Intramolecular C-H insertion reactions of boroxy Fischer carbene complexes.: Regio- and diastereoselective modification of terpenes

In-situ-generated dialkylboroxy and diaminoboroxy Fischer carbene complexes lead to oxaborolane or oxazaborolidine derivatives via an intramolecular C-II insertion reaction. Further oxidation of these intermediates yields 1,3-diol or 1,2-amino alcohol derivatives. Diastereoselectivities as high as 99% are reached when starting from boroxy Fischer carbene complexes derived from terpenes, which represents a regio- and diastereoselective modification of this type of natural product. The influence of the C-beta substituents relative to the boron atom on the reaction path is studied, and a mechanism is proposed after identification of an intermediate by NMR. In addition, theoretical calculations show the presence of a boron-metal interaction, which could result as key step to the C-H insertion reaction.