Mechanism of Grignard reagent formation. Comparisons of D-model calculations with experimental product yields

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

“…Firstly, although the route going through step a suggests the continuous presence of the magnesium surface in close proximity to the formed aryl radical, a better representation would be a short-distance shuttle of this radical between the metallic surface and the solution. [81,82] Scheme 4 emphasises the average proximity of the metallic surface, to contrast this situation with that in which intramolecular redox catalysis takes place.…”

Reactions of Lithium With Benzaldehyde in Solution

“…The nature of the reactions that occur at the catalyst surface is a matter of intense research at present. [7][8][9][10] We have recently provided substantial evidence that the first step of the complex mechanism of the solution reaction of phenyllithium with CO is electron transfer from PhLi to CO. 11 This paper reports a mechanistic study of the reduction of benzaldehyde by lithium metal. Evidence for a common radical intermediate for both reactions is presented.…”

“…Newcomb and co-workers 23,24 reported similar results in reactions when a ␤-hydride ion is transferred from LDA to benzophenone to produce benzhydrol. To our knowledge, the first analytical steady-state solution for a somewhat similar system is that developed by Garst et al, 25 called a simplified 'D model,' which was applied to describe the mechanism of Grignard reagent formation. A similar, albeit more simplified, approach will be followed here; it includes first-order surface reaction steps and first-and second-order solution steps.…”

“…The observed adsorption phenomena (symbolized by PhCOH/ Li) are considered as the first step and the overall rate of adsorption measured by k 1 =8ϫ10 Ϫ 4 s Ϫ 1 at 48°C [Garst et al 25 called it 'flux' instead of velocity in order to distinguish ) from the usual volumebased rate; for the sake of simplicity they are not differentiated in the present paper. ]…”

“…On the other hand, when the radicals are formed at a solid surface, they tend to 'pile up' near it, even if they are freely diffusing in solution. 28 This can result, during the electron transfer, in a steady-state concentration near the surface that approaches or exceeds 10 25 at which concentration the rate of coupling is about 50 times that of reaction with the solvent. In the present study, the observed values for k c and k S at 25°C are 7·25ϫ 10 consistent with the small amounts of coupling products found and a negligible accumulation of 5 on the lithium surface.…”