Scott E.
Denmark
* and
Christopher R.
Butler
Roger Adams Laboratory, 600 S. Mathews Avenue, Urbana, IL 61801, USA. E-mail: denmark@scs.uiuc.edu; Fax: 217-333-3984; Tel: 217-333-0066
First published on 27th August 2008
Functionalized styrenes are extremely useful building blocks for organic synthesis and for functional polymers. One of the most general syntheses of styrenes involves the combination of an aryl halide with a vinyl organometallic reagent under catalysis by palladium or nickel complexes. This Feature Article provides the first comprehensive summary of the vinylation methods currently available along with a critical comparison of the efficiency, cost and scope of the methods.
Scott E. Denmark was born in Lynbrook, New York on 17 June 1953. He obtained an S.B. degree from MIT in 1975 (working with Richard H. Holm and Daniel S. Kemp) and his D.Sc.Tech. (under the direction of Albert Eschenmoser) from the ETH Zürich in 1980. That same year he began his career at the University of Illinois. He was promoted to associate professor in 1986, to full professor in 1987 and since 1991 he has been the Reynold C. Fuson Professor of Chemistry. His research interests include the invention of new synthetic reactions, exploratory organoelement chemistry and the origin of stereocontrol in fundamental carbon–carbon bond forming processes. Professor Denmark is currently on the Board of Editors of Organic Syntheses and has served on many editorial advisory boards (including Chemical Communications). He is Co-Editor of Topics in Stereochemistry and was an Associate Editor of Organic Letters. In 2008 he became Editor in Chief and President of Organic Reactions. |
Christopher R. Butler was born in Peoria, IL on 22 October 1978. He obtained a B.A. degree from Illinois Wesleyan University in 2000 (working with Ram S. Mohan and Jeffery A. Frick). He then worked as a research associate in Medicinal Chemistry for Johnson and Johnson, PRD in La Jolla, California. In the fall of 2003, he began his graduate studies at the University of Illinois (under the direction of Scott E. Denmark). His thesis work has focused on the development of vinylation reactions using organosilicon reagents. After completing his Ph.D., he will resume his medicinal chemistry career at Pfizer in Groton, CT. |
Styrenes, a subclass of α-olefins in which the alkene bears a single aryl substituent, are useful building blocks for fine chemical synthesis and the polymer industry.3,4 Moreover, these substrates are often workhorses for the optimization of new synthetic methods, often those involving catalytic, asymmetric transformations.5 Hence, the development of efficient, mild, selective, and high-yielding methods for the preparation of styrenes will continue well into the future.
Classically, the installation of a terminal double bond occurs by one of the following strategies: (1) elimination of activated leaving groups, (2) carbonyl olefination (by phosphorus, silicon, or titanium-based reagents), or (3) the partial reduction of a terminal alkyne . A more recent development involves palladium-catalyzed, cross-coupling reactions that employ, as precursors, independent aryl and vinyl units. The features that distinguish each of these approaches include the number of bonds formed, the nature of the precursors needed and the reactions that connect them (Fig. 1).
Fig. 1 Methods used to form a terminal alkene . |
The utility of each of the three approaches can be evaluated by considering the ease of access and stability of the required substrates, as well as the functional group tolerance of the key olefin-forming event. For case A (eliminations), both carbon atoms already must be present, which shifts the problem to the often non-trivial introduction of a functionalized ethyl group. The precursors are generally stable, as leaving group activation is required to effect elimination. Because the reaction conditions for eliminations can involve elevated temperatures and strong bases, only a limited subset of functional groups are compatible. Carbonyl olefination reactions (B) require an aldehyde and ylide starting materials. Aldehydes are readily available precursors and phosphorus ylides are equally accessible. On the other hand both aldehydes and ylides are reactive functions. Most importantly, carbonyl olefination is generally associated with poor atom economy. Finally, each of these disconnections (A and B) is a two-step sequence. The cross-coupling strategy (C), avoids these concerns. In general, the required aryl (or vinyl) halide substrates are commercially available. Aryl halides are inert to most synthetic transformations and can be carried through a multiple reaction sequence as a placeholder for a vinyl group. Further, with the recent development of milder reaction conditions and expanded scope of electrophiles (vide infra) the functional group tolerance of vinylation reactions (Scheme 1) is superior to methods A or B. Therefore, these new methods offer significant strategic advantages over the classical preparations.
Scheme 1 Transition metal-catalyzed, vinylation reaction. |
The cross-coupling disconnection can be further subdivided into four pairwise combinations of aryl and vinyl units (Scheme 2): (1) vinylmetallic donor and aryl halide (or pseudohalide), (2) arylmetallic donor and vinyl halide, (3) aryl halide and vinyl halide (with a reductant), and (4) arylmetallic donor and vinylmetallic donor (with an oxidant). Of these, the first two follow the normal cross-coupling strategy (donor/acceptor) and are therefore the most easily adapted. The latter two are inherently less efficient because the reactants are not oxidation state matched and require stoichiometric amounts of either reductants or oxidants. Moreover, the additional complication of cross, vs. homocoupling products is introduced.
