Olefin metathesis

An entirely chemical-synthetic approach to linear long-chain α,ω-functionalized compounds is provided by olefin metathesis. In the last two decades major improvements have been achieved in metathesis catalyst performance, and olefin metathesis has evolved as a powerful tool for oleo-chemistry.^57-62 Metathesis products like unsaturated diacids, diesters, or mixed α,ω-functionalized compounds in principle can be used for polycondensation conversions after hydrogenation of the carbon-carbon double bonds. However, as double bond isomerization⁶³ is a generic issue in olefin metathesis and ultimately will result in the formation of α,ω-difunctional compounds of various chain-length, which likely cannot be separated completely, this side reaction is a crucial issue for the generation and isolation of well-defined α,ω-functionalized polycondensation monomers in pure form (> 99 %).

General introduction Self-metathesis reactions

An early example of self-metathesis of unsaturated fatty esters into mono-unsaturated hydrocarbons and α,ω-diesters was reported by van Dam et al. in 1974.⁶⁴ Using WCl6/SnMe4 as a catalyst system, methyl oleate (and methyl elaidate) were converted to dimethyl-octadec-9-ene-1,18-dioate and octadec-9-ene, reaching equilibrium conversion within a few hours (Scheme 1.11).

Scheme 1.11: Self-metathesis of methyl oleate.

Subsequently, several other classical in situ catalyst systems and also heterogeneous catalysts were found to convert unsaturated fatty acids and oil substrates.^65,66 However, their performance in these reactions is limited due to their (partially) insufficient tolerance toward the substrates’

carboxylic acid or ester groups.⁵⁹ This issue was advanced with the development of more functional group tolerant, defined metal alkylidene metathesis catalyst precursors.^67,68

In particular, ruthenium alkylidenes, most prominently [(PCy3)2Cl2Ru=CHPh] (Grubbs first generation catalyst) and [(PCy3)(η-C-C3H4N2Mes2)Cl2Ru=CHPh] (Grubbs second generation catalyst) have been found to be very active catalyst precursors. High productivities of up to several 10⁵ turnovers have been reported for self-metathesis of methyl oleate, applying these catalyst precursors.⁶⁹ Nevertheless, molar conversions are limited (at its best ≈ 50 % in homo-geneous solution), since such reactions typically are subjected to thermodynamic control. This limitation can be overcome applying solvent-free self-metathesis of monounsaturated fatty acids.

Under these bulk conditions, the diacid products formed during the reaction are not soluble in the reaction medium and precipitate from the mixture. This removal of a product shifts the equilibrium mixture. Thus, conversion of the starting material and yields of the diacid products increase. In this way even carbon numbered, monounsaturated dicarboxylic acids can be obtained.⁷⁰ By comparison, self-metathesis of mixtures containing multiple unsaturated fatty acid methyl esters (FAMEs), such as methyl linoleate or methyl linolenate, gives rise to more complex product mixtures, including linear polyenes, monoesters, diesters and cyclopolyenes.⁷¹ This issue becomes even more relevant considering the self-metathesis of common plant oils, which typically contain triglycerides of fatty acids of different chain lengths and, in particular often possess significant amounts of their multiply unsaturated analogues, additionally to the monounsaturated compounds.⁷²

General introduction

Another potential route to linear α,ω-difunctional compounds is self-metathesis of ω-unsaturated compounds. Self-metathesis of methyl undec-10-enoate, which can be obtained by pyrolysis of methyl ricinoleate from castor oil, for example yields internally unsaturated dimethyl-eicos-10-ene-1,20-dioate (Scheme 1.12).⁷³ By removing the volatile by-product ethylene from the reaction mixture, the equilibrium can be shifted toward the long-chain α,ω-diester and thus the reaction can proceed to completion.

Scheme 1.12: Self-metathesis of methyl undec-10-enoate.⁷³ Cross metathesis reactions

In the recent past, especially cross metathesis reactions of readily available plant oils with low molecular weight unfunctionalized olefins have attracted interest. Cross metathesis with ethylene (ethenolysis) splits the fatty acid derivatives at their internal double bonds to terminally unsaturated compounds,^74-84 which are potential platform chemicals for polymers^74,75 and lubricants.⁸⁵ In this way for example methyl oleate can be converted with very low amounts of Grubbs first generation catalyst precursor (0.02 mol-% or less) and high conversions to methyl dec-9-enoate and 1-decene (Scheme 1.13).⁷⁶ The resulting ω-unsaturated fatty acid methyl esters can be further functionalized at their double bonds or dimerized in a self-metathesis reaction, leading to unsaturated long-chain α,ω-diesters (vide supra).⁵⁷

Scheme 1.13: Ethenolysis of methyl oleate.

Starting from methyl oleate or methyl erucate, respectively, in a two-step ethenolysis/self-metathesis process (or directly from methyl undec-10-enoate) a range of long-chain, symmetrically unsaturated α,ω-esters (C18, C20 and C26), suitable for polyester synthesis can be prepared.74,75,86,87 Applying this two-step procedure can be advantageous, particularly on a laboratory scale, even though the longer chain products can be obtained more directly via self-metathesis of the initial fatty acid substrate (cf. Scheme 1.11), as product separation and isolation due to the lower molecular weights and boiling points of the intermediate ethenolysis products is facilitated. In practice, ethenolysis reaction conditions comprise a trade-off between selectivity and catalyst productivity in terms of substrate turnover.

Cross metathesis of fatty acid derivatives with short-chain, internally unsaturated olefins can be performed with significantly higher turnovers. For the cross metathesis of methyl oleate with 2-butene, productivities of up to 5 × 10⁵ turnovers have been reported.^88,89,90 To achieve this

General introduction catalytic performance, careful destillative purification of the oleate substrate as well as the utilization of pure, butadiene-free 2-butene is required. Moreover, in this context also a direct butenolysis of triglycerides of different natural occurring plant oils has been reported.

Figure 1.2: Scheme of an alkenolysis process that converts plant oil triglycerides to medium- and long-chain linear olefins and esters.⁹¹

Butenolysis with 1-butene can be considered a compromise between the desired terminal olefin products and catalyst performance, which also appears to be feasible on a larger scale.⁹¹ A biorefinery by Elevance Renewable Science and Wilmar International in Gresik, Indonesia for the large scale conversion of palm oil by cross metathesis with 1-butene has started operation in 2013.^92-95 Due to the specifics of this process and of olefin metathesis in general, also a significant amount of self-metathesis will occur as a side reaction to yield in particular octadecene-1,18-dioate (Figure 1.2). Hydrogenation and hydrolysis of this by-product yields octadecane-1,18-dioic acid, which is marketed currently by Elevance Renewable Science under the trade name Inherent^TM C18 Diacid in pilot quantitaties.⁹⁶

By contrast to the biotechnological routes discussed (cf. Chapter 1.3.1), the preparation of α,ω-difunctional compounds by metathesis chemistry generally utilizes only half of the fatty acid chain. Furthermore, stoichiometric amounts of less valuable byproducts are formed. An alternative chemical catalytic conversion addressing these problems is isomerization functionalization.