Highly Chemoselective Methylation and Esterification Reactions with Dimethyl Carbonate in the Presence of NaY Faujasite.The Case of Mercaptophenols,Mercaptobenzoic Acids,and Carboxylic Acids Bearing OH Substituents

摘要

In the presence of NaY faujasite,the reactions of dimethyl carbonate (DMC) with several ambident nucleophiles such as o- and p-mercaptophenols (la,b),o- and p-mercaptobenzoic acids (2a,b),o- and p-hydroxybenzoic acids (3a,b),mandelic and phenyllactic acids (4,5),have been explored under batch conditions.Highly chemoselective reactions can be performed: at 150 deg C,compounds 1 and 2 undergo only a S-methylation reaction,without affecting OH and CO_2H groups;at 165 deg C,acids 3-5 form the corresponding methyl esters,while both their aromatic and aliphatic OH substituents are fully preserved from methylation and/or transesterification processes.Typical selectivities are of 90-98% and isolated yields of products (S-methyl derivatives and methyl esters,respectively) are in the range of 85-96%.A comparative study with K_2CO_3 as a catalyst is also reported.Although the base (K_2CO_3) turns out to be more active than the zeolite,the chemoselectivity is elusive: compounds 2a,b undergo simultaneous S-methylation and esterification reactions,and acids 3-5 yield complex mixtures of products of O-methylation,O-methoxycarbonylation,and esterification of their OH and CO_2H groups,respectively.Overall,the combined use of a nontoxic reagent/solvent (DMC) and a safe promoter (NaY) imparts a genuine ecofriendly nature to the investigated synthesis.