Alcohols are generally poor leaving groups in organic chemistry, which necessitates their conversion into better leaving groups. One effective method for this transformation is the use of sulfonate esters, which are highly stable and excellent at stabilizing the negative charge once they leave. To convert an alcohol into a sulfonate ester, a sulfonyl chloride is employed. The general structure of a sulfonate ester includes a sulfur atom bonded to two double-bonded oxygen atoms and an alkyl or aryl group (R). The specific name of the sulfonate ester varies depending on the R group; for instance, a methyl group yields a mesylate, a benzene ring with a methyl group results in a tosylate, and a trifluoromethyl group produces a triflate.
A sulfonyl chloride is similar to a sulfonate ester but has a chlorine atom in place of one of the oxygen atoms. The mechanism begins with the alcohol's oxygen acting as a nucleophile, attacking the electrophilic sulfur atom in the sulfonyl chloride. This results in the formation of a sulfonate ester intermediate, where the oxygen remains attached to the hydrogen while forming a bond with sulfur. The chlorine atom can then leave, leading to the stable sulfonate ester structure.
The sulfonate ester is characterized by its stability, as it has no formal charges, allowing it to be stored and used later in reactions. When a nucleophile is introduced, it can perform a backside attack on the sulfonate ester, facilitating further reactions. Importantly, the conversion from alcohol to sulfonate ester retains the stereochemistry of the original alcohol, as the oxygen atom does not undergo any positional change during the reaction. This retention of configuration is a key distinction from other reactions involving thionyl chloride or phosphorus bromide, which typically result in inversion of configuration.
In summary, sulfonate esters serve as excellent leaving groups due to their stability and the ability to retain stereochemistry during their formation from alcohols. This makes them valuable intermediates in various organic reactions.
