Highly Active Ammonium-Tagged Olefin-Metathesis Catalyst for Simplified Purification

Abstract

When an 2-isopropoxy-substituted benzylidene ligand of a Hoveyda-type Ru-metathesis complex is modified by an additional 4-diethylamino group and treated wit PTSA it yields a highly active precatalyst suitable for RCM, cross metathesis, and enyne metathesis. The important property of this ammonium tagged Rucomplex is associated with the possibility of its easy removal by a simple filtration protocol.During recent years, olefin metathesis using modern ruthenium catalysts has become a key reaction in organic chemistry. 1 A principal problem for applications outside academic laboratories is associated with heavy-metal impurities in the products and their efficient removal. 2 Ruthenium hydride complexes, formed after decomposition of metathesis catalysts can also lead to unwanted side reactions during product isolation and purification. 3 Several protocols such as scavengers, biphasic extraction, and silica gel chromatography have been proposed to overcome these problems. Another strategy to make Ru-based olefin-metathesis technology more economical is the immobilisation of these homogeneous catalysts on a solid phase or in ionic liquids. 4 We demonstrated that the 5-and 4-nitro-substituted complexes 6a,b (Figure 1) initiate olefin metathesis dramatically faster than the parent Hoveyda-Grubbs catalyst 3b. 5 We proposed that the electron-withdrawing group (EWG) in the benzylidene fragment of 6 decreases the electron density on the chelating oxygen of the isopropoxy group and weakens the O→Ru coordination, facilitating faster initiation of the catalytic cycle.In accordance with this assumption, we observed that complex 7 (Scheme 1), bearing the electron-donating (EDG) diethyl amino group shows low or no activity in olefin metathesis. 6a However, in a striking contrast, quaternisation of the nitrogen atom either by alkylation or protonation leads to ammonium cations with electronwithdrawing properties, such as 8 and 10. Both these salts show excellent activity, surpassing the parent HoveydaGrubbs complex 3b in terms of initiation speed. 6a,b In addition, the conversion of the Et 2 N group into polar quaternary ammonium salt, yielded the resulting catalyst 8 (R = Me, X = I), is suitable for applications in aqueous solvents. 7 Unfortunately, quaternisation of 7 with MeI or

Cationic Bis‐N‐Heterocyclic Carbene (NHC) Ruthenium Complex: Structure and Application as Latent Catalyst in Olefin Metathesis

“…[23] However, under acidic conditions, a faster conversion rate was observed with the precatalyst 2 c, which led in situ to an activated benzylidene ether based catalyst by means of the electron-donating to electronwithdrawing switch through the protonation of the basic dimethylamine function. [24] It is important to note that this type of acid-mediated metathesis catalytic activation has only recently been described with a ruthenium complex that bore two nonidentical NHC ligands, in which one is a mesoionic carbene (MIC) that possesses an unsubstituted nitrogen atom prone to react with a molecule of acid. [22b, 25] In the present example, the strong potential for protonolysis could be attributed to the more basic cycloalkylamine moiety.…”

Easily removable olefin metathesis catalysts

“…1). 12,13,14 It was shown that simple filtration of the reaction mixture through silica gel results in lowering the amount of catalyst impurities in the crude products, however in the most cases the residual Ru level was still too high for pharmaceutical applications. 12 This problem is probably related to placing of the quaternary ammonium group in the benzylidene part which dissociates during metathesis act.…”

Cross metathesis of unsaturated epoxides for the synthesis of polyfunctional building blocks

“…To date, 1 has been scarcely used in olefin cross metathesis transformations. In some examples, Grela used 1 as a test substrate to evaluate the efficiency of new catalysts [ 26 ], and Cossy prepared vinyl functionalized oxazoles [ 27 ]. To our knowledge, the cross metathesis of 1 with electron-deficient olefins has not been reported.…”