1 Introduction

With the depletion of fossil fuels and increasing emission of greenhouse gases, the importance of renewable raw materials has been increasing by the day and sustainable development has become a key ideal of the 20th century.[1] The utilization of renewable raw materials can meet the principles of green chemistry such as built-in design for degradation or low toxicity of the resulting products.[2] Vegetable oils with unsaturated fatty acids form promising, renewable and cheaper feedstocks for generating a number of oleo chemicals to develop a sustainable future.[3,4,5,6] Numerous fatty acids are today available which makes them attractive for synthesis and as raw materials for the chemical industry.[5] Hitherto, industrial oleo chemistry has concentrated predominantly on the carboxyl functionality of fatty acids but, more recently, modern synthetic methods have been applied extensively to fatty compounds for the selective functionalization of the alkyl chain.[5] Radical, electrophilic, nucleophilic and pericyclic as well as transition metal catalyzed additions to the C-C double bond, as seen in case of oleic acid, an unsaturated fatty acid, is readily accessible to a large number of novel fatty compounds with many interesting properties. Reports say that 90% of the oleo chemical reactions are derived by the modification at the fatty acid carboxylic group and less than 10% by transformations of the alkyl chain. However, future technologies on the development of various industrial chemicals depend on the modification of the alkyl chain. Hence, oils and fats offer possibilities for providing chemistry with a wealth of reaction products which are of great value in the future by conducting a number of organic reactions across the double bonds of vegetable oil fatty acids like oxidations, reductions, polymerizations,[5, 7] etc. However, olefin metathesis has been a modern versatile tool for the functionalization of plant oil derived chemical intermediates. The approach contributes to sustainable development and reduction in \(\hbox {CO}_{2}\) emission as the raw materials employed are safer, and less toxic. Metathesis reactions involve redistribution of fragments of alkenes by scission and regeneration of C-C double bonds (Figure 1).[8, 9]

Fig. 1
figure 1

Olefin metathesis.

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Fig. 2
figure 2

Types of olefin metathesis reactions.

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Depending on the participation of the unsaturated fatty acids, metathesis reactions can be classified into self-metathesis (SM), cross-metathesis (CM), ring-closing metathesis (RCM), ring-opening metathesis (ROM), ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis polymerization (ADMET) (Figure 2).

The mechanism for the olefin metathesis reaction was proposed by Chauvin (Figure 3). This involves a [2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallacyclobutane intermediate, which then breaks up in the reverse approach to give a new alkylidene and a new olefin and the process is repeated sufficiently till an equilibrium mixture of olefins are obtained.

Transition metal catalyzed metathesis[9,10,11,12,13,14,15,16] of olefins is used in petrochemistry and polymer chemistry for the production of special olefins and is applicable to unsaturated fatty acid esters. Metathesis of oil-based feedstocks such as vegetable oil, fish oil and animal fat leads to a number of industrially important chemicals e.g: waxes, plastics, cosmetics, biofuels, etc.[6] The researchers have employed different homogenous and heterogeneous catalysts like ruthenium-based,[17] molybdenum[18] tungsten[16] and rhenium-based[19,20,21,22] for metathesis reactions. Further, the researchers also observed that good conversions were obtained using high catalyst loadings. However, modern heterogeneous catalysts namely Grubbs’ first and second generation catalysts, Hoveyda-Grubbs catalyst (Figure 4) were found to be more efficient in generating platform chemicals which can replace most of the petroleum-based chemicals.[23] Grubbs’ first and second generation catalysts are used in organic synthesis as the first generation catalysts with higher activity. Recent advances in the development of metathesis catalysts in the Grubbs’ group resulted in a family of ruthenium carbene catalysts which have significant advantages over the traditional tungsten based catalysts.[24] Grubbs’ catalysts were shown to be insensitive to the presence of oxygen, and with the modifications to the ligand system, these catalysts were actually shown to be active in water.[25]

Fig. 3
figure 3

Chauvin mechanism for olefin metathesis reaction.

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Fig. 4
figure 4

Structures of Grubbs’ first, second generation and Hoveyda catalysts.

