Vivian Marlowe
Tosylates are an important method for the conversion of alcohols to tosylamides, which are used in the synthesis of many natural products and drugs. The conversion of an alcohol to a tosylate is a well-established process in organic chemistry, with tosylates acting as excellent leaving groups in nucleophilic substitution reactions. This is due to the resonance delocalization of the developing negative charge on the leaving oxygen. Tosylates are formed by treating alcohols with tosyl chloride, an amine, and a proper catalyst in an organic solvent. While this process is effective for primary and secondary alcohols, it does not always work for benzyl alcohols, where the presence of an electron-withdrawing group is necessary to improve the reaction activity of benzyl tosylates.
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What You'll Learn
- Tosylates are used to make alcohols into good leaving groups
- Tosylates can be used to convert primary alcohols into tosylated products
- Tosylates can be used to protect secondary alcohols
- Tosylates are used in the nucleophilic substitution of benzyl tosylates
- Tosylates are used in the functional group transformation of organic derivatives
Tosylates are used to make alcohols into good leaving groups
Tosylation involves treating an alcohol with tosyl chloride (TsCl). The alcohol performs a substitution reaction on sulfur, leading to the formation of O-S and breakage of S-Cl. Then, deprotonation of the charged alcohol leads to the neutral mesylate or tosylate. The stereochemistry of the alcohol remains unchanged during this process.
Tosylates are excellent leaving groups and can be used in substitution and elimination reactions. They are versatile substrates for substitution reactions and can be readily prepared from tosyl chloride and an alcohol, allowing the replacement of the hydroxy group by many good nucleophiles.
The advantage of tosylation is that it turns some liquid alcohols into solids, which are sometimes preferred as they are easier to handle. Additionally, the aromatic ring of the tosylates allows for better visualization on a TLC plate. However, because it is a larger molecule, its preparation is slower, and it is not as reactive in substitution reactions.
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Tosylates can be used to convert primary alcohols into tosylated products
Tosylates are a versatile substrate for substitution reactions, containing an excellent leaving group. They can be readily prepared from tosyl chloride and an alcohol, allowing the replacement of the hydroxy group by many good nucleophiles. The preparation of the tosylate is similar to the reaction of an alcohol with thionyl chloride, SOCl2.
Alcohols can be converted into tosylates using tosyl chloride and a base to "mop up" the HCl byproduct. The -OH group reacts first as a nucleophile, attacking the electrophilic centre of the tosylate and displacing a chloride ion, Cl-. Tosylates have a much better leaving group than the original alcohol. The tosylate ester undergoes subsequent reactions (typically SN1 or SN2) as part of a multiple-step synthesis.
The poor leaving group of alcohols can be overcome by converting the hydroxyl group to a tosylate ester. This is an excellent leaving group, and the conversion proceeds with retention of configuration at the electrophilic carbon. The nucleophilic substitution of the tosyl group by Cl- will not occur when no electron-withdrawing inductive effect exists in tosylates, such as benzyl tosylate. However, benzyl tosylate can be converted to benzyl iodide by a much better nucleophile, I-.
In the laboratory synthesis of isopentenyl diphosphate, the 'building block' molecule used by nature for the construction of isoprenoid molecules such as cholesterol and b-carotene, the first step is to convert the alcohol into an organic tosylate. This is followed by displacing the tosylate group with an inorganic pyrophosphate nucleophile.
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Tosylates can be used to protect secondary alcohols
Tosylates are used to protect secondary alcohols. The OH group can be converted into a much better leaving group through conversion to a sulfonate group such as p-toluenesulfonyl ("tosyl", abbreviated Ts) or methanesulfonyl ("mesyl", abbreviated Ms). Installing these groups does not affect the stereochemistry of the alcohol. The OTs and OMs groups can participate in substitution and elimination reactions.
The preparation of the tosylate is similar to the reaction of an alcohol with thionyl chloride, SOCl2. Treatment of alcohols with tosyl chloride does not always lead to the formation of tosylates. However, the reactions of benzyl alcohols with TsCl usually gave the corresponding tosylates initially. The nucleophilic substitution of the tosyl group by Cl− would not occur when no electron-withdrawing inductive effect exists in tosylates, such as benzyl tosylate.