Scheme 2 Cross-coupling disconnections. |
Despite the vast number of newly-developed, transition metal-catalyzed, cross-coupling reactions,6 only a small fraction of these accommodates the attachment of a simple vinyl unit. At first glance, the coupling of a vinyl group appears to be no different than that of more elaborate alkenyl groups. However, the coupling of a vinyl group and an aryl group presents significant differences. The first consideration is cost and atom efficiency.7 Unlike larger and more complex donors and acceptors, the vinyl unit is almost always smaller (lower molecular weight) than the non-transferable group (–SnBu3, –B(OR)2, –BF3, –SiR3, or Br, I). Therefore, the relative size of the non-transferable group is much more pertinent to the overall reaction efficiency in comparison to the alkenyl- or arylmetallic congeners.
The second consideration is the reactivity of the educts and products under the reaction conditions. The vinylmetallic donor (or acceptor) can react in two ways, either in the desired cross-coupling reaction (Scheme 3), or alternatively participate in a Heck reaction8 that leaves the MLn unit intact. Stewart and Whiting,9 and Jeffery10 have independently capitalized upon this disparate reactivity to develop two sets of conditions that are selective for either of these pathways using vinylboronic esters or vinyltrimethylsilane, respectively, and their results will be discussed later.
Scheme 3 Competitive reaction pathways in vinylation. |
In addition, the products of the reaction, by definition, contain a terminal vinyl group, either as a styrenyl or dienyl unit that can serve as substrates for subsequent Heck reactions. Therefore, a successful vinylation reaction must display high selectivity for the primary vinylation process over a secondary Heck process. Finally, the polymerization of the styrenyl and dienyl products is known to occur in the presence of bases and transition-metal catalysts, especially at elevated temperatures. Therefore, mild conditions must be employed to achieve high yields of the desired products.
In the past decade, a number of new vinylation methods have been developed that have dramatically increased the scope of this reaction. Thus, the purpose of this Feature Article is to provide a comprehensive overview of vinylation methods with an emphasis on these recent advances and to evaluate the relative merits of each. The presentation will follow the organization outlined in Scheme 2, beginning with the coupling of vinylmetallic donors and aryl halides. The discussion of this strategy will be organized by the nature of the metal/metalloid on the vinyl donor, following their location in Groups 2–14 in the Periodic Table. The scope and limitations for each of these methods will be discussed, and where applicable, we will specifically illustrate the strategies used to address the aforementioned challenges of the introduction of a vinyl group.
Entry | R | Ethylene/psi | Solvent | t/h | Yield of 2 (%) | Yield of 3 (%) |
---|---|---|---|---|---|---|
a 3-Bromopyridine. | ||||||
1 | 2-CH3 | 20 | CH3CN | 20 | 54 | 34 |
2 | 2-CH3 | 100 | CH3CN | 7 | 83 | 10 |
3 | 2-CH3 | 120 | CH3CN | 18 | 86 | 4 |
4 | 2-NO2 | 120 | CH3CN | 2 | 55 | 5 |
5 | 4-NHAc | 120 | DMF | 23 | 59 | 20 |
6 | 2-NH2 | 200 | CH3CN | 30 | 45 | |
7 | a | 200 | CH3CN | 66 | 52 | |
8 | 3-COOH | 200 | CH3CN | 4 | 51 | 12 |
9 | 2-Br | 200 | CH3CN | 15 | 78 |
Scheme 4 Vinylation using vinylmagnesium bromide (4). |
Entry | R | Pd (mol%) | Yield (%) |
---|---|---|---|
a 2-Bromo-6-methoxynaphthalene. b 3-Bromoquinoline. | |||
1 | 4-t-Bu | 0.4 | 87 |
2 | 4-NMe2 | 1.0 | 89 |
3 | 4-MeC(O) | 0.1 | 85 |
4 | 4-MeC(O) | 0.01 | 24 |
5 | 4-HC(O) | 0.4 | 100 |
6 | 4-CN | 0.4 | 100 |
7 | a | 0.4 | 80 |
8 | 2-MeC(O) | 0.1 | 84 |
9 | 2-HC(O) | 1.0 | 85 |
10 | 2-CN | 0.4 | 87 |
11 | 2,4,6-Me3 | 0.4 | 88 |
12 | 2,4,6-i-Pr3 | 5.0 | 22 |
13 | b | 0.1 | 87 |
Scheme 5 Competitive reactions of pinacol vinylboronic esters. |
Whiting and co-workers subsequently showed that the dioxaborinane 8 efficiently delivers a vinyl group to aryl halides selectively and in good yield.23 This reagent, in conjunction with various activators (potassium hydroxide, potassium tert-butoxide and silver oxide) provides good yields of styrenes in reactions with aryl iodides (Table 3, entries 1–5). Whereas each of the three activators does facilitate the coupling with aryl iodides, silver oxide does not work in reactions that employ aryl bromides. With potassium tert-butoxide, however, aryl bromides can be successfully vinylated, albeit in modest yields in most cases (entries 6–10). Potassium hydroxide also provides some of the styrene products, but was not very effective. Aryl chlorides are unreactive toward 8 regardless of activator . The authors report that they observed the styrene product exclusively in preference to the alkenylborinane product.