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Fig. 5
figure 5

Synthesis of (Z)-cycloheptadec-9-en-1-one (civetone).

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The article reviews the various metathesis reactions and catalysts used to generate platform chemicals useful for a number of industrial applications.

1.1 Self-metathesis (SM)

Metathesis reaction in which both double bonds are part of the same molecule is termed as self-metathesis.

1.1.1 Self-metathesis of fatty acids

Self-metathesis of long-chain unsaturated fatty acid esters was first reported by Boelhouwer and co-workers.[26] This route in the presence of a suitable catalyst provides a convenient and highly selective route to produce unsaturated diesters, high-value intermediates for a number of oleo chemicals. Self-metathesis of the following fatty acids or their esters was carried out to develop a variety of platform chemicals.

Fig. 6
figure 6

Self-metathesis of fatty acid methyl esters.

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The most studied fatty acid ester is methyl oleate (methyl cis-9-octadecenoate), an ester of oleic acid, a readily available naturally occurring fatty acid. Methyl oleate upon metathesis yields 50% (equilibrium conversion) of 9-octadecene (a lube oil range hydrocarbon intermediate) and dimethyl 9-octadecendioate (used for the production of macrocyclic compounds). The diester of methyl or ethyl oleate on subjecting to Dieckmann condensation leads to cycloheptadec-9-en-1-one (civetone), an important base material used in the perfumery industry (Figure 5).[27,28,29]

Self-metathesis of many long-chain unsaturated fatty acids such as methyl elaidate, methyl 11-eicosenoate, methyl palmitoleate, methyl vaccinate, methyl erucate (major fatty acid of high erucic rapeseed oil and crambe oil), methyl petroselenate (major fatty acid of coriander oil) and methyl 5-eicosenoate (methyl gadoleate, major fatty acid of meadow foam oil), methyl ricinoleate (major fatty acid of castor oil), methyl undecenoate (second generation product of castor oil) and long chain fatty acids such as methyl oleate was carried out (Figure 6). The various intermediates produced from these sources are polyenes, monoesters, diesters and cyclopolyenes.[30] Ricinoleic acid, a hydroxyl fatty acid present in 87–90% in castor oil upon self-metathesis leads to dihydroxy unsaturated hydrocarbon and an unsaturated diester.[31] Also, the second generation product of castor oil, undecenoic acid upon self-metathesis leads to unsaturated diester.[32] These molecules with unusual chain lengths and functionalities which are otherwise difficult to be synthesized by conventional methods can be obtained by employing metathesis route. These can be exploited for lubricant, surfactant and other polymeric applications.

The effect of Grubbs’ second-generation catalyst on the product formation was studied by Vyshnavi et al., varying the catalyst concentrations from 0.03 mmol to 0.18 mmol.[33] At 0.06 mmol concentration only two products namely 9-octadecene and dimethyl-9-octadecenedioate were observed. Lower and higher concentrations resulted in more number of products such as cyclopropaneoctanoic acid methyl ester, 11-eicosenoic acid methyl ester, methyl-9-octadecenedioate, and 9-octadecene. Hence, the desired products can be synthesized by manipulating the catalyst concentration.

Table 1 Cross metathesis of vegetable oils and fatty acid.
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1.2 Cross metathesis (CM)

Metathesis between two different molecules is termed as cross-metathesis. Cross-metathesis of unsaturated fatty acid esters with alkenes is an elegant way for synthesizing homologues of these esters, extending greater versatility in the field of oleo chemistry.

