Since the first investigations in the 1960s, olefin metathesis reactions have become a powerful tool in both organic chemistry and industrial processes (e.g. Phillips Triolefin Process,18 Shell Higher Olefin Process,19 Elevance Process20) to catalytically convert olefinic compounds. The mechanism was revealed by Hérisson and Chauvin, proposing the formation of a metal alkylidene and a metalla-cyclobutane as intermediates of the olefin metathesis equilibrium (Figure 1.5).21
Figure 1.5. Mechanism of the olefin metathesis reaction proposed by Hérisson and Chauvin.21
Figure 1.6. Common olefin metathesis catalyst precursors used for olefin metathesis reactions.26
While first catalytic systems were based on early transition metals like tungsten and molybdenum salts with co-catalysts like tin or aluminum alkyls, Schrock and coworkers developed well-defined W and Mo based alkylidene catalyst precursors (Figure 1.6).22,23 However, the commonly high oxophilicity of these early transition metal based catalysts makes them sensitive toward oxygen and moisture, limiting conversion of functional group containing monomers (though recently examples of Mo based catalyst precursors tolerating also functionalized substrates have been reported24). Grubbs and coworkers synthesized the first well-defined metathesis-active ruthenium alkylidene complexes from the L2X2Ru=CHR family,25 with commercially available Grubbs 1st generation alkylidene (G1) as the most
prominent representative, allowing for the conversion of functionalized olefins. The introduction of N-heterocyclic carbenes (substituting one phosphine ligand) further improved the performance of the complexes, resulting in increased functional group tolerance and reactivity (Grubbs 2nd generation alkylidene, G2). By substitution of a phosphine ligand via intramolecular coordination of the ether oxygen atom of an ortho-isopropoxyphenylmethylene alkylidene moiety (Hoveyda-Grubbs 1st and 2nd generation alkylidenes, HG1 and HG2), the chelating effect on the one hand decreases the initiation rate (compared to e.g. G2), but also improves thermal stability as well as oxygen and moisture tolerance of the reactive species within the olefin metathesis reaction.26
Figure 1.7. Common olefin metathesis reactions.
Various chemical olefin transformations are possible via metathesis reactions.27 In ring-opening metathesis (ROM) reactions, cyclic olefins are converted with e.g. 1-olefins to yield linear dienes, while the corresponding backward reaction in the equilibrium is the ring-closing metathesis (RCM) reaction (Figure 1.7). Applying cyclic olefins with relevant ring strains, ring-opening metathesis polymerization (ROMP) can yield polymers with molecular weights on the order of 105 g mol-1. Here, the reduction of ring stain is the driving force in the polymerization reaction. In general cross metathesis (CM) reactions, two substituted olefins are converted to yield an equilibrium of various substituted olefins. In this context, particular reactions of interest are self-metathesis (SM) reaction of just one substituted olefin (R1 = R3
and R2 = R4) and ethenolysis, in the case of conversion of an internal olefin with ethylene (R3, R4 = H) to yield 1-olefins. Since the olefin metathesis catalysts typically show different reactivities toward the substrates under particular reaction conditions, the selection of the appropriate olefin metathesis catalyst precursor has to be made individually.
Figure 1.8. General mechanism of ADMET polymerizations.
A further particular olefin cross metathesis reaction is acyclic diene metathesis (ADMET) polymerization of ,ω-diene monomers,28 commonly yielding polymers with molecular weights on the order of 104 g mol-1. Usually, ADMET polymerizations are performed under solvent-free conditions in the neat molten monomer to disfavor ring formation via ring-closing metathesis reactions. The polymerization mechanism is displayed in Figure 1.8.
During the initiation step, the diene monomer coordinates to the Ru-alkylidene complex (shown for a common Grubbs-type catalyst precursor, A), reacting in a [2+2]-cycloaddition to yield the Ru-cyclobutane derivative B. After [2+2]-cycloreversion, the Ru-methylidene species C is generated, which is active in the catalytic polymerization circle. During the initiation step, the phenyl group (originating from the catalyst precursor Ru=CHPh) is transferred to the diene compound, forming an end group in the ADMET polymer. In the polymerization cycle, the Ru-methylidene C reacts with the ,ω-diene monomer in a [2+2]-cycloaddition to the cyclobutane derivative D, followed by a [2+2]-cycloreversion with the release of ethylene to yield E. Further reaction with a diene molecule in the cycloaddition yields F, which is again converted to the Ru-methylidene species C in the following cycloreversion, releasing the ,ω-diene G as a dimer compound. The polymerization reaction is generally performed under low pressure conditions to remove the ethylene byproduct, shifting the reaction equilibrium to the polymer side. With ongoing polymerization, first dimer and short-chain oligomer species are generated, while with continuing reaction polymer species are formed increasingly. ADMET polymerizations are step-growth reactions (comparable to polycondensation reactions of e.g. diacid and diol compounds, generating polyesters).
Figure 1.9. Relationship between conversion and degree of polymerization in ideal step-growth polymerizations following ‘Carother’s Equation’.
Figure 1.10. Isomerization side reactions occurring during self-metathesis of 1-hexene with HG2 as the olefin metathesis catalyst precursor (in chloroform at 45 °C).33
In an ideal step-growth polymerization, the ‘Carothers’ Equation’ describes the relationship between the conversion (p) and the degree of polymerization (DPn),29 according to
𝐷𝑃𝑛 = 1
1 − 𝑝. (1)
Consequently, sufficiently high degrees of polymerization (and desirably high molecular weights) are only possible in ADMET polycondensations, if high conversions of the vinyl groups are achieved (Figure 1.9).
A relevant side reaction during olefin metathesis is carbon-carbon double bond isomerization, arising from highly reactive ruthenium hydride species formed by decomposition reactions of Grubbs-type catalysts.30,31 Already at moderate reaction temperatures, this behavior is pronouncedly observed for second generation type catalysts, while G1 is normally only affected at high temperatures.32 The significance of the isomerization side reaction becomes obvious regarding the self-metathesis of 1-hexene using HG2 as the cross metathesis catalyst precursor (in chloroform at 45 °C, Figure 1.10).33 Besides the main product 5-decene, also formation of several side-products with extended and
truncated chain lengths is observed. By reaction of 5-decene with the Ru-hydride decomposition product, 4-decene is generated after -hydride elimination. Self-metathesis of this compound forms 4-octene and 6-dodecene, while 4-nonene and 5-undecene may be formed via secondary metathesis with 5-decene, yielding in the end a series of products.
If carbon-carbon double bond isomerization reactions occur during ADMET