Ethylene-Promoted versus Ethylene-Free Enyne Metathesis

Abstract

The role of ethylene in promoting metathesis of acetylenic enynes is probed within the context of ring-closing enyne metathesis, using first- and second-generation Grubbs catalysts. Under inert atmosphere, rapid catalyst deactivation is observed by calibrated GC-FID analysis for substrates with minimal propargylic bulk. MALDI-TOF mass spectra reveal a Ru(enyne)(2) derivative that exhibits very low reactivity toward both enyne and ethylene. Under ethylene, formation of this species is suppressed. Enynes with bulky propargylic groups are not susceptible to this catalyst deactivation pathway, even under N(2) atmosphere.

5.27 Ene–Yne Metathesis

“…For instance, alkynes can react exclusively with the initiator in a oligomerization reaction). 44,45 Early mechanistic studies of RCEYM suggested an 'alkylidene-first' reaction mechanism based on NMR studies. In RCM depicted in equation 8, Hoye recognized that there were two possibilities: reaction of [Ru]¼ CH 2 with the alkyne directly or initial transalkylidenation to generate an alkylidene 9 (Scheme 3).…”

“…55 Further insight into the ethylene effect has been provided by observations made using matrix-assisted laser desorption/ionization (MALDI) mass spectral analysis under inert atmosphere conditions. 44 Fogg and coworkers had developed mass spectral analysis as a tool for studying intermediates and decomposition products of ruthenium carbenes relevant to metathesis. The RCEYM of the malonate shown in Scheme 5 was considered in terms of both the ene-then-yne and the yne-then-ene pathway.…”

“…This pathway leads to higher molecular weight products that are intractable and explains the low mass balance observed in these reactions. (Fogg et al 44 also showed that the oligomerizations are not reversible using the first-generation Grubbs carbene 1. The observation of higher molecular weight oligomers obtained from alkyne oligomerization provides critical insight into the dichotomy of enyne metathesis versus alkyne oligomerization.)…”

“…Under conditions without ethylene ( Table 1), comparison of the first-and the second-generation Grubbs catalyst shows that each is susceptible to alkyne oligomerization (entries 1 and 2). 44 Alkyne oligomerization has been observed in EYM using the Hoveyda-Blechert catalyst. 45 Low conversions were seen after 24 h and remarkably very low yields of dienes were found, suggesting that little conversion to product had occurred.…”

“…68 Normally, geminal (1,1-disubstituted) alkenes are nonreactive in EYM because they do not initiate with 2 and cannot participate in the alkylidene-first mechanism. According to the alkylidene-first mechanism of enyne metathesis, 44,[46][47][48][49]51,69 the alkene must be able to react with the Grubbs catalyst. Yet by the Grubbs model, 70 1,1-disubstituted alkenes are unreactive with the second-generation Grubbs carbene.…”