1.2.1 Cross-metathesis of fatty acids

Cross metathesis of long-chain unsaturated fatty acids \(\hbox {C}_{16}\hbox {-C}_{18}\) with lower olefins leads to less abundant medium chain lengths \(\hbox {C}_{10}\hbox {-C}_{14}\), especially \(\hbox {C}_{12}\), highly demanded compounds in the surfactant industry. For example methyl oleate with 2-butene, 2-pentene and a mixture of 2-hexene and 3-hexene, all giving rise to \(\hbox {C}_{11}\hbox {-C}_{13}\) esters.[34,35,36] Apart from cross metathesis of methyl oleate, cross metathesis of \(\hbox {C}_{4}\hbox {-C}_{6}\) acids with methyl linoleate and a mixture of \(\hbox {C}_{18 }\) unsaturated methyl esters derived from tallow and sunflower,[37] rapeseed and soybean oils,[38, 39] palm oil[40] also leads to a mixture of medium chain fatty acids. A pheromone precursor is produced by the cross metathesis of ethyl oleate with 5-decene.[41] Ethenolysis, cross metathesis of unsaturated esters with ethene leads to short chain \(\omega \)-unsaturated fatty acid esters with a broad range of applications,[42]\(^{\mathrm{a,b}}\) and is particularly attractive due to low cost and abundant supply of ethylene.[43]\(^{\mathrm{a,b}}\)

Ethenolysis of methyl oleate results in methyl 9-decenoate, a hypothetical source for the production of a number of polymers and copolymers (Table 1). The yields and selectivity of the ethenolysis products with respect to methyl oleate was studied employing Hoveyda-Grubbs’ second-generation catalyst immobilized on silica \((\hbox {HG/SiO}_{2})\). The ratio of \(\hbox {PC}_{2}\hbox {H}_{4}/\hbox {methyl}\) oleate influenced the selective formation of methyl-9-decenoate and 1-decene proportionately. However, the ratio on higher side deactivated the catalyst. By careful tuning of the conditions, Nieres et al.,[44] observed an optimum ratio of \(\hbox {PC}_{2}\hbox {H}_{4}/\hbox {MO}\) as 2.5. Ethenolysis of oleic acid serves as an access to olefin blends with mean chain lengths below 18 carbons while reaction with hex-3-enedioic acid gives unsaturated dicarboxylic acids with adjustable mean chain lengths as major products which is due to isomerisation of the double bonds.[45] Ethenolysis with high turnover numbers (340,000) has been achieved using Grubbs, CAAC catalyst.[46] Oleic acid upon cross metathesis with 9-decen-1-ol in the presence of Grubbs’ second generation catalyst results in (E)-Didec-9-enyl octadec-9-enedioate and branched derivatives with lubricating and wax-like properties.[47] Another industrially valuable polymer, nylon 12 and nylon 13 were obtained from microalgae by cross metathesis of methyl oleate with allyl cyanide or homo allyl cyanide with 12-cyano-9-dodecenoate as intermediate (Table 1).[48] The route was also employed in developing hydrophobizing agents used in the manufacture of lignocelluloses composite products.[49] High-weight esters \((< \hbox { or } = \hbox {C}_{24})\), acids and their derivatives were prepared from low molecular weight unsaturated esters via cross metathesis of methyl 9-decenoate, used for the preparation of nylon-10. These esters find several polymer applications such as polyurethanes, Nylon -9, etc.[50]

Long chain di-unsatuarted monoesters were prepared by the cross-metathesis of an unsaturated monoester or diester with cyclic olefins. It also produces long chain dicarboxylic acids and diols, fragrances such as 9-decen-ol and civetone,[51] prostaglandins, etc.[52] Pederson et al.,[53] reported a typical metathesis reaction to yield the pheromone, 11-tetradecenyl acetate from a long chain olefinic ester and 3-hexene catalysed by Grubbs’ catalyst. Materia medica, an international drug company has produced pheromones: (E)-5-decenyl acetate, a pheromone of peach twig borer (Anarsia lineatella), a mixture of (E)-and (Z)-11- tetradecenyl acetate of sparganothis fruit worm, pest of cranberries and blueberries employing metathesis route.[54]

Methyl oleate-based estolides and oligomeric base oils were prepared from terminal unsaturated fatty acids by cross metathesis of oleic acid with terminal unsaturated hydrocarbons such as butene catalysed by Grubbs’ second generation catalyst (Table 1).[55] Biokerosene hydrocarbons from \(\hbox {C}_{9}\hbox {-C}_{16}\) and maleinized ester derivatives were also prepared employing cross metathesis, which are otherwise difficult to prepare using conventional methods.[56] A number of catalysts were employed for the cross metathesis of unsaturated fatty acid esters for example: Wampler et al., and Bruneau et al.,[57]\(^{\mathrm{a,b}}\) has conducted a series of metathesis reactions catalyzed by ruthenium alkylidene complex and alkali metals developed by Robert H. Grubbs and also Molybdenum and Tungsten-based catalyst developed by Richard R. Shrock on unsaturated fatty acid esters and vegetable oils to develop diesters and polyol esters. The researchers’ observed that TON of the catalyst can be improved by the pretreatment of the substrate, as the contaminants may reduce the efficiency of the catalyst.

