Impact of Ethylene on Efficiency and Stereocontrol in Olefin Metathesis: When to Add It, When to Remove It, and When to Avoid It

Abstract

Ethylene is the byproduct of olefin metathesis reactions that involve one or more terminal alkenes. Its volatility is one reason why many cross‐metathesis or ring‐closing metathesis processes, which are reversible transformations, are efficient. However, because ethylene can be converted to a methylidene complex, which is a highly reactive but relatively unstable species, its concentration can impact olefin metathesis in other ways. In some cases, introducing excess ethylene can increase reaction rate owing to faster catalyst initiation. Ethylene and a derived methylidene complex can also advantageously inhibit substrate or product homocoupling, and/or divert a less selective pathway. In other instances, a methylidene's low stability and high activity may lead to erosion of efficiency and/or kinetic selectivity, making it preferable that ethylene is removed while being generated. If methylidene decomposition is so fast that there is little or no product formation, it is best that ethylene and methylidene complex formation is avoided altogether. This is accomplished by the use of di‐ or trisubstituted alkenes in stereoretentive processes, which includes adopting methylene capping strategy. Here, we analyze the different scenarios through which ethylene and the involvement of methylidene complexes can be manipulated and managed so that an olefin metathesis reaction may occur more efficiently and/or more stereoselectively.

Ring‐Closing Olefin Metathesis Catalyzed by Well‐Defined Vanadium Alkylidene Complexes

“…Both syn ‐ and anti ‐alkylidene signals of 1 d and the new alkylidene slowly disappeared over a few hours, suggesting decomposition of the active species. Catalytic reactions were conducted in open vials to allow escape of ethylene gas from the reaction mixture [34] . Reactions in closed vials exhibited a lower conversion of 2 to 3 in all cases (entries 1–5, Table 1), confirming the catalyst's limited stability in the presence of ethylene.…”

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

“…It is however noteworthy that in few cases the presence of ethylene can be used to accelerate the metathesis of internal olens. 191 This is particularly pronounced for the tungsten oxo cationic catalysts that contain a bulky NHC stabilizing ligand, where ethylene is thought to help interconversion of the metal centre between resting and active states. 126 Moreover, detailed NMR studies have revealed the parallel between the NMR spectroscopic signatures of molecular and silica-supported TBP and SP metallacyclobutanes, pointing in particular to the observation of very deshielded a-carbon and shielded b-carbon in TBP structures.…”

“…It is however noteworthy that in few cases the presence of ethylene can be used to accelerate the metathesis of internal olefins. 191 This is particularly pronounced for the tungsten oxo cationic catalysts that contain a bulky NHC stabilizing ligand, where ethylene is thought to help interconversion of the metal centre between resting and active states. 126 …”

The Merger of Isocyanide‐Based Multicomponent Reaction and Ring‐Closing Metathesis (IMCR/RCM)

“…Interestingly, the best results for RCM were obtained when peptide solutions of 120 c and 120 d were added to the refluxing catalyst solution under the N 2 stream to give the product 121 c and 121 d in 51 and 73 % respectively. It was also found that the ethylene, a byproduct of olefin metathesis, can impact the rate of RCM negatively, [62] thus its removal with the help of N 2 stream played a notable role in the yield of RCM. The yield of 121 c was increased from < 5 % to 34 % by bubbling nitrogen gas through the reaction.…”