Efficient Removal of Ruthenium Byproducts from Olefin Metathesis Products by Simple Aqueous Extraction

Link: Org Lett ASAP

Soon Hyeok Hong and Robert H. Grubbs*

The Arnold and Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125

The article presents a new way for removal of Ru-catalyst from an RCM reaction. Catalysts 3 and 4 were used to perform RCM reaction of 5 to give 6 and then level of Ru was measured against other known methods for removal of ruthenium. Catalyst 2 was also used in comparison of reactivity for substrate 5.

Catalyst 4 was the modified version of 3 where NHC part of the species was supported by poly(ethylene glycol) (PEG). Possessing PEG-bound NHC, 4 became soluble in water in addition to being soluble in typical RCM solvents such as toluene and CH2Cl2, while it was insoluble in other organic solvents such as ethers, isopropyl alcohol, and hexanes. This provided the basis for aqueous extraction to be more effective in removal of catalyst from product.

Before proceeding, reactivity of catalysts 2-4 was tested with diene 5 to make sure that having PEG-bound NHC in 4 would not affect the reactivity in RCM. Ans this was found to be the case, that all catalysts performed RCM in comparable capacities.

RCM of 5 was then performed with both catalysts 3 and 4 and the level of Ru in 6 was measured. The results are summarized below.

As seen in Table 1, entry 5, after RCM of 5 using 4, the use of aqueous washes alone removed Ru better than any other known methods. When aqueous washes were used in conjunction with tris(hydroxymethyl)phosphine (THMP) or activated carbon, level of Ru was effectively reduced to a small trace (entries 6 and 7). The use of PEG separately in the aquoues washes in RCM of 5 using catalyst 3 did not reduce level of Ru, indicating that PEG-bound NHC of 4 remained intact after the reaction and was crucial in assisting the removal of the catalyst.

The workup was very simple, after the RCM was complete, the reaction mixture was taken up in ether which was simply washed with water to give clear ether layer and brown aqueous layer. The crude product was further treated with other removal techniques to further reduce the trace of ruthenium species.