Activator | |||||
---|---|---|---|---|---|
Entry | X | R | t-BuOK | KOH | Ag2O |
a 1-Iodonaphthalene. b 1-Bromonaphthalene. | |||||
1 | I | H | 62 | 73 | 51 |
2 | I | 4-Me | 75 | 68 | 83 |
3 | I | 4-MeO | 95 | 66 | 90 |
4 | I | 4-CF3 | 87 | 75 | 76 |
5 | I | a | 65 | 74 | 96 |
6 | Br | H | 56 | 36 | 0 |
7 | Br | 4-Me | 52 | 35 | 0 |
8 | Br | 4-MeO | 65 | 28 | 0 |
9 | Br | 4-CF3 | 71 | 50 | 11 |
10 | Br | b | 41 | 31 | 39 |
11 | Cl | H | 0 | 0 | 0 |
Entry | X | R | Pd (mol%) | Yield (%) |
---|---|---|---|---|
1 | Br | NO2 | 1 | 74 |
2 | I | NO2 | 1 | 77 |
3 | I | NHAc | 1 | 73 |
4 | Br | CN | 1 | 79 |
5 | Br | CHO | 1 | 77 |
6 | Br | Ph | 1 | 84 |
7 | Br | 2-NHBoc-5-F | 1 | 78 |
8 | Br | NHBoc | 1 | — |
9 | Br | NHBoc | 5 | 80 |
10 | Br | Me | 1 | — |
11 | Br | Me | 5 | 68 |
12 | Br | OMe | 5 | 70 |
Entry | R | t/min | Yield (%) |
---|---|---|---|
1 | 4-OMe | 20 | 81 |
2 | 2-Me | 20 | 78 |
3 | 4-COOEt | 120 | 88 |
4 | 2-COOEt | 20 | 70 |
5 | 4-COOH | 15 | 72 |
6 | 3-C(O)Ph | 10 | 81 |
7 | 4-NO2 | 15 | 84 |
8 | 4-Br | 30 | 69 |
9 | 4-OTf | 60 | 75 |
10 | 3-I | 60 | 76 |
Although the aryldiazonium salts are easily prepared from the corresponding anilines (comprising a large pool of available substrate precursors), this additional step detracts from the overall efficiency of the method. To address this limitation, Molander and Rivero extended the scope of these coupling reactions to engage aryl bromides, aryl triflates (OTf) and activated chlorides.25b These reactions are carried out with triethylamine as the base in refluxing 2-propanol.
More recently, Molander and Brown have further optimized the reaction conditions for a wide range of aryl electrophiles by using cesium carbonate in THF–water, 9 : 1).25c Under these conditions, potassium vinyltrifluoroborate provides high yields of the corresponding styrenes for electron-deficient (entries 1–5), electron rich (entries 6–10), and somewhat sterically-hindered (entries 11–15) aryl bromides (Table 6). Electron-deficient substrates react significantly faster than electron-rich substrates, and numerous heterocyclic substrates are competent in the vinylation reaction. Additionally, the triflate derived from 4-hydroxyacetophenone is converted to the corresponding styrene in an 82% yield. Under the standard conditions, 2-bromomesitylene is not completely converted to the styrene and further optimization was required. Among many ligands tested, dicyclohexyl(2-(2′,6′-diisopropoxy)biphenyl)phosphine (RuPhos)27 provides a preparatively useful ratio of the desired product and the stilbene product (arising from a secondary Heck reaction). The employment of the RuPhos ligand also facilitates the coupling of an activated aryl chloride, 4-chloroacetophenone, to provide the corresponding styrene in 65% yield. The reaction setup (refluxing THF–H2O in a sealed tube (85 °C)) is not ideal for large-scale processes, but is comparable to the conditions developed for alternative vinylboron donors. The broad substrate scope and functional group tolerance, however, clearly highlight the advantages of this method.
Entry | Aryl bromide | Yield (%) |
---|---|---|
a 6% RuPhos used. | ||
1 | 4-Bromobenzonitrile | 83 |
2 | 4-Bromoacetophenone | 85 |
3 | 4-Bromobenzotrifluoride | 64 |
4 | Methyl 4-bromobenzoate | 87 |
5 | 4-Bromonitrobenzene | 84 |
6 | 4-Bromoanisole | 72 |
7 | 4-Bromoacetanilide | 78 |
8 | 4-Bromotoluene | 76 |
9 | 4-Bromobenzyl alcohol | 82 |
10 | N,N-Dimethyl 4-bromoaniline | 93 |
11 | 2-Bromotoluene | 82 |
12a | 2-Bromomesitylene | 81 |
13 | 1-Bromonaphthalene | 82 |
14 | 2-Bromobenzonitrile | 82 |
15 | 2-Bromoanisole | 71 |
Although the implementation of organoborane reagents in vinylation reactions was hampered by the instability of vinylboronic acid, the development of vinylboronic esters, cyclic vinylboroxane and potassium vinyltrifluoroborate has addressed this instability and propelled these reagents to the forefront of vinylation reactions.
Scheme 6 Vinylation reactions using vinylaluminium complex 11. |
Oshima and co-workers have shown that vinylgallium dichlorides (derived from the hydroalumination of alkynes and transmetalation to gallium trichloride) are capable of transferring a vinyl group to aryl iodides (and in some cases, aryl bromides).29 The standard reaction conditions require no external activation and are conducted in THF/DMSO at refluxing temperatures (Scheme 7). The electrophile scope demonstrates excellent functional group compatibility and the substrates are transformed to the desired products in good to excellent yields.
