Radical Clocks, Solvated Electrons, and Magnesium. Heterogeneous versus Homogeneous Electron Transfer. Selectivity at Interfaces

Abstract

Ammonia solutions of solvated electrons may be prepared by electrolyzing the solvent in the presence of Mg(BF4)2 and Mg++ provided by a magnesium anode. The reactions of these solutions (−40 °C) with the two radical clocks 1-bromo-2-(3-butenyl)benzene 1 and 1-(allyloxy)-2-bromobenzene 7 (0.007−0.008 M) are examined. Both probes yield very high percentages of cyclized products (>97%) when the concentration of solvated electrons is low (0.028 M). These percentages diminish when the metallic character of the solution is increased (0.183 M concentration in solvated electrons). These results strongly contrast with those obtained for the reactions of these radical probes with magnesium turnings or potassium lumps in THF. Under these conditions, the percentages of cyclized products are lower than 4%. We propose that the main cause of this contrasting behavior is the higher reducing efficiency of the heterogeneous electron transfer with respect to the homogeneous electron transfer. This holds even if the reducing agent in the bulk is one of the strongest known reducing agents (solvated electron). The causes for the striking difference between heterogeneous and homogeneous electron transfer are discussed in terms of selectivity within a double-layer perspective. These differences allow the rationalization of an apparent discordance. The behavior of MgX2 (X = halogen) under pulse radiolysis conditions (Mostafavi's group) shows that MgX2 are so weakly oxidizing that in most solvents they do not react with the solvated electron. Nevertheless, the reduction of MgX2 by alkali metals (Rieke's method) is the basis for the preparation of very active slurries of magnesium. This principle has been widely extended for the preparation of various metallic nanoparticles. The difference between homogeneous and heterogeneous electron transfer to MgX2 lies at the heart of the problem as explained in the framework of molecular electrochemistry at the cathode surface. Kinetic treatment of the selectivity at the metal−liquid interface is contrasted with selectivity obtained in homogeneous solution.

Clear-cut difference in the rearrangement of 1-bromo-2-(2-phenyl-3-butenyl)benzene under anionic or radical conditions

“…The following program temperature was used: injector : 280 °C; interface: 280 °C; 150 °C (3 min) to 250 °C (5 min) at 5 °C min -1 . As in previous works, [2][3][4] 1-Methyl-2-phenylindane 6 and (E)-1,2-diphenyl-2-butene 8 were isolated from reaction mixtures (see experimental part). 2-phenyltetralin 7 was isolated among other reaction products, this structure was proposed by NMR ( 1 H, 13 C) and GC-MS comparisons with literature and database data (NIST98.L).…”

“…Our interest in the mechanism of formation of Grignard reagents and homogeneous versus heterogeneous electron transfers focused our attention on 1-bromo-2-(3-butenyl)benzene 1. [1][2][3][4] Cyclization of o- (3-butenyl)phenyl radical 2r (kcy = 5 x 10 8 s -1 , 30 °C) to 1-methylindanyl radical 3r and related rearrangements have been used to detect radical intermediates in several reactions. [5][6][7][8] However, cyclization of the o- (3-butenyl)phenyl anion 2c has to be considered (Scheme 1).…”

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

“…[44,51] In a previous report we showed that the linear/cyclised selectivity is best explained by use of the equations proposed by Saveant's group to interpret the selectivity observed in the yields of rearranged products in the vicinity of a cathode. [46,47,52] In these equations, the diffusion coefficients associated with the reactive species play a definite role. Several reports have suggested that the diffusion coefficients of isomers may slightly depend on the shapes of these isomers (here Z vs. E isomers).…”

“…In previous reports, we proposed that the reductive strength of the magnesium surface was stronger than most of the homogeneous reductants (including the solvated electron). [46,47] As a consequence, an aryl radical created in close vicinity to this metallic surface should have less chance to cyclise because its rate of reduction into a carbanion would be very high. In contrast, when the radical is created at a greater distance from this metal surface, it would have better chances to cyclise.…”

“…We have previously shown that the participation of the radical route to the overall selectivity increases under such conditions. [45][46][47] …”