On the other hand as pure fatty acids and the catalysts involved are expensive in turn increasing the production cost, attempts were made to prepare the above products taking vegetable oils rich in unsaturation and subjecting them to metathesis employing different catalysts based on Mo and W.[58, 59] Cross-metathesis of triglycerides on butenolysis result in oligomeric and polymeric esters with high viscosity useful for lubricant applications. In this context, the company Elevance Renewable Sciences 180,000 MT, a joint venture biorefinery is worth mentioning for its 400 million pounds a year that has been operating in Gresik, Indonesia since 2013. The plant uses a second generation Grubbs catalyst (Figure 4), palm oil and 1-butene, due to its increased solubility and higher catalyst turnover number compared to ethylene. The refinery was initially operated using metathesis of palm oil, and later, on a variety of renewable oil feedstocks such as soybean, mustard, jatropha and algal oils. Metathesis of these oils result in personal care products, detergents and cleaners, lubricants and additives, engineered polymers and other speciality chemical markets.[60, 61] The existing industrial utilization of linear internal olefins in the range of \(\hbox {C}_{11}\) to \(\hbox {C}_{14}\) by Shell Higher Olefin Process result in linear non petroleum –derived waxes, while hydrogenation of the entire olefin stream yields n-alkanes (\(\hbox {C}_{8}\hbox {-C}_{18})\) suitable for diesel or jet fuels.[62] Table 1 shows some of the cross-metathesis reactions employing a variety of molecules, different reactions and different reaction conditions employed and the industrial applications of the products generated.

1.3 Self and cross-metathesis of vegetable oils

Self-metathesis of vegetable oils plays a vital role in the polymer industry. Self-metathesis of natural, unsaturated rich fats and oils proceed intra and intermolecularly to produce polymeric glycerides (mainly dimers and trimers). Self-metathesis of plant oils with high unsaturation results in \(\hbox {C}_{18}\) diesters, which on subsequent hydrogenation is converted to diols useful for the synthesis of long-chain polyester in the presence of 1, 5, 7-triazobicyclodecane as catalyst.[63] Self-metathesis of a mixture of sunflower oil and moringa oil resulted in cyclo-hexa-1, 4-diene, alkenes, mono and dicarboxylic acids.[37] In the case of soybean, rapeseed, tall oils self-metathesis leads to polycondensation, which in turn results in branching with increased molecular weight, thereby the viscosity and drying properties.[64]

Heterogeneous catalysts \(\hbox {Re}_{2}\hbox {O}_{7}/\hbox {SiO}_{2}\), \(\hbox {Al}_{2}\hbox {O}_{3}/\hbox {SnBu}_{4 }\) were employed for the metathesis of different fatty acid esters of South African sunflower oil, which resulted in alkene, mono and diesters of varying chain lengths.[65] Self- metathesis of palm oil was carried out by Nordin et al.,[66] in the presence of homogenous catalysts \(\hbox {WCl}_{6}/\hbox {Me}_{4}\hbox {Sn}\) to yield low molecular weight products, 6-pentadecane and 9-octadecene due to the intramolecular reaction between oleate and linoleate chains in the triglycerides. Anti-microbial agents were developed using Copper and Nickel carboxylates of castor seed oilvia metathesis in the presence of ethanol.[67] Pillai et al.,[68] observed that methyltrioxorhenium (MTO) was a better catalyst for bulky functionalized olefin metathesis such as triglycerols and edible oils compared to Grubbs’ second generation catalyst. Unsaturated fatty alcohol alkoxylates were prepared by reducing metathesis-derived monounsaturated alkyl ester of oils followed by alkoxylation of the metathesized esters. These products find an important place as agricultural solvents, non-ionic emulsifiers for agricultural compositions, hard surface cleaners, etc.[69]

