Solutions Grignard reagent formation

Figure 7 D-model for Grignard reagent formation. Since R, Ri, and S were assumed to have identical properties, they could be treated as a single intermediate T. which may either return to the surface and react with magnesium or undergo coupling in solution. Differences in the nature of the radical intermediates R. R,., and S were ignored.

It is unreasonable to believe that a model that assumes that the radicals diffuse freely in solution could account for such results. On the other hand, a surface-bound radical is fully in accord with them. Compound (S)-(-l-)-18 behaves similarly when exposed to metal surfaces of alkali metals in hydroxylic solvents [94]. As in the Grignard reagent formation, when a solution of (S)-(-f)-l-bromo-l-methyl-2,2-diphenylcyclopropane 18 in methanol, isopropyl alcohol, or r-butenol was exposed to an alkali metal, such as lithium, sodium, or potassium, the resulting hydrocarbon (K)-( —)-l-methyl-2,2-diphenylcyclopropane 13 was shown to be optically active and with retained configuration, as shown in Table 19 and Scheme 40. 

Since the 2-norbornyl radical [117] and the 5-norbornenyl radical [118] have been thoroughly studied and their behavior in solution is well known, studies of Grignard reagent formation using the 2-norbornyl and norbornenyl systems seemed well worthwhile [116]. Relative to the nature of these radicals, they are planar /r-radicals. 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

The addition of vinylmagnesium bromide to methyl (S)-3-benzyloxy-4-oxobutanoate (5) in tetrahydrofuran proceeded with a slight preference for the nonchelation-controlled reaction product (40 60)5°. A reversal of the diastereoselectivity (80 20) could be observed when the Grignard reagent, as a solution in tetrahydrofuran, was added to a dichloromethane solution of the aldehyde which had been precomplexed with one equivalent of magnesium bromide. The almost exclusive formation of the chelation-controlled reaction product 6 was achieved when tetrahydrofuran was completely substituted by dichloromethane the presence of tetrahydrofuran interferes with the formation of the chelate complex, which is a prerequisite for high chelation-controlled diastereoselection. 

Specifically, it has recently been found 149) that diarylthallium tri-fluoroacetates may be converted into aromatic iodides by refluxing a solution in benzene with an excess of molecular iodine. Yields are excellent (74-94%) and the overall conversion represents, in effect, a procedure for the conversion of aromatic chlorides or bromides into aromatic iodides via intermediate Grignard reagents. The overall stoichiometry for this conversion is represented in Eq. (10), and it would appear that the initial reaction is probably formation of 1 mole of aromatic iodide and 1 mole of arylthallium trifluoroacetate iodide [Eq. (8)] which subsequently spontaneously decomposes to give a second mole of aromatic iodide and thallium(I) trifluoroacetate [Eq. (9)]. Support for this interpretation comes from the... 

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