Scheme 7 Vinylation using vinylgallium reagent 12. |
Entry | R | 15 (equiv.) | TBAF (equiv.) | Pd(dba)2 (mol%) | t/h | Yield (%) |
---|---|---|---|---|---|---|
a 4-Bromoacetophenone used as substrate. b [allylPdCl]2 (2.5 mol%) used as catalyst. c 40 °C. d AsPh3 (10 mol%) added to the reaction. e 1-Iodonaphthalene used as substrate. | ||||||
1 | 4-COOEt | 1.2 | 2.0 | 1 | 1 | 93 |
2 | 4-C(O)Me | 1.2 | 2.0 | 1 | 1 | 85 |
3 | 4-C(O)Mea | 1.2 | 3.0 | 2.5b | 0.5c | 75 |
4 | 4-NO2 | 1.2 | 2.0 | 1 | 1 | 90 |
5 | 4-CN | 1.2 | 2.0 | 1 | 1 | 87 |
6 | 4-OMe | 1.5 | 4.5 | 5d | 4 | 74 |
7 | 3-NO2 | 1.2 | 2.0 | 1 | 1 | 92 |
8 | 3-COOEt | 1.2 | 2.0 | 3 | 1 | 90 |
9 | 3-CH2OH | 1.2 | 2.0 | 5d | 7.5 | 79 |
10 | 2-NO2 | 1.2 | 2.0 | 1 | 1.5 | 86 |
11 | 2-COOEt | 1.2 | 3.0 | 5d | 14 | 85 |
12 | 2-Me | 1.2 | 3.0 | 5d | 16 | 70 |
13 | 2-OMe | 1.5 | 4.5 | 5d | 10 | 75 |
14 | e | 1.2 | 3.0 | 5 | 4 | 76 |
However, DVDS itself can be used as a donor for vinylation reactions. Denmark and Butler have shown that potassium trimethylsilanolate (KOSiMe3) is capable of activating DVDS toward vinylation reactions through a “silanolate exchange” in DMF to generate two equivalents of potassium vinyldimethylsilanolate (17) and one equivalent of the innocuous hexamethyldisiloxanein situ (Scheme 8).39 Therefore, each of the two vinyl groups on DVDS is available for transfer, increasing the efficiency of these reactions.
Scheme 8 Silanolate exchange of DVDS and KOSiMe3. |
The in situ generated vinyldimethylsilanolate reacts with a range of aryl iodides at room temperature (Table 9). Good yields are obtained in all cases, and some functional groups are tolerated (entries 2, 5 and 6). The reactions are generally fast (<3 h), although 2-iodoanisole requires 14 h. The successful vinylation of ethyl 4-iodobenzoate is notable, as the ester survives the reaction even in the presence of potassium trimethylsilanolate, which is capable of cleaving esters to the corresponding acids.40
Entry | R | Pda (mol%) | t/h | Yield (%) |
---|---|---|---|---|
a 1.0 equiv. Ph3PO per Pd(dba)2. b 1-Iodonaphthalene. | ||||
1 | 4-OMe | 5 | 1.5 | 80 |
2 | 4-COOEt | 2.5 | 0.5 | 81 |
3 | 4-OBn | 5 | 1.5 | 74 |
4 | 4-t-Bu | 5 | 4 | 69 |
5 | 4-CN | 2.5 | 0.5 | 81 |
6 | 3-NO2 | 2.5 | 1 | 76 |
7 | 2-OMe | 5 | 14 | 68 |
8 | b | 5 | 2.5 | 80 |
Avoiding the requirement of fluoride-based activators is of great value, as fluoride reagents are generally expensive,41 capable of etching glass reaction vessels, and are incompatible with silicon-based protecting groups. In contrast, the combination of DVDS and KOSiMe3 does not suffer from these limitations; both reagents are inexpensive and widely available.42 Thus, “fluoride-free” activation significantly enhances the scope and utility of these reactions.43
Aryl bromides also succumb to the vinylation conditions with only minor modifications.39 A simple increase in reaction temperature (to 70 °C) and a solvent change to THF is sufficient to engage a number of aryl bromides (Table 10). This modification has a significantly narrower substrate scope than the aryl iodides, although good yields are obtained in some cases (entries 1 and 3–5). Substrates bearing strongly electron-withdrawing substituents (entries 2, 6, and 7) generally give diminished yields due to competing polymerization of the products under the reaction conditions.
Entry | R | t/h | Yield (%) |
---|---|---|---|
a 3-Bromoquinoline. b 2-Bromo-6-methoxynaphthalene. | |||
1 | 4-OMe | 1 | 71 |
2 | 4-Cl | 3 | 55 |
3 | 4-PhC(O) | 3 | 93 |
4 | a | 3 | 91 |
5 | b | 3 | 70 |
6 | 4-NMe2C(O) | 3 | 34 |
7 | 2-CF3 | 3 | 52 |
To improve the scope of this reaction with aryl bromides, a second, milder protocol was developed that employs potassium triethylsilanolate (KOSiEt3) in place of KOSiMe3. The superiority of KOSiEt3 is related to its increased steric bulk that allows for silanolate exchange without concomitant attack at the palladium center. The combination of di-tert-butyl(2-biphenyl)phosphine (BPTBP, 18) and [allylPdCl]2 in DMF allows for successful vinylation reactions at much lower temperatures than the initial modification (Table 11). Thus, electron-rich (Table 11, entries 1–2 and 7), electron-deficient (entries 3–5 and 9–12), and sterically encumbered aryl bromides (entries 7–9) are vinylated at or just above room temperature. The functional group tolerance is significantly increased, as amino, amido, carboalkoxy, and silyloxy groups all participate and are converted to the corresponding styrene in good yields. Unfortunately, a bulkier ester (tert-butyl) is required as cleavage of an ethyl ester is observed over the longer reaction times (entry 5). A divinylation of 1,4-dibromobenzene can also be accomplished, albeit in significantly diminished yields, likely due to competing polymerization of the divinylbenzene product (entry 12).
