INTERNATIONAL JOURNAL OF PHARMACEUTICAL AND CHEMICAL SCIENCES ISSN: 22775005

Review Article A Review on Catalyzed Coupling Reactions

Megha sahu* and Prabodh Sapkale JZMDS COP, Jalgaon, Maharastra, India. ______ABSTRACT Palladium compounds are used as catalyst, usually as homogeneous catalyst in many coupling reactions. Unoptimized reactions typically use 10-15 mol% of palladium; whereas optimized, catalyst loadings can be to the order of 0.1 mol %. Palladium readily absorbs hydrogen at room temperature forming palladium hydride - PdHx, with ‘x’ below 1. While this property is common to many transition metals, palladium is unique by the high absorption capacity, and by that it does not lose its ductility until high x values. This property has been investigated for designing an efficient, yet inexpensive hydrogen storage material (palladium itself is prohibitively expensive for this purpose)

Keywords: Homogeneous Catalyst, Absorption capacity, Unoptimized reaction, Ductility.

INTRODUCTION1-7 of 0.1 mol %. Palladium readily absorbs Palladium compounds are used as catalyst, hydrogen at room temperature forming usually as homogeneous catalyst in many palladium hydride - PdHx, with ‘x’ below 1. coupling reactions. Unoptimized reactions Palladium compounds are used as a catalyst in typically use 10-15 mol% of palladium; whereas many coupling reactions, usually as a optimized, catalyst loadings can be to the order homogeneous catalyst8, 9, 10. Examples include:

Heck Reaction Suzuki Coupling

Buchwald Hartwig

Palladium Fukuyama cataysed Coupling coupling

reaction

Sonogashira Hiyama Coupling Coupling

Negishi Coupling

Fig. 1: Flow chat of palladium catalysed

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Properties of Palladium  Pd (I), Pd (III) and Pd (IV) complexes are also Atomic number 46 -1 known, though less thoroughly, with Pd (IV) Atomic mass 106.42 g.mol species essential in C-H activation mechanisms. Density 11.9 g.cm-3 at 20°C Melting point 1560 °C Boiling point 2927 °C Mechanism of Palladium catalysed 14, 15 Isotopes 9 coupling reaction Vander waals radius 0.065 nm (+2) • Preactivation of catalyst: When a Pd(II) source Ionic radius 0.137 nm is used in a reaction, it must be reduced to Pd(0) Electronic shell [ Kr ] 4d10 5s0 -1 before entering the catalytic cycle reduction by Energy of first ionization 703 kJ.mol Energy of second ionization 1870 kJ.mol -1 Phosphines. Energy of third ionizsation 3177 kJ.mol -1 • : Oxidative addition proceeds Standard potential + 0.85 V (Pd2+/ Pd ) with retention of stereochemistry with vinyl Discovered by William Wollaston halides, while giving inversion of stereochemistry with allylic and benzylic Palladium is used for coupling because halides. The oxidative addition initially forms the 11, 12  Its electronegativity , which leads to cis-palladium complex, which rapidly isomerizes relatively strong Pd-H and Pd-C bonds, and also to the trans-complex. develops a polarised Pd-X bond. • Reductive elimination: Using deuterium-  It allows easy access to the Pd (II) and Pd (0) labelling16, the reductive elimination proceeds oxidation states, essential for processes such as with retention of stereochemistry and relative oxidative addition, transmetalatio13 and reductive reactivity of different metal complexes in the C-C elimination. reductive17, 18 elimination was established: Pd(IV), Pd(II) > Pt(IV), Pt(II), Rh(III) > Ir(III), Ru(II), Os(II).

Fig. 2: Mechanism of palladium catalysed coupling reaction

Palladium catalyzed reactions were palladium-catalyzed cross-coupling reactions, developed by many scientist19- 21 this reaction is of great importance, as it allows : The Heck20 reaction (also called one to do substitution reactions on planar the Mizoroki-Heck reaction) is the chemical centers. It is named after Tsutomu Mizoroki and reaction of an unsaturated halide (or triflate) with Richard F. Heck. Heck21, 22 was awarded the an alkene and a base and palladium catalyst or 2010 Nobel Prize in Chemistry for the discovery palladium nanomaterial-based catalyst to form a and development of this reaction. substituted alkene. Together with the other

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triphenylphosphine23-25 or BINAP. The base is triethylamine, potassium carbonate or sodium acetate.

