Transition Metal Catalyzed SP3 C-H Bond Activation Reactions

Transition Metal catalyzed C-C Bond Activation Reactions

Chemistry-Proverbs (Important aspects of Chemistry research)

10 seconds labeling a sample saves 30 minutes identifying it later.

Knowing the chemistry of a reaction before running it once is better than running it ten times without knowing anything.

Just because you found it in literature doesn't mean it's correct

The more you learn, the dumber you feel

Troubles overcome are good to tell

One chemist's side product/decomp is another chemists target.

Cleanliness is next to success

A day in the library can save a week in the hospital

The more you get,the more you want

Let it sit, and it goes to sh*&.

Gram in hand is worth two in the flask

Rearrangements happen.

Garbage in, garbage out

If you don't get what you like, try to like what you get.

The more you try the luckier you get.

Change or you will be changed

Too much analysis leads to paralysis

It's not the cage that sings, it's the bird inside.

The flask only falls when you are not there to catch it.

If it is highly colored, you will spill it on yourself.

The worse it smells, the greater chance that it will bump.

Chemistry is trying

The more you give the more you get

An hour a week on basics can yield advanced ideas

Education is the path from cocky arrogance to miserable insecurity

Love your chemistry, chemistry will love you!

The work isn't done until the pigs are fed, watered and ready to fly!

A week in the lab can save you a day in the library.

BOC-Pipirazines WILL foam!

When in doubt, throw it out.

Iterative Suzuki Miyaura Coupling Reaction

Iterative Suzuki Miyaura Coupling Reactions

     The Suzuki-Miyaura cross-coupling reaction of boronic acids is one of the most important and highly utilized reactions in the organic chemistry toolbox, with applications in polymer science as well as in the fine chemicals and pharmaceutical industries. However, many boronic acids are extremely unstable and susceptible to decomposition that renders them inefficient in coupling reactions or makes long-term storage difficult.

    Iteration (lat. “iterare”=to repeat) is a powerful strategy employed in the biosynthesis of complex molecules. In these controlled iterative reactions, di- and multifunctional building blocks are employed that contain only one reactive functional group (“ON”), while all other groups are unreactive (“OFF”) thereby suppressing uncontrolled polymerization (Scheme 1). After the selective coupling of the reactive group, another, previously unreactive functional group is activated/deprotected (“ON”) and the coupling sequence repeated, thus allowing the efficient formation of defined oligomers from readily available building blocks. This enables even nonexperts to synthesize complex molecules in a short time, and promotes the rapid investigation and application of these compounds in chemistry and biology. An ideal iterative coupling would meet the following criteria:

ü  Many differently substituted building blocks are readily available and inexpensive;

ü  Coupling and activation/deprotection step are high yielding, are tolerant of many different functional groups, and do not require nor produce toxic compounds;

ü  Handling, separation, and purification are facile;

ü  The iterative coupling sequence is reliable and predictable, which are important aspects for applications in natural product synthesis and in industry;

ü  The sequence is suitable for solid phase synthesis and automation.

Here you may get the recent literature on Iterative Suzuki Miyaura Coupling reaction.

CSIR-GATE Model Questions

Name Reactions in Organic Chemistry

Dear Chemistry lovers, 

As a chemist we need to remember at least 100-200 name reactions. You no need to worry to find all name reaction collections. you can easily find from the following sites. These are very nice sites for your name reaction searches. 

Named Organic Reactions (By Prof. Doug Taber) 

Named Organic Reactions (by Prof. Michael Smith) 

Named Organic Reactions (by Hilton Evans) 

Named Organic Reaction (by Marcus Brackeen) 

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Chemistry of Benzylic Compounds
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Amines : Preparation and Reactivity
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Chemistry Of Carboxilic Acids
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Enols. Enolates, Enals, and Enones

Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Unsaturated Aldehydes and Ketones

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Carbonyl Compounds: Aldehydes and Ketones
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Directing Group Effects in Aromatic Electrophilic Substitution Reactions
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Electrophilic Aromatic Substitutions
Its a very nice video Class by prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Benzene and Aromaticity
Its Very Nice Video Class By prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Diels-Alder and Electrocyclic Reactions
Its Very Nice Video Class By prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Delocalized Pi system : Proppenyl and Extended Conjucation
Its Very Nice Video Class By prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Organic Chemistry: Structure and Reactivity - Video Class By Peter Vollhardt

Delocalized Pi Systems - Propenyl and Butadiene

Its Very Nice Video Class By prof. Peter Vollhard  from Berkeley University. Its a good resource for student as well lecturers.

