From Resting State to the Steady State: Mechanistic Studies of Ene–Yne Metathesis Promoted by the Hoveyda Complex

Abstract

The kinetics of intermolecular ene-yne metathesis (EYM) with the Hoveyda precatalyst (Ru1) has been studied. For 1-hexene metathesis with 2-benzoyloxy-3-butyne, the experimental rate law was determined to be first-order in 1-hexene (0.3-4 M), first-order in initial catalyst concentration, and zero-order for the terminal alkyne. At low catalyst concentrations (0.1 mM), the rate of precatalyst initiation was observed by UV-vis and the alkyne disappearance was observed by in situ FT-IR. Comparison of the rate of precatalyst initiation and the rate of EYM shows that a low, steady-state concentration of active catalyst is rapidly produced. Application of steady-state conditions to the carbene intermediates provided a rate treatment that fit the experimental rate law. Starting from a ruthenium alkylidene complex, competition between 2-isopropoxystyrene and 1-hexene gave a mixture of 2-isopropoxyarylidene and pentylidene species, which were trappable by the Buchner reaction. By varying the relative concentration of these alkenes, 2-isopropoxystyrene was found to be 80 times more effective than 1-hexene in production of their respective Ru complexes. Buchner-trapping of the initiation of Ru1 with excess 1-hexene after 50% loss of Ru1 gave 99% of the Buchner-trapping product derived from precatalyst Ru1. For the initiation process, this shows that there is an alkene-dependent loss of precatalyst Ru1, but this does not directly produce the active catalyst. A faster initiating precatalyst for alkene metathesis gave similar rates of EYM. Buchner-trapping of ene-yne metathesis failed to deliver any products derived from Buchner insertion, consistent with rapid decomposition of carbene intermediates under ene-yne conditions. An internal alkyne, 1,4-diacetoxy-2-butyne, was found to obey a different rate law. Finally, the second-order rate constant for ene-yne metathesis was compared to that previously determined by the Grubbs second-generation carbene complex: Ru1 was found to promote ene-yne metathesis 62 times faster at the same initial precatalyst concentration.

Comparison of the Spectroscopically Measured Catalyst Transformation and Electrochemical Properties of Grubbs’ First- and Second-Generation Catalysts

“…When Grubbs' catalysts 1 and 2 are dissolved in a suitable solvent, the phosphine (PCy 3 ) ligand dissociates during a slow kinetic process to eventually reach a dynamic equilibrium with the re-association of the phosphine 60 (see Scheme 1 for the reactions of 1 as an example (with additional transformations)). 72 This conversion of 1 and 2 in solution was studied by UV−vis spectroscopy. The experiments were conducted at 25 °C using 0.10 mM solutions of 1 and 2 in dry CH 2 Cl 2 under Ar.…”

Cp*RuCl‐Vinyl Carbenes: Two Faces and the Bifunctional Role in Catalytic Processes

“…Unlike typicalG rubbs-type vinyl carbenes,D FT calculations show that Cp*RuCl h 3 -vinylcarbenes are significantly more stable than its h 1 counterpart ( Figure 4), probably owing to a combination of geometrical restrictions imposed by the pianostool disposition of the ligands, the intramolecular steric repulsion between alkyl chains in the donor-donorc arbene and the increased stabilityo fa n1 8e À complex compared with an unsaturated 16 e À complex.A tt he same time, h 3 -coordination saturates the metal and would prevent (or seriously hamper) furthere volution throughm etathesis with other olefins or alkynes. [22,29] Having established the unique geometricala nd electronic properties of Cp*RuCl h 3 -vinylcarbenes, we now focus our at-Scheme2.Preparation of aR uh 3 -vinylcarbeneb yC AM. tention on their divergentr eactivity towards alkynals with the elucidation of the mechanistic pathways that lead to vinyl dihydrooxazines 2 or vinyl epoxypyrrolidines 3 depending on the aza-alkynal 1 used (Scheme1).…”

Ene-yne Cross-Metathesis for the Preparation of 2,3-Diaryl-1,3-dienes

“…Because it is a straightforward atom economic reaction, ene-yne cross metathesis has been applied as an efficient method for the construction of 1,3-dienes [4,[42][43][44][45]. Using ethylene as olefinic substrate allows the production of buta-1,3-dienes with two terminal methylene groups according to a catalytic cycle initially proposed by M. Mori [46] and further studied in more details by S. T. Diver [47,48]. However, if many examples involving ruthenium-catalysis have been described starting from aliphatic including benzylic and propargylic alkynes [49][50][51][52], much less data are available from aromatic alkynes in the Elimination reactions such as the thermal decomposition of 3,4-diaryldihydrothiophene-1,1-dioxides [27] and the acid-catalyzed dehydration of 1,2-diol derivatives [28] have been used for the synthesis of 2,3-diaryl-buta-1,3-dienes of type A. But-2-yn-1,4-diol biscarbonates have been efficiently coupled with aryl and heteroaryl boronic acids in the presence of a palladium catalyst to give 2,3-diarylated 1,3-dienes [1, 29,30].…”

“…Because it is a straightforward atom economic reaction, ene-yne cross metathesis has been applied as an efficient method for the construction of 1,3-dienes [4,[42][43][44][45]. Using ethylene as olefinic substrate allows the production of buta-1,3-dienes with two terminal methylene groups according to a catalytic cycle initially proposed by M. Mori [46] and further studied in more details by S. T. Diver [47,48]. However, if many examples involving ruthenium-catalysis have been described starting from aliphatic including benzylic and propargylic alkynes [49][50][51][52], much less data are available from aromatic alkynes in the literature.…”