Cyclodimerization of Alkynes with Phosphine-Free Ruthenium Carbene Complexes:  Carbene Consumption by a Shunted Alkyne Oligomerization

Abstract

A new cyclodimerization of ruthenium carbenes and alkynes is reported. The cyclodimerization produces substituted cyclopentadienes and consumes the carbene complex. Cyclodimerization of alkynes is predominant at low alkene concentrations and was found to be competitive with enyne metathesis. The presence of the ortho alkyl ether in the ruthenium carbene complex appears to be a critical determinant for the formation of cyclopentadienes. At higher alkene concentration, the observed cyclodimerization can be suppressed, favoring enyne metathesis.

5.27 Ene–Yne Metathesis

“…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. Monosubstitution on the alkyne propargylic position enhanced conversion, but did not appreciably increase the yield (entries 3 and 4).…”

“…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).…”

“…The higher reactivity of the catalysts can lead to alkyne oligomerization either at low alkene concentrations or with alkenes that are not very reactive with the Grubbs complex. 45 (This can be gauged from inspection of the Grubbs selectivity model for alkene metathesis. The less reactive alkenes are in the type III and type IV family.…”

Polymerization of hydroxyacetylenes by ruthenium alkylidene complexes

“…The 1 H NMR spectrum of the product also showed additional signals that might be confirmed the formation of this type of products. The cyclodimerization of alkynes with phosphine-free ruthenium alkylidene complexes was previously observed by Diver and coworkers [27]. Unfortunately, separation of poly(2) from the cyclic products by reprecipitation was not feasible due to the similar solubility of the compounds.…”

Enyne‐Metathesis‐Based Tandem Processes

“…[58] Recently, Diver et al reported the ruthenium-carbene-mediated formation of cyclopentadiene 142 from terminal alkynes by two successive alkyne insertions that gave intermediate 141 (Scheme 34). [59] In the cyclopolymerization of 1,6-diynes, Buchmeiser and co-workers also found that dipropargylethylamine cyclodimerized to benzene derivative 143 in the presence of trifluoroacetate-modified Grubbs-Hoveyda catalyst 144 (Scheme 35). [60] Scheme 33.…”