Femtosecond laser

Femtosecond laser system 
  A detailed schematic of the femtosecond laser facility is shown in Figure 1. A femtosecond mode-locked seed beam of 14.5 nm bandwidth, pulse energies in the nanojoule range and repetition rate of 80 MHz is emitted from a Ti:sapphire oscillator pumped by a diode laser. A pulsed Nd:YLF operating at repetition rate of 1 kHz pumps the seed beam through a regenerative amplifier. Using the chirped pulse amplification technique, ultra-short pulses are generated with a FWHM pulse width of about 83 fs, 800 nm wavelength and 1 mJ maximum pulse energy.

To more information, read this PPT .

CSIR - Chemistry Study Materials - IV

CSIR and GATE exams are considered as gate way to do Phd in India in various field ranging from biology to engineering ,even foreign university expecting students applying from India to be csir cleared. Candidates are advised to go through the syllabus, Exam Pattern, Sample Papers for better preparation. Here you may get study materials for CSIR Exam paper-II. It is also good for all competitive exams.

CSIR - Chemistry Study Materials - III

CSIR and GATE exams are considered as gate way to do Phd in India in various field ranging from biology to engineering ,even foreign university expecting students applying from India to be csir cleared. Candidates are advised to go through the syllabus, Exam Pattern, Sample Papers for better preparation. Here you may get study materials for CSIR Exam paper-II. It is also good for all competitive exams.

CSIR - Chemistry Study Materials - II

CSIR and GATE exams are considered as gate way to do Phd in India in various field ranging from biology to engineering ,even foreign university expecting students applying from India to be csir cleared. Candidates are advised to go through the syllabus, Exam Pattern, Sample Papers for better preparation. Here you may get study materials for CSIR Exam paper-II. Its also good for all chemistry compettive Exams.

CSIR - Chemistry Study Materials - I

CSIR - Chemistry Study Materials for Paper-II

CSIR and GATE exams are considered as gate way to do Phd in India in various field ranging from biology to engineering ,even foreign university expecting students applying from India to be csir cleared. Candidates are advised to go through the syllabus, Exam Pattern, Sample Papers for better preparation. Here you may get study materials for CSIR Exam paper-II.

Synthesis of (Diacetoxyiodo)benzene

(Diacetoxyiodo)benzene; Large-Scale Synthesis:

K2S2O8 (100 mmol) was slowly added portion-wise over 20 min to a stirred solution of an iodobenzene (5.10 g, 25 mmol) in AcOH (125 mL) with concd H2SO4 (100 mmol) at r.t. (25 °C), and the mixture was stirred at r.t. for 4 h. The solution was then concentrated to half its volume by evaporation of AcOH under reduced pressure, and H2O (100 mL) was added. The precipitate formed was collected by filtration, washed with H2O (200 mL), and dried in air. A second crop of product was obtained by extraction of the filtrate with CH2Cl2 (3 × 25 mL); the combined extracts were dried over anhyd Na2SO4, filtered, and concentrated under reduced pressure. The combined crude products were purified by recrystallization from AcOH–hexane; yield: 7.15 g (88.7%).

Reference:

Synthesis 2005, No. 12, 1932–1934

Synthesis of Palladium tetrakis(triphenylphosphine)

Chemicals Used

Palladium dichloride (1g, 1 equiv.) Triphenylphosphine (7.4g, 5 equiv.) Dimethylsulphoxide (distilled, 12mL/mmol, 68mL) Hydrazine hydrate (1.1mL, 4 equiv.)