Recent application of Heck reaction The reaction is performed in the presence of an 26, 27 In synthesis of Rhazinal organo palladium catalyst. The halide (Br, Cl) or • Rhazinal is an alkaloid obtained from the stem triflate is an aryl, benzyl, or vinyl compound and extract of Kopsia teoi. the alkene contains at least one proton and is • Which is Antimitotic agent. often electron-deficient such as acrylate ester or • It is a promising starting point for anticancer an acrylonitrile.The catalyst can be tetrakis agent. (triphenylphosphine) palladium(0), palladium chloride or palladium(II) acetate. The ligand is

Ionic liquid Heck reaction Suzuki coupling31, 32 In the presence of an ionic liquid a Heck reaction Since the discovery of the over proceeds in absence of a phosphorus ligand28, a century ago, in 1901, the transition-metal- 29. In one modification palladium acetate and the catalyzed cross-coupling reaction has played an ionic liquid PF6 are immobilized inside the important role in the synthesis of C-C bonds. In cavities of reversed-phase silica gel. In this way 1981, Suzuki discovered a novel Pd-catalyzed the reaction proceeds in water and the catalyst cross-coupling reaction of aryl boronic acids and is re-usable. aryl halides, which has been applied widely and for which he received the 2010 Nobel prize in chemistry. This reaction has become an extremely powerful process for the synthesis of biaryls, which have a diverse spectrum of applications, ranging from pharmaceuticals to materials scienc The given scheme shows the first published Amino-Heck reaction Suzuki Coupling, which is the palladium- In the amino-Heck30, 31 reaction a nitrogen to catalysed cross coupling between carbon bond is formed. In one example, an organoboronic acid and halides. Recent catalyst oxime with a strongly electron withdrawing group and method developments have broadened the reacts intramolecularly with the terminal end of a possible applications enormously, so that the diene to a pyridine compound. The catalyst is scope of the reaction partners is not restricted to tetrakis(triphenylphosphine)palladium(0) and the aryls, but includes alkyls, alkenyls and alkynyls. base is triethylamine. Potassium trifluoroborates and organoboranes

or boronate esters may be used in place of boronic acids33. Some pseudohalides (for example triflates) may also be used as coupling partner.

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Recent application of Suzuki coupling34, 35 A Highly Efficient Microwave-Assisted Suzuki Coupling Reaction of Aryl Perfluoro- octylsulfonates with Boronic Acids.

C8H17 O Suzuki microwave S R O O 1 F-SPE separation R2 R1 Cyclic sulfamide HIV-1 protease inhibitor36, 37, with side chains spanning from P2/P2′ to P1/P1:-Molecular modeling suggested that this could be achieved with appropriate ortho-substitution of the P2/P2′ benzyl groups in our cyclic sulfamide inhibitors.

Suzuki–Miyaura cross coupling correlation between ligand alkyl chain length and The Suzuki–Miyaura cross-coupling38, 39 of aryl catalytic efficiency, however, as their Dalton halides with organoboronic acids is an important Transactions, the Pd(II) complexes are pre- tool for synthetic organic chemistry. A range of catalysts which generate in situ real catalytic palladium complexes with different ligands can species of approx. 3 nm Pd(0) nanoparticles, been used to catalyse this interesting reaction. protected by organo selenium species. Their remarkable results indicate there is a

Fig. 3: Palladium catalyst used in Suzuki coupling

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Negishi coupling40, 41 an exceedingly useful alternative to other cross-coupling procedures, as well as carbon– carbon bond formation.

Recent application of Negishi coupling42 1) Synthesis of beta-carotene

2) A Highly Efficient Microwave-Assisted Suzuki Coupling Reaction of Aryl Perfluoro- octylsulfonates with Boronic Acids43, 44.

3) Suzuki coupling sequence to the stereoselective synthesis of E-trisubstituted olefins.45, 46

Hiyama-coupling47- 49 It is a palladium-catalyzed cross-coupling reaction of organosilanes with organic halides used in organic chemistry to form carbon-carbon bonds (C-C bonds). This reaction was discovered in 1988 by and Recent application of Hiyama Coupling Yasuo Hatanaka as a method to form carbon- 1) Many modifications to the Hiyama- carbon bonds synthetically with chemo- and coupling50- 52 have been developed that avoid regioselectivity. the use of a fluoride activator base. Using This reaction has been applied to the synthesis organo chlorosilanes, Hiyama found a coupling of natural products such as Papulacadin D, an scheme utilizing NaOH as the basic activator. anti-fungal agent. Modifications using alkoxy silanes have been reported with the use of milder bases like NaOH

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and even water. Study of these mechanisms Denmark coupling which utilize organosilanols have led to the development of the Hiyama- as coupling partners.

