undergo a concerted mechanism, whereas those in the s-trans conformation undergo a stepwise mechanism. α-diazo ketones with better migratory groups prefer a concerted mechanism. However, for all substrates except cyclic α-diazo ketones that exist solely in the s-cis conformation, products come from a combination of both pathways. Transition metal mediated reactions are quite varied; however, they generally prefer forming the metal carbene intermediate. The complete mechanism under photolysis can be approximated in the following figure: Migratory trends The mechanism of the Wolff rearrangement is dependent on the aptitude of the migratory group. Migratory abilities have been determined by competition studies. In general, hydrogen migrates the fastest, and alkyl and aryl groups migrate at approximately the same rate, with alkyl migrations favored under photolysis, and aryl migrations preferred under thermolysis. Substituent effects on aryl groups are negligible, with the exception of NO2, which is a poor migrator. In competition studies, electron deficient alkyl, aryl, and carbonyl groups cannot compete with other migrating groups, but are still competent. Heteroatoms, in general, are poor migratory groups, because their ability to donate electron density from their p orbitals into the π* C=O bond decreases migratory ability. The trend is as follows: Photochemical reactions: H > alkyl ≥ aryl >> SR > OR ≥ NR2 Thermal reactions H > aryl ≥ alkyl (heteroatoms do not migrate) Preparation of α-diazocarbonyl compounds While known since 1902, the Wolff rearrangement did not become synthetically useful until the early 1930s, when efficient methods became available to synthesize α-diazocarbonyl compounds. The primary ways to prepare these substrates today are via the Arndt-Eistert procedure, the Franzen modification to the Dakin-West reaction, and diazo-transfer methods. Arndt-Eistert procedure The Arndt–Eistert reaction involves the acylation of diazomethane with an acid chloride, to yield a primary α-diazo ketone. The carbon terminus of diazomethane adds to the carbonyl, to create a tetrahedral intermediate, which eliminates chloride. The chloride then deprotonates the intermediate to give the α-diazo ketone product. These α-diazo ketones are unstable under acidic conditions, as the α-carbon can be protonated by HCl and SN2 displacement of nitrogen can occur by chloride. Franzen modification to Dakin-West reaction The Dakin–West reaction is a reaction of an amino acid with an acid anhydride in the presence of a base to form keto-amides. The Franzen modification to the Dakin–West reaction is a more effective way to make secondary α-diazo ketones. The Franzen modification nitrosates the keto-amide with N2O3