Tosylation of hydroxyl-functionalized substrates is an important transformation to activate hydroxyl groups, thus yielding substrates for further nucleophilic substitution in various fields of organic synthesis. Tosylation of alcohols followed by nucleophilic substitution is a well-established process in organic chemistry.
In one example, a molecule consisting of a secondary alcohol and a secondary amide was used. The idea was to protect the alcohol as OMs or OTs to make it a better leaving group, which would then react with the amide group and close the structure (cyclization) to form a 2,5-substituted pyrrolidine.
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Tosylates are used in the nucleophilic substitution of benzyl tosylates
Tosylates are derivatives of p-toluenesulfonic acid and are used as good leaving groups in nucleophilic substitution reactions. They are often used as a substitute for alkyl halides, which are also good substrates for nucleophilic substitution reactions.
The OH group in alcohols can be converted into a better leaving group by installing a sulfonate group, such as p-toluenesulfonyl ("tosyl", Ts). This process does not affect the stereochemistry of the alcohol. Alcohols are poor substrates for substitution reactions because the hydroxyl group is a strong base and a poor leaving group.
Tosylates are good substrates for substitution reactions, reacting with nucleophiles similarly to alkyl halides. The advantage of using tosylates is that the substitution reactions are not under strongly acidic conditions. Tosylates are mostly used for 1o and 2o ROH (hence SN2 reactions). The -OH group in the alcohol reacts first as a nucleophile, attacking the electrophilic centre of the tosylate and displacing a chloride ion, Cl-.
The nucleophilic substitution of the tosyl group by Cl− will not occur when no electron-withdrawing inductive effect exists in the tosylate, such as benzyl tosylate. However, benzyl tosylate can be converted to benzyl iodide by a better nucleophile, I−. The reaction activity of benzyl tosylates is improved when there is an electron-withdrawing group, such as a nitro group, attached to the benzene ring. This is due to the presence of the electron-withdrawing inductive effect, which allows the nucleophilic substitution of these activated benzyl tosylates by Cl− to occur smoothly, despite the relatively poor nucleophilicity of Cl− in the reaction mixture.
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Tosylates are used in the functional group transformation of organic derivatives
The OH group can be converted into a much better leaving group through conversion to a sulfonate group such as p-toluenesulfonyl (“tosyl”, abbreviated Ts) or methanesulfonyl (“mesyl”, abbreviated Ms). The resulting sulfonic esters can then be transformed into many other groups with an appropriate nucleophile, such as alkali azides, thiocyanates, sulfonates, and amines.
The preparation of the tosylate is similar to the reaction of an alcohol with thionyl chloride, SOCl2. The reactions of benzyl alcohols with TsCl usually give the corresponding tosylates initially. However, treatment of alcohols with tosyl chloride does not always lead to the formation of tosylates. In some cases, the corresponding chlorides are formed instead.
Tosylates are also used in substitution and elimination reactions. Tosylates and mesylates are always reacted with a salt such as NaCl or NaBr and not with acids. This is because the use of salts ensures that halogens are in the form of good nucleophiles, as they are not suppressed by protons. Tosylates are also used to convert OH into a better leaving group, which can then participate in any substitution and elimination reaction.
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Frequently asked questions
Tosylates are the product of treating alcohols with tosyl chloride, an amine, and a catalyst in an organic solvent.
Tosylates are used to convert the hydroxyl group in alcohols into a better leaving group. Primary and secondary alcohols have hydroxyl groups attached to a carbon atom that has at least one hydrogen atom attached to it. This makes them more reactive than tertiary alcohols, which lack this hydrogen atom, and therefore better candidates for tosylation.
Tosylates can be prepared by treating alcohols with tosyl chloride, an amine, and a proper catalyst in an organic solvent.
Tosylates are used in organic chemistry to activate hydroxyl groups, yielding substrates for further nucleophilic substitution. This process is used in the synthesis of many natural products and drugs.