Entry | R | KOSiEt3 (equiv.) | T°/C | t/h | Yield (%) |
---|---|---|---|---|---|
a 2.0 equiv. of DVDS used. b 1-Bromonaphthalene. c 2-Bromo-6-methoxynaphthalene. d 3-Bromoquinoline. e 2.6 equiv. of DVDS used. | |||||
1 | 4-OMe | 4a | 35 | 4 | 74 |
2 | 4-NMe2 | 4a | 40 | 24 | 70 |
3 | 4-Cl | 3 | 40 | 3 | 50 |
4 | 4-Me2NC(O) | 3 | 40 | 2 | 69 |
5 | 4-CO2tBu | 3 | 40 | 2 | 60 |
6 | 3-TBSOCH2 | 4a | 40 | 24 | 64 |
7 | 2-OMe | 4a | 40 | 24 | 72 |
8 | 2,4,6-Me3 | 4a | 40 | 24 | 99 |
9 | b | 3 | 25 | 12 | 80 |
10 | c | 3 | 25 | 4 | 82 |
11 | d | 3 | 25 | 2 | 79 |
12 | 4-Br | 6e | 40 | 2 | 48 |
Entry | R | 19 (equiv.) | TBAF (equiv.) | t/min | Yield (%) |
---|---|---|---|---|---|
a Pd(dba)2 (1 mol%) used. b Ph3As (10 mol%) added to the reaction. c 24 h. d 1-Iodonaphthalene. | |||||
1 | 4-C(O)Me | 0.3 | 2.0 | 10 | 88 |
2 | 4-COOEt | 0.3 | 2.0 | 10 | 85 |
3a | 4-COOEt | 0.3 | 2.0 | 60 | 83 |
4b | 4-OMe | 0.375 | 3.0 | 360 | 63 |
5 | 3-NO2 | 0.3 | 2.0 | 10 | 87 |
6 | 3-CH2OH | 0.3 | 2.0 | 480 | 59 |
7b | 2-OMe | 0.375 | 3.0 | c | 72 |
8 | 2-COOMe | 0.3 | 2.0 | 480 | 83 |
9 | d | 0.3 | 2.0 | 180 | 64 |
The reaction of 19 with aryl bromides has also been developed48 but requires the use of phosphine18 as a ligand for palladium at elevated temperatures (50 °C). Additionally, a slight increase in the loading of D4V is needed to suppress the formation of the symmetrical stilbene. With these adaptations, a wide range of bromides participates in the reaction, including electron-rich (Table 13, entries 6–9 and 13), electron-deficient (entries 1–5 and 11), and sterically-encumbered substrates (entries 7, 9, 11 and 12). Bromides containing nitrogen functions (entries 12–14) afford good to excellent yields of the corresponding styrenes, although substrates bearing free –OH and –NH2 groups give lower yields. This method has also been recently shown to work with vinyl halides in the synthesis of Diels–Alder precursors.49
Entry | R | t/h | Yield (%) |
---|---|---|---|
a 1-Bromonaphthalene. b 2-Bromonaphthalene. c 3-Bromoquinoline. | |||
1 | 4-C(O)Me | 3 | 91 |
2 | 4-COOEt | 5 | 83 |
3 | 2-COOEt | 5 | 86 |
4 | a | 3 | 71 |
5 | b | 3 | 81 |
6 | 2-Et | 17 | 75 |
7 | 2,4,6-Me3 | 48 | 72 |
8 | 4-OMe | 10 | 86 |
9 | 2-OMe | 20 | 80 |
10 | 4-CH2OH | 14 | 54 |
11 | 2-NO2 | 2 | 85 |
12 | 4-NHAc | 12 | 77 |
13 | 2-NMe2 | 24 | 74 |
14 | c | 3 | 89 |
Scheme 9 Vinylation using vinyltrimethoxysilane (20). |
Although both of these methods employ inexpensive52vinyltrimethoxysilane, TBAF is required as an activator and therefore the methods suffer from the drawbacks discussed earlier. Recently, Alacid and Najera have developed vinylation conditions using 20 or vinyltriethoxysilane (23) that do not require fluoride activation.53 The authors found that both 20 and 23 could engage in cross-coupling with aryl halides in the presence of sodium hydroxide in water at 120 °C, using either conventional or microwave (μW) heating (Table 14). Both palladium acetate and palladacycle 24 are able to effect the reaction (Table 14, entries 1 and 2). Better results are obtained for reactions with 25 mol% of tetrabutylammonium bromide (TBAB) (entry 3 cf. 2), and in those cases, the catalyst loading could be decreased to 0.01 mol% (entry 5), although most reactions require at least 0.1 mol%. The reaction conditions are general for a moderate scope of aryl iodides and aryl bromides. The reactions that employ conventional heating provide similar (and sometimes superior) yields, although those reactions are significantly slower (entry 10 cf. 11). Activated aryl chlorides participate in the reaction, but provide diminished yields of the corresponding styrenes (entries 13 and 14). The functional group tolerance is limited, likely due to the use of aqueous hydroxide at elevated temperatures, although ketone-bearing and pyridine substrates are coupled in high yield (entries 1–3,12 and 13).