Procedure

Palladium chloride is placed in a 3 necked round bottomed flask which is fitted with a thermometer and a filter stick attached to a second inverted 3 necked round bottomed flask. DMSO is added, followed by triphenylphosphine. The mixture is heated to 140C using an oil bath with stirring in order to dissolve all the solid (a small amount of solid does not seem to dissolve). The oil bath is then removed and the mixture stirred for a further 15 minutes. Hydrazine hydrate is rapidly added causing evolution of nitrogen. The resulting dark solution is immediately cooled with a water bath until crystallisation begins to occur (~125C) at which point it is allowed to cool without external cooling. Once the mixture has reached room temperature, the apparatus is inverted in order to filter and the solid is washed succesively with absolute ethanol (2 x 10 mL) and dry ether (2 x 10 mL) (this is achieved by adding the solvent from a syringe through a side arm). The resulting yellow solid is then dried under vacuum for 3-4 hours.

Note:

All glassware should be oven or flame dried prior to use, and the entire procedure should be carried out under nitrogen. If care is taken in the preparation, the material should have a bench life of many months (>8), and should be stored under nitrogen in a freezer. 5-6g of catalyst is atttainable starting from 1g palladium dichloride. Analysis of the compound is not really required; if it's a yellow solid, it should work (although melting point determination should give a decomposition point of 115C, but this has not been checked)! On exposure to air, the material will slowly turn orange, this seems to be a result of reaction with water.

Reference:

D.R Coulson, Inorganic Syntheses XIII, p.121

Iron-catalyzed oxidative homo-coupling of indoles via C–H cleavage

Iron-catalyzed oxidative homo-coupling of indoles via C–H cleavage

The authors, Tianmin Niu and Yuhong Zhang from Zhejiang University, peoples republic of China reported a cheap Fe catalysed Oxidativecoupling of N-H indoles. Its a new method for the homo-coupling of indoles has been developed by the use of FeCl3 as catalyst and molecular oxygen as the only oxidant. The protocol provides a practical and straightforward approach toward 3,3′-biindolyls.

For original Article:
doi:10.1016/j.tetlet.2010.10.088

Transition Metal Free C-H Activation and C-C Bond Formation – Is that Really Metal Free?

Transition metal catalysed direct arylation of aromatic C-H bonds is emerging as a valuable and efficient alternative to traditional cross-coupling in the construction of biaryl compounds. Selective functionalization of aromatic C–H bonds is now an important aspect of this rather general field due to the universal existence of aromatic functionalities in nature and the synthetic world. ‘Fenton’ chemistry and Friedel–Crafts reactions are early examples of transformations of aryl C–H bonds to different functionalities.

Many reports have demonstrated that direct arylation of heterocycles, arenes with directing groups, and electron-deficient arenes(1-7). As well a completely unactivated arene, benzene has been directly arylated by a few efficient transition-metal-catalyzed methods (8-12).

In 2003, Leadbeater et.al,. reported a transition metal free Suzuki coupling reaction in water using sodium carbonate as a base(13-14)(Scheme-1). Later, the same group reported the transition metal free sonogashira type reaction (15)(Scheme-2). After that they discovered that the reaction was in fact metal-mediated — by palladium contaminants of as little as 50 ppb that were present in the sodium carbonate base used. In 2009, Buchwald reported that iron catalysed cross coupling reaction has been done by copper catalyst which is an impurity in Iron source(16).

Scheme-1

Scheme-2

In 2008, Daugulis, reported transition-metal-free, base-mediated intramolecular arylation of phenols with aryl halides. The sp2 C-H bond functionalization occurs via a benzyne intermediate. At this point, a phenolate activating group is essential for the arylation(17)(Scheme-3). In the same time, Itami et al. reported a transition metal free direct C-H arylation electron deficient nitrogen heterocycles using haloarenes. As well, they also reported a transition-metal-free systems for the cross coupling reactions of nitrogen heteroaromatics and alkanes (18-19)(Scheme-4). Itami et al. proposed a radical pathway for the sole KOBut promoted direct arylation of electron-deficient nitrogen heterocycles with aryl iodides.

Scheme-3

Scheme-4

Recently, the most notable examples are three ‘transition matal free’ methods for pereparing biaryls by C-H activation. 

