The Claisen rearrangement – Part 1 : mechanisms and transition states, revisited with quantum mechanical calculations and ultrashort pulse spectroscopy

The Claisen rearrangement is an ideal candidate for mechanistic and transition-state analysis. Recently, physical-chemistry investigations, in particular relying on quantum mechanics and modern fs-pulse (‘short-time') laser spectroscopy, have given totally new insights here. At the same time, flow chemistry has emerged, offering considerable processing advances over batch processing. It is also known to be much powerful with inline flow sensing techniques. Yet, both key innovations have not been bridged so far and thus there is an eminent knowledge gap in mechanistic analysis of reactions. This two-review compilation is a first step to close this gap. Part 1 focuses on the insight which quantum-mechanical calculations can add to the existing knowledge based on heuristic, handy interpretation of reaction experiments in the light of electrodensity distributions. Quantum-mechanical calculation can provide geometric key data and short-time laser spectroscopy can verify this and deepen it in the sense of a comprehensive vibration analysis. In this way, prior structural assumptions can be confirmed or turned to be falsified and there is potential to resolve the uncertainty.