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Color profile: Generic - CMYK US Negative ProofingComposite Default screen The Heck reaction in the production of fine
chemicals

Johannes G. de Vries
Abstract. An overview is given of the use of the Heck reaction for the production of fine chemicals. Five commercial
products have been identified that are produced on a scale in excess of 1 ton/year. The herbicide Prosulfuron™ is pro-
duced via a Matsuda reaction of 2-sulfonatobenzenediazonium on 3,3,3-trifluoropropene. The sunscreen agent 2-
ethylhexyl p-methoxy-cinnamate has been produced on pilot scale using Pd/C as catalyst. Naproxen™ is produced via
the Heck reaction of 2-bromo-6-methoxy-naphthalene on ethylene, followed by carbonylation of the product. Monomers
for coatings are produced via a Heck reaction on 2-bromo-benzocyclobutene. A key step in the production of the
antiasthma agent Singulair™ is the use of the Heck reaction of methyl 2-iodo-benzoate on allylic alcohol (18) to give
ketone (20). The high cost of palladium has spurred much research aimed at the development of more active palladium
catalysts. Ligandless catalysts are very attractive for production, but work only on reactive substrates. Palladacycles are
much more stable than Pd–phosphine complexes and can be used at higher temperatures. The same effect has been
reached with pincer ligands. Bulky ligands lead to coordinatively unsaturated Pd-complexes, which are highly active for
the Heck reaction. Recycle of palladium catalysts is also very important to reduce cost. Immobilization of catalysts by
attaching ligands to solid support is not very useful, because of leaching and reduced activity. In ligandless Heck reac-
tions the catalyst can be precipitated on carriers such as silica, dicalite, or celite. This material can be restored to its
original activity by treatment with I2 or Br2.
Key words: homogeneous catalysis, palladium, arylation, olefination, ligandless, catalyst recycle.
Résumé : On présente une revue de l’utilisation de la réaction de Heck dans la production de produits chimiques fins.
On a identifié cinq produits commerciaux qui sont produits sur une échelle supérieure à une tonne par année.
L’herbicide Prosulfuron® est produit par le biais d’une réaction de Matsuda du 2-sulfonatobenzènediazonium sur le
3,3,3-trifluoropropène. L’agent solaire 4-méthoxycinnamate de 2-éthylhexyle a été produit à l’échelle pilote à l’aide
d’un catalyseur de Pd/C. Le Naproxen® est produit par le biais d’une réaction de Heck du 2-bormo-6-
méthoxynaphtalène sur l’éthylène, suivie d’une carbonylation du produit. Des monomères de couches protectrices sont
produits par le biais d’une réaction de Heck sur le 2-bromobenzocyclobutène. Une étape clé dans la production de
l’agent antiasthmatique Singulair® est l’utilisation de la réaction de Heck du 2-iodobenzoate de méthyle sur l’alcool al-
lylique 18 pour obtenir la cétone 20. Le prix élevé du palladium a provoqué beaucoup de recherches dans le but de dé-
velopper des catalyseurs du palladium qui soient plus actifs. Les catalyseurs sans ligands sont très attrayants pour la
production, mais ils ne sont utiles qu’avec des substrats réactifs. Les cycles palladiés sont beaucoup plus stables que
les complexes Pd/phosphine et ils peuvent être utilisés à des températures plus élevées. On peut obtenir le même effet
avec des ligands en forme de pinces. Les ligands encombrés conduisent à des complexes à coordination insaturée du
Pd qui sont très actifs pour la réaction de Heck. Le recyclage des catalyseurs de palladium est important afin de mini-
miser les coûts. L’immobilisation des catalyseurs en attachant des ligands à un support solide n’est pas très utile en rai-
son du lessivage et de la réduction de l’activité. Dans les réactions de Heck sans ligand, il est possible de précipiter le
catalyseur sur des porteurs, comme la silice, la dicalite ou la célite; ce matériel peut être ramené à son activité origi-
nale par traitement avec du I2 ou du Br2.
