The stereoselective addition of 2-(perfluorohexyl)ethyllithium to moderately hindered diimines led to racemic diamines, which were further transformed to light or heavy fluorous analogues of Hoveyda−Grubbs second-generation precatalysts. The complex bearing the NHC ligand modified with four polyfluoroalkyl ponytails represents the first known example of an alkene metathesis precatalyst retaining its heavy fluorous properties in the active catalytic form. The synthesized complexes match the activity and stability of a commercial Hoveyda−Grubbs second-generation precatalyst in model RCM reactions forming tri-and tetrasubstituted double bonds. The fluorophilic catalyst was successfully recycled using heavy fluorous separation techniques.
■ INTRODUCTIONFluorous chemistry is firmly established as an essential part of organic and organometallic chemistry. It uses orthogonal properties of polyfluorinated ponytails for the recycling of homogeneous catalysts, and the separation of products or byproducts of organic reactions, reagent scavengers, etc. 1 The recycling of ruthenium metathesis precatalysts remains a controversial issue, with some authors supporting 2 and other contesting 3 the essential release−return mechanism. However, heavy fluorous ruthenium precatalysts (i.e., the complexes with higher solubility in perfluorinated solvents than in common organic solvents) can benefit from greater hydrophobicity of the catalytic center, a lower ruthenium content in metathesis products, and better separation of catalyst decomposition byproducts and have potential applications in flow systems. Although several authors attempted to synthesize heavy fluorous alkene metathesis catalysts based on a Grubbs or Hoveyda−Grubbs second-generation precatalyst framework, they found that even three polyfluoroalkyl ponytails of average length are not sufficient to provide the desired properties. 4 The first heavy fluorous precatalyst known thus contains a phosphine ligand modified with three (perfluorodecyl)ethyl ponytails (Chart 1). 5 A two-phase perfluorocarbon/hydrocarbon solvent system can be used to accelerate the initiation of the precatalyst. However, the active form of this precatalyst is not fluorophilic. We recently reported that substitution of polyfluoroalkyl chains for polyfluoropolyoxaalkyl chains results in a surprisingly higher fluorophilicity in both imidazolium saltbased ionic liquids 6 and silver NHC complexes. 7 Accordingly, we found that Hoveyda−Grubbs second-generation precatalyst analogues bearing one perfluoroalkyl and two perfluoropolyoxaalkanoate chains have heavy fluorous properties (Chart 1). 8 However, the activity and stability of these precatalysts proved to be significantly inferior to those of the parent precatalysts, and we hence concentrated on the fluorous modifications of the NHC imidazolidinylidene ring.