Now, these reports getting a lot of attention, and a lot of raised eyebrows. The authors claim that they can couple aryl iodides with  unfunctionalized aromatic compounds with nitrogen bidentate ligands as catalysts - and no transition metals at all - just potassium or sodium t-butoxide as base. Organic chemists will recognize that this is a very unusual reaction indeed, since carbon-carbon bonds between aryl groups are not supposed to be so easy to form. This reaction, in fact, would suggest that a lot of the palladium-catalyzed work is some sort of odd detour to get to a process that happens fairly easily anyway.

The authors suggest that since they're using iodides that a free radical mechanism is operating. Addition of radical scavengers, they say, shuts the reaction down. The fact that they don't get regioisomers, that rules out another possible mechanism through benzyne intermediates.

It is also worth noting a paper from earlier this year reporting the use of an iron catalyst (20) (Scheme-5) — iron acetate combined with 10 mol% bathophenanthroline (4, 7-diphenyl-1, 10-phenanthroline) as a ligand — for the same sorts of couplings. In this case, the researchers actually tried the reaction of iodotoluene and benzene in the absence of the iron salt (but in the presence of the bathophenanthroline) and obtained no product. Could it be that the two ‘transition-metal-free’ methodologies are in fact catalysed by very low levels of iron? This in turn begs the question: What does it take to prove that a transformation is transitionmetal free?

Scheme-5

The authors Liu, W.; as well Sun, C.L.; mentioned that, In order to eliminate the possibility of the presence of trace transition metal elements in the commercially available potassium tert-butoxide that would potentially affect our investigation, they purified the KOBut by sublimation prior to their examination. Almost the same results were obtained between the nonpurified and purified base.

Could trace amounts of transition metal have contaminated the experiment? 'Obviously this is one of the most important factors,' says Lei. 'We have checked the contamination of trace amounts of transition metals by ICP [inductively coupled plasma atomic emission spectroscopy] and excluded the involvement of small amounts of transition metals in this transformation.'

Commenting on the work, Carsten Bolm, an organic synthesis expert from Aachen University in Germany, says, 'To be able to prepare cross-coupling products without the use of transition metals is an important scientific advance. Although at the present stage the substrate scope is by far too limited to make the process synthetically attractive, the findings illustrate that new reaction paths in direct C-H arylations are still to be discovered, and as such this work will be highly stimulating to the community.'

http://www.rsc.org/chemistryworld/News/2010/August/11081002.asp

References:

