Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

Abstract

AbstractA large class of radical clocks is based on the intramolecular trapping of a reactive radical by a suitably located unsaturated system. Depending on the substituents present on this unsaturated system, the rate of cyclisation may vary drastically. This property has been repeatedly used to diagnose the participation of very short‐lived radicals in the mechanisms of a wide variety of reactions. For reactions occurring in homogeneous solution, a phenyl substituent capable of stabilizing the radical formed during the act of trapping has been one of the most widely used tools of this type. During study of the mechanisms of formation of Grignard reagents – reactions that occur at the interface of the metal and the solution – the phenyl substituent displayed a specific new behaviour pattern. Besides its stabilizing role, it was also able to play the role of mediator in redox catalysis of electron transfer. In this case, the first events on the pathway to the Grignard reagents involve a cascade of three (one intermolecular followed by two intramolecular) electron transfers. Introduction of a p‐methoxy substituent on the phenyl ring, making the phenyl group a poorer electron acceptor, suppresses this specific second role. Applied to the mechanism of Grignard reagent formation, this p‐methoxy effect is consistent with a triggering mechanistic act of electron transfer from the metal to the aryl halide rather than with a concerted oxidative addition. A similar change in selectivity is observed when a p‐methoxy group is introduced onto a phenyl group that also bears a halogen, but its origin is different: this effect is associated with the shortening of the lifetime of the radical anion formed by the triggering electron transfer. These observations reemphasise our earlier proposals to use concepts originating from electrochemical kinetics to explain the selectivities of reactions occurring at metal/solution interfaces. This conjecture could possibly hold for any interface where the diffusion of reactive species plays a role in the settling of selectivity. These concepts emphasise the necessity to consider, for each reactive species, their average distance of diffusion away from the metal/solution interface.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)