Diastereoselective synthesis of cis‐1,2‐limonene oxide using dimeric Salen‐Mn (III) complexes as reusable catalysts

Diastereoselective epoxidation of R‐(+)‐limonene using achiral and racemic dimeric Salen‐Mn (III) complexes as catalysts ((1a) and (1b)) and in situ generated dimethyldioxirane (DMDO) as an oxidizing agent was explored. The best reaction parameters were: (i) KHSO5/R‐(+)‐limonene molar ratio = 0.25; (ii) R‐(+)‐limonene, catalyst molar ratio = 20, (iii) absence of nitrogenous bases (axial ligands), (iv) ambient temperature (20°C), (v) racemic dimeric catalyst, and (vi) low amount of acetone (4 mL). Under these reaction conditions isolated yield to 1,2‐(+)‐limonene oxide and diastereomeric excess (d.e), and diastereomeric yield excess (d.y.e) to major diastereomer (cis‐epoxide) was 96%, 77%, and 72%, respectively. Moreover, the catalyst was segregated into a solid phase, while products remained in the liquid phase, allowing the easy separation of the catalyst and reaction products. Consequently, the catalyst could be recycled up to three times without appreciable loss of its initial catalytic activity. We report the asymmetric epoxidation of R‐(+)‐limonene using a dimeric racemic Salen Mn (III) complex (5 mol%) in a mixture of KHSO5 and substrate in the ratio 1:4, respectively, in acetone at ambient temperature to give cis‐1,2‐(+)‐limonene oxide with d.e and d.y.e of 77% and 72%, respectively. Remarkably, the catalyst was easily separated and reused up to four cycles without appreciable loss of catalytic activity in the absence of co‐catalysts and nitrogenous bases.