What grignards are reagents
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Organic Chemistry Portal: Grignard Reaction
The Grignard reaction was originally understood as the addition of organomagnesium compounds as a nucleophile to carbonyl compounds as an electrophile to produce alcohols:
Reactions with formaldehyde produce primary alcohols, secondary aldehydes and tertiary ketones.
The reaction of sterically hindered Grignard reagents usually takes place radically (SET: single electron transfer). The tert-Butyl group stabilizes the intermediate radical:
If chiral carbonyls are used, it can be predicted which diastereomer is preferentially formed (Cram's rule), since the attack of the nucleophile preferably takes place via sterically unhindered sites.
A competitive reaction is the formation of an enol - the Grignard compound reacts as a base due to steric hindrance:
In a further side reaction, the Grignard reagent acts as a reducing agent. The prerequisites for this are the presence of & # 946 hydrogen atoms on the organyl and the formation of a six-membered transition state:
Grignard compounds enter into a large number of other electrophilic reactions, the following conversions are valuable:
With acid chlorides to ketones:
With a second equivalent of the Grignard compound, the reaction to the tertiary alcohol proceeds:
The reaction of esters with Grignard reagents can be carried out less selectively, since esters are less reactive than ketones. However, with 2 equivalents of Grignard reagent, they provide tertiary alcohols (carbinols) in good yields:
Nitriles are reduced to the imine, which after hydrolysis yields ketones:
The implementation with CO2 (preferably on powdered dry ice) leads to carboxylic acids:
Chain-extended alcohols can be produced by reacting with epoxides. The nucleophile attacks the more accessible point:
The literature section of the English-language page also describes enantioselective conversions in the presence of sparteine and the use of various additives that accelerate the reaction.
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