two aspects of the mechanism can be agreed upon. First, α-diazocarbonyl compounds are in an equilibrium of s-cis and s-trans-conformers, the distribution of which may influence the mechanism of the reaction. Generally, under photolysis, compounds in the s-cis conformation react in a concerted manner due to the antiperiplanar relationship between the leaving and migrating groups, whereas compounds in the s-trans conformation react stepwise through a carbene intermediate or do not rearrange. Second, regardless of the reaction mechanism, the rearrangement gives a ketene intermediate, which can be trapped by a weakly acidic nucleophile, such as an alcohol or amine, to give the corresponding ester or amide, or an olefin, to give a [2+2] cycloaddition adduct. Strong acids do not rearrange, but rather protonate the α-carbon and give SN2 products. Stereochemistry of α-diazo ketones 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 R1 is large and R2 is hydrogen, s-cis is sterically favored. If R1 and R2 are large, s-trans is sterically favored; if both substituents 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. Concerted mechanism When the α-diazo ketone is in the s-cis conformation, the leaving group (N2) and the migrating group (R1) 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. 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. Stepwise mechanism 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.