Cross-metathesis of fatty oils offers an attractive route for the conversion of long-chain high molecular weight oils into fatty oils of low molecular weight (Figure 7, Table 1).[70] Ethenolysis of triolein was studied using two catalytic systems \(\hbox {Re}_{2}\hbox {O}_{7}/\hbox {Al}_{2}\hbox {O}_{3}\hbox {-SnBu}_{4}\) and \(\hbox {Re}_{2}\hbox {O}_{7}/\hbox {SiO}_{2}.\hbox {Al}_{2} \hbox {O}_{3}\hbox {-SnBu}_{4}\)[37] which proceeds via three consecutive metathesis reactions to yield tridecyl glycerol. The second generation ruthenium-based catalysts were found to be the best catalysts to catalyze metathesis reactions involving internal double bonds.[71] Cross metathesis of vegetable oils such as sunflower, soybean, rapeseed and safflower oils rich in mono-, di-and tri-unsaturated rich fatty acid chains with an excess amount of olefins like 3-hexene, 5-decene and 7-tetradecene was carried out to produce triglycerides with desirable chain lengths. The triglycerides upon transesterification with ethanol, followed by reduction and on subsequent oxidation or acetylation results in aldehydes or acetates.[72] Ethanolysis of oils from first generation (rapeseed and sunflower), second generation (waste coffee oil) and third generation (microalgae and yeast) products serve as feedstocks for the production of fuels and chemicals.[73] Thus, metathesis of biologically sourced oils with ethylene has excellent potential to produce multiple fuels from single oil. Cross metathesis of vegetable oils such as coconut, rapeseed, corn, cottonseed, olive, palm, peanut etc and animal fats with ethylene or butene results in polyol ester oligomers (Figure 7, Table 1). The oligomers on partial amidation of some of these esters with ammonia or higher organic amines such as dodecyl amine or other fatty amines can be used in personal care formulations (Table1).[74] Natural oils such as canola oil, rapeseed oil, coconut oil, soybean oil, etc., yellow grease and fish oil-based monomers, dimers, trimers, tetramers, pentamers and higher order oligomer acrylates were prepared to employ the said route.[75] The polyols synthesized via cross metathesis of 1-butene with palm oil (PMTAG) is mostly employed for the preparation of rigid foams. This route offers an advantage of reducing the dangling chain effects associated with the omega unsaturated fatty acids.[76]

Fig. 7
figure 7

Cross metathesis of triolein.

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Natural oils were also converted into greases in the presence of metathesis catalysts.[77] Cross metathesis has been a versatile route in preparing polyethylene mimicking molecules generated by metathesis and copolymerization of fatty acid-based \(\upalpha \),\(\upomega \)-dienes.[78]\(^{\mathrm{a,b}}\) Bio-based unsaturated acids were also prepared from octadec-9-enedioic acid in the presence of ruthenium catalysts within 3 h and in good yields 77% employing cross metathesis route.[79] Table 1 shows various products obtained employing cross-metathesis reactions of different triglycerides and their applications.[80] Cross-metathesis of fatty acid derivatives with reaction partners bearing different functionalities can create a library of internal olefins leading to statistical mixtures of different substituted olefins.[81,82,83]\(^{\mathrm{a,b}}\) Cross-metathesis has also opened up a new avenue for the synthesis of important peptide-sugar complexes. For example, bioconjugates can be prepared by cross metathesis route taking an amino acid, a fatty acid and a sugar moiety.[84]

Fig. 8
figure 8

Self and cross metathesis of mixed fatty acid methyl esters prepared from vegetable oils.

