Mg(ClO₄)₂ as a Powerful Catalyst for the Acylation of Alcohols under Solvent-Free Conditions

Abstract

A trace amount of magnesium perchlorate (from 0.1 mol% to 1 mol%) is able to promote quantitative acylation, with anhydrides, of a large variety of functionalized alcohols in short times, at room temperature and under solvent-free conditions.

acylations - alcohols - magnesium perchlorate - anhydrides - solvent-free reactions

References

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All experiments were carried out in air. Identification of all products was performed by comparison with literature data. Chemicals were obtained from Aldrich and Fluka and were used without further purification, with the exception of Mg(ClO₄)₂. In fact, Mg(ClO₄)₂ from Aldrich resulted to be very active only when a freshly opened bottle was used. In other cases, a lowering in activity was observed, perhaps owing to the increase in water content of the salt by exposure to airborne moisture. For sake of reproducibility of experimental data, a prior drying of Mg(ClO₄)₂ was considered necessary. Hence, in spite of the reported recommendations, [21] 0.200 g of commercial Mg(ClO₄)₂ were carefully dried in vacuo (0.1 Torr) for 2 h at a maximum of 140 °C to avoid the possibility of decomposition. [16] After cooling, 9.26 mL of Ac₂O were added and stirred at room temperature until the salt was completely dissolved. This solution was used until depletion for a series of experiments without loss of activity.
Typical acylation experiment: To 1 mL of the 0.096M solution of Mg(ClO₄)₂-Ac₂O the substrate (9.6 mmol) was added dropwise, when liquid, or in small portions, when solid. The reaction was monitored by GC. After the reaction was complete, aqueous NaHCO₃ was added and the product was extracted with Et₂O. The organic layer was dried over MgSO₄ and the solvent was evaporated to give the pure ester.

<A NAME="RG28802ST-14">14</A>

The control on the occurrence of possible racemization and epimerization processes was made by comparing the [α]_D values of the final esters with those reported in the literature.

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For example LiClO₄ is used in large excess as an activator or co-activator in various chemical processes, such as Friedel-Crafts acylation (see ref. 18), Diels-Alder cycloaddition (see ref. 19) and addition to carbonyl compounds (see ref. 20).

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<A NAME="RG28802ST-18C">18c</A> Kawada A. Mitamura S. Matsuo J. Tsuchiya T. Kobayashi S. Bull. Chem. Soc. Jpn.  2000,  2325 

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<A NAME="RG28802ST-19B">19b</A> Forman MA. Dailey WP. J. Am. Chem. Soc.  1991,  113:  2761 

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<A NAME="RG28802ST-21">21</A> Casaschi A. Desimoni G. Faita G. Gamba Invernizzi A. Lanati S. Righetti PP. J. Am. Chem. Soc.  1993,  115:  8002