Mots clés : catalyse hétérogène, palladium, arylation, oléfination, sans ligand, recyclage du catalyseur.
Received October 13, 2000. Published on the NRC Research Press Web site at http://www.canjchem.nrc.ca on July 14, 2001.
Dedicated to Brian James, a great scientist and a superb lecturer with a keen sense of humour, on the occasion of his 65thbirthday.
J.G. de Vries. DSM-Research, Life Sciences-Chemistry & Catalysis, P.O. Box 18, 6160 MD Geleen, The Netherlands. (Telephone:
+31-46-4761572; fax: +31-46-4767604. e-mail: hans-jg.vries-de@dsm-group.com).
Can. J. Chem. 79: (2001)
Color profile: Generic - CMYK US Negative ProofingComposite Default screen Scheme 1. The Heck reaction.
The Heck reaction
The Heck arylation reaction (Scheme 1), invented inde- pendently by Mizoroki and Heck (10) in 1970 establishes a bond between olefins and aromatic rings. Initially, reportsonly mentioned the use of aryl bromides and iodides asarylating agent. Later, variants were developed using aro-matic triflates (11), aroyl chlorides (12), aryl sulfonyl chlo-rides (13), aromatic diazonium salts (14) (the Matsuda reaction), aroyl anhydrides (15), aryl chlorides (16–20), and arylsilanols (21). Consequently, the number of commerciallyavailable aromatic substrates is very high. The diazoniumsalts work at room temperature or below and thus are suit-able for reactions with thermally labile olefins. In situ prepa- ration of the diazonium salt from the aniline during the Heck X = I, Br, Cl, COCl, OSO2R, SO2Cl, N2 X-, reaction has also been reported (22). The variant using aro- matic anhydrides only produces the aromatic acid as side product, which can be recycled. In addition it does not needphosphine ligands or a base (15). Hence, this reaction is saltfree.
Most heteroaromatic halides and pseudohalides can be used in the Heck reaction, though sometimes substrates withhalide ortho- to the heteroatom cause problems (10).
Introduction
The Heck reaction works best with alkenes containing electron-withdrawing groups and in most cases gives the β- Fine chemicals are produced on a scale of roughly be- arylated products exclusively. Olefins with electron-donating tween 1 and 10 000 tons per year. They comprise groups give rise to mixtures of α- and β-arylated products. If pharmaceuticals, agrochemicals, polymer additives, flavours palladium complexes with bidentate ligands are used the and fragrances, food and feed additives, and chemical inter- regioselectivity can be determined by the choice of leaving mediates to name the most important classes. With relatively groups. Noncoordinating anions like triflate lead mainly to the α-arylated products, whereas halides predominantly give stoichiometric chemistry as this is easily scaled up and does the β-products (10e). Simple olefins may suffer from Pd- not need specialized equipment. Unfortunately, these pro- catalyzed isomerization reactions leading to mixtures.
duction methods also lead to relatively large amounts of Acetylenes may also be used and are generally more reactive Use of homogeneous catalysis has a number of obvious Pd-catalysts are used with very few exceptions, usually advantages (1–3): (i) less waste; (ii) lower cost, in particular PdCl2 or Pd(OAc)2 alone or in combination with Ph3P or o- if shortcuts in total syntheses can be achieved; (iii) high Tol3P (2 or 3 equiv). Almost all Heck reactions require the chemo- and regioselectivity, easily tuned by the ligands; presence of a base, which is often triethylamine. The combi- (iv) asymmetric catalysis for the single step production of nation Pd(OAc)2, MHCO3 (M = Na, K), KOAc or K2HPO4 with a phase transfer salt is also often used (Jeffery condi- If a product grows to a large volume, use of catalysis in a tions) (10f). The ligand, the counter ion, the base, the phase second-generation process becomes attractive because low transfer salt, and the solvent all have a profound influence on the rate and the selectivity of the reaction; many of these effects are related to the oxidation state and the coordination carbonylation) (6), malonate esters (carbonylation) (7), and chemistry of the catalyst (23). It is also possible to use heter- Metolachlor™ (asymmetric hydrogenation) (8).
