Upgrading of limonene and other terpenes by palladocatalyzed C–H activation

From a structural point of view, limonene – like other terpenes – is composed only of C–C and C–H bonds, making it an ideal biosourced substrate for the development of C–H activation reactions. In the literature, two C–H functionalization reactions of limonene – but not involving a CDC reaction – have been reported. Watson's group in 2018 describes a palladocatalyzed (boryl-Heck) C–H borylation of the exocyclic limonene alkene. The mechanism of this reaction is thought to involve an oxidative addition of the B bond–Br followed by carbopalladation of the resulting Pd(II) complex onto the alkene. This is followed by β-H elimination and reductive elimination of the Pd(II) hydride formed.