Hydration and acetoxylation of monoterpenes catalyzed by heteropoly acid

Abstract

The liquid-phase hydration and acetoxylation of limonene ( 1), β-pinene ( 2) and α-pinene ( 3) catalyzed by dissolved or silica-supported heteropoly acid H 3PW 12O 40 (PW) in acetic acid and acetic acid/water solutions have been studied. All three substrates give α-terpineol ( 4) as the main product along with α-terpenyl acetate ( 5). The reaction rate increases in the order: limonene<α-pinene<β-pinene. Synthetically useful homogeneous and heterogeneous acetoxylation and hydration of 1, 2 and 3 into 4 and 5 have been developed. At room temperature under optimized conditions, 2 and 3 form a mixture of 4 and 5 with 85% selectivity at 90% substrate conversion. 1 gives 4 and 5 with 85% selectivity at 50% conversion, with the main product being ester 5 in acetic acid ( 4/ 5≈30/70 ) and alcohol 4 in HOAc/H 2O (90/10 v/v) solutions ( 4/ 5≈85/15 ). Virtually no oligomerization of monoterpenes occurs under the optimized conditions. The catalyst can be separated without neutralization and may be reused. The PW shows a much higher catalytic activity than conventional acid catalysts such as H 2SO 4 and Amberlyst-15.