Metathesis in Oleochemistry

Me tá te se é uma re a ção atra en te para a con ver são de óle os e gor du ras em pro du tos quí mi cos úte is. Assim, a me tá te se de és te res de áci dos gra xos in sa tu ra dos (cons ti tu in tes bá si cos de óle os e gor du ras) é uma rota con ve ni en te para a sín te se de diés te res in sa tu ra dos, que po dem ser usa dos como in ter me diá ri os na pro du ção de po lí me ros e na quí mi ca fina. A me tá te se cru za da com ole fi nas sim ples per mi te al te rar o com pri men to da ca de ia car bo na da de áci dos gra xos e de seus de ri va dos; a me tá te se cru za da com ete no (ete nó li se) for ne ce com pos tos con ten do li ga ções du plas ter mi na is, que pos su em uma ex ten sa gama de apli ca ções. A ete nó li se de óle os gra xos (tri gli ce rí de os) per mi te a trans for ma ção de tri gli ce rí de os de áci dos gra xos de lon gas ca de i as car bo na das em óle os gra xos de me nor mas sa mo le cu lar. Di ver sos sis te mas ca ta lí ti cos ho mo gê ne os ou he te ro gê ne os -, al ta men te se le ti vos, se mos tra ram ca pa zes de efe tu ar a me tá te se de és te res in sa tu ra dos. Ca ta li sa do res he te ro gê ne os à base de rê nio ou de mo lib dê nio têm atra í do mu i ta aten ção. Con si de ran do os pro ce di men tos de ati va ção e de re ge ne ra ção do ca ta li sa dor, os sis te mas ca ta lí ti cos à base de rê nio são pre fe ri dos.

and as raw ma te ri als for the ole o che mi cal in dustry.About 15% of the world pro duc ti on of fats and oils (81 mil li on t/a in 1990, 105 mil li on t/a ex pec ted in 2000) is used in the ole oche mi cal in dustry as star ting ma te ri als for a wide ran ge of che mi cal pro ducts 1 .The most im por tant group from the point of view of cost-effectiveness is that of the long-chain vege ta ble oils, such as soy be an oil, sun flo wer oil and new ra pe se ed oil (all con sis ting ma inly of un sa tu ra ted C 18 fatty acid cha ins) and palm oil (con ta i ning both C 16 and C 18 chains).The ole o che mi cal in dustry pre fers a high con tent of ole ic acid (mo no-unsaturated C 18 fatty acid) be ca u se this is an im por tant star ting ma te ri al for many con se cu ti ve che mical re ac ti ons.Short-and me di um-chain fatty oils, such as palm-kernel oil and co co nut oil, con sist ma inly of la u ric (C 12 ) and myris tic (C 14 ) acid and are im por tant sour ces for the pro duc ti on of de ter gents, so aps, emul si fi ers, etc. Ani mal fats, such as tal low (con ta i ning 40-45% ole ic acid), are also in de mand as raw ma te ri al, ma inly be ca u se of the ir low price.Some ot her ve ge ta ble oils are the sour ce of ole o che micals on a smal ler sca le.An exam ple is cas tor oil, con sis ting ma inly (85-95%) of ri ci no le ic acid, which has a wide ran ge of in dus tri al uses.
The fe eds tocks for ve ge ta ble oils and fats come from diffe rent parts of the world.Soy be ans are pro du ced chi efly in the USA, Bra zil, Chi na and Argen ti na, ra pe se ed in Chi na, Eu ro pe, Ca na da and India, and sun flo wers in East & South Eu ro pe and Cen tral & South Ame ri ca.Co co nut and palm ker nel oil come from Sout he ast Asi an Coun tri es.
Fatty acid es ters are ge ne rally ob ta i ned by tran ses te ri fica ti on of na tu ral oils and fats with a lo wer al co hol, e.g.metha nol.Although more than 95% of all che mi cal re ac ti ons (e.g.con ver si on into fatty al co hols and fatty ami nes) of fatty acid es ters (or acids) are car ri ed out at the car boxy func ti on, trans for ma ti on of un sa tu ra ted fatty acid es ters by re ac ti ons of the car bon-carbon dou ble bond, such as hydro ge na ti on, epo xi da ti on, ozo noly sis, hydro form yla ti on and di me ri za tion, are be co ming in cre a singly of in dus tri al im por tan ce.Metat he sis is anot her re ac ti on of the car bon-carbon dou ble bond.

Me tat he sis
Ole fin me tat he sis is a ca taly tic ex chan ge re ac ti on bet ween two al ke ne mo le cu les du ring which two alk yli de ne groups are in ter chan ged.For nor mal ole fins this can be repre sen ted by re ac ti on (1).

