Metal carbenes are involved in cycloadditions, metatheses and C–H functionalizations. However, their main precursors are hazardous diazo compounds, which makes them unfit for industrial use. Recently, however, synthetic strategies overcame this limitation — enabling access to metal carbenes from aldehydes, carboxylic acids and alcohols. Carbenes are an exception to the octet rule: they are neutral species containing a divalent carbon atom with six valence electrons, making them highly reactive. The synthesis of carbenes has traditionally required diazoalkanes as precursors. These compounds contain a diazo functional group that releases nitrog...