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Most of the work was carried out either on edible oils or unusual fatty acids/esters. However, in some Asian countries like India, which are importing huge quantities of edible oils and have more than a hundred non-edible oil bearing material containing trees can be exploited in generating a variety of platform chemicals. Some of these sources are rubber, palash and tobacco. In case of rubber (80.7% unsaturation), palash (65.1% unsaturation) and tobacco (88.2% unsaturation), the oils were found rich in unsaturation and were subjected to self-metathesis employing Grubbs’ second generation catalyst. The authors observed that very low concentration of the catalyst (0.03 mmol, based on the weight of the substrate) could bring about metathesis resulting in metathesized products such as cyclodecacyclododecene, 6-pentadecene, 3-dodecene, 9-octadecenoicacid methyl ester, cyclopropaneoctanoic acid methyl ester along with diesters and 9-octadecene for the first time. The possible mechanism involved in the self and cross metathesis of major fatty acid methyl esters such as oleic, linoleic and linolenic acid methyl esters resulting in the above products were also discussed in this study. (Figure 8).[85,86,87]

Thus, self-metathesis of fatty acids and vegetable oils can be exploited for the generation of alkenes, mono and diesters with high molecular weight intermediates in high TON. The route was employed to prepare methyl oleate with TON 2500 and even 440,000[88]\(^{\mathrm{a,b}}\) employing Grubbs’ first and second generation catalysts. With the advent of development of more and more heterogeneous catalysts, the commercial applications of these reactions can be designed for a wide range of applications. For instance, Materia Inc and Cargill announced the partnership in 2005 to develop metathesis processes for converting biobased oils to industrial chemicals, feedstocks and consumer products.[89]

On the other hand, cross-metathesis helps in functionalizing fatty acid derivatives with functional groups (ROH, \(\hbox {R-NH}_{2}\), \(\hbox {RCO}_{2}\hbox {H}\), etc.) required for the efficient synthesis of high-value products. It can be a source for multiple fuels from single oil feedstocks, while producing valuable hydrocarbon side products for the polymer industry, forming the basis for a sustainable oil bio-refinery.

Another important application of olefin metathesis is polymerization of unsaturated vegetable oils or fatty acids employing ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) polycondensation.

1.4 ADMET

Acyclic diene metathesis (ADMET) is a well-defined polycondensation reaction that allows synthesis of unique polymer architectures by simple monomer design, which is not possible using other polymerization methods.[90] High molecular oligomers can be obtained by acyclic diene metathesis of vegetable oils using Grubbs’ ruthenium catalyst, (\(\hbox {Cy}_{3}\hbox {P})_{2}\hbox {Cl}_{2}\hbox {Ru=CHPh}\) (Figure 4) and these can be separated from unreacted oils and lower molecular weight alkene by-products easily. The development of robust metathesis catalysts allowed polymerization of dienic monomers in the presence of several functional groups very easily. For example, the application of Grubbs’ ruthenium catalyst (\(\hbox {Cy}_{3}\, \hbox {P})_{2}\, \hbox {Cl}_{2}\, \hbox {Ru = CH Ph}\) to the acyclic diene metathesis (ADMET) of vegetable oils resulted in a more environmentally friendly, convenient process to produce metathesized vegetable oils.[91]

ADMET polymerization proceeds in the presence of heteroatoms, as long as the terminal olefins are far away from each other.[92] ADMET has been extensively used by Wagener since 1987[93] for the synthesis of polyolefins having regularly spaced functional group branches and high thermal stability and crystallinity.[94] ADMET is also used in preparing a number of polymeric materials containing in-chain functionalities. Many reactions employing ADMET of heteroatom-containing \(\upalpha \), \(\upomega \)-dienes, as functionalizing the backbone of the polymers providing different properties (Figure 9) have been reported.[95, 96]

Fig. 9
figure 9

ADMET polymerization of polymerization of \(\upalpha , \upomega \)-dienes.

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Fig. 10
figure 10

ADMET polymerization of \(\upalpha , \upomega \) dienes containing secondary amides.