Entry | R1 | R2 | X | Pd cat (mol%) | Heat source | t/min | Yield (%) |
---|---|---|---|---|---|---|---|
a TBAB (25 mol%) added. b 1-Bromonaphthalene. c 2-Bromo-6-methoxynaphthalene. d TBAB (200 mol%). e 1 day. f 3-Bromopyridine. | |||||||
1 | Me | 4-MeC(O) | Br | Pd(OAc)2 (0.5) | μW | 10 | 97 |
2 | Et | 4-MeC(O) | Br | 24 (0.5) | μW | 10 | 90 |
3 | Me | 4-MeC(O) | Br | 24 (0.1)a | μW | 10 | 99 |
4 | Me | 4-MeO | I | 24 (0.1) | μW | 10 | 93 |
5 | Me | 4-MeO | I | 24 (0.01)a | Δ | 240 | 89 |
6 | Me | 3,5-(MeO)2 | I | Pd(OAc)2 (0.1)a | μW | 15 | 83 |
7 | Me | 4-MeO | Br | 24 (1)a | μW | 10 | 97 |
8 | Me | b | Br | 24 (1)a | μW | 20 | 92 |
9 | Me | 4-Cl | Br | 24 (1)a | μW | 15 | 71 |
10 | Me | c | Br | Pd(OAc)2 (0.5)d | Δ | e | 89 |
11 | Me | c | Br | Pd(OAc)2 (0.5)a | μW | 10 | 89 |
12 | Me | f | Br | 24 (1)a | μW | 15 | 97 |
13 | Me | 4-MeC(O) | Cl | 24 (2)a | μW | 25 | 71 |
14 | Me | 4-PhC(O) | Cl | 24 (2)a | μW | 25 | 65 |
A number of silicon-based reagents have addressed the challenges of developing a mild and efficient vinylation reaction. Fluoride activation is needed with trialkylsilanes, polyvinylsiloxanes, and trialkoxysilanes. The recently introduced methods that employ non-fluoride activators have considerably enhanced the utility of these reagents. Vinyltrialkoxysilanes can be activated by aqueous hydroxide at high temperatures, whereas the combination of DVDS and KOSiMe3 generates the vinyldimethylsilanolate in situ. Both of these methods are able to engage a range of aryl bromides in good yield without the need for fluoride.
In 1986, Scott and Stille described the first successful cross-coupling of enol triflates using 25.57 Less than a year later, a second report detailed the vinylation of a wide range of aryl bromides. In the reactions with aryl bromides, no external activation is required, thus simplifying the reaction protocol and facilitating a broad substrate scope and functional group tolerance (Table 15). Aryl bromides bearing nitro, formyl, 1,3-dicarbonyl, keto and carboalkoxy groups in the para position are all tolerated (entries 2, 4 and 6–9). 1,4-Dibromobenzene can undergo a mono- (entry 5) or divinylation (12 h, 73% yield) by using 1 or 2.2 equiv. of 25, respectively.
Fu and co-workers have introduced an improvement that allows aryl bromides to be vinylated at room temperature.58a By the use of the bulky, electron-rich ligand tri-tert-butylphosphine in combination with Pd2(dba)3 a wide range of bromides undergo cross-coupling with 25 in 66–92% yield (Table 16). In general, the substrate scope is good and electron-deficient (entries 1, 2 and 5), electron-rich (entries 3, 4 and 8), and sterically hindered substrates (entries 5–9) are vinylated with similar degrees of success.
Entry | R | Yield (%) |
---|---|---|
a 2-Bromonaphthalene. b Reaction carried out in Et2O. c 1-Bromonaphthalene. d 9-Bromoanthracene. | ||
1 | 4-MeC(O) | 88 |
2 | a | 88 |
3 | 4-PhO | 85 |
4 | 4-OH | 85b |
5 | 2-COOEt | 92 |
6 | 2-Ph | 76 |
7 | c | 91 |
8 | 2,4-(OMe)2 | 89 |
9 | d | 66 |
Shirakawa and Hiyama extended the scope of this reaction to include aryl chlorides by using nickel catalysis.59 These reactions require the preformation of a nickel hydride complex derived from Ni(acac)2, Ph3P and DIBAL-H. Aryl chlorides are converted to the corresponding styrenes at 80 °C in dioxane in 9–96 h using this catalyst. Reaction yields range from 37–91% (Table 17) and electron-deficient substrates afford higher yields in shorter reaction times (entries 1–4). Substrates bearing sulfur- and nitrogen-containing substituents also undergo cross-coupling albeit in diminished yields. In general, steric encumbrance does not affect the yield or reaction rate significantly (entries 1 vs. 2 and 6 vs. 8), although 2-bromobenzonitrile reacts more slowly and in poorer yield than does 4-bromobenzonitrile. This tendency was confirmed by competition experiments, which showed that the relative rate of the vinylation of 2-bromobenzonitrilevs.4-bromobenzonitrile was significantly lower than similar comparisons with other functional groups.
