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In the year 1864 Hugo Schiff invented Schiff’s bases. Schiff’s bases are carbon-nitrogen double bond (-CH=N-, azomethine or imine group) containing compound synthesized by condensation of primary amine with carbonyl compounds (aldehyde or ketone) (Scheme 01). The imine group of Schiff’s base plays an important role for showing excellent biological activities 1.  Scheme 01: Synthesis of Schiff base.The reactions involved in forming carbon-carbon bonds are unquestionably the most important processes in organic chemistry, as they represent key steps in the building of more complex molecules from simple precursors. Of the many commonly used coupling reactions, such as Heck, Suzuki, Sonogashira, Stile, amination and Negishi (Scheme 02) have been widely used because of their selective and versatile applications. Scheme 02 : Various Coupling Reactions.In the last few decades there is a development of new and better transition metal complexes as a catalyst with high efficiency, lowest loading ratios or less toxicity, etc. Transition metal complexes based on Schiff base ligands have got great attention because of their diverse structural features. Oshin Sebastian and Ashwin Thapa reviewed the Schiff base metal complexes of Nickel and Palladium and their catalytic activity for Suzuki reaction 2.It is worth pointing out that many articles are reported on coupling reactions only a selection of the most significant developments in catalysts and applications in coupling reactions are presented here and for simplicity the details on reaction conditions (base, solvent, temperature) not discussed more though these factors were essential for outcome of reaction, focusing mostly in coupling partners, ligands, metals and products.Palladium Schiff Base ComplexesW. N. W. Ibrahim, M. Shamsuddin reported the synthesis of Symmetrical Palladium (II) N,N,O,O-Schiff Base Complex and application in Heck and Suzuki reaction. the Schiff base ligands are prepared from 2,2-dimethyl-1,3-propanediamine and aldehyde orketone derivatives the resultant ligand acted as N,N,O,O-tetradentate ligand and have bonded to the palladium atom through the azomethine nitrogen atom (C = N) and the phenolic oxygen atom. These complexes were then subjected in catalytic Heck and Suzuki reaction of iodobenzene gives conversion up to 100% using triethylamine as base at temperature 120?C – 140?C for both Heck and Suzuki reactions of iodobenzene 3. In 2015 M. K. Shah gave a synthesis of palladium co-ordination metal complex of cyano-acetohydrazide Schiff base (Scheme 03) and its Catalytic application in Efficient and convenient Suzuki cross-coupling reaction 4. Scheme: 03: Palladium co-ordination metal complex of cyano-acetohydrazide Schiff base.Palladium (II) complexes containing ONO tridentate hydrazine (Scheme 04) for Suzuki Miyaura coupling of aryl chlorides in aqueous-organic media 5. Distorted square-planar geometry around palladium (II) ion satisfied by ONO tridentate coordination of hydrazone ligands and a triphenylphosphine as a fourth ligand. One of the new complexes acted as a fabulous catalyst for Suzuki-Miyaura coupling of p-chlorobenzonitrile or 1-chloro-4-nitro-benzene with substituted aryl boronic acids in water-toluene media up to 99% yield. Where R = 2-furyl or 2-thiophenyl or 4-pyridylScheme 04: Palladium (II) complexes containing ONO tridentate hydrazone.Palladium (II) complex Pd (L1)2 derived from Schiff base ligand HL-1 (Scheme 05) displays highly catalytic activities in the Suzuki coupling reaction of 4-bromoanisole with phenylboronic acid, which are also very sensitive to the choice of base and solvent 6. Scheme 05: Schiff base ligand used for preparation of Pd (L1)2 Complex.Rupesh Narayana Prabhu and Samudranil Pal reported synthesis of a square-planar mononuclear palladium (II) complex bearing N, S-donor 1-pyrenaldehyde 4-methyl-3-thiosemicarbazonate (L) as ligand and the complex is Pd(L)Cl(PPh3) (Scheme 06). The complex has been found to be an active and efficient homogeneous catalyst for the copper free Sonogashira coupling reactions of phenylacetylene with various aryl halides (bromides and chlorides) at room temperature under aerobic conditions 7. Scheme 06:  Synthesis of Pd(L)Cl(PPh3).In 2015 M. keles et al. prepared bidentate Schiff bases and their Pd(II) complexes (Scheme 07)  8.  