1. McGlacken, G. P.; Bateman, L. M. Chem. Soc. Rev. 2009, 38, 2447–2464.

2. Daugulis, O.; Do, H. Q.; Shabashov, D. Acc. Chem. Res. 2009, 42, 1074–1086.

3. Fagnou, K. & Lautens, M. Chem. Rev. 2003,103, 169–196 .

3. Kuninobu, Y., Nishina, Y., Takeuchi, T. & Takai, K. Angew. Chem. Int. Ed. 2007 , 46, 6518–6520.

4. Lersch, M. & Tilset, M. Chem. Rev. 2005, 105, 2471–2526.

5. Li, Z., Brouwer, C. & He, C. Chem. Rev. 2008, 108, 3239–3265.

6. Chen, X., Hao, X.-S., Goodhue, C. E. & Yu, J.-Q. J. Am. Chem. Soc. 2006, 128, 6790–6791.

7. Gandeepan, P.; Parthasarathy, K.; Cheng, C.-H. J. Am. Chem. Soc. 2010, 132, 8569.

8.  Fujita, K.; Nonogawa, M.; Yamaguchi, R. Chem. Commun. 2004, 1926– 1927.

9.  Lafrance, M.; Fagnou, K. J. Am. Chem. Soc. 2006, 128, 16496– 164.

10. Kobayashi, O.; Uraguchi, D.; Yamakawa, T. Org. Lett. 2009, 11, 2679–2682.

11.  Vallee, F.; Mousseau, J. J.; Charette, A. B. J. Am. Chem. Soc. 2010, 132, 1514–1516.

12.  Liu, W.; Cao, H.; Lei, A. Angew. Chem., Int. Ed. 2010, 49, 2004–2008.

13. Leadbeater, N. E. & Marco, M. Angew. Chem. Int. Ed. 42, 1407–1409 (2003).

14. Leadbeater, N. E. & Marco, M. J. Org. Chem.2003 68, 5660–5667.

15. Leadbeater, N. E., Marco, M. &Tominack, B. J. Org. Lett. 2003, 5, 3919–3922.

16. Buchwald, S. L. & Bolm, C. Angew. Chem. Int. Ed. 2009,  48, 5586–5587.

17. Bajracharya, G. B.; Daugulis, O. Org. Lett. 2008, 10, 4625–4628.

18. Yanagisawa, S.; Ueda, K.; Taniguchi, T.; Itami, K. Org. Lett. 2008, 10, 4673–4676.

19. Deng, G. J.; Ueda, K.; Yanagisawa, S.; Itami, K.; Li, C. J. Chem. Eur. J. 2009, 15, 333–337.

20. Valle, F., Mousseau, J. J. & Charette, A. B. J. Am. Chem. Soc. 2010, 132, 1514–1516.

Palladium Catalyzed Cross Coupling Reactions

General Introduction:

During the second half of the 20th century, transition metals have come to play an important role in organic chemistry and this has led to the development of a large number of transition metal-catalyzed reactions for creating organic molecules. Transition metals have a unique ability to activate various organic compounds and through this activation they can catalyze the formation of new bonds. One metal that was used early on for catalytic organic transformations was palladium. One event that stimulated research into the use of palladium in organic chemistry was the discovery that ethylene is oxidized to acetaldehyde by air in a palladium-catalyzed reaction and this became the industrially important Wacker process. Subsequent research on palladium-catalyzed carbonylation led to new reactions for the formation of carbon-carbon bonds. In general, transition metals, and in particular palladium, have been of importance for the development of reactions for the formation of carbon-carbon bonds. In 2005 the Nobel Prize in chemistry was awarded to metal-catalyzed reactions for the formation of carbon-carbon double bonds. This year(2010) the Nobel Prize in chemistry is awarded to the formation of carbon-carbon single bonds through palladium-catalyzed cross-coupling reactions.

Palladium catalyzed cross coupling reactions:

In general, palladium-catalyzed cross couplings is that two molecules are assembled on the metal via the formation of metal-carbon bonds. In this way the carbon atoms bound to palladium are brought very close to one another. In the next step they couple to one another and this leads to the formation of a new carbon-carbon single bond. Thus, palladium catalysis has gained widespread use in industrial and academic synthetic chemistry laboratories as a powerful methodology for the formation of C-C and C-Heteroatom bonds.

There are many palladium catalyzed coupling reactions are developed with various coupling partners.

1. SUZUKI-MIYAURA

2. STILLE

3. NEGISHI

4. KUMADA

5. HIYAMA

6. SONOGASHIRA

7. HECK

8. BUCHWALD-HARTWIG

9. CYANATION

10. CARBONYLATION

Most palladium catalysed reactions are believed to follow a similar catalytic cycle.

The Suzuki-Miyaura coupling

In 1979, A. Suzuki and N. Miyaura reported the stereoselective synthesis of arylated (E)-alkenes by the reaction of 1- alkenylboranes with aryl halides in the presence of a palladium catalyst. The palladium-catalyzed cross-coupling reaction between organoboron compounds and organic halides or triflates provides a powerful and general method for the formation of carbon-carbon bonds known as the Suzuki cross-coupling.

There are several advantages to this method:

1) mild reaction conditions;

2) commercial availability of many boronic acids;

3) the inorganic by-products are easily removed from the reaction mixture, making the reaction suitable for industrial processes;

4) boronic acids are environmentally safer and much less toxic than organostannanes (see Stille coupling);

5) starting materials tolerate a wide variety of functional groups, and they are unaffected by water;

6) the coupling is generally stereo- and regioselective;  

7) sp3-hybridized alkyl boranes can also be coupled by the B-alkyl Suzuki-Miyaura cross-coupling.