Entry | R | t/h | Yield (%) |
---|---|---|---|
1 | 4-PhC(O) | 23 | 91 |
2 | 2-PhC(O) | 23 | 80 |
3 | 4-CHO | 9 | 86 |
4 | 4-CN | 10 | 78 |
5 | 2-CN | 96 | 37 |
6 | 4-OMe | 91 | 51 |
7 | 3-OMe | 40 | 69 |
8 | 2-OMe | 30 | 66 |
9 | 4-NH2 | 67 | 40 |
10 | 2-NH2 | 67 | 51 |
11 | 4-SMe | 37 | 65 |
Littke and Fu have developed a procedure for the vinylation of aryl chlorides using palladium catalysis.58b Two aspects of the reaction conditions are crucial to the success of the method. First, the reactions require the use of the bulky, electron-rich tri-tert-butylphosphine as described above. Second, a fluoride source is needed, presumably to facilitate the transmetalation from tin to palladium. Cesium fluoride is optimal for this catalyst/ligand system. Using these conditions, aryl chlorides can be converted to the corresponding styrenes in good to excellent yield at either 80 °C or 100 °C in dioxane (Table 18). Amino and keto groups are tolerated (entries 1 and 4), and sterically encumbered substrates react in good yield (entry 5).
Nolan and co-workers confirmed the beneficial aspects of fluoride additives in Stille couplings by observing hypercoordinate aryl- and vinylstannate intermediates with 19F-NMR spectroscopy .60 These observations translated to preparative utility, as Nolan is able to vinylate aryl bromides using vinyltributyltin and TBAF. These reactions occur at lower temperatures (80 °C) than previously reported by Stille, again using their NHC ligands, although reaction times for unactivated bromides were rather long (Table 19, entries 1–4). Whereas 4-chloroacetophenone is vinylated under similar reaction conditions in good yield (entry 5), less activated substrates did not provide satisfactory yields (entries 6 and 7).
Entry | R | X | t/h | Yield (%) |
---|---|---|---|---|
1 | 4-MeC(O) | Br | 3 | 92 |
2 | 4-OMe | Br | 48 | 69 |
3 | 2,4,6-Me3 | Br | 48 | 25 |
4 | 4-Me | Br | 48 | 98 |
5 | 4-MeC(O) | Cl | 3 | 83 |
6 | 4-MeO | Cl | 24 | 15 |
7 | 4-Me | Cl | 12 | 41 |
Charette and co-workers have recently developed recyclable triarylphosphonium-supported tin reagents.61 The application of these reagents under the conditions described above provides equivalent, in some cases superior, results (Scheme 10). More importantly, the tin byproducts are removed by precipitation and filtration, thus minimizing the toxic waste stream.
Scheme 10 Vinylation reaction with a supported tin reagent. |
Scheme 11 Vinylation using vinyl chloride. |
Three methods have been developed recently to further enable this disconnection and circumvent these challenges. The first method, developed by Lando and Monteiro, employs the reaction of arylboronic acids and dibromoethane.66 The coupling of these reagents in methanol provides the corresponding styrenes in moderate to excellent yields (Table 20). The reaction involves the dehydrobromination of 1,2-dibromoethane using 4 equiv. of KOH to generate vinyl bromide in a separate step prior to the introduction of the arylboronic acid and the catalyst (Pd(OAc)2). The reaction tolerates a free carboxylic acid (entry 6) and a range of halogen substituents (entries 3–5). Electron rich (entries 1 and 7) and sterically encumbered substrates (entries 7–9) provide the corresponding styrenes in similar yield. Because the roles of the donor and the acceptor are reversed in this strategy, the nucleophilicity of the arylboronic acids plays a role in these reactions, and, not surprisingly, the electron-deficient (and thus less nucleophilic ) substrates (entries 3–6) afford lower yields. Although this method addresses the challenge of using vinyl bromide by generating it in situ, the loss of two moles of HBr significantly lowers its atom efficiency. Unfortunately, 1.5 equiv. of the more expensive arylboronic acid are required, thus making this a less economical process.
Entry | Aryl | Yield (%) |
---|---|---|
1 | 4-Methoxyphenyl | 94 |
2 | 4-Tolyl | 86 |
3 | 4-Trifluoromethylphenyl | 58 |
4 | 4-Fluorophenyl | 62 |
5 | 4-Chlorophenyl | 58 |
6 | 4-Benzoyl | 69 |
7 | 2-Methoxyphenyl | 89 |
8 | 2-Mesityl | 63 |
9 | 1-Naphthyl | 100 |
10 | 2-(6-Methoxynaphthyl) | 72 |
The second strategy, developed by Skrydstrup and co-workers, employs vinyl tosylate (derived from the fragmentation of tetrahydrofuran) and arylboronic acids.63 The combination of these two addends with potassium phosphate and the commercially available catalyst SK-CCO2-A (26) provides corresponding styrenes in 60–99% yields (Table 21). Electron-rich (entries 4, 5 and 7) and electron deficient (entries 2, 8 and 9) substrates are transformed in similar yields. Excellent functional group tolerance is observed, and includes cyano, acetamide, aldehyde and thio ether groups (entries 2–4 and 8–9). Additionally, the use of aryl- and heteroarylboronic esters (pinacol and neopentyl glycol) increases the substrate scope to include pyridyl and quinolyl precursors, and even those bearing secondary amines.