R-CH=CH
A sim ple exam ple is the me tat he sis of pro pe ne (R = H; R' = CH 3 ) into et he ne and but-2-ene.The for ward re ac ti on is of ten cal led self-metathesis , whi le the re ver se re ac ti on, i.e. a re ac ti on bet we en two dif fe rent al ke ne mo le cu les, is cal led cross-metathesis .In fact, many ole fi nic subs tra tes can un -der go me tat he sis in the pre sen ce of a su i ta ble ca talyst, re sulting in a wide va ri ety of pos si ble pro ducts.The se subs tra tes in clu de acy clic al ke nes, di e nes, pol ye nes, cyclic al ke nes, and also func ti o nally subs ti tu ted al ke nes, such as un sa tu rated es ters, ni tri les, ha lo gens etc.
At the mo ment the re are va ri ous com mer ci al ap pli ca tions of ole fin me tat he sis: (i) the pro duc ti on of poly me ri sa tion-grade pro pe ne via cross-metathesis bet we en et he ne and but-2-ene; this is, in fact, the re ver se pro cess of the first indus tri al ap pli ca ti on of the ole fin me tat he sis re ac ti on, viz. the me tat he sis of pro pe ne into high-purity et he ne and but-2-ene, cal led the Phil lips Tri o le fin Pro cess, which was in ope ra ti on from 1966 till 1972; (ii) the pro duc ti on of ne o he xe ne (an impor tant in ter me di a te in the ma nu fac tu re of synthe tic musks) via cross-metathesis of di-isobutene with et he ne; (iii) the Shell Hig her Ole fins Pro cess (SHOP), a lar ge-scale in dus trial pro cess in cor po ra ting ole fin me tat he sis, for con ver ting ethe ne to de ter gent-range ole fins; (iv) the pro duc ti on of se ve ral kinds of poly mers pro du ced via the me tat he sis of cyclic ole fins, such as cyclo oc te ne, nor bor ne ne, and dicyclo pen ta di e ne.A pro mi sing synthe tic ap pli ca ti on for the me tat he sis re ac ti on con cerns the me tat he sis of acy clic olefins con ta i ning he te ro a tom func ti o nal groups.This would al low sin gle-step synthe ses of va ri ous mo no-and di func ti onal de ri va ti ves of hydro car bons with well-defined struc tures.
Con si de ring the ex ten si ve pos si bi li ti es of fe red by the me tat he sis of un sa tu ra ted hydro car bons wit hin che mi cal synthe sis, it is not sur pri sing that much re se arch is be ing carri ed out into the de ve lop ment of ac ti ve and se lec ti ve catalysts for this re ac ti on.The me tat he sis re ac ti on can be ca taly sed by both he te ro ge ne ous and ho mo ge ne ous catalysts.A wide va ri ety of tran si ti on me tal com pounds will ca taly se the re ac ti on, the most im por tant ones be ing ba sed on W, Mo, Re and Ru.He te ro ge ne ous ca talysts ge ne rally con sist of a tran si ti on me tal oxi de, or an or ga no me tal lic precur sor, de po si ted on a high-surface-area sup port (an inor ganic oxi de).Exam ples are Re 2 O 7 /Al 2 O 3 , MoO 3 /SiO 2 , WO 3 /SiO 2 , and ( π-C 4 H 7 ) 4 W/SiO 2 .In par ti cu lar a sup por ted Re 2 O 7 ca talyst shows pro noun ced ac ti vity and high se lec tivity even at room tem pe ra tu re; when pro mo ted with R 4Sn (R = Me, Et, Bu) it be co mes also ac ti ve for the me tat he sis of unsa tu ra ted es ters and ot her func ti o na li zed ole fins.Ho mo gene ous ca talysts ma inly con sist of (i) a com bi na ti on of the tran si ti on me tal com pound (usu ally a chlo ri de, such as WCl 6 , WOCl 4 and ReCl 5 ) and an or ga no me tal lic compound, e.g.R 4Sn (R = alkyl) or EtAlCl 2, as co ca talyst, or (ii) a well-defined alk yli de ne (car be ne) com plex of a tran si ti on me tal, e.g.Ru(=CHCH=CPh 2(Cl) 2 (PCy 3)2.Only a re la tively small num ber of the many me tat he sis ca talyst systems are able to bring about the me tat he sis of func ti o na li zed olefins; the se will be dis cus sed in Ca talysts sec ti on.
Me tat he sis is a tran salk yli de na ti on re ac ti on and it is gene rally ac cep ted that the re ac ti on pro ce eds via the so-called me tal-carbene me cha nism.The pro pa ga ti on re ac ti on in volves a tran si ti on-metal car be ne as the ac ti ve spe ci es with a vacant co-ordination site at the tran si ti on me tal.The al ke ne co-ordinates at this va cant site, and sub se quently a me tallacy clo bu ta ne in ter me di a te is for med.The me tal lacy cle is uns ta ble and cle a ves to form a new me tal car be ne com plex and a new al ke ne, re ac ti on (2).The ini ti al me tal car be ne can be for med by a re ac ti on bet we en the ca talyst and the co catalyst, if pre sent, or by in te rac ti on of the subs tra te al ke ne with the tran si ti on me tal cen tre.When using an ac tu al me tal car be ne as ca talyst a me tal-carbene for ming step is, of course, not ne ces sary.A de ta i led mo no graph de a ling with all aspects of ole fin me tat he sis has re cently ap pe a red 2 .

