Polar mechanisms, Grignard carbonyl additions

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

If the addition of a Grignard reagent to a carbonyl compound leads to the formation of such an unexpected alcohol, it is clear that this reaction cannot have proceeded according to the polar mechanism outlined in the upper row of Figure 10.27. If the respective addition does not produce anything hut the expected alcohol, the polar mechanism provides the easiest explanation. Nevertheless, the radical mechanism could also have applied—provided, though, that the isomerization H —> iso-H would have been much slower than the radical annihilation reaction H — I. 

For the mechanism of the metal-mediated Grignard-type reactions in water, Li previously proposed a carbanion/allylmetal/radical triad, in which the specific mechanism of the reaction is dependent on the metal being used (Fig. 4.6). Recently, mechanistic studies of the aUylation detected secondary deuterium kinetic isotope effects in the metal-mediated aUylation ofbenzaldehyde in aqueous media.The inverse SDKIE observed for the indium and tin cases are consistent with the polar addition mechanism. For magnesium and antimony, normal SDKIE were observed. These were interpreted as single electron transfer processes on metal surface in the magnesium case, or between the allyhnetal and the carbonyl compound in the antimony case. 

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