Scheme 07: Synthesis of Pd(II) complexes 1a–3a.The complexes were tested as catalysts for Mizoroki–Heck and Suzuki–Miyaura reactions. High conversions were obtained in the Suzuki–Miyaura reaction when carried out at 100°C in K2CO3  media.Claudia A. Contreras-Celedón and co-workers gave the development of simple and efficient catalytic system based on a Pd complex of 4-aminoantipyrine, 4-AAP–Pd(II) (Scheme 08) 9.4-AAP–Pd(II) complex work as efficient catalyst for Suzuki  cross-coupling reaction of  aryl iodides and bromides with substituted phenylboronic acids  in EtOH, in the presence of air, with low catalyst amount, by heating at reflux conditions within short reaction times afford biaryl compounds in good to excellent yield. Scheme 08: Synthesis of 4-aminoantipyrine-Pd(II) complex.Rupesh Narayana Prabhu and Rengan Ramesh reported A convenient synthesis of square planar dinuclear palladium(II) terephthaldehyde bis(thiosemicarbazone) complex (Scheme 09). The new complex acts as an active recyclable homogeneous catalyst for the Mizoroki-Heck reaction of electron deficient (activating) and electron rich (deactivating) aryl halides with various olefins 10. Scheme 09: Synthesis of dinuclear Pd(II) bis(thiosemicarbazone) complex.In 2016 a new chitosan bead supported Pd(II) catalyst was prepared (Scheme 11) and used for Suzuki coupling reaction to synthesize biaryl compounds under microwave heating and solvent free media, it shows good conversion reaction yields and high TON and TOF values. The catalyst is recyclable for seven runs 11. Scheme 10: Synthesis of Schiff base (glyoxal cross-linked to chitosan). Scheme 11: Synthesis of chitosan bead supported Pd(II) catalyst.Farook Adam and Salih Hamza Abbas synthesized symmetric palladium(II)(4-OH)2salen complex ((4-OH)2salen=N,N?-bis(4-hydroxysalicylidene)ethylenediimine) immobilised on MCM-41 (Scheme 12).The prepared catalyst gave excellent catalytic activity towards the Suzuki-Miyaura reaction of bromobenzene and phenylboronic acid. The highest conversion of more than 98% was obtained when 0.05 g of catalyst and 1:1 mole ratio of reactants in 8:1 ml of mixed solvent DMF:H2O at 130°C for 2 h 12. Scheme 12: Synthesis of MCM Salen Pd catalyst.In same year synthesis of two novel salen-palladium(II) complexes M SB-1 and M SB-2 Scheme 13).The  catalysts were found to be efficient catalyst for Mizoroki Heck reaction of 4-bromonitrobenzene with methyl acrylate 13. The percentage conversion of 4-bromonitrobenene to the coupled products were 100% in 3 hrs. Scheme 13: Synthesis of Salen-palladium(II) complexesS. Sobhani et al. reported a palladium-Schiff base complex immobilized covalently on magnetic nanoparticles (Pdimino-Py-c-Fe2O3) was synthesized by the reaction between chloro-functionalized c-Fe2O3 with iminopyridine followed by the reaction with palladium acetate (Scheme 14) 14.  Scheme 14: Synthesis of Pd-imino-Py-c-Fe2O3Pd-iminoPy-c-Fe2O3 was successfully used as a magnetically recyclable heterogeneous catalyst in Heck and Suzuki cross-coupling reactions of various aryl halides (iodide, bromides and chlorides substrates) with olefins and phenylboronic acid. The synthesized catalyst was separated easily by using an external magnet and recycled eight runs without loss of catalytic activity and leaching.A water soluble and efficient imidazolium compound-supported palladium complex (Scheme 15) was prepared and anlyzed its catalytic activity for the Suzuki reactions in aqueous media. The catalyst showed high catalytic activity and excellent yields. The catalyst shows more stability and reusability, recycled six times without any decrease in activity 15. Scheme 15: Imidazolium ionic compound -supported palladium complexA series of self-assembled Pd(II)–alkyl Schiff base having different chain lengths were synthesized (Scheme 16). Catalyst 3 with a moderate alkyl chain exhibited highly active and recyclable ability for Suzuki coupling reaction, which indicated that the length of the alkyl chain significantly affects the catalytic activity due to suitable orientation and moderate electron-rich environment which can make easy formation of palladium– ligand bonds, stabilize the active species and facilitate the insertion of this metal into the Ar–X bond. Those complexes shows a true heterogeneous catalysis and catalysis occurred on the surface 16. Scheme 16: Preparation of Pd(II)–alkyl Schiff base complexes on silicon supports.Magnetic and easily recyclable nanocatalyst developed by using [email protected]/Schiff base/Pd complex (Scheme 17) for rapid and effective N-arylation of carbamates in good to excellent yields 17. Scheme 17: Synthesis of [email protected]/Schiff base/Pd complexThis is new heterogeneous method for the arylation of various carbamates with aryl halides. The separation of catalyst is easy by an external magnetic field and reused for at least 6 runs.Recently in 2017 preparation of Schiff base ligands (L1 and L2) by the reaction between ferrocene-carboxaldehyde and sulfated/selenated amine. The