Self-metathesis
Unsa tu ra ted fatty acid es ters and fatty oils are very promi sing and che ap fe eds tocks for me tat he sis.For this re a son, the me tat he sis re ac ti on is of in te rest to the ole o che mi cal industry.
The first suc cess ful me tat he sis con ver si on in this area was the se lec ti ve trans for ma ti on of methyl ole a te (methyl cis -oc ta dec-9-enoate), a re a dily ava i la ble es ter, into equimo lar amounts of oc ta dec-9-ene and di methyl oc tadec-9-enoate in the pre sen ce of a WCl 6 /Me 4 Sn ca talyst system 3 , re ac ti on (3).
Be ca u se the free ent halpy chan ge in this type of re ac ti on is vir tu ally zero, the re sult at equi li bri um is a ran dom dis tribu ti on of the alk yli de ne groups.Thus, star ting with methyl ole a te, the equi li bri um mix tu re con sists of 50 mol% of the star ting ma te ri al and 25 mol% of each of the pro ducts oc tadec-9-ene and di methyl oc ta dec-9-enoate.The cis/trans ratio of the re ac ti on pro ducts is also in ac cor dan ce with ther mody na mics.This de mons tra tes that -in the pre sen ce of a su i ta ble ca talyst -the me tat he sis of un sa tu ra ted fatty acid es ters pro vi des a con ve ni ent and highly se lec ti ve rou te to un sa tu ra ted di es ters.Unsa tu ra ted di es ters can be used for the pro duc ti on of use ful che mi cal pro ducts such as ma crocyclic com pounds.For ins tan ce, the di es ter ob ta i ned by me tathe sis of ethyl ole a te has been sub jec ted to a two-step re ac ti on se quen ce, i.e. a Di eck mann con den sa ti on fol lo wed by hydroly sis-decarboxylation to give 9-cycloheptadecen-1-one, who se cis form, ci ve to ne , is an im por tant base ma te ri al in the per fu me in dustry 4,5 , re ac ti on (4).Mo re o ver, un sa tu ra ted di car boxy lic es ters and acids are in te res ting star ting ma te ri als for the ma nu fac tu re of pol yesters and pol ya mi des 6 .On the ot her hand, the pro duct oc tadec-9-ene can be di me ri zed and hydro ge na ted to 10,11-dioctyleicosane, a lu be-oil ran ge hydro car bon in terme di a te 7 .
Many ot her un sa tu ra ted fatty acid methyl es ters of the ge ne ral for mu la Me(CH 2 ) n CH=CH(CH 2 ) m COOMe have been shown to un der go me tat he sis, such as methyl pal mi tole a te ( n = 5, m = 7), methyl eru ca te ( n = 7, m = 11) and methyl pe tro se le na te ( n = 10, m = 4).Anot her exam ple is methyl undec-10-enoate, which can be ob ta i ned from cas tor oil via pyroly sis cle a va ge of the ri ci no le ic acid.This re ac ti on proce eds to com ple ti on when the by-product et he ne is con ti nuously re mo ved du ring the re ac ti on, re ac ti on (5).It should be no ted that ole fins with an OH-containing func ti o nal group, such as COOH and CHO, de ac ti va te most ca talysts which are ac ti ve for the me tat he sis of es ters 8 .

CH 2=CH(CH 2)8COOMe
MeOOC(CH 2)8CH=CH(CH 2)8COOMe + CH 2=CH 2 (5)   For the me tat he sis of methyl ole a te a very pure subs tra te is re qui red.In an al ter na ti ve pro cess for the synthe sis of cive to ne, methyl ole a te is first con ver ted to the doubly-unsaturated ke to ne pen ta tri a conta-9,26-dien-18-one, ole on ( 1), which can be se pa ra ted in pure form from the re ac ti on mix tu re.Ole on is then con ver -ted into 9-cycloheptadecen-1-one ( 2) via an in tra mo le cu lar me tat he sis re ac ti on (see Sche me 1), at room tem pe ra tu re in the pre sen ce of a Re 2 O 7 ca talyst sup por ted on SiO 2 -Al 2 O 3 and pro mo ted with Bu 4 Sn.To re du ce the pos si bi lity of inter mo le cu lar me tat he sis bet we en two ole on mo le cu les it is ne ces sary to carry out the re ac ti on un der high di lu ti on con diti ons 9 .
Me tat he sis of pol yun sa tu ra ted fatty acid es ters, e.g.methyl li no le a te and methyl li no le na te, le ads to a va ri ety of re ac ti on pro ducts, in clu ding pol ye nes, mo no es ters, di es ters and cyclo pol ye nes 10 .

Cross-metathesis
Cross-metathesis of un sa tu ra ted fatty acid es ters with a nor mal al ke ne is an ele gant way of synthe si sing ho mo lo gues of the se es ters, and gre atly ex tends the ver sa ti lity of the metat he sis re ac ti on in the fi eld of ole o che mistry.From most indus tri al ve ge ta ble oil crops fatty acid es ters are ob ta i ned with a pre do mi nant cha in length of 18 car bon atoms.Shor tening the se es ters to me di um-chain fatty acid es ters (C 10 -C 14 ) is pos si ble via cross-metathesis with lo wer ole fins, see e.g.re ac ti on (6) [11][12][13] .
A lar ge ex cess of the nor mal al ke ne can for ce the re ac tion to the pro duct side.Fatty acid de ri va ti ves of me di um chain length -es pe ci ally C 12 -are in high de mand for the in dus tri al pro duc ti on of sur fac tants.With re gard to the ir cha in length the se es ters are qui te si mi lar to tho se of the fatty acids de ri ved from palm ker nel oil and co co nut oil.The by-product al ke nes, with the dou ble bond near the end of the cha in, can be used for exam ple for the pro duc ti on of C 12 -C 14 al co hols by hydro form yla ti on.
On the ot her hand, ins te ad of shor te ning the car bon cha in of un sa tu ra ted es ters, it is pos si ble to leng then it, as il lus tra -ted for the cross-metathesis bet we en methyl un dec-10-enoate and hex-3-ene in Eq. 7.
CH 2 =CH(CH 2 ) 8 COOMe + EtCH=CHEt EtCH=CH(CH 2 ) 8 COOMe + CH 2 =CHEt (7)   From a synthe tic po int of view, cross-metathesis re ac tions are very pro mi sing in ope ning new rou tes for the synthesis of de ri va ti ves that of ten can hardly be ob ta i ned by ot her me ans.An exam ple is the is the synthe sis of 1-triacontanol, CH 3 (CH 2 ) 28 CH 2 OH, a plant growth sti mu lant, in a re la tively sim ple two-step pro cess by cross-metathesis bet we en methyl eru ca te and oc ta dec-1-ene in the pre sen ce of a WCl 6 /Me 4 Sn ca talyst, re ac ti on (8), fol lo wed by hydro ge nati on of the es ter thus ob ta i ned 14  Anot her exam ple of or ga nic synthe sis via cross-metathesis is the synthe sis of bi o lo gi cally ac ti ve un satu ra ted com pounds such as in sect phe ro mo nes.Use of such phe ro mo nes of fers an ef fec ti ve and se lec ti ve pest con trol met hod.Thus, cross-metathesis of ethyl ole a te with dec-5-ene re sults in a cis-trans mix tu re of ethyl te tradec-9-enoate, a phe ro mo ne pre cur sor 15 .Ethyl te tradec-9-enoate is also ob ta i ned by cross-metathesis of mix tu res of un sa tu ra ted C 18 ethyl es ters (ole ic, li no le ic, li nole nic) de ri ved from oli ve, sun flo wer or lin se ed oil with excess dec-5-ene 16 .Other exam ples are sum ma ri sed el sew he re 8,17 .
Cross-metathesis of an un sa tu ra ted es ter with a cyclic ole fin le ads to long-chain li ne ar di-unsaturated es ters.Thus, 1-triacontanol is also ob ta i ned by cross-metathesis bet we en methyl ole a te and cyclo do de ce ne, re ac ti on (9), fol lo wed by hydro ge na ti on of the un sa tu ra ted di es ter pro duct 18 .