ogeneous palladium catalysts such as Pd/C (24). Palladium The lack of homogeneous catalysis in first generation pro- clusters have also shown to be active (25); in fact Pd-clusters cesses is very apparent in the production of pharmaceuticals.
seem to form in most phosphine free Heck reactions. There This is related to the limited duration of patent protection in- is an ongoing debate whether the clusters themselves are cat- ducing a very strong time-to-market incentive. Because of alytically active or if it is a monomeric soluble form of Pd this it is not possible to drastically change complex synthesis that is the active catalyst (10i, 26). Obviously, to increase ac- routes. Fortunately, there is an increasing interest in the use tivity it is of importance to keep the size of these of homogeneous catalysis among medicinal chemists.
nanoclusters small. This can be achieved by the addition of The history of homogeneous catalysis fine chemicals was tetraalkylammonium salts (Jeffery conditions) (10f) or with initially largely dominated by asymmetric catalysis, particu- weakly binding polymers (27) or dendrimers (28). In most larly enantioselective hydrogenations (9). Recently, palla- cases the active species is based on Pd(0), though it can be dium catalysed aromatic substitution reactions have been anionic (23). Proposals have also been advanced for Pd(II)– used increasingly for the production of fine chemicals. In the past few years, five new processes based on the use of the Typical solvents for the Heck reaction are dipolar non- protic solvents like DMF and NMP. An asymmetric variant Color profile: Generic - CMYK US Negative ProofingComposite Default screen Scheme 2. Synthetic scheme for the production of the herbicide
Scheme 3. Sunscreen agent via the Heck reaction.
4 Prosulfuron
of the Heck reaction has been developed (32). The scope ofthis transformation has been limited thus far.
4%. Presumably, some palladium dissolves during thereaction, but after consumption of the starting materials all Use of the Heck reaction for the production
palladium precipitates, allowing easy catalyst recovery. The of fine chemical intermediates
reaction mixture can contain up to 15% of water, which ac-tually has an accelerating effect. This process, which was The herbicide Prosulfuron™
developed at the IMI institute for R & D in Israel, has been The first reported example of industrial use of the Heck used on pilot scale to produce several tons of sunscreen reaction was for the production of Prosulfuron™, a new and agent in yields ranging from 75–92%.
highly active herbicide, by Ciba–Geigy (now Novartis) (33).
In this instance, the Matsuda variant was used to great ad- Naproxen™ via Heck reaction and
vantage as the substrate also contains a sulfonate group. The hydroxycarbonylation
diazotization of the aniline, is a neutral compound and rela- Albemarle, developed a new process for the production of tively stable. The Heck reaction of this betaine on 3,3,3- Naproxen™, based on the Heck reaction of 2-bromo-6- trifluoropropene was performed at 15°C in HOAc using Pd2(dba)3 (0.5–1 mol%) as catalyst (Scheme 2). It was not carbonylation to Naproxen™ (Scheme 4) (6, 35, 36). The necessary to isolate the Heck product. After addition of bromide already was an intermediate for the existing produc- some active carbon to deposit the palladium on, the double tion of Naproxen™ and hence was available at a low price bond was hydrogenated. Not only was the catalyst used in two consecutive steps, this method also allowed the catalyst The key to the commercial success was finding a catalyst to be reclaimed by filtration in 95% yield. The reaction is for the Heck reaction that was sufficiently active. This was performed in a single reactor without isolation of the inter- accomplished by screening ligands based on their steric and mediates. The average yield per step is in excess of 90%.
electronic properties. From these results it was found that aphosphine ligand with steric and electronic properties in the Sunscreen agent via a ligandless Heck reaction
middle of those screened would be optimal. These properties Companies producing bromine and aromatic bromides are were found in neomenthyldiphenylphosphine. Because of of course in an excellent position to apply the Heck reaction.
the high activity of the catalyst it was possible to use a sub- This advantage is not limited to the raw material position, strate:catalyst ratio of between 2000 and 3000 with the reac- but also involves the ability to recycle the bromide salts that tion going to completion at 95–105°C within a few hours.
