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 non-toxic isopropenyl acetate allowed an initial irreversible mono-transesterification 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 by-product. 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 p...