Ethe noly sis
Cross-metathesis of an ole fi nic com pound with et he ne is cal led et he noly sis.Ethe noly sis of un sa tu ra ted fatty acid esters al lows the synthe sis of shor ter-chain ω-un sa tu ra ted esters which have a bro ad ran ge of ap pli ca ti ons.An ex cess of et he ne can ea sily be ap pli ed ( e.g. by using an et he ne pres su -res of 30-50 bar) to sup press self-metathesis of the es ter and to for ce the con ver si on to com ple ti on.The et he noly sis of methyl ole a te will lead to methyl dec-9-enoate, to get her with dec-1-ene 19,20 , re ac ti on (10).
It is an in te res ting che mi cal bu il ding block for the synthe sis of re le vant che mi cal pro ducts; af ter hydroly sis and hydro gena ti on it yi elds de ca no ic acid or de ca nol, subs tan ces used in lu bri cants and plas ti ci zers.In ad di ti on, fra gran ces can be obta i ned (such as 9-dec-1-enol and ci ve to ne), as can phe ro mones, pros ta glan dins etc. 8,17 , which can ea sily be iso la ted in pure form.Dec-1-ene, like ot her alk-1-enes, is an im por tant in ter me di a te in or ga nic synthe ses, and has a va ri ety of end uses in poly mers, sur fac tants and lu bri cants.Ethe noly sis of methyl eru ca te gi ves anot her ω-un sa tu ra ted es ter, methyl tetra dec-13-enoate, which could have ap pli ca ti ons ana lo gous to tho se of methyl dec-9-enoate.
For ef fi ci ent pro duc ti on of the di es ter of methyl ole a te a two-step pro cess can be con si de red 21 .First, methyl ole a te un der go es et he noly sis to dec-1-ene and methyl dec-9-enoate; high con ver si ons can be ob ta i ned by using a high et he ne pres su re.After pro duct se pa ra ti on, methyl dec-9-enoate un der go es self-metathesis to et he ne and dimethyl oc ta dec-9-enoate.In the lat ter case the equi li bri um can be shif ted to the pro duct side by con ti nu ally re mo ving the vo la ti le et he ne.In this way more than 50% con ver si on can be ob ta i ned in both re ac ti on steps, and the re are no big pro blems in se pa ra ting the re ac ti on pro ducts.

Struc tu re and re ac ti vity of es ters in me tat he sis
In dif fe rent stu di es the re la ti on bet we en the struc tu re of fatty acid es ters (and re la ted com pounds) and the ir re ac ti vity in me tat he sis has been in ves ti ga ted with two types of catalyst systems: WCl 6 /Me 4 Sn at 70 °C 11 and Re 2 O 7 / Al 2 O 3 /Me 4 Sn at 20 °C 12 .It ap pe a red that the re ac ti on works best if the es ter group is not too clo se to the car bon-carbon dou ble bond.The pre sen ce of one or more CH 2 groups betwe en the dou ble bond and the es ter group is suf fi ci ent to ensu re that me tat he sis ta kes pla ce.

Me tat he sis of Na tu ral Fats and Oils
Me tat he sis of es ters of glyce rol, i.e. un sa tu ra ted fatty oils, pro ce eds in tra-as well as in ter mo le cu larly, the lat ter reac ti on strongly pre do mi na ting, re ac ti on (11).Thus, oli ve oil, which con sists ma inly of glyceryl tri o le a te, yi elds oc tadec-9-ene and poly me ric glyce ri des (prin ci pally di mers and tri mers).Me tat he sis of oils such as oli ve oil, soy be an oil and lin se ed oil (con ta i ning trigl yce ri des of ole ic, li no le ic and lino le nic acid) re sults in vis cous high-molecular-weight oils, so-called stand oils, with outs tan ding drying pro per ti es due to the ir pro noun ced un sa tu ra ted cha rac ter 17,22 .
Ethe noly sis of dis tinct fatty oils (oli ve, soy be an, ra pe seed, cot ton seed) al lows, in prin ci ple, the trans for ma ti on of long-chain fatty acid glyce ri des into glyce ri des of lo wer mole cu lar we ight, e.g. the pro duc ti on of tri ca prin from oli ve oil (ma inly tri o le in) by et he noly sis and sub se quent hydro ge nati on, re ac ti on (12), and of trimy ris tin from tri e ru ca in along a si mi lar rou te.As va lu a ble by-products cor res pon ding amounts of ter mi nal ole fins are pro du ced.A study of the ethe noly sis of tri o le in (glyceryl tri o le a te) as mo del subs tra te with a sup por ted Re 2 O 7 ca talyst ac ti va ted with Bu 4 Sn showed that the re ac ti on con sists of a se quen ce of three con secu ti ve et he noly sis re ac ti ons, from tri o le in to tri de cenylglyce rol, and pro ce eds with high con ver si on at room tem pe ra tu re and an et he ne pres su re of 30 bar 23 .
Ethe noly sis of pol yun sa tu ra ted es ters, as pre sent e.g. in soy be an oil, with an ex cess of et he ne in the pre sen ce of the ca talyst system Re 2 O 7 /Al 2 O 3 /Et 4 Sn at 20 °C le ads to good yi elds of hept-1-ene or but-1-ene (from li no le ic or li no le nic es ters, res pec ti vely), pen ta-1,4-diene, de ca-1,4-diene, methyl dec-9-enoate and methyl tri de ca-9,12-dienoate, as il lus tra ted for methyl li no le a te in Eq. 13 24 .

