N-Heterocyclic carbene–ruthenium complexes synthesized by a hydrogen-bonding-assisted strategy for dehydrogenative rearrangement of esters

Abstract

Simplifying the synthetic procedures of transition-metal complexes is of great significance for homogeneous catalysis. Herein, a new, efficient, and practical method has been developed for synthesizing N-heterocyclic carbene (NHC)–ruthenium complexes by reacting benzimidazolium salts bearing iodine-anions with 1,2-butylene oxide. This method has the following advantages: (1) it avoids the dehydration procedure of organic solvents by employing unpurified 1,2-butylene oxide as a hydrogen-bond initiator, (2) it lowers operational requirements by enabling the reaction to proceed under an air atmosphere, and (3) it avoids the use of bases and other additives compared with traditional methods, making it atom efficient, low-cost, and sustainable. Mechanism investigation via electrospray ionization-high resolution mass spectrometry (ESI-HRMS) and in situ infrared spectroscopy revealed that benzimidazolium salts bearing iodine-anions react with epoxides to generate hydrogen-bonded adducts, and these adducts then split into NHC and 1-iodobutan-2-ol by-products. The generated NHC then coordinates with the ruthenium or iron precursors to form NHC–ruthenium complexes. The NHC–ruthenium complexes were effective in the dehydrogenation–rearrangement reaction of esters. This protocol exhibits a wide scope of ester substrates, including aryl, alkyl, and cyclic groups, producing various 1,3-diketones in high yields.