Beyond the Woodward-Hoffman Rules: What Controls Reactivity in Eliminative Aromatic Ring-Forming Reactions?

Abstract

2The Mallory (photocyclization) and Scholl (thermal cyclohydrogenation) reactions

Predicting the Outcome of Photocyclisation Reactions: A Joint Experimental and Computational Investigation

“…Such reactions are referred to as oxidative eliminative photocyclisations. On the other hand, reactions in which X is a suitable leaving group (alkoxy groups or halogens) are referred to as non‐oxidative eliminative cyclisation since the elimination step can proceed spontaneously without an oxidising agent . In this paper, our focus will be on oxidative eliminative cyclisation reactions because they do not require substituted starting materials to be prepared and so are synthetically more convenient.…”

“…On theo ther hand, reactions in which Xi sasuitable leaving group (alkoxy groups or halogens) are referred to as non-oxidative eliminativec yclisation since the elimination step can proceed spontaneously without an oxidising agent. [30] In this paper,o ur focus will be on oxidative eliminativecyclisation reactions because they do not require substituted starting materials to be prepared and so are synthetically more convenient. Further,the use of oxidising agenttopromote elimination allows an additional degree of synthetic control and further potentialf or optimising reaction conditions to promote product formation.…”

“…Most experimental studies focus on optimising reaction conditionssolvent, oxidant, temperature-inana ttempt to identify experimentalc onditions that will lead to product formation.H owever,r ecent computational studies have shown that the elimination step is thermodynamically favourable and can be kinetically enhanced by optimising reaction conditions, regardless of whether elimination proceeds in the excited state or following internal conversion backt ot he ground state. [30] Further, if internal conversion is coupled to elimination, the whole process will proceeds pontaneously and exothermicallyf ollowing photoexcitation and photoplanarisation. [30] In either case, photoplanarisation is an ecessary pre-condition for elimination, so it is important to be ablet op redict and/or control the process.…”

“…[30] Further, if internal conversion is coupled to elimination, the whole process will proceeds pontaneously and exothermicallyf ollowing photoexcitation and photoplanarisation. [30] In either case, photoplanarisation is an ecessary pre-condition for elimination, so it is important to be ablet op redict and/or control the process. Controlling the outcome of the photoplanarisation step is far harder than fore limination, because both the thermodynamic and kinetic favourability are determined primarily by changes in the molecule's electronic structureu pon photo-excitation.…”