Magnesium-Carbon Bond

We saw in Section 17.5 that treatment of an aldehyde or ketone with a Grignard reagent, RMgX, yields an alcohol by nucleophilic addition of a carbon anion, or carbanion. A carbon-magnesium bond is strongly polarized, so a Grignard reagent reacts for all practical purposes as R - +MgX. 

The only data omitted in this analysis are those for vinyl- (determined in THF) and cyclopentadienylmagnesium bromide. Including the data point for cyclopentadienyl in the analysis worsens the correlation. This may be caused by a difference in carbon bonding to the magnesium for cyclopentadienide compared to the other carbon-magnesium bonds. This bonding will be mentioned in a later section on magnesium sandwich compounds. 

Organomagnesium compounds undergo fast intermolecular carbon-magnesium bond exchange in solution. One such process in THF solution, (equation 14) was studied by NMR line-shape analysis ... 

Electrophilic functional groups in ortho-position to the carbon-magnesium bond allow two sequential alkylations. Starting from ort/io-iodobenzyl chloride 104, the benzannulated heterocycles 105 and 106 are obtained after the reaction with appropriate electrophiles (Scheme 8) . ... 

The reactivity of the 1,1 -diorganometallics (61) has been extensively investigated by Knochel and co-workers.9293 Since the reactivity of a carbon-lithium or a carbon-magnesium bond is considerably different from that of a carbon-zinc bond, a selective reaction of (61) with two different electrophiles is often possible. The general reaction pathways are summarized in Scheme 33. The addition of allylic zinc... 

Grignard reagent (organomagnesium halide) (Section 18.5) An oiganometallic species with a carbon—magnesium bond RMgX. 

The most important determination is normally the concentration of carbon-magnesium-bonded species in solution. For routine estimation of this concentration for freshly prepared solutions of organomagnesium compounds, an aliquot of the test solution may be added to an excess of standard acid, and then back-titrated with sodium hydroxide. However, this simple determination of total base will give a high estimate of organomagnesium content if products of hydrolysis or oxidation are present. Analytical methods based on the determination of the hydrocarbon formed on hydrolysis of the organomagnesium compound... 

In addition to molecular oxygen, a number of reagents have been developed for introduction of oxygen into a caibon-4ithium or carbon-magnesium bond. These include peroxides, molybdenum peroxide-pyridine-hexamethylphosi ioramide (MoOPH), sulfonyloxaziridines, nitrobenzene" and oxidation of the boronate" after transmetallation. 

The electron pair in the carbon-magnesium bond of phenylmagnesium bromide is the nucleophile, and the carbonyl carbon of the ketone is the electrophile. Also, magnesium is an electrophile and the carbonyl oxygen is a nucleophile, so that the salt of an alcohol is the product of the reaction. The alcohol itself is generated by an acidic workup. 

Other examples are summarized in Table 1. Terminal olefins nicely participated in this reaction as shown in Eq. (1) and entries 1 and 2 in Table 1. Styrene showed a reversal of regioselection relative to the position of the newly formed carbon-magnesium bond. However, sterically hindered olefins afforded an increasing amount of hydrogenated product, even after treatment of the reaction mixture with D2O (see Table 1, entries 3 and 4). Finally, no deuterated hydrocarbons were obtained from trans- and trisubstituted olefins, even though the starting materials were almost completely consumed (see Table 1, entries 5 and 6). 

The Grignard reagents can be divided into three classes according to the strength of the carbon-magnesium bonds [54]. 

The greater electronegativity of the earlier halides in the periodic table is expected to cause an increase in the polarity of the carbon magnesium bond and, hence, to increase its reactivity. 

Apart from differences in polarity of the carbon-magnesium bond, there may be other reasons why organomagnesium chlorides react faster than the other organomagnesium halides. This, then, may also cause a change in the composition of the reaction products, since slower reactions have no chance to develop, as in the reaction of neopentylmagnesium chloride with benzophenone in Scheme 4. 

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