The Pinner reaction, originally discovered by Pinner and Klein in 1877, involves the conversion of nitriles into imidate hydrochlorides upon treatment with strong protic acids such as hydrogen chloride. This classical method has long been limited by harsh conditions, including the use of gaseous HCl, which poses safety and handling challenges. Over time, various modifications have been developed to circumvent these drawbacks. Among them, the use of Lewis acids offers a milder and more selective alternative for promoting the acylation of alcohols with nitriles to form carboxylic esters.
In this study, we report a highly efficient Lewis acid-promoted Pinner reaction utilizing trimethylsilyl triflate (TMSOTf) as a catalyst. The reaction proceeds smoothly under ambient conditions when two equivalents of TMSOTf are employed, with the nitrile serving simultaneously as both reactant and solvent. Primary aliphatic alcohols and aliphatic or benzylic nitriles yield good to excellent results, providing the corresponding esters in high purity and yield. For instance, the reaction of 9H-fluoren-9-ylmethanol with acetonitrile affords the desired ester in 83% yield, while benzyl cyanide reacts efficiently with primary alcohols to give products in up to 90% yield. Notably, the synthesis of acrylates from acrylonitrile and primary alcohols is also feasible, achieving yields comparable to or better than those reported in previous methods.
One of the most significant advantages of this approach is its chemoselectivity. Functional groups such as free carboxylic acids and phenolic hydroxy moieties remain intact under the reaction conditions. This allows for the direct transformation of complex substrates without requiring protection steps. For example, the reaction of p-hydroxymethylbenzoic acid with acetonitrile proceeds smoothly to yield the ester product without interference from the carboxylic acid group.
However, complications arise when using benzyl alcohols. Under identical conditions, variable amounts of amides are formed via a Ritter-type side reaction. This competing process likely occurs through the formation of a benzylic carbocation after silylation, followed by nucleophilic attack at the nitrogen of the nitrile.M-CSF Protein, Humansupplier The presence of electron-withdrawing substituents—such as nitro groups—enhances the efficiency of the main Pinner pathway, possibly due to their radical scavenging ability, which suppresses undesired decomposition pathways.Nevirapine MedChemExpress Indeed, addition of nitrobenzene or hydroquinone improves yields in some cases.PMID:35112275
The reaction is not effective with secondary or tertiary alcohols, which fail to produce any ester under these conditions. Phenols are completely unreactive, confirming the high chemoselectivity for aliphatic alcohols over aromatic ones. Furthermore, polyfunctional alcohols like hexane-1,6-diol afford moderate yields of diesters, along with substantial amounts of monoester byproducts, indicating partial reactivity.
This methodology was successfully applied in the first total synthesis of monaspilosin, an aromatic ester isolated from *Monascus pilosus*, a fungus known for its antioxidant properties. The one-step synthesis from 4-(2-hydroxyethyl)phenol and benzyl cyanide provided the target compound in 73% yield, demonstrating the practical utility of this protocol in natural product synthesis.
In conclusion, the Lewis acid-promoted Pinner reaction using TMSOTf presents a robust, mild, and selective route to carboxylic esters from readily available alcohols and nitriles. Its compatibility with sensitive functional groups and operational simplicity make it a valuable tool in synthetic organic chemistry, particularly in the context of complex molecule assembly and late-stage functionalization.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com