cyclopalladated complexes (1-3) have been synthesized from the Schiff base ligands L1 and L2 (Scheme 18)  18. Scheme 18: Synthesis of Schiff base ligands and Cyclopallaated complexes The cyclopalladated complexes have been used for the O – arylation of phenol and Suzuki-Miyura coupling of phenyl boronic acid with several activated and deactivated aryl bromides. The complexes 1-3 as catalyst show good conversion with yield up to 93 % of Ar-Br to coupled product with low catalyst loading 0.5 and 0.01 mol% for C-O and Suzuki-Miyura coupling respectively in 3-6 h.A new efficient, simple and cost effective protocol was introduced for carbon-carbon bond formation via Heck-Mizoroki coupling reaction of aryl halides with olefins in high to excellent yields in short reaction times by employing [email protected]/Schiff base/Pd(II) complex (Scheme 19) as a catalyst 19.  Scheme 19: [email protected] Nanoparticles Decorated with Schiff base Complex of Pd(II) and                    Heck reaction The catalyst was dispersible, magnetically recoverable and recyclable for several times. Prepaaration of N,N,O-Tridentate ligands by the condensation of 1-(Benzothiazole 2-yl)  hydrazine with substituted salicyldehydes or different aromatic ketones and palladium metal  complex (Scheme 20).The catalyst were tested for Suzuki–Miyaura cross-coupling reaction 20. Scheme 20: Preparation of Schiff base ligand and Pd complex.The yield of Suzuki cross coupling reaction was obtaining good/moderate. In screening of different bases the triethylamine was got a good yield, as compared to other bases. The benefit of catalyst is easy to handle, recoverable and also cheap, it work in aqueous media.Nickel Schiff Base ComplexesIn 2006 Synthesis of series of nickel-salicylideneimines complexes (Scheme 21) for efficient and high yielding aryl Kumada cross-couplings, employing aryl halides and Grignard reagents. N-substituted groups could affect the activities of the Ni-catalysts 21. .Scheme 21: Synthesis of nickel-salicylideneimines complexes.M. Muthu Tamizh and  R. Karvembu reported Synthesis of triethylphosphite complexes of nickel(II) and palladium(II) with tridentate Schiff base ligand (Scheme 22) for catalytic application in Kumada–Tamao–Corriu cross coupling reactions and Suzuki-Miyaura cross coupling reactions 22. Scheme 22: Synthesis of nickel(II) and palladium(II) complexes.The nickel(II) and palladium(II) complexes are air stable and show good catalytic activity for Kumada–Tamao–Corriu cross coupling reactions and Suzuki-Miyaura cross coupling reactions.P. K. Suganthy et al. described convenient synthesis of a new square planar nickel(II) naphthaldehyde thiosemicarbazone complex (Scheme 23). The new complex acts as an active homogeneous catalyst for the Mizoroki–Heck reaction of electron deficient (activating) and electron rich (deactivating) aryl bromides with various olefins 23. Scheme 23: Synthesis of nickel(II) thiosemicarbazone complex.Panneerselvam Anitha et al. gave synthesis Nickel(II) complexes containing thiosemicarbazone ligands (Scheme 24). The analysis revealed that the complexes possess a distorted octahedral geometry with the ligand coordinating in a uni-negative tridentate ONS fashion 24.  Scheme 24: Synthesis of Ni(II) 9,10-phenanthrenequinone N-substituted thiosemicarbazone complexes.The catalytic activity of the complexes was investigated towards the C-C coupling reactions such as Kumada-Corriu, Suzuki-Miyaura and Sonogashira coupling reactions. The results show best catalytic system to C-C coupling. Kumada Corriu C-C coupling to be the best as compare to the Sonogashira and Suzuki-Miyaura reactions and this catalytic system is remarkably simple, convenient and efficient. The results also showed that steric effects in the ligands play a more important role than electronic effects in the catalytic activity of the complexes. That is, in all the reactions, complex 1 has been proven to be an efficient and versatile catalyst compared to complexes 2 and 3 due to the presence of bulky groups (Methyl and Phenyl) on the terminal part of thiosemicarbazone ligands in complex 2 and 3.MIRI et al. in 2015 reported an efficient C–C cross-coupling reactions by (isatin)-Schiff base functionalized magnetic nanoparticle-supported Cu(II) acetate (Scheme 25) as a magnetically recoverable catalyst 25. Scheme 25: The Preparation of copper catalyst.These catalysts showed high efficiency for phosphine-free Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions with variety of substrates. The catalysts could be easily recovered and reused several times without a significant loss in their catalytic activity and stability.

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