Synthesis of Heavy Fluorous Ruthenium Metathesis Catalysts Using the Stereoselective Addition of Polyfluoroalkyllithium to Sterically Hindered Diimines

Abstract

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.

Nickel and palladium complexes with fluorinated alkyl substituted α-diimine ligands for living/controlled olefin polymerization

“…2,6-Dimethyl-4-(1H,1H,2H,2H-perfluorooctyl)aniline (A1) and 2,6-dimethyl-4-perfluorohexylaniline (A2) were obtained according to the method described in the literature. 36,37 Methanol and ethanol for the reaction were purchased from Penta s.r.o. and used without any other purification or drying.…”

“…Aniline A1 with an ethylene spacer between the aromatic ring and a C 6 F 13 moiety was prepared according to the known procedure in a four-step synthesis starting from commercial 2,6-dimethylaniline. 36 The same starting aniline was also modified in the para-position with the C 6 F 13 moiety in a two-phase metal free reaction 37 to obtain A2. Fluorine atoms in the α-position of the perfluoroalkyl chain attached in the ortho-or para-position of aniline are known to be highly reactive against nucleophilic substitution.…”

Synthesis of metathesis catalysts with fluorinated unsymmetrical N,N’-diaryl imidazoline-based NHC ligands

“…Meanwhile, the number of reports on metathesis catalysts decorated with fluorinated NHC ligands is extremely limited [64][65][66][67]. For instance, Fürstner et al described an unsymmetrical complex with C6F13(CH2)2 group at one of the imidazolyl nitrogen to increase the solubility of the catalyst in supercritical CO2 [68].…”

Octa‐1,7‐diene‐4,5‐diamine Derivatives: Useful Intermediates for the Stereoselective Synthesis of Nitrogen Heterocycles and Ligands for Asymmetric Catalysis

“…Significant examples of carbene complexes obtained through the preliminary addition of alkyl-, vinyl and aryllithium and magnesium reagents to glyoxal diimines are shown in Figure 1. [8][9][10][11][12][13] In all these cases, the C-substituents in the imidazolium ring have a trans relationship, which is a consequence of the moderate to excellent diastereoselectivity occurring in the organometallic additions to the precursors diimines. Surprisingly, the addition of allylmagnesium chloride to the diimine 10 (Mes = 2,4,6-trimethylphenyl) provided, in moderate yield; the mesodiamine 11, which precipitated from the reaction mixture upon standing at room temperature.…”

“…The vicinal diamines are then converted into carbene complexes by a sequence of steps involving the formation of imidazolium salts and metal complexation. Significant examples of carbene complexes obtained through the preliminary addition of alkyl‐, vinyl and aryllithium and magnesium reagents to glyoxal diimines are shown in Figure …”