are formed as waste. Because of the presence of large depos- An ethylene pressure of around 3 kPa was used. This pro- its of bromide, Israel has a flourishing organobromide indus- cess is run on a scale of 500 tons per year. The bromide is try. This has lead to the development of a new process for the production of 2-ethylhexyl p-methoxy-cinnamate, the The hydroxycarbonylation process is catalyzed by a mix- most common UV-B sunscreen (34). The process involves the Heck reaction of p-bromoanisole with 2-ethylhexylacrylate (Scheme 3). In this process, palladium on carbon is Monomers for coatings of electronic components
used as the catalyst without any ligands. This has the disad- The mild conditions of the Heck reaction are also highly vantage of relatively low reactivity as compared to most ho- suitable for making carbon—carbon bonds with aromatic mogeneous catalysts, necessitating a reaction temperature of compounds that are thermally labile. This was the key to 190°C. Also, as a result of the high reaction temperature success in the production of benzocyclobutene-containing some diarylation is found, mainly the 3,3′-isomer in up to monomers. These are used to form coatings (known as Color profile: Generic - CMYK US Negative ProofingComposite Default screen Scheme 4. Naproxen™ via Heck reaction and
Scheme 6. Mechanism of formation of the stilbene side product.
was found after screening a range of phosphorus ligands at 95°C. Tri-ortho-tolylphosphine gave the best results of thecommercially available ligands in terms of product yield(83%). No correlation was found between ligand parameters on one hand, and conversion of bromide, yield of product, orregioselectivity on the other hand. The ligand parameters 10 Naproxen
used were cone angle for bulkiness and pKa for electronicproperties.
A major side product (3–11%) in these reactions is the stilbene (17), which is formed by desilylation of the initially
Scheme 5. Monomers for coatings via the Heck reaction.
formed monoarylated disiloxane (15) followed by Heck re-
action on the vinyl group (Scheme 6). Initial results using 3N as base gave much higher yields of this side product.
This Heck reaction is performed by Dow on a scale of The Heck reaction as the key step in the production of
an antiasthma agent

An important aspect of the attractiveness of the Heck re- action is the ability to form carbon—carbon bonds without the use of strongly basic reagents such as Grignards andlithiated carbon nucleophiles. Hence, the functional grouptolerance of the Heck reaction is very wide allowing its use in the latter stages of a total synthesis. This aspect is very
important in the production of Merck’s LTD4 antagonist
Singulair™ (22) (39) that has been introduced on the market
as an antiasthma agent. A synthesis has been published for
the closely related L-699 392 (21) (Scheme 7) (40). The key
step in the synthesis is the Heck reaction of methyl 2-
iodobenzoate with allylic alcohol (18). Because of the high
reactivity of these substrates it was possible to use ligandless
Pd(OAc)2 as catalyst with Et3N as base in CH3CN as sol- benzocyclobutene groups form ortho-quinone-dimethanes vent. With 1 mol% of catalyst the reaction is complete in upon heating to 180°C, which may react with the double 1 h. A minor by-product was formed via arylation of the 2- bond introduced by the Heck reaction, thus leading to cross- position. However, the product could be obtained in pure The monomers are formed by Heck reaction of 4-bromo- In this variant, reaction on the allylic alcohol leads to the benzocyclobutene with tetramethyldivinyldisiloxane cata- formation of a ketone, because β-hydride elimination of the lyzed by Pd(OAc)2–o-Tol3P in DMF–H2O using KOAc as palladium–alkyl intermediate leads to the preferential forma- base (Scheme 5) (38). The optimal catalyst for this reaction Color profile: Generic - CMYK US Negative ProofingComposite Default screen Scheme 7. The Heck reaction in the production of antiasthma
Scheme 8. A practical catalyst recycle for ligandless Heck reac-
theme are the use of bulky phosphite (18, 41) or phosphoramidite ligands (42), carbene ligands (43, 44), andpincer ligands (31, 45). Another interesting, cheap, and very active class of catalysts are the palladacycles based on aro- matic compounds with side chains containing N (41, 46), O (47), or S (48, 49). The more active catalysts also have al- lowed the use of chloroarenes as arylating agents, which istremendously important because of the much wider commer- cial availability and the lower costs of these compounds(16–20).