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For example \(\upalpha \), \(\upomega \)-dienic monomers containing hydroxyl or phosphorous moieties in addition to ester groups lead to improved flame retardance properties of thermoplastic and thermosetting polyesters.[97]\(^{\mathrm{a,b,}}\)[98] Natural plant oils and fatty acids are the most important renewable feedstocks processed in the chemical industry and in the preparation of bio-based functional polymers and polymeric materials.[99,100,101] Such monomers with different functionalities were prepared from 10-undecenoic acid and its derivatives employing a three-step procedure. The first step involved reduction of 10-undecenic acid to 10-undecenate followed by condensation of the aldehyde to \(\upalpha \), \(\upomega \) diene under basic conditions. The ADMET polymerization of the monomer, undecenate and allyl alcohol obtained at \(80\, ^{\circ }\hbox {C}\) in the presence of several metathesis catalysts lead to poly allyl alcohols which could be further functionalized or cross linked. Hence, ADMET polymerization plays a key role in the above reaction (Figure  10).[102]

Acyclic diene metathesis (ADMET) has been employed in bulk polymerization of glyceryl triundec-10-enoate for producing biorenewable branched polymers. ADMET was also used to polymerize high oleic sunflower oil to afford highly branched and polymerized polyesters.[103] ADMET polymerization of \(\upalpha \), \(\upomega \)-dienes containing two secondary amides[104] (Figure 10) results in polyamides useful for the preparation of a number of oleochemicals. Thus, ADMET is the most efficient method for developing high molecular weight and branched polymers which are otherwise difficult to be produced by conventional methods. The molecular weight of polymers can be varied by varying the ratio of triglyceride used.

1.5 Ring-opening metathesis polymerization (ROMP)

ROMP is a type of olefin metathesis chain-growth polymerization, uses strained cyclic olefins to produce stereoregular and monodisperse polymers and copolymers. This provides a wide range of polymers with unique structures and functionalities useful for a number of industrial applications. This type of metathesis converts cyclic olefins into linear polymers containing olefins in the main chain in a three-step process. The first step is the coordination of the cyclic olefin to a metal alkylidene complex, subsequent [2+2] cycloaddition produces a metallacyclobutane intermediate for a growing polymer which undergoes cycloreversion to afford a new metal alkylidene. Strained cyclic olefins having no bulky groups around the double bonds are used as ROMP monomers. The most commonly used cyclic olefin is norbornene and its derivatives due to their high ROMP activity and easy incorporation of substituents on the ring. Norbornene monomers functionalized with a series of fatty acids of chain lengths varying from \(\hbox {C}_{6}\hbox {-C}_{18}\) were synthesized and polymerized via ROMP in presence of Grubbs’ third generation catalyst.[105] The length of the fatty acid chain strongly determines the terminal properties of the polymers. Some other rings used for ROMP were cyclobutene, cyclopentene and cis-cyclooctene.[106, 107]

Fig. 11
figure 11

Ring opening metathesis.

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ROMP are mostly catalyzed using transition metal chlorides as transition metal complex with organic ligands, which helps the reactions to proceed homogenously and are easier to control. In this context, titanacyclobutane complexes are the first examples of single-component catalysts capable of catalysing living ROMP.[108] On the other hand, titanium and tantalum complex catalysts face limited applicability, as they are incompatible with most heteroatom-containing functional groups. Molybdenum-based alkylidene complexes are also used in ROMP as they are stable towards decomposition and tolerant to a broad range of monomers functionalized with ester, amide, imide, ketal, ether, cyano, trifluoromethyl and halogen groups. Molybdenum catalysts also have the ability to provide stereoregular polymers. The 2,3-disubstituted-2,5-norbornadienes polymerize in a living fashion to give highly tactic polymers with \(>98\%\) trans-olefin geometry.[103] Today a number of metathesis-active Ru-carbene complexes have been developed. Among them, the Grubbs’ first generation catalyst synthesized from (\(\hbox {PPh}_{3})_{3}\hbox {RuCl}_{2}\) by the reaction with diazobenzylidene and subsequent phosphine exchange[109] has been developed as a new class of Ru-based metathesis catalysts.[110] \(^{\mathrm{a,b}}\) Lactonic sophorolipids are prepared by ROMP. These are a variety of amino acid and peptide-based polymers, which are biocompatible and biodegradable, are expected to help in building a society with an environmentally sound material-cycle.[111, 112] Polymerizations require compatibility with the polar functional groups of these bioactive ligands. Ru complex catalysts for ROMP are tolerant of various functionalities such as carboxyl, hydroxyl, amide and ester groups. To design artificial analogues to biologically active peptides, an amino acid derived carboxyl groups; ROMP using Grubbs’ first-generation catalyst is the best method.[113]