Mechanism:

References

1. Galardon,E.; Ramdeehul, S.; Brown, J.M.; Cowley, A.; Hii, K.K.; Jutand, A.; Angew. Chem, Int. Ed. 2002 41, 1760-1763

2. Tolman, C. A. Chem. Rev., 1977, 77, 313–348

3. For a review see: Hillier, A.C.; Grasa, G. A.; Viciu, M.S.; Lee, H. M.; Yang, C; Nolan, S. P. J. Organomet. Chem. 2002, 69-82

Stille coupling

The Pd(0)-catalyzed coupling reaction between an organostannane and an organic electrophile to form a new C-C sigma bond is known as the Stille cross coupling. The Stille reaction is an extremely versatile alternative to the Suzuki reaction. It replaces the organoboron reagents with organostannanes. As the tin bears four organic functional groups, understanding the rates of transmetallation of each group is important. Relative rate of transmetallation: Alkynyl > vinyl > aryl > allyl ~ benzyl >> alkyl

The reaction also has the advantage that it is run under neutral conditions making it even more tolerant of different functional groups than the Suzuki reaction.The precursor organotin compounds have many advantages because they:

1) are tolerate a wide variety of functional groups;

2) are not sensitive to moisture or oxygen unlike other reactive organometallic compounds; and

3) are easily prepared, isolated, and stored.

The main disadvantages are their toxicity and the difficulty to remove the traces of tin by-products from the reaction mixture.

Mechanism:

References

1. Jung, D; Shimogawa, H.; Kwon, Y.; Mao, Q.; Sato, S.-I. Kamisuki, S.; Kigoshi, H.; Uesugi, M. J. Am. Chem. Soc. 2009, 131, 4774-4782.

2. van Niel, M. B.; Wilson, K.; Adkins, C. H.; Atack, J. R.; Castro, J. L.; Clarke, D. E.; Fletcher, S.; Gerhard, U.; Mackey, M. M.; Malpas, S.; Maubach, K.; Newman, R.; O’Connor, D.; Pillai, G. V.; Simpson, P. B.; Thomas, S. R.; MacLoed, A. M. J. Med. Chem. 2005, 48, 6004-6011.

3. Giri, R.; Maugel, N; Li, J.J; Wang, D.-H.; Breazzano, S. P.; Saunders, L. B.; Yu J.-O. J. Am. Chem. Soc. 2007, 129, 3510-3511.

4. Molander, G.A.; Canturk, B. Angew. Chem. Int. Ed. 2009; 48; 9240-9261

Stille-Kelly coupling.

The Pd-catalyzed intramolecular biaryl coupling of aryl halides or aryl triflates in the presence of distannanes is known as the Stille-Kelly coupling.

Mechanism:

Reference:

Yue, W. S.; Li, J. J. Org.Lett. 2002, 13, 2201-2204.

Negishi coupling

The Negishi coupling utilises organo-zinc reagents as starting materials to cross couple with organohalides and equivalents. The method is compatible with a good range of functional groups on the organohalide including ketones, esters, amines and nitriles. The organo-zinc reagent can be prepared in situ by a variety of methodologies, such as transmetallation of the corresponding organo-lithium or Grignard reagent, or via oxidative addition of activated Zn(0) to an organohalide.

Mechanism:

Reference:

1) Krasovskiy, A.; Krasovskaya, V.; Knochel, P. Angew. Chem. Int. Ed. 2006, 45, 2958-2961.

2) Prasad, A. S. B.; Stevenson, T. M.; Citineni, J. R.; Nyzam, V.; Knochel, P. Tetrahedron 1997, 53, 7237-7254

Kumada coupling

The cross coupling of organohalides with Grignard reagents is known as the Kumada coupling. Although it suffers from a limited tolerance of different functional groups, the higher reactivity and basicity of the Grignard reagent allows viable reactions to take place under mild conditions.

The Mechanism is similar as Negishi coupling.

Reference:

1) Anastasia, L., Negishi, E.-i. Palladium-catalyzed aryl-aryl coupling. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 1, 311-334.