Separation of the product can sometimes be hampered by 21 L 699,392
the presence of a phosphine ligand. Therefore, ligandlessHeck reactions are preferred where possible, as shown inthree of the above five cases.
Because of the steadily rising cost of palladium, catalyst recovery and recycle has become a major issue. Recovery is easy in the case of the ligandless Heck reactions as the cata- lyst precipitates completely once all the substrate has been used up. If ligands are used this can be a problem, thoughsometimes water-soluble ligands can be used to allow for two-phase catalysis (50–53). Ionic liquids have also been re-ported as an aid in catalyst recycle (54–56). The recoveredcatalyst is usually returned to the catalyst manufacturer for 22 SingulairTM
reclaiming as catalyst activity is greatly diminished. Recy-cling methods based on the immobilization of ligands areusually flawed, as the instability of the palladium complexeswill invariably lead to leaching (10i). In addition, reactivity Development aspects
ligandless palladium has some merit (57). Even though A major aspect in the development of a homogeneous cat- leaching occurs extensively, the palladium precipitates on alyzed reaction is the cost of the catalyst. This in turn is the support at the end of the reaction. However, these cata- strongly dependent on two important parameters: (i) The ac- tivity of the catalyst expressed as turnover frequency (TOF: Recently, a new method for catalyst recycle was reported mol of product/mol of catalyst h); (ii) The stability of the from our laboratories (58, 59). In this method, Pd(OAc)2 is catalyst expressed as total turnover number (this might in- used as ligandless catalyst. At the end of the reaction, the clude recycling; TON = mol of product/mol of catalyst).
catalyst precipitates in >99% as Pd(0) on an inexpensive car- For this reason, much effort has been put into the develop- rier material like silica, dicalite, or celite. This catalyst is ment of new, more active, and more stable palladium cata- about 10 times less active than the Pd(OAc)2 that is used ini- lysts. The palladacycle developed by Herrmann et al. (17) is tially. However, full catalytic activity can be restored by the a good example. Because of its increased stability, the cata- addition of a few equivalents of I2 or Br2 (Scheme 8). These lyst can be used at higher temperatures, thus producing halogens oxidize Palladium(0) to Pd(II) which is then re- higher reaction rates. In addition, unreactive arylating agents duced again during the next cycle to form highly active cata- like aryl chlorides can be used. More variations on this Color profile: Generic - CMYK US Negative ProofingComposite Default screen Conclusion
12. H.-U. Blaser and A. Spencer. J. Organomet. Chem. 233, 267
The Heck reaction is finding increasing use for the produc- 13. M. Miura, H. Hashimoto, K. Itoh, and M. Nomura. J. Chem.
tion of fine chemicals on a scale of 1–500 tons per year. At least five published cases are known of products that have 14. K. Kikukawa and T. Matsuda. Chem. Letters, 159 (1977).
been produced on a scale of more than 1 ton using the Heck 15. M.S. Stephan, A.J.J.M. Teunissen, G.K.M. Verzijl, and J.G. de reaction. Advantages when compared to more classical meth- Vries. Angew. Chem. Int. Ed. Engl. 37, 662 (1998).
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shorter route than the original stoichiometric route. This has 17. W.A. Herrmann, C. Brossmer, C.-P. Reisinger, T.H. Riermeier, spurred renewed activity in the area of development of new, K. Öfele, and M. Beller. Chem. Eur. J. 3, 1357 (1997).
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2 leads to full regeneration of activity.
20. A.P. Littke and G.C. Fu. J.Org.Chem. 64, 10 (1999).
Acknowledgements
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Source: http://stratingh.eldoc.ub.rug.nl/FILES/root/2001/CanJChemdeVriesJG/2001CanJChemdeVriesJG.pdf

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