2) Hiyama-Denmark coupling53, 54]: The Kenneth Stille and David Milstein, a post- Hiyama-Denmark coupling is the modification of doctorate in his laboratory. Stille reactions were the Hiyama-coupling that does not require a used in 50% of all cross-coupling reactions fluoride additive to utilize organosilanols and published in 1992. The reaction continues to be organic halides as coupling partners. The exploited industrially, especially for general reaction scheme is shown below, pharmaceuticals. The reaction is widely used in showcasing the utilization of a Brønsted base55 organic synthesis. X is typically a halide, such as as the activating agent as opposed to fluoride, Cl, Br, I. Additionally, X can be a pseudohalide - phosphine ligands are also used on the metal such as a triflate, CF3SO3 . center. R-Sn(R) R' RR' X Sn(R) 3 X 3

Recent application of Stille coupling59, 60 1) Use of the Stille Coupling Reaction on Heteroaromatic Cations Synthesis of Substituted Quinolizinium Salts. We describe the first efficient application of the Stille coupling reaction on heteroaromatic cations. In 56-58 Stille coupling the presence of Pd(0)/CuI, the reaction of bromo The Stille reaction (also known as Stille quinolizinium salts and various tributy lstannyl61- Coupling) is a chemical reaction coupling an 63 compounds proceeds in satisfactory yield 2 organotin compound with an sp -hybridized under mild reaction conditions, affording organic halide catalyzed by palladium. The Stille different substituted quinolizinium salts. reaction was discovered in 1977 by John

2) In the realm of green chemistry a Stille reaction64- 66 is reported taking place in a low melting and highly polar mixture of a sugar such as mannitol, a urea such as dimethylurea and a salt such as ammonium chloride67, 68. The catalyst system is tris (dibenzylideneacetone) dipalladium(0) with triphenylarsine69-72.

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Fukuyama coupling73- 75 discovered by Tohru Fukuyama et al. in 1998. It is a coupling reaction taking place between a Advantages are high chemoselectivity, mild thioester and an organozinc halide in the reaction conditions and the use of less-toxic presence of a palladium catalyst. The reaction reagents. product is a ketone. This reaction was

Recent application of Fukuyama coupling dichlorobis79-81 (triphenylphosphine) palladium(II) 1) Synthesis of multi-functionalized [PdCl2(PPh3)2] provides highly functionalized ketones76-78 ketones in excellent yields. The reaction is Fukuyama and co-workers have recently featured by unusually high chemoselectivity, developed a highly efficient synthetic method for mild reaction conditions and the use of less-toxic the generation of ketones. Treatment of thiol reagents83-85. esters with zinc reagents in the presence of

2) The reaction has been used in the synthesis of biotin86, 87.

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Buchwald–Hartwig amination88- 90 expanding the repertoire of possible C–N bond It is a chemical reaction used in organic formation. chemistry for the synthesis of carbon–nitrogen bonds via the palladium-catalyzed cross- X R R Pd catalyst., 3 coupling of amines with aryl halides. Though HN 3 N R2 publications with similar focus were published as R2 Base Ligand R R1 early as 1983, credit for its development is 1 typically assigned to Stephen L. Buchwald and John F. Hartwig, whose publications between X= Cl, Br,I,OTf R2= Alkyl, Aryl, H 1994 and the late 2000s established the scope R3= Alkyl, Aryl of the transformation. The reaction's synthetic utility stems primarily from the shortcomings of Application of Buchwald–Hartwig amination typical methods (nucleophilic substitution, 1) A palladium catalyzed C–N cross-coupling reductive amination. reaction was published in 1983 by Migita and For the synthesis of aromatic C–N bonds,with coworkers and described a reaction between most methods suffering from limited substrate several aryl bromides and N,N-diethylamino- scope and functional group tolerance. The tributyltin using 1 mol% PdCl2[P(o-tolyl)3]2. development of the Buchwald–Hartwig 91- 93 Though several aryl bromides were tested, only reaction allowed for the facile synthesis of electronically neutral, sterically unencumbered aryl amines, replacing to an extent harsher substrates94-96 gave good yields. methods (the Goldberg reaction, nucleophilic aromatic substitution, etc.) while significantly

2) In 1984, Dale L. Boger and James S. Panek reported an example of Pd(0)-mediated C–N bond formation in the context of their work on the synthesis of lavendamycin97- 99 which utilized stoichiometric Pd(PPh3)4.

3) Sonogashira reaction100: It is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It makes use of a palladium catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or .

Recent application of A -Free Sonogashira Coupling Reaction in Ionic Liquids100 and Its application to a Microflow System for Efficient Catalyst Recycling. The PdCl2(PPh3)2-catalyzed Sonogashira coupling reaction, in

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good to high yields, was performed in an ionic liquid ([BMIm][PF6]) in the absence of a copper salt. The use of an ionic liquid allows for the facile separation and recycling of the catalyst.

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