2) Anctil, E. J. G., Snieckus, V. The directed ortho metalation-cross coupling symbiosis. Regioselective methodologies for biaryls and heterobiaryls. Deployment in aromatic and heteroaromatic natural product synthesis. J. Organomet. Chem. 2002, 653, 150-160.

3) Banno, T., Hayakawa, Y., Umeno, M. Some applications of the Grignard cross-coupling reaction in the industrial field. J. Organomet. Chem. 2002, 653, 288-291.

4) Hayashi, T. Palladium-catalyzed asymmetric cross-coupling. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 1, 791- 806.

Hiyama coupling

The Hiyama Coupling is the palladium-catalyzed C-C bond formation between aryl, alkenyl, or alkyl halides or pseudohalides and organosilanes. This reaction is comparable to the Suzuki Coupling and also requires an activating agent such as fluoride ion or a base.

Mechanism:

Crucial for the success of the Hiyama Coupling is the polarization of the Si-C bond. Activation of the silane with base or fluoride ions (TASF, TBAF) leading to a pentavalent silicon compound is a first necessary step. The use of a silanol as the organosilane is one recent method that has managed to negate the requirement for the reaction to contain fluoride as an activator. This has helped to enlarge the substrate scope available to organic chemists.

Reference:

1) Li, J.-H.; Deng, W.-J.; Liu, Y.-X. Synthesis 2005, 3039-3044.

2) For a recent review on silanols in the Hiyama coupling see: Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008, 41, 1486-1499.

Sonogashira coupling

The Sonogashira reaction offers an extremely useful route into aryl- and alkenyl-alkynes. The alkyne moiety is usually introduced via its copper salt. This is generated in situ from a Cu(I) salt, such as CuI or CuCN, and a terminal alkyne in the presence of an amine base. Recent improvements in this reaction have led to the development of copper and amine free couplings.

The general features of the reaction are:

1) the coupling can usually be conducted at or slightly above room temperature, and this is a major advantage over the forcing conditions required for the alternative Castro-Stephens coupling;

 2) the handling of the shock-sensitive/explosive copper acetylides is avoided by the use of a catalytic amounts of copper(I) salt;

3) the copper(I) salt can be the commercially available CuI or CuBr and are usually applied in 0.5-5 mol% with respect to the halide or alkyne;

4) the best palladium catalysts are Pd(PPh3)2Cl2 or Pd(PPh3)4;

5) the solvents and the reagents do not need to be rigorously dried. However, a thorough deoxygenation is essential to maintain the activity of the Pd-catalyst;

6) often the base serves as the solvent but occasionally a co-solvent is used;

7) the reaction works well on both very small and large scale (>100g);

8) the coupling is stereospecific; the stereochemical information of the substrates is preserved in the products;

9) the order of reactivity for the aryl and vinyl halides is I ≈ OTf > Br >> Cl;

10) the difference between the reaction rates of iodides and bromides allows selective coupling with the iodides in the presence of bromides;

11) almost all functional groups are tolerated on the aromatic and vinyl halide substrates.

Mechanism:

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the CuI catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd(0) species from a Pd(II) complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd(0) then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(I)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle.

References

1) Cassar, L. J. Organomet. Chem. 1975, 93, 253-257.

2) Dieck, H. A., Heck, F. R.  J. Organomet. Chem. 1975, 93, 259-263.      

3) Sonogashira, K., Tohda, Y., Hagihara, N. Tetrahedron Lett. 1975, 4467-4470.

4)  Thorand, S.; Krause, N. J. Org. Chem. 1998, 63, 8551-8553.

5)  Liang, Y.; Xie, Y.-X.; Li, J.-H. J. Org. Chem. 2006, 71, 379-380.

6) Schiedel, M.-S., Briehn, C. A., Bauerle, P. C-C J. Organomet. Chem. 2002, 653, 200-208.

Heck reaction

In the early 1970s, T. Mizoroki and R.F. Heck independently discovered that aryl, benzyl and styryl halides react with olefinic compounds at elevated temperatures in the presence of a hindered amine base and catalytic amount of Pd(0) to form aryl-, benzyl-, and styryl-substituted olefins. Today, the palladium-catalyzed arylation or alkenylation of olefins is referred to as the Heck reaction. Since its discovery, the Heck reaction has become one of the most widely used catalytic carbon-carbon bond forming tools in organic synthesis.