Me(CH
A va ri ety of un sa tu ra ted fatty oils may ser ve as ea sily obta i na ble and re la ti vely che ap raw ma te ri als of this type.Thus, good re sults have also been ob ta i ned in the et he noly sis of methyl es ters from oli ve oil and palm oil 25 .

Ca talysts
Be ca u se of the po ten ti al in dus tri al im por tan ce of the reac ti on, much ef fort has been de vo ted to the de ve lop ment of ca talysts able to bring about the me tat he sis of un sa tu ra ted fatty acid es ters.A pro blem is the de ac ti va ti on of ca taly tic si tes by the func ti o nal group re sul ting in tur no ver num bers which are much lo wer than tho se ob ta i ned for the me tat he sis of ana lo gous sim ple ole fins.Exam ples of ca talyst systems which can with stand the ne ga ti ve ef fect of the po lar group to a cer ta in ex tent are gi ven in the Ta ble 1.

Ho mo ge ne ous systems
Undoub tedly the most ac ti ve ho mo ge ne ous ca talyst systems are the well-defined me tal alk yli de ne com ple xes synthe si sed in the la bo ra to ri es of Schrock 29,33,34 and Basset 32 .Such systems are cha rac te ri sed by a me tal (W, Mo or Re) in its hig hest oxi da ti on sta te con ta i ning in its co-ordinating sphe re both alk yli de ne and bulky electron-withdrawing li gands (ary lo xi des, flu o ro al ko xi des, etc.); exam ples are the com ple xes I, II and III .The bul kiness of ary lo xi des 32 and imi do or alk ylidy ne li gands 29,33,34 would pre vent di me ri za ti on of the se elec tron-deficient or gano me tal lic com ple xes and, to some ex tent, the co-ordination of the func ti o nal group to the me tal atom 35 .
On the ot her hand, two-component ca talyst systems, mainly WOCl 4 and WCl 6 with a su i ta ble co ca talyst (an alk ylating agent such as Me 4Sn), have been wi dely em plo yed on a la bo ra tory sca le.The se are che a per and ea si er to hand le than the alk yli de ne com ple xes.In this sen se, the use of  W(OAr) 2 Cl 4 has the ad van ta ge of al lo wing its ma ni pu la ti on un der air 30 .The role of the co ca talyst would be a dou ble alkyla ti on of the me tal cen tre, fol lo wed by an α-H-elimination le a ding to the for ma ti on of an alk yli de ne li gand.A buty li dene li gand has in de ed been ob ser ved du ring in situ 1 H-NMR stu di es of the system W(OC 6H3-Ph 2-2,6) 2Cl 4/Bu 4Pb 30 .

He te ro ge ne ous systems
He te ro ge ne ous ca talysts seem more su i ta ble for in dus trial ap pli ca ti ons, be ca u se they are more fa vou ra ble with respect to the se pa ra ti on of the ca talyst from the re ac ti on pro ducts, ca talyst re ge ne ra ti on and ap pli ca ti on in con ti nuous pro ces ses.The most ac ti ve systems are ba sed on rhe nium or moly bde num oxi des.They are usu ally pre pa red by im preg na ti on of the sup port with an aque ous so lu ti on of the am mo ni um salt of the tran si ti on me tal.After drying in air at 110 °C and cal ci na ti on at the de si red tem pe ra tu re (usu ally bet we en 500 and 550 °C), the ca talysts are com monly formu la ted as Re 2O7/sup port or MoO 3/sup port.Ho we ver, in or der to be able to bring about me tat he sis of func ti o na li zed ole fins, the se ca talysts need eit her the ad di ti on of a pro mo ter as co ca talyst (Re 2O7/sup port + R 4Sn or R 4Pb) or to un der go a spe ci al ac ti va ti on pro ce du re (MoO 3 /sup port).Se ve ral para me ters can af fect the per for man ce of the se ca taly tic systems, such as (i) the tran si ti on me tal lo a ding; (ii) the kind of sup port; (iii) the cal ci na ti on pro ce du re; (iv) the tran si ti on me tal/co ca talyst mo lar ra tio; etc.The ta ble pre sents the best per for man ces re por ted for each system.The very high se lecti vity (> 97%) en coun te red in the me tat he sis of fatty acid esters might be due to the com ple xa ti on of the es ter group to the Bröns ted acid si tes on the sup port, pre ven ting side re ac tions such as dou ble bond shift.

