Olefin metathesis is a powerful carbon-carbon bond formation reaction in both polymer and small molecule synthesis.¹ In particular, the recent development of ruthenium olefin metathesis catalysts, which show high activity and functional group tolerance, has expanded the scope of olefin metathesis. However, performing olefin metathesis in aqueous media is still challenging due to the lack of a stable and active catalyst soluble in water. Aqueous olefin metathesis has the economic, environmental, and processing benefits of both homogeneous aqueous catalysis and aqueous two-phase catalysis.² Further, aqueous olefin metathesis is critical for some biological applications of olefin metathesis.

With the goal of developing a homogeneous catalyst that displays increased activity and stability, our group has reported several water-soluble catalysts, such as 1,³ 2,³ and 3.⁴ Catalysts 1-3 are unable to mediate the ring-closing metathesis (RCM) reaction of simple ,-dienes in water and show limited RCM activity in methanol. Moreover, they do not show any activity in cross-metathesis (CM) reactions in protic media. The recently developed catalyst 3 containing an N-heterocyclic carbene (NHC)-based ligand shows improved activity in aqueous ring-opening metathesis polymerization (ROMP) reactions.⁴ Appending poly(ethylene glycol) (PEG) to the nondissociating NHC ligand allows catalyst 3 to remain in solution throughout the entire metathesis reaction. However, having the PEG-carbamoyl-benzyl group as a pendant group of NHC limited the stability of the complex 3. Earlier studies have shown that 1,3-diaryl group of NHCs in (NHC)(L)(Cl)₂Ru=CHPh-type complexes are important for catalyst stability.⁵ As part of the ongoing effort to use PEG as a water solubilizing moiety, we have developed the novel water soluble catalyst 4 which shows improved stability and activity in water. Appending PEG on the backbone of saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (H₂IMes) ligand renders catalyst 4 soluble in organic solvents, such as dichloromethane and toluene, as well as water, with maintaining stability and activity of well-known H₂IMes-based ruthenium metathesis catalysts.⁶

Catalyst 4 was prepared in three steps from readily available starting materials. PEG-attached diamine 7 was synthesized using an S_N2-type reaction between N,N'-dimesityl-2,3-diamino-1-propanol 5⁷ and PEG mesyl methyl ether 6. The diamine 7 was subsequently converted to the corresponding imidazolium salt 8 through condensation with triethyl orthoformate in the presence of ammonium tetrafluoroborate. Deprotonation of 8 with potassium bis(trimethylsilyl)amide (KHMDS) followed by the addition of Ru complex 9 generated the desired catalyst 4 (Scheme 1). Catalyst 4 was purified by column chromatography followed by precipitation from dichloromethane into diethyl ether. Attempts to synthesize a phosphine-containing version of this catalyst were unsuccessful.

Scheme 1. Synthesis of Catalyst 4

Complex 4 is stable in water. In the ¹H NMR recorded in D₂O, no signal corresponding to the benzylidene proton (Ru=CHPh, 16.4 ppm, CD₂Cl₂) was observed. Initially, this was believed to be either due to deuterium exchange of the benzylidene proton^3d or from rapid decomposition of 4 in D₂O. However, upon extracting the catalyst with CD₂Cl₂ from the D₂O solution, the benzylidene peak reappeared. Even after 1 week in D₂O, the ¹H NMR spectra, after CD₂Cl₂ extraction, were not significantly altered, showing stability of this catalyst in water.⁸ This type of solvent-dependent NMR behavior has been reported in micelle-type complexes.⁹ We believe that catalyst 4 aggregates could form a micelle-like structure in D₂O due to the hydrophilic PEG chain and hydrophobic ruthenium center.¹⁰

As an activity comparison, we examined the ROMP of endo-norbornene monomer 10 with catalysts 2, 3, and 4.¹¹ As shown by Figure 1, catalyst 4 showed much improved activity when compared to other water-soluble catalysts.¹² This is consistent with past results as saturated NHC-based ruthenium olefin metathesis catalysts are known to be more active than phosphine-based and unsaturated NHC-based catalysts.^5,6

Figure 1 A comparison of the ability of water-soluble catalysts to polymerize endo-monomer 10 (data for catalyst 2 and 3 are obtained from ref 4).

RCM reactions of water-soluble ,-dienes have been highly challenging due to instability toward water of the Ru methylidene species generated after the first catalytic turnover.^3b There have been a few reports of RCM of ,-dienes in water.¹³ However, the reported reactions either involved water-insoluble substrates or water-insoluble catalysts.¹³ The actual metathesis reactions in these systems are believed to occur in organic-friendly environments, such as inside solid supports, as a decrease in activity is observed with water-soluble ,-dienes. The best reported conversion of RCM of diallylamine hydrochloric acid salt 16 in water was just 11% at 45 C.^13b In homogeneous systems, there has been no report of the RCM of the ,-dienes in aqueous media.¹⁴

Catalyst 4 showed unprecedented RCM activity with water-soluble ,-dienes in water yielding the corresponding five- and six-membered rings in good to excellent yields (Table 1). RCM of 12 and 14 produced the corresponding five-membered and six-membered ring compounds, 13 and 15, quantitatively (entries 1 and 2). In RCM of 16, cycloisomerized product 18 was observed along with the major metathesis product 17 (entry 3). This type of cycloisomerization has previously been observed during olefin metathesis, presumably by ruthenium hydrides from catalyst decomposition.^13b,15 For the other substrates (entries 1, 2, 4, and 5), the corresponding cycloisomerized products were not observed. Allyl-2-methylallylamine hydrochloride 19 was cyclized to generate a trisubstituted olefin 20 with relatively lower yield (entry 4). For reasons not yet fully understood, RCM of diallyldimethylamine chloride 21 was not successful (entry 5).

Cross-metathesis is also challenging in aqueous media. To the best of our knowledge, there have been no reports of homogeneous cross-metathesis in water. The Blechert group demonstrated homodimerization of allyl alcohol 23 in D₂O up to 80% conversion using the aforementioned heterogeneous catalyst system.^13b Catalyst 4 shows excellent activity in homodimerization of 23 and the self-metathesis of cis-2-butene-1,4-diol 24 in water (Table 2). However, cross-metathesis reaction with catalyst 4 is highly substrate dependent. 4 is unable to homodimerize vinylacetic acid, allylamine hydrochloride, and other water-soluble olefins derived from carboxylic acid and quaternary ammonium salts. Variations of pH using DCl or NaOD solutions did not improve the cross-metathesis activity of catalyst 4.

A novel water-soluble catalyst which is active and stable in water has been developed. This catalyst shows unprecedented activity in ROMP, RCM, and CM in aqueous media.

Acknowledgment

We would like to thank Jason P. Jordan for generous donation of substrate 12 and helpful discussions. The National Institutes of Health (5R01GM068647) is acknowledged for financial support.