The Heck reaction follows a slightly different pathway to other palladium catalysed couplings. For intermolecular reactions with monosubstituted olefins, the olefin insertion step is usually directed by steric hindrance. This intermediate then undergoes β-hydride elimination under thermodynamically controlled conditions, leading to preferential formation of the E product.

Reference:

1) Heck, R. F. J. Am. Chem. Soc. 1968, 90, 5518- 5526.

2) Mizoroki, T., Mori, K., Ozaki, A.  Bull. Chem. Soc. Jpn. 1971, 44, 581.

3) Heck, R. F., Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320-2322.

4) Shibasaki, M., Miyazaki, F. Asymmetric Heck reactions. in Handbook of Organopalladium Chemistry for Organic Synthesis (eds. Negishi, E.-i.,De Meijere, A.), 1, 1283-1315 (Wiley-Interscience, New York, 2002).

5)  Dounay, A. B., Overman, L. E. Chem. Rev. 2003, 103, 2945-2963.

6) Braese, S., de Meijere, A. Cross-coupling of organic halides with alkenes: The Heck reaction. Metal-Catalyzed Cross-Coupling Reactions (2nd Edition) 2004, 1, 217-315.

Buchwald-Hartwig coupling

Palladium catalysis has also been expanded to the formation of C-N bonds. In 1995 Buchwald and Hartwig independently reported the palladium catalysed coupling of aryl halides with amine nucleophiles in the presence of stoichiometric amounts of base.

Mechanism:

The first step in the catalytic cycle is the oxidative addition of Pd(0) to the aryl halide (or sulfonate). In the second step the Pd(II)-aryl amide can be formed either by direct displacement of the halide (or sulfonate) by the amide via a Pd(II)- alkoxide intermediate. Finally, reductive elimination results in the formation of the desired C-N bond and the Pd(0) catalyst is regenerated. Below is the catalytic cycle for the formation of an arylamine.

References

1)  a) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem. Int. Ed. 1995, 34, 1348-1350. b) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609-3612.

2). Hillier, A.C.; Grasa, G. A.; Viciu, M.S.; Lee, H. M.; Yang, C; Nolan, S. P. J. Organomet. Chem. 2002, 69-82.

3)  Shen, Q.; Hartwig, J. J. Am. Chem. Soc. 2006, 128, 10028-10029.

Palladium catalysed cyanation

The palladium catalysed cyanation of aromatic halides offers a convenient alternative to the Rosemund-Von Braun reaction, which often employs harsh reaction conditions and can have a labour intensive workup. As the cyanide nucleophile is a strong σ-donor and can poison the catalyst, it is necessary to keep its concentration low during the reaction. To achieve this Zn(CN)2 is often employed as the cyanide source as its solubility in DMF (a common solvent for this reaction) is limited.

An alternative, non-toxic, source of cyanide has also been reported. K4[Fe(CN)6] can be used in combination with palladium catalysts to synthesise aryl nitriles from their corresponding halides.

Reference:

1) Schareina, T.; Zapf, A.; Beller, M. Chem. Comm. 2004, 1388-1389

2) Weissman, S. A.; Zewge, D.; Chen, C. J. Org. Chem. 2005, 70, 1508-1510

Palladium catalysed carbonylation

As with most palladium mediated C-C bond forming reactions palladium catalysed carbonylation is compatible with a range of functional groups. This gives it significant advantages over standard organolithium and Grignard chemistry for the synthesis of aryl aldehydes, acids, esters and amides.

References

1) Beller, M.; Magerlein, W.; Indolese, A. F.; Fischer C. Synthesis, 2001, 1098-1110.

2) Kumar K.; Zapf, A.; Michalik, D; Tillack, A.; Heinrich, T.; Bottcher, H.; Arlt, M.; Beller, M. Org. Lett., 2004, 6, 7-10.

3) Ashfield, L.; Barnard, C. F. J.; Org. Process Res. Dev., 2007, 11, 39-43.