Noah Vinter
Esters can be converted to tertiary alcohols through two types of reduction reactions. One method involves reacting esters with two equivalents of a Grignard reagent or organolithium reagent, resulting in the formation of tertiary alcohols. This process occurs under aerobic conditions in Deep Eutectic Solvents or bulk water. Alternatively, esters can undergo reduction by a strong reducing agent such as LiAlH4 to produce primary alcohols. These reactions are essential in understanding the transformation of organic compounds and have applications in various fields, including green and sustainable chemistry.
| Characteristics | Values |
|---|---|
| Conversion of esters to tertiary alcohols | Two equivalents of Grignard reagent or organolithium |
| Reaction type | Nucleophilic addition |
| Reaction time | 20 seconds |
| Yield | 60-98% |
| Reagents | Organometallic compounds of s-block elements |
| Solvent | Biodegradable choline chloride/urea eutectic mixture or water |
| Ester type | Carboxylic ester |
| Ester formation | Combination of alcohol and acid with acid catalysis |
| Acid catalysts | Sulfuric acid, Tosic acid, hydrochloric acid |
| Acid type | Inorganic or organic |
| Ester byproducts | Alkene, cyclic ester, anhydride |
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What You'll Learn
Using Grignard reagent
Grignard reagents are formed by the reaction of magnesium metal with alkyl or alkenyl halides. They are extremely good nucleophiles and strong bases. They react with carbonyl compounds such as aldehydes, ketones, and esters.
To form a tertiary alcohol from an ester using a Grignard reagent, the reaction requires two equivalents of the Grignard reagent. The Grignard reagent, a source of carbanions, functions as a nucleophile and attacks the carbonyl carbon of the ester to form a tetrahedral intermediate. This is the first step of the reaction, which results in the formation of a carbon-carbon bond and an alkoxide (the conjugate base of an alcohol).
Next, the carbonyl group is reconstructed with the departure of an alkoxide ion as a leaving group to yield a ketone. This is followed by the addition of a second equivalent of the Grignard reagent, which attacks the ketone, forming an alkoxide intermediate.
Finally, protonation of the alkoxide yields tertiary alcohol as the final product. This is done by adding acid at the end of the reaction (the "workup" step). The acid used should be strong enough to protonate the negatively charged oxygen without causing the formation of a carbocation. For example, NH4+ Cl- is sometimes used instead of H+ for the workup step.
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Using organolithium reagent
In the field of organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. They are highly reactive and nucleophilic in nature. They are widely used in organic synthesis, especially in the synthesis of pharmaceutical compounds.
To form a tertiary alcohol from an ester using organolithium reagents, a nucleophilic addition reaction can be carried out. This involves the nucleophilic attack of the organolithium reagent on the ester, resulting in the formation of a carbon-carbon bond and the subsequent conversion to a tertiary alcohol. The reaction can be carried out using bulk water or ChCl/urea as a solvent, working under air and room temperature conditions. The reaction proceeds quickly, usually within 20 seconds, and can deliver tertiary alcohols in good yields, ranging from 60% to 98%.
It is important to note that organolithium reagents are strong bases and, therefore, cannot be used as nucleophiles on compounds containing acidic hydrogens. If they are used in such cases, they will act as a base and deprotonate the acidic hydrogen instead of reacting as a nucleophile. However, they can be used as nucleophiles on compounds such as aldehydes and ketones to produce tertiary alcohols.
Additionally, organolithium reagents have certain advantages over Grignard reagents, which are also commonly used in organic synthesis. Organolithium reagents often perform similar reactions with increased rates and higher yields. They are also less likely to reduce ketones and can be used to synthesize substituted alcohols.
In summary, the formation of tertiary alcohol from an ester using organolithium reagents involves a nucleophilic addition reaction, which can be carried out under mild conditions, resulting in the efficient and quick synthesis of tertiary alcohols. The reactivity and versatility of organolithium reagents make them a valuable tool in organic chemistry, especially in the synthesis of pharmaceutical compounds.
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Using nucleophilic addition
Tertiary alcohols can be formed from esters through nucleophilic addition reactions. One method involves the use of Grignard reagents, which are carbon-based nucleophiles that react with esters to form tertiary alcohols. Grignard reagents add to the ester twice, first through a nucleophilic acyl substitution to form a ketone intermediate, and then through a nucleophilic addition to form the tertiary alcohol product. This reaction involves the formation of two C-C bonds on the ester's original carbonyl carbon. The OR leaving group from the ester is replaced by the R group from the Grignard reagent during the nucleophilic acyl substitution step.
