The role of molecular geometry in the Wolff rearrangement of α-diazocarbonyl compounds — Conformational control or structural constraints?

Relaxed scans of potential energy surfaces for the loss of nitrogen from four different diazocarbonyl compounds: 3-diazo-2-butanone (1), 2-diazocyclohexanone (2), methyl diazomalonate (3), and diazo Meldrum's acid (4), were conducted at the B3LYP/6-31+G(d,p) level. The geometries of species and transition states involved in the process were optimized at the B3LYP/6-311+G(3df,2p) level, while electronic energies were computed using the MP2(full)/aug-cc-pVTZ method. These calculations suggest that the rigidity of cyclic molecules, rather than the conformational structure of the starting diazocarbonyl compounds, defines the pathway of the dediazotization reaction. In acyclic diazocarbonyl compounds, loss of nitrogen results in the formation of a carbene, which is stabilized by the overlap of the system of carbonyl group and the unshared electron pair of a singlet carbene. On the contrary, in small- to medium-sized cyclic systems, carbonyl carbenes are unable to attain a stabilizing orthogonal conformation. Consequently, cyclic carbonyl carbenes are less stable, and the concerted Wolff rearrangement becomes the predominant process. Transition states for the concerted Wolff rearrangement and for the formation of carbonyl carbenes have a very similar geometry.Key words: diazocarbonyl compounds, Wolff rearrangement, conformation, carbene, ketene.