Kinetic and Thermodynamic Analysis of Processes Relevant to Initiation of Olefin Metathesis by Ruthenium Phosphonium Alkylidene Catalysts

Abstract

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

Computational study of productive and non-productive cycles in fluoroalkene metathesis

“…Computational chemistry proved to be extremely valuable in the study of reaction mechanisms. In particular, the use of time-efficient DFT methods for the theoretical study of alkene metathesis has been extensively reviewed [ 9 – 11 ] and computational results have been found to agree well with recent experimental mechanistic studies based on easily initiating ruthenium precatalysts [ 12 – 13 ]. A theoretical approach has been also employed in attempts to gain a better insight into the complex structure of intertwined productive and non-productive catalytic cycles of alkene metathesis [ 14 ].…”

Olefin Metathesis, p‐Cresol, and the Second Generation Grubbs Catalyst: Fitting the Pieces

“…31 P NMR analysis of a β-methylstyrene (2 b) -methyl acrylate (1 b) (1 : 1) reaction mixture after being heated for 1 hour at 40°C in the presence of GII and p-cresol (12) (4 eq. relative to GII), revealed the presence of zwitterion 11 at δ P 31.5, protonated 11 at δ P 32.3 (Figure 1c) and a ruthenium benzylidene ( 13) complex with PCy 3 (δ P 28.9) (Figure 2, part-7, Figure 3 18), [28] was observed at m/z 307 (Supplementary Information). The latter indirectly confirmed the formation of the ruthenium methylidene (15) (Figure 3).…”

Diastereocontrol in Olefin Metathesis: the Development of Z ‐Selective Ruthenium Catalysts

“…Second, ruthenacycles (Chapter 8) are not structurally well defined compared to tungsta-and molybdacycles, which makes it difficult to design ligands that can influence the geometry of these important intermediates. Finally, related to the second point, ruthenacycles are highly dynamic, even at low temperatures, which further complicates ligand design [12][13][14][15][16][17][18]. Despite these challenges, notable successes in Z-selective metathesis using a large ligand strategy have recently been achieved by the Grubbs, Hoveyda, and Jensen groups [19][20][21][22] through the use of thiolate, sulfonate, and phosphate ligands (Scheme 3.2).…”