Another method involves the use of organolithium reagents, which can also convert esters to tertiary alcohols. This reaction can be carried out in bulk water or ChCl/urea, working under air and at room temperature. The nucleophilic addition of organolithium reagents to esters results in the formation of tertiary alcohols with high yields (60-98%) and short reaction times (20 seconds).
Additionally, nucleophilic addition/thioetherification processes starting from esters have been shown to be feasible in water, allowing the isolation of pharmaceutically relevant S-trityl-l-cysteine derivatives through the formation of tertiary alcohols.
It is important to note that Grignard reagents will not react with alkyl halides through an SN2 reaction and are also incompatible with carboxylic acids and alcohols. The choice of reaction conditions is crucial to ensure the desired product is favoured.
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Using organometallic compounds
Tertiary alcohols are important structural subunits in chemical building blocks and are common among biologically active compounds. One of the most efficient and direct routes to accessing tertiary alcohols is through the addition of organometallic compounds of s-block elements (typically organolithium and Grignard reagents) to ketones or esters.
Grignard reagents, for example, can be added to esters to form tertiary alcohols. In this reaction, the Grignard reagent adds twice. The first addition produces a ketone, which then undergoes a second reaction to form the tertiary alcohol.
Organolithium reagents can also be used in the nucleophilic addition to esters to form tertiary alcohols. For example, one equivalent of n-BuLi (2.0 M in cyclohexane) can be added to a suspension of methyl benzoate in water at room temperature under air, with vigorous stirring to generate an emulsion. This reaction proceeds quickly and smoothly, delivering tertiary alcohols in good yields (60-98%).
It is important to note that Grignard reagents are destroyed by H+. Additionally, ketones are more reactive than esters towards Grignard reagents, which means they will be consumed more quickly.
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Using Fischer esterification
Fischer esterification is a widely practiced process for ester synthesis. It involves refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. The reaction was first described by Emil Fischer and Arthur Speier in 1895.
In Fischer esterification, an ester is formed when a carboxylic acid is treated with an alcohol and an acid catalyst. The acid catalyst protonates the carbonyl oxygen of the carboxylic acid, forming an oxonium ion. This protonated carbonyl is a stronger electrophile than a neutral carbonyl carbon. The second step is the addition of a neutral nucleophile (ROH) to the protonated carboxylic acid, forming a C-O bond and breaking a C-O (pi) bond. This results in a tetrahedral intermediate. The next two steps, known as "proton transfer," involve the movement of H+ from one oxygen to another. The O-H from the alcohol is deprotonated, followed by the protonation of the O-H oxygen, forming two new O-H bonds and generating a molecule of water. The elimination of water forms a new C-O (pi) bond and breaks a C-O bond, giving a protonated ester. Finally, the deprotonation of the ester yields the neutral ester product and water.
To drive the reaction towards the formation of the ester, it is important to use a large excess of alcohol, preferably as the solvent. The byproduct, water, should be removed as it forms to slow down the reverse reaction. The removal of water can be achieved through the use of a drying agent (desiccant) or a Dean-Stark type apparatus. By removing water, the equilibrium is shifted towards the formation of the ester product. This can also be facilitated by adding anhydrous salts, such as copper(II) sulfate or potassium pyrosulfate, which sequester water by forming hydrates.
While most carboxylic acids are suitable for Fischer esterification, the alcohol should generally be primary or secondary. Tertiary alcohols are prone to elimination, and the reaction may not yield the desired ester product. Common choices for the acid catalyst include sulfuric acid (H2SO4), tosic acid (TsOH), hydrochloric acid (HCl), and tetrabutylammonium tribromide (TBATB).
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Frequently asked questions
One way to form a tertiary alcohol from an ester is by reacting them with two equivalents of a Grignard reagent. Another way is by reacting them with two equivalents of an organolithium reagent.
The first step involves the nucleophilic attack of the Grignard reagent, forming a C-C bond and shifting the electrons of the π bond to the oxygen. The second equivalent is needed because the product of the first addition-elimination reaction to the carbonyl is a ketone, which is then converted into a tertiary alcohol.
The conversion of esters to tertiary alcohols has applications in the synthesis of pharmaceutically relevant compounds, such as S-trityl-l-cysteine derivatives. It also enables the preparation of important intermediates, such as in the synthesis of Polmacoxib.
