Fischer carbene

This tetraalkylammonium salt could be acylated to give a highly electrophilic mixed anhydride-like Fischer carbene which could undergo nucleophilic substitution with alcohol.

Treating the intermediate with excess amount of trimethylsilyl chloride yields this particular group of Fischer carbene complex.

[6] In 2021, Alvarez et al reported that a Fischer carbene could be effectively prepared from a decarbonylative process of a metal carbenoid derived from a stabilized diazo compound.

A methoxy chromium carbene with a methyl side chain has a pKa of 12.5 in aqueous acetonitrile (1:1 volume ratio).

For instance, the Diels–Alder reaction between methyl acrylate and isoprene is completed in 7 days at room temperature with low para-meta selectivity.

On the other hand, the Fischer carbene counterpart finished in 3 hours at room temperature with much higher para-meta selectivity.

If the side chain of Fischer carbene bears an α-proton, it could be reversibly deprotonated with a weak base such as pyridine.

This facilitates formation of chromium hydride species, which can undergo reductive elimination to give a cis-enol ether.

With ketene reactivity, the species could be trapped by several nucleophiles such as alcohols and amines, or could react in [2+2] cycloaddition with alkenes, imines, or aldehyde yielding the corresponding cyclobutane, β-lactam, and β-lactone adducts.