Rhe ni um Ca talysts
Re 2O7/Al 2O3/Me 4Sn was the first he te ro ge ne ous catalyst system found to be ef fec ti ve for the me tat he sis of olefi nic es ters 36 .The ca taly tic per for man ce of this system has sin ce been im pro ved in se ve ral ways by (i) the in cor po ra ti on of a third me tal oxi de such as V 2O5, MoO 3 or WO 3   37,38   ; (ii) the use of a mo di fi ed sup port, e.g.SiO 2-Al 2O3 39 , Al 2 O 3 -B 2 O 3 40 , phosp ha ted alu mi na 41

Struc tu re x ca taly tic ac ti vity
Many spec tros co pic stu di es have been car ri ed out in order to cha rac te ri se the sur fa ce struc tu re of Re2O7/Al2O3 ca talysts42.The system is ge ne rally des cri bed as a mo no la - yer ca talyst; the rhe ni um oxi de spe ci es which are ca talyst site pre cur sors are bon ded to the alu mi na sur fa ce as mo nome ric spe ci es, pre sen ting a C 3v symmetry.The struc tu re of the rhe ni um oxi de pha se pre sent on SiO 2-Al 2O3 has been stu di ed less.Ho we ver, at le ast up to a 3 wt% Re 2O7 lo a ding, the rhe ni um oxi de sup por ted on SiO 2-Al 2O3 has pre do minantly a mo no me ric struc tu re; at hig her lo a dings it might also be pre sent as three-dimensional clus ters 43 .
The ca taly tic ac ti vity of Re2O7/Al2O3 in cre a ses with the rhe ni um lo a ding.A po si ti ve cor re la ti on bet we en the cataly tic ac ti vity and the sur fa ce aci dity (ma inly the Bröns ted aci dity) of such systems has been ob ser ved 44,45 .The ac ti vity of Re 2 O 7 /Al 2 O 3 ca talysts can be ex pla i ned on the ba sis of the ac ti vity of the sur fa ce ReO 4 groups which have re ac ted with Le wis acid si tes and with the dif fe rent alu mi nium-bonded OH groups du ring the pre pa ra ti on of the catalyst 41 .The most ac ti ve si tes ari se from the re ac ti on bet ween ReO 4 -ions and the most aci dic OH groups on the alu mi na sur fa ce to form Al-bonded ReO 4 spe ci es.Such re ac ti ons are fa vou red only af ter the ba sic and ne u tral OH groups have reac ted to some ex tent.
On the ot her hand, when SiO 2 -Al 2 O 3 is used as the support, the spe ci fic ca taly tic ac ti vity de cre a ses as the rhe ni um lo a ding is in cre a sed 46 .By ana logy with the ex pla na ti on given abo ve for Al 2 O 3 it is thought that the re are two types of hydroxyl groups on SiO 2 -Al 2 O 3 : hydroxyl groups at ta ched to a Si atom and brid ging hydroxyl groups at ta ched to both a Si and an Al atom.The re ac ti on of ReO 4 -ions with the bridging hydroxyl groups would re sult in elec tro po si ti ve rhe nium cen tres (ReO 4 -te tra he dra) , the ac ti ve si tes.Thus, hig her rhe ni um lo a dings would fa vour the for ma ti on of inac ti ve sites of the type ⇐Si-O-ReO 3 , as it is known that Re 2 O 7 /SiO 2 has no ac ti vity in ole fin me tat he sis 47 .It should be po in ted out, ho we ver, that a gre at dif fi culty in the cha rac te ri sa ti on of the se systems lies in the fact that only ca.1% of the rhe ni um si tes are ac ti ve in me tat he sis 48,49 .

Role of the pro mo ter
Although rhe ni um-based systems are only ac ti ve for the me tat he sis of func ti o na li zed ole fins when pro mo ted with an alkyltin or alkylle ad com pound, the role of the lat ter is still not well un ders to od.Re duc ti on of the rhe ni um atom, mo difi ca ti on of the ac ti ve site (by ad di ti on of a tin li gand) and forma ti on of the ini ti a ting me tal-alkylidene spe ci es ( via a dou ble alk yla ti on fol lo wed by an α-H-abstraction) have been pos tu la ted as pro mo ti on me cha nisms 50 .Ho we ver, only a few stu di es have been car ri ed out con cer ning the na tu re of the in te rac ti on bet we en the tin com pound and the sur fa ce rhe ni um spe ci es [51][52][53][54][55] .Evi den ce for the pre sen ce of Re-O-Sn bonds co mes from Ra man spec tros co pic stu di es of Re 2 O 7 /Al 2 O 3 /Me 4 Sn 53 .Ba sed on 119 Sn Möss ba u er and 13 C and 119 Sn MAS NMR ex pe ri ments with Re 2 O 7 /SiO 2 -Al 2 O 3 /R 4 Sn, re ac ti ons as de pic ted in Sche me 2 have been pro po sed 54 .The R li gand on spe ci es A might still un der go an elec trop hi lic at tack from a re ma i ning OH -group from the sur fa ce with re le a se of an al ka ne.If a dou ble alk ylati on in vol ving only one rhe ni um site ta kes pla ce, a co or di nati vely sa tu ra ted spe ci es, B, would be for med.This spe ci es might un der go an α-H-abstraction, spon ta ne ous or sur face-induced, le a ding to a rhe ni um-alkylidene spe ci es, C.Although an alk yli de ne li gand in the rhe ni um co-ordination sphe re could not be spec tros co pi cally iden ti fi ed, che mi cal re ac ti vity sug gests its pre sen ce in mi nor amounts.Whi le spe ci es A would pre do mi na te for Re 2 O 7 /SiO 2 -Al 2 O 3 /R 4 Sn systems, in par ti cu lar when R = Et, for Re 2 O 7 /Al 2 O 3 /Bu 4 Sn the re would only be spe ci es C or D 55 .Ta king into ac count that (i) for Re 2O7/SiO 2-Al 2O3/R 4Sn systems the hig hest cataly tic ac ti vity per rhe ni um atom is ob ta i ned when R = Et 8 , for which spe ci es A ap pe ars in hig her con cen tra ti on than for R = Bu or Me, and (ii) spe ci es C and/or D are the only ones pre sent on the sur fa ce of Re 2 O 7 /Al 2 O 3 /Bu 4 Sn, which is less ac ti ve, it se ems that with the se systems the pre sen ce of a pre-formed alk yli de ne li gand may not be a pre re qui si te for me tat he sis to take pla ce.

