<P> Understanding the stereochemistry of α - diazo ketones is essential in elucidating the mechanism of the Wolff rearrangement . α - diazocarbonyl compounds are generally locally planar, with large rotational barriers (55--65 kJ / mol) due to C-C olefin character between the carbonyl and α - carbon, illustrated in the rightmost resonance structure . Such a large barrier slows molecular rotations sufficiently to lead to an equilibrium between two conformers, an s - trans and s - cis - conformer . s - cis - Conformers are electronically favored due to Coulombic attraction between the oxygen with a partial negative charge and the cationic nitrogen, as seen in the rightmost resonance structure . If R is large and R is hydrogen, s - cis is sterically favored . If R and R are large, s - trans is sterically favored; if both substitents are sufficiently large, the steric repulsion can outweigh the Coulombic attraction, leading to a preference for s - trans . Small and medium cyclic substrates are constrained in the s - cis conformation . </P> <P> When the α - diazo ketone is in the s - cis conformation, the leaving group (N) and the migrating group (R) are antiperiplanar, which favors a concerted mechanism, in which nitrogen extrusion occurs concurrently with 1, 2 - alkyl shift . There is evidence this mechanism occurs in both thermolytic and photolytic methods, when the s - cis - conformer is strongly favored . </P> <P> CIDNP studies show that photochemical rearrangement of diazoacetone, which largely exists in the s - cis - conformer, is concerted . Product ratios from direct and triplet - sensitized photolysis have been used as evidence for proposals that claim that concerted products arise from the s - cis - conformer and stepwise products occur through the s - trans - conformer . </P> <P> s - trans - α - Diazo ketones do not have an antiperiplanar relationship between the leaving and migrating group, and thus are thought to generally rearrange stepwise . The stepwise mechanism begins with nitrogen extrusion, forming an α - ketocarbene . The α - ketocarbene can either undergo a 1, 2 - alkyl shift, to give the ketene product, or can undergo a 4π electrocyclic ring closure, to form an antiaromatic oxirene . This oxirene can reopen in two ways, to either α - ketocarbene, which can then form the ketene product . </P>

Diazomethane decomposes in presence of light to give