Ultrafast time‐resolved studies of the photochemistry of diazo carbonyl compounds

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The rich photochemistry of diazo carbonyl compounds in solution has been extensively studied by ultrafast transient absorption spectroscopy with UV–Vis and mid‐IR detection. Both, stepwise (carbene mediated) and concerted Wolff rearrangement (WR) mechanisms have been confirmed for cyclic and acyclic diazo carbonyl precursors. Cyclic diazo‐carbonyl compounds are structurally locked in a syn conformation and mainly follow the concerted mechanism, however, a stepwise pathway does slightly contribute to the overall WR process, contrary to Kaplan's rule. With acyclic diazo ester carbonyl compounds, the WR process is dominated by the stepwise mechanism. A carbene–carbene interconversion process and singlet to triplet carbene intersystem crossing (ISC) processes are discussed in this review. The growth of secondary species (cations, ylides, OH insertion product) emerging from an interaction between a singlet carbene and solvent molecules has been characterized by picosecond time‐resolved UV–Vis transient absorption spectroscopy. Carbene reactivity results in shortening of the singlet carbene lifetime relative to more inert solvents. The quantum yield of diazo decomposition can be deduced from partial ground state bleach recovery using ultrafast time‐resolved IR spectroscopy.