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  Reaction with Organometallic Reagents Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions with Organometallic Reagents

The facile addition of alkyl lithium reagents and Grignard reagents to aldehydes and ketones has been explained. These reagents, that are ready from aryl and alkyl halides, are powerful nucleophiles and extremely strong bases. The Reaction of an excess of these reagents with acyl chlorides, anhydrides and esters leads to alcohol results, in similar way like the hydride reductions. As depicted by the following equations (shaded box in the diagram), this takes palce by sequential addition-elimination-addition reactions and ends with hydrolysis of the resulting alkoxide salt. A general bonding pattern is found in all these carbonyl reactions. The organometallic reagent is a basis of a nucleophilic alkyl or aryl group (colored purple in the disgram), which bonds to the electrophilic carbon of the carbonyl group (colored orange in the disgram). Substituent Y (colored green in the disgram) is eliminated from the tetrahedral intermediate as its anion. The aldehyde or ketone result of this elimination then adds a second corresponding of the reagent.

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Reactions of this type are significant synthetic transformations, Since they allow simple starting compounds to be joined to create more complex structures. Esters are the most general carbonyl reactants, because they are cheaper and less hazardous to use than acyl chlorides and anhydrides. Many esters react with organometallic reagents to give 3º-alcohols; but formate esters (R=H) give 2º-alcohols. Some instances of these reactions are provided in the diagram. As illustrated by the last equation, lactones go through ring opening and yield diol products.

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The acidity of the carboxylic acids and 1º & 2º-amides acts for convert Grignard and alkyl lithium reagents to hydrocarbons (see equations), thats why these functional groups should be avoided when these reagents are employed.

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Because acyl chlorides are more reactive than esters, isolation of the ketone intermediate created in their reactions with organometallic reagents becomes an attractive possibility. To get this selectivity we require to convert the highly reactive lithium and Grignard reagents to less nucleophilic species. Two such type of modifications that have proven effective are the Gilman reagent (R2CuLi) and organocadmium reagents (prepared in the manner shown).

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Particular examples of ketone synthesis using these reagents are existed in the diagram. The second equation illustrates the low reactivity of organocadmium reagents, inasmuch as the ester function is not changed. Other related approach to this transformation is demonstrated by the third equation. Grignard reagents add to the nitriles, forming the comparatively stable imino derivative that can be hydrolyzed to a ketone. Imines themselves do not respond with Grignard reagents.

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