Organometallic Compounds

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Chapter: Organic Chemistry : Carbon-Carbon Bond Formation Between Carbon Nucleophiles and Carbon Electrophiles

Organometallic compounds such as Grignard reagents and organolithium reagents are very powerful nucleophiles which react with a wide variety of carbonyl compounds.


ORGANOMETALLIC COMPOUNDS

Organometallic compounds such as Grignard reagents and organolithium reagents are very powerful nucleophiles which react with a wide variety of carbonyl compounds. In general, organolithium reagents are more reactive than Grig-nard reagents. Both types of reagents react rapidly with aldehydes and ketones and yield secondary and tertiary alcohols after aqueous workup. The new car-bon–carbon bond joins the organometallic fragment with the carbonyl carbon.


The tetrahedral intermediate produced by addition of organolithiums or Grig-nard reagents to esters collapses to a ketone which is more reactive than the original ester electrophile. A second equivalent adds to give a tertiary alcohol as the product. This process cannot be controlled by temperature or by mode of addition since the intermediate product is more reactive than the starting material.


Collapse of the tetrahedral intermediate can be prevented, however, by reaction of a carboxylic acid with two equivalents of an organolithium (or a carboxylate salt with one equivalent of the organolithium). Addition to the carboxylate gives a dianionic tetrahedral intermediate which has the ketone oxidation state but is stable to further addition under the reaction conditions. The ketone is revealed only upon hydrolysis. This is a widely used method for the preparation of ketones by the formation of a carbon–carbon bond, but it is restricted to organolithium reagents as the carbon nucleophile. Grignard reagents do not react with car-boxylates, illustrating they have a slightly reduced nucleophilicity relative to organolithiums.



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