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

A 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

Chemical Bonding and the Equilibrium Composition of Grignard Reagents in Ethereal Solutions

“…It is experimentally well established that Grignard reagent formation reactions involve a series of electron transfers (ET). ,,, It was remarked that in the study of this mechanism, it is essential to consider, for each of the reactive species, the extent of diffusion away from the metal/solution interface. One way to rationalize this very complex chemical landscape is to consider that the possible species presented in the medium are dependent on the predominance of one ET step or two ET steps before reaching the equilibrium.…”

“…These types of experiments do not provide an accurate picture of the species present in solution. It is certain that ionic and radical species are present in Grignard reagent solutions, as shown by electrochemical studies. − ,− These additional species probably play a pivotal role in the determination of the different degrees of association found experimentally. In the following paragraphs, we briefly review the electrochemically based models for the Grignard reagent formation.…”

Reaction mechanisms : Part (i) Radical and radical ion reactions

“…For example, Mattalia et al used radical cyclizations to probe the mechanism for the formation of Grignard reagents from aryl bromides. 40 Guijarro and coworkers probed the existence of S N 2 vs. single electron transfer (SET) pathways in the reaction of biphenyl radical anion with alkyl fluorides. 41 This section begins with a discussion of radical addition reactions that result in C-C bond formation.…”