Efficient isomerization of α-pinene oxide to campholenic aldehyde promoted by a mixed-ring analogue of molybdenocene

Abstract

The MoIV complex [(eta5-indenyl)(eta5-cyclopentadienyl)Mo(MeCN)2](BF4)2 (1) has been used to promote two acid-catalyzed epoxide ring-opening reactions under ambient conditions. The alcoholysis of styrene oxide in neat ethanol gave 2-ethoxy-2-phenylethanol in quantitative yield within 10 min. The use of an ionic liquid (IL) as cosolvent benefitted catalyst solubility and recycling while not impairing catalytic performance. Complex 1 in 1,2-dichloroethane was effective for the isomerization of alpha-pinene oxide to campholenic aldehyde (CPA), leading to 87% yield at 1 h reaction. The same yield could be achieved within 1 min by using the IL [Choline][NTf2] as solvent. CPA yields at 1 min reached near-quantitative values (98%) upon recycling of the catalyst/IL mixture, demonstrating an unparalleled combination of activity, selectivity and recyclability for this commercially important reaction. Considering the catalytic features of the 1/IL system, a CPA process flow diagram is proposed and compared to patented technology.