developed, 2,3-butadienone is cleaved after addition to the thiazolium catalyst to release ethyl acetate and generate the Breslow intermediate necessary for the Stetter reaction to proceed.

In addition, they showed the atom economy and utility of using a cyclic α-diketone to generate the Stetter product with a tethered ethyl ester. The reaction precedes through the same mechanism as the acyclic version, but the ester generated by attack of ethanol remains tethered to the product. However, the conditions only allow for the generation of ethyl esters, due to the necessity of ethanol as solvent. Substitution of ethanol with tert-butanol resulted in no product. The authors speculate this is due to the difference in acidity between the two alcoholic solvents.

In 2004, Scheidt and coworkers introduced acyl silanes as competent substrates in the Stetter reaction, a variation they termed the "sila-Stetter reaction." Under their reaction conditions, the thiazolium catalyst induces a Brook rearrangement, which is followed by desilylation by an isopropanol additive to give the common Breslow intermediate of the traditional Stetter reaction. The desilylation step was found to be necessary, and the reaction does not proceed without an alcoholic additive. Acyl silanes are less electrophilic than the corresponding aldehydes, preventing typical benzoin-type byproducts often observed in the Stetter reaction.