Entry | Aryl | Yield (%) |
---|---|---|
1 | 1-Naphthyl | 99 |
2 | 4-Cyanophenyl | 89 |
3 | 3-Acetamidophenyl | 89 |
4 | 4-Thiomethoxyphenyl | 90 |
5 | 3′-(1,3-Benzodioxolyl) | 85 |
6 | 2-(6-Methoxynaphthyl) | 60 |
7 | 1-Dibenzofuryl | 89 |
8 | 2-Formylphenyl | 63 |
9 | 4-Formylphenyl | 90 |
Dunet and Knochel have also described a method that implements this strategy in the coupling of arylcyanocuprates and vinyl nonaflates (n-C4F9SO3) to provide styrenes in 64–72% yields.67
A third strategy, involving the coupling of vinyl acetates and aryl halides catalyzed by a cobalt(II) complex, has been reported by Gosmini and co-workers.68 This example represents the third motif described in the introduction, wherein both vinyl and aryl groups are introduced as acceptors, and 10 equiv. of manganese (per aryl chloride) is used to reduce the cobalt catalyst and complete the catalytic cycle. Aryl bromides and chlorides are converted to the corresponding styrenes in 37–81% yield (Scheme 12).
Scheme 12 Vinylation of aryl halides using a cobalt catalyst. |
Vinyl reagent | Vinyl donor | MW | Equiv. requireda | Nominal atom efficiencyb (%) | Actual atom efficiencyc (%) | Actual atom efficiency (+ activator )d (%) | $ mol−1e | $ per mol vinyl transferredf | Electrophile scope |
---|---|---|---|---|---|---|---|---|---|
a Equivalents of vinyl donor required to effect vinylation. b MW C2H3/MW donor. c Nominal efficiency/equiv. required. d MW C2H3/((MW donor X equiv.) + (MW activator X equiv.)). e Based upon 2007–2008 Aldrich catalog. f ($ mol−1)/required equivalents. | |||||||||
4 | VinylMgBr | 131.25 | 2 | 21 | 11 | 11c | 61 | 132 | Poor |
5 | VinylB(OH)2 | 71.87 | 3 | 38 | 13 | 5.5 (K2CO3) | — | — | Good |
8 | VinylB(OR)2 | 154.01 | 1.2 | 17 | 15 | 8.4 (KOt-Bu) | 3578 | 4293 | Moderate |
9 | (VinylBO)3·Pyr | 240.67 | 0.5 | 11 | 22 | 10.4 (K2CO3) | 4615 | 2307 | Excellent |
10 | VinylBF3K | 133.95 | 1 | 20 | 20 | 2.4 (Cs2CO3) | 1958 | 1958 | Excellent |
11 | VinylAl(OR)NR | 338.2 | 1 | 8 | 8 | 8 | — | — | Moderate |
12 | VinylGaCl2 | 167.67 | 1 | 16 | 16 | 16 | — | — | Excellent |
13 | VinylSiMe3 | 100.27 | 1.3 | 27 | 21 | 9 (KF) | 615 | 799 | Moderate |
15 | Vinylsiletane | 112.24 | 1.2 | 24 | 20 | 3.6 (TBAF) | — | — | Excellent |
16 | DVDS | 186.40 | 0.75 | 14 | 19 | 4.6 (KOSiMe3) | 236 | 177 | Good |
19 | D4V | 344.66 | 0.3 | 8 | 26 | 3.4 (TBAF) | 345 | 103 | Excellent |
20 | VinylSi(OMe)3 | 148.23 | 2 | 18 | 9 | 6.8 (NaOH) | 29 | 58 | Good |
25 | VinylSnBu3 | 317.10 | 1.08 | 9 | 8 | 8 | 3675 | 3969 | Excellent |
Another important criterion is the relative cost of the commercially available reagents. In this comparison, the silicon-based reagents ($29–615 mol−1) are considerably more attractive than the corresponding vinylboron and vinyltin donors ($1958–4615 mol−1). In many cases, these cost and atom efficiencies will greatly impact a synthetic strategy and therefore must be considered when evaluating alternative vinylation methods.
In this Feature Article, we have described advances that have enhanced the substrate scope and utility of the palladium-catalyzed vinylation reaction. The three main developments that have been highlighted are: (1) the preparation of specifically-tuned vinyl donors, stable for storage but reactive under palladium (or nickel) catalysis, (2) the incorporation of newly developed ligands that facilitate various components of the catalytic cycle and allow for reactions to occur under milder conditions, and (3) the elimination of toxic reagents and by-products from the reactions. Collectively, the methods that arose from these developments provide access to a wide range of styrene derivatives from multiple classes of aryl electrophiles. These methods encompass considerable overlap, thus affording many options (vinyl donors and conditions) for a specific vinylation. From the perspective of scope and utility, the current state of the art is deemed acceptable. However, as clearly highlighted in Table 22, considerable room for improvement remains, particularly to provide solutions that are more amenable to scalable processes. Thus, future work should focus upon the development of more cost- and atom-efficient vinyl donors to address these limitations.
Footnote |
† Dedicated to the memory of Prof. Makoto Kumada, a pioneer in cross-coupling chemistry. |
This journal is © The Royal Society of Chemistry 2009 |