Ring‐Closing Metathesis of Aliphatic Ethers and Esterification of Terpene Alcohols Catalyzed by Functionalized Biochar

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Ring-closing C-O/C-O metathesis of ethers with primary aliphatic alcohols

In canonical organic chemistry textbooks, the widely adopted mechanism for the classic transetherifications between ethers and alcohols starts with the activation of the ether in order to weaken the C-O bond, followed by the nucleophilic attack by the alcohol hydroxy group, resulting in a net C-O/O-H σ-bond metathesis. In this manuscript, our experimental and computational investigation of a Re₂O₇ mediated ring-closing transetherification challenges the fundamental tenets of the traditional transetherification mechanism. Instead of ether activation, the alternative activation of the hydroxy group followed by nucleophilic attack of ether is realized by commercially available Re₂O₇ through the formation of perrhenate ester intermediate in hexafluoroisopropanol (HFIP), which results in an unusual C-O/C-O σ-bond metathesis. Due to the preference for the activation of alcohol rather than ether, this intramolecular transetherification reaction is therefore suitable for substrates bearing multiple ether moieties, unparalleled by any previous methods.