Acid-Catalyzed Reactions of Isopropenyl Esters and Renewable Diols: A 100% Carbon Efficient Transesterification/Acetalization Tandem Sequence, from Batch to Continuous Flow

A new acid-catalyzed tandem sequence was investigated for the upgrading of renewable 1,2-diols such as propylene glycol (PG) and ethylene glycol (EG), with isopropenyl esters. For example, at 50 °C and in the presence of Amberlyst-15, the reaction of PG with nontoxic isopropenyl acetate allowed an initial irreversible monotransesterification of the diol, releasing acetone which then promoted acetalization on a second molecule of the glycol. The overall protocol was 100% carbon efficient, affording water as the sole byproduct. The reaction scope was extended to higher homologues of enol esters as isopropenyl-octanoate and phenylbutyrate. Additionally, the tandem sequence was successfully transferred in the continuous-flow (CF) mode where the catalyst (Amberlyst-15) could be used virtually indefinitely without loss of performance, and the solvent (THF or CPME) was quantitatively recovered and reused. Under CF conditions, the reaction of PG with isopropenyl acetate could be run at 30 °C and atmospheric pressure with a (nonoptimized) productivity up to 9.7 mmol gcat-1 h-1, 3 times higher than that achieved in the batch mode. When ethylene glycol was used, a lower tandem selectivity was observed due to predominance of transesterification products, mono- and diesters, over the acetal compound.