Oxidising agent and catalyst chirality effect on epoxidation of R-(+)-Limonene using Jacobsen-type catalysts

Abstract

The oxidising agent effect on R-(+)-Limonene epoxide diastereoselectivity using chiral and achiral Jacobsen’s type catalysts is presented. The type of oxidising agent strongly influences diastereoselectivity. Moderate diastereomeric excesses were achieved in the presence of oxidising agents prepared in situ but in the absence of catalyst (23% for DMD and 29% for O2/pivalaldehyde). Similar diastereomeric excesses were obtained with both chiral and achiral catalyst when the oxidising agents were prepared in situ; 56% and 50% excesses were obtained when using DMD for the chiral and achiral catalyst (respectively) and 38% using O2/pivalaldehyde for both catalysts. Diastereomeric excesses were not observed in the absence of catalyst when using commercial oxidising agents; the chiral catalyst presented larger diastereomeric excesses than its achiral counterpart: 65% and 38% excess using NaOCl for the chi-ral and achiral catalyst, respectively, and 79% and 39% using m-CPBA for the chiral and achiral catalyst, respective-ly. This suggests that at least one oxidant species, different from the conventionally accepted (MnV(oxo)), might be in-volved in this catalytic process. A modification of the traditional catalytic cycle is proposed considering the type of o-xidising agent. The catalyst’s chiral centre appears to govern asymmetric induction when commercial oxidising a-gents are used, whereas the R-(+)-Limonene chiral centre appears to govern asymmetric induction in the presence of in situ-prepared oxidising agents. On the other hand, the chemical stability of Jacobsen’s catalyst improved when in situ produced DMD was used as oxidising agent.

Keywords

• diastereoselectivity
• epoxidation
• R-(+)-Limonene
• oxidising agent type
• asymmetric induction
• chiral
• achiral