Orga no me tal lic mo dels of ac ti ve si tes
Sin ce Re 2 O 7 re acts with Me 4 Sn to form CH 3 ReO 3 56 , it has been sug ges ted that the lat ter com plex is an in ter me di a te in the he te ro ge ne ous system.In so lu ti on CH 3 ReO 3 is ac ti ve for the me tat he sis of cyclic and acy clic ole fins, but only in the pre sen ce of both a Le wis acid and an alk yla ting agent.On the ot her hand, when sup por ted on Al 2 O 3 , SiO 2 -Al 2 O 3 or Nb 2 O 5 , it be co mes an ef fi ci ent me tat he sis ca talyst -even for func ti o na li zed ole fins -wit hout a pro mo ter 57,58 .Although 13 C-NMR stu di es show that the or ga nic li gand re ma ins at tached to the rhe ni um atom af ter in te rac ti on bet we en CH 3 ReO 3 and Nb 2 O 5 58 , ex chan ge re ac ti ons with a symmetri cal ole fin using a 13 C la bel led sam ple sho wed that the first me tal-alkylidene spe ci es does not ari se from the methyl group, but from the ole fin it self 59 .The main role of the support would be to in cre a se the Le wis aci dity of the rhe ni um cen tre.
Anot her ap pro ach to pre pa re sur fa ce-anchored alk yli dene spe ci es is ba sed on the fact that in so lu ti on Schrock-type tung sten alk yli de ne com ple xes (which are ex pec ted to have the same che mi cal be ha vi our as the ir rhe ni um ana lo gues) can be synthe si sed from the cor res pon ding alk ylidy ne comple xes via pro to na ti on by Bröns ted acids 60  sur fa ce spe ci es, at le ast one of them be a ring an alk yli de ne ligand 63 .Ho we ver, me tat he ti cal ex chan ge ex pe ri ments suggest that the most ac ti ve systems are tho se that can re act with ole fins and form the ini ti a ting me tal-alkylidene spe ci es via a π-allyl me cha nism 62,63 .It has been pro po sed that sur fa ce Lewis acid si tes in du ce the for ma ti on of a te tra co or di na ted ca tio nic ne o pent yli de ne tung sten(VI) com plex (Sche me 3), which might well be the pre cur sor of the most ac ti ve spe ci es due to its high elec tro nic and co or di na ti ve un sa tu ra ti on 63 .The pre sen ce of such a spe ci es could ac count for both the high iso me ri za ti on (through dou ble-bond mi gra ti on) ob served for ter mi nal ole fins and the lack of ca taly tic ac ti vity of the se he te ro ge ne ous systems to wards func ti o na li zed olefins 64 .
All re sults dis cus sed abo ve show that sur fa ce or ga no metal lic ap pro a ches to the synthe sis of well-defined he te ro gene ous me tat he sis ca talysts have so far fa i led to pro vi de a good pic tu re of the ac tu al ac ti ve site, the ho mo ge ne ous systems be ing not an ade qua te mo del for the ir he te ro ge neous coun ter parts.

Ca talyst de ac ti va ti on and re ge ne ra ti on
The ac ti vity of any ca taly tic system in the me tat he sis of ole fi nic es ters is una vo i dably li mi ted by the pre sen ce of the func ti o nal group 23 .Be si des an in trin sic de ac ti va ti on me chanism (re duc ti ve eli mi na ti on of the me tal lacy clo bu ta ne in terme di a te) 65 , a Wit tig-type re ac ti on bet we en the car bonyl group and the me tal-alkylidene may also be res pon si ble for a de cre a se in ca taly tic ac ti vity.The se systems are also air sensi ti ve. 119Sn Möss ba u er spec tros co pic stu di es of Re 2 O 7 /SiO 2 -Al 2 O 3 /R 4 Sn systems sug gest that air ex po su re does not ca u se hydroly sis of Sn-O-Re bonds; such bonds might, ho we ver, be we a ke ned owing to a chan ge in the co-ordination sphe re of the rhe ni um atom 54 .
After de ac ti va ti on, the ca talyst can be re ge ne ra ted many ti mes via cal ci na ti on fol lo wed by ad di ti on of anot her amount of the pro mo ter.Ho we ver, the re is a li mit to the num ber of ti mes the ca talyst can be re ge ne ra ted in this way 66 . 119Sn Möss ba u er spec tros co pic stu di es re ve a led af ter cal ci na ti on the for ma ti on of both an SnO 2 pha se and a polyme ric io nic Sn(IV) spe ci es, pro bably bon ded to the rhe ni -um 54,55 , which even tu ally would pre vent the for ma ti on of new ac ti ve si tes.

Moly bde num Ca talyst
Only two of the many Mo-based he te ro ge ne ous me tathe sis ca talyst systems have been re por ted to bring about the me tat he sis of func ti o na li zed ole fins, viz.MoCl 5/SiO 2/R 4Sn 15 and Mo/SiO 2, pho to re du ced in CO at room tem pe ra tu re and sub se quently ac ti va ted with cyclopro pa ne.The lat ter system has been syste ma ti cally stu di ed and will be dis cus sed here.

