Olefin Metathesis in Water at Room Temperature using …
Olefin Metathesis - Chemistry LibreTexts
As a highly effective means for creating carbon–carbon bonds, olefin metathesis is a privileged reaction in the armamentarium of synthetic and polymer chemists. Readily available and highly active ruthenium catalysts 1–4 have popularized metathesis chemistry in organic solvents ()., On the other hand, metathesis in aqueous solvents largely remains the domain of catalysts designed expressly for use in water ().,, Despite the allure of aqueous olefin metathesis for biological applications and green chemistry, the synthesis of these water-soluble ligands and complexes imposes barriers to their wider use. Developing aqueous reaction conditions suitable for catalysts such as 1–4 is an alternative approach. In the initial work with well-defined catalysts in aqueous systems, Grubbs and coworkers performed ring-openingmetathesis polymerization (ROMP) with 1 in aqueous emulsions, a method that allowed Kiessling and coworkers to synthesize biologically active glycopolymers. For ring-closing metathesis (RCM) and cross metathesis (CM), Blechert and coworkers employed commercially-available 2 and complex 6c, a relative of commercially-available 4, in water–methanol and water–DMF mixtures. Although they achieved high conversion in the ring-closure of N-tosyldiallylamine (8) in 3:1 water/methanol and 3:1 water/DMF over an extended reaction time of 12 h, these reactions were not accomplished in homogenous systems: neither the substrate nor the catalyst was dissolved completely in the aqueous phase. In most cases detailed in their report, the ruthenium complex was only sparingly soluble in the aqueous reaction mixture.
Olefin Metathesis: Theory and Practice
Metathesis in homogeneous aqueous systems would likely be faster and more versatile than in these heterogeneous systems. An effective system for metathesis with commercially available catalysts in homogeneous aqueous media would not only make this chemistry more accessible, but also highlight the limitations of the standard catalysts in water, informing catalyst-design efforts. Yet, reports of the use of common metathesis catalysts in an aqueous context are limited. For these reasons, we chose to test the capabilities of catalysts 1–4 in homogeneous aqueous media, and we report the results of our exploration herein. First, we screened various organic solvents as co-solvents for RCM of 8 in a homogeneous aqueous solution (). The solvents typically used for olefin metathesis reactions, such as CH2Cl2, 1,2-dichloroethane, and toluene, are immiscible with water, so we resorted to water-miscible solvents. Tetrahydrofuran (THF), used previously as a solvent for ROMP and acyclic diene metathesis with varying success, failed as a cosolvent for RCM. On the other hand, the ethylene glycol ether-based solvents dimethoxyethane (DME or glyme) and poly(ethylene glycol) (PEG) were excellent co-solvents. Their improved results with respect to THF and dioxane could relate to their better coordinating ability. Able to coordinate the tetracoordinate ruthenium complexes of the metathesis catalytic cycle, these ethers could more ably protect them from detrimental coordination by water. Grubbs and coworkers suggest that decomposition of metathesis intermediates in water results from water coordination at ruthenium in the methylidene-propagating species. Interestingly, these results show that the protective environment of the PEG-bearing ligand in 6a can also be provided by ethylene glycol ethers in the bulk solvent.