ROMP is mostly employed to develop thermosets. Castor oil containing hydroxy fatty acid, ricinoleic acid (87–90%) is the most frequently used substrate to develop thermosets.[32, 114] Castor oil is reacted with commercially available bicyclic anhydride bicycle [2.2.1] hept-5-ene-2,3-dicarboxylic anhydride to give a norborneyl- functionalized bicyclic castor oil derivative (BCO).[114] Neat BCO undergoes ROMP in the presence of cyclooctene under catalytic conditions affording a transparent rubbery thermoset with Tg values ranging from 14 to \(1\, ^{\circ }\hbox {C}\). Further norbornenyl- functionalized fatty alcohols derived from soya bean oil and castor oil with fatty acid chains have been used in developing environmentally friendly bioplastics with high performance[115] (Figure 11). Thus, ROMP plays a very crucial role in developing thermosets with improved thermophysical and mechanical properties with high thermal stabilities.

1.6 Ring-closing metathesis (RCM)

RCM is a widely used olefin metathesis route in organic chemistry for the synthesis of various unsaturated rings via intramolecular metathesis of two terminal alkenes, which results in cycloalkene as the E- or Z- isomers. Ethylene is used widely for the preparation of medium-large rings into complex drug molecules,[116] supra molecular assemblies[21] or small molecule libraries.[117, 118] Asymmetric RCM reactions[18] using molybdenum imido-supported alkylidenes offer advantages such as greater activity and superior enantioselectivity. However, most commonly employed catalysts are Grubbs’, ruthenium-based catalysts. The later offers high tolerance of functional groups, air stability and easy handling. RCM is usually used in conjugation with other metathesis reactions. For example, some vegetable oils such as algal oils, animal fats, tall oils, canola or soybean oils undergo self, ring opening metathesis, ring-closing metathesis and ADMET to generate fuel range products and intermediates for the preparation of speciality chemicals[119] (Figure 12). Renewable fatty waxes can be prepared by ROMP, RCM of vegetable oils with low molecular weight olefins such as styrene and vinyl cyclohexane.[119]

Fig. 12
figure 12

Ring closing metathesis.

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The approach can be applied in the synthesis of most important perfumery chemicals such as \(\upgamma \)- and \(\delta \)- lactones which are produced in TONS and used in cosmetic industry. These are produced either by chemical synthesis or biotechnological methods which are expensive routes. Thus, this route creates new synthetic opportunities for the incorporation of oleochemicals, as renewable feedstocks into the chemical value chain particularly for the cosmetic industry.

2 Conclusions

The review is focused on the importance of various metathesis reactions on vegetable oils and fatty acids and the potential products developed from them. Highly branched, viscous intermediates useful for a number of polymeric and lubricant applications can be produced by self-metathesis of vegetable oils and fatty acids. Cross-metathesis of unsaturated fatty acids with alkenes serves an elegant way for synthesizing homologues of their esters, useful for the preparation of a wide variety of oleochemicals. Cross-metathesis of unsaturated fatty acid esters with ethylene is the most attractive reaction due to low cost. These reactions result in \(\upomega \)-unsaturated fatty acid esters useful for a number of polymeric, lubricant and perfumery applications. Cross-metathesis of vegetable oils results in polyethylene mimicking molecules which are useful intermediates for polymeric applications. Perfumery chemicals, such as civetone and pheromones such as unsaturated fatty acid acetates have been synthesized employing cross metathesis. ROMP (ring opening metathesis polymerization) and ADMET (acyclic diene metathesis) offer a potential route for generating polymeric compounds from unsaturated fatty acids and vegetable oils. ROMP reaction is one of the potential routes for developing thermosets, where castor oil is the most exploited raw material. RCM (ring closing metathesis) and ADMET metathesis are the most employed reactions to generate fuel range products and intermediates. Among the ruthenium, tungsten, molybdenum and rhenium based catalysts, ruthenium-based Grubbs’ catalysts are most widely used in the metathesis reactions. Thus, metathesis of vegetable oils and fatty acids is a novel attractive technique that offers possibilities for future developments in the oleochemical industry leading to a sustainable future.