Pho to re du ced moly bde num ca talysts
Pho to re duc ti on of cal ci ned 1 wt% MoO 3 /SiO 2 with a Hg lamp in a CO at mosp he re le ads ma inly to Mo(IV) ions 68,69 .The re sul ting system is ac ti ve for the me tat he sis of pro pe ne.Tre at ment of this system with cyclo pro pa ne re sults in a sharp in cre a se of its spe ci fic ac ti vity 70 .Mo re o ver, the catalyst be co mes ac ti ve for the me tat he sis of ole fi nic es ters 16 .From IR and UV/vis spec tros co pic stu di es it was con clu ded that a moly bde nacy clo bu ta ne in ter me di a te is for med, which de com po ses into moly bde num-alkylidene and Mo(IV)π-ole fin com ple xes; Sche me 4 [70][71][72] .The moly bdenum-alkylidene com ple xes were cha rac te ri sed by an absorp ti on band at 450-480 nm in the elec tro nic spec trum 70 .They pro ved to be ther mally sta ble up to 430°C and oxy gen re sis tant at room tem pe ra tu re.Inte res tingly, the pro po sed [Mo]=CH 2 spe ci es does not un der go a Wit tig-type re ac ti on with ace to ne 70 , which is cha rac te ris tic of Schrock-type alkyli de ne com ple xes.
An al ter na ti ve ac ti va ti on pro ce du re, con sis ting of pho tore duc ti on of MoO 3 /SiO 2 in CO with a la ser beam of 308 nm ins te ad of a Hg lamp, fol lo wed by cyclo pro pa ne tre at ment le ads to a ca talyst of even hig her ac ti vity 23,67 .

Ca talyst de ac ti va ti on
Stu di es con cer ning the sta bi lity of the ca talyst du ring con ti nu ous me tat he sis of pro pe ne sho wed a loss of ac ti vity in de pen dent of the pro pe ne flow rate, sug ges ting an in trin sic de ac ti va ti on me cha nism 73 .Due to the high sta bi lity of both [Mo]=CH 2 and [Mo]=CHCH 3, the de ac ti va ti on of the catalyst is as sig ned to iso me ri za ti on of the in ter me di a te me tallacy clo bu ta ne com ple xes, le a ding to inac ti ve π-com ple xes, in a way ana lo gous to that de pic ted in Sche me 4.This hypothe sis is sup por ted by in situ UV/vis spec tros co pic stu di es 73 .

Pers pec ti ves
Des pi te its con si de ra ble po ten ti al, the me tat he sis of unsa tu ra ted fatty oils and the ir de ri va ti ves has not yet been adop ted by the che mi cal in dustry.The ac ti vity of the catalysts is de cre a sed by the func ti o nal group and thus ca talyst costs are high, be ca u se high ca talyst con cen tra ti ons are requi red.Be si des the in trin sic de ac ti va ti on re ac ti ons, ca talyst Sche me 3.
de ac ti va ti on might in prac ti ce also take pla ce due to im pu riti es in the subs tra te, com ple xa ti on of the func ti o nal group to the ac ti ve site or ad sorp ti on of pro duct mo le cu les (when using a so lid ca talyst), which can block ac ti ve si tes.
The ques ti on of ac ti vity con cerns not only the tur no ver num ber that can be at ta i ned, but also the re ac ti on rate (in other words, how long the re ac ti on ta kes to re ach an ac cep table con ver si on).For the well-defined ho mo ge ne ous ca talyst com ple xes a con si de ra ble amount of time and mo ney may be ex pen ded in just the ca talyst pre pa ra ti on alo ne, which me ans that from an in dus tri al po int of view they are very ex pen sive 74 .The re fo re, in view of the pre sent know led ge, re ge ne rable he te ro ge ne ous ca talysts are su pe ri or.
Although rhe ni um is a pre ci ous me tal, rhe ni um-based ca talysts are to be pre fer red over moly bde num ones be ca u se the ir ac ti va ti on is much sim pler and re ge ne ra bi lity is much bet ter.After the ca talyst has de ac ti va ted, the ques ti on is not only whet her, but also how many ti mes it can be re ge ne ra ted.He te ro ge ne ous rhe ni um oxi de ca talysts can be re ge ne ra ted many ti mes wit hout any loss of ac ti vity, whi le the rhe ni um can be re co ve red from the se ca talysts af ter to tal de ac ti va tion.Mo re o ver, me tat he sis can be car ri ed out with the se catalysts un der mild re ac ti on con di ti ons and with high se lec ti vity.This me ans that the re are fa vou ra ble pros pects for its ap pli ca ti on in ole o che mistry.
. Thus, Schrock-type tung sten alk ylidy ne com ple xes, pre cur sors of the ho mo ge ne ous ca talysts I and II , have been re ac ted with the sur fa ce of SiO 2 61,62, Nb 2O5, Al 2O3 and SiO 2-Al 2O3 63, res pec ti vely, le a ding in all ca ses to very ac ti ve ca talysts for the me tat he sis of sim ple ole fins.Che mi cal and spec tros copic stu di es of the se systems po int to the pre sen ce of se ve ral Sche me 2. .
2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOMe + Ta ble 1. Exam ples of ca talyst systems for the me tat he sis of un sa tu ra ted es ters.
or bo ra ted si li -ca-alumina 21 ; (iii) the use of anot her pro mo ter, such as Bu 4 Sn, Et 4 Sn or R 4 Pb 8 .