Development of thiosemicarbazone-based transition metal complexes as homogeneous catalysts for various organic transformations
Graphical abstract
This review summarizes the catalytic applications of transition metal (Ru, Pd, Ni, Cu, Mo and V) complexes containing thiosemicarbazone ligands in various organic transformations.
Introduction
In recent years, research on homogeneous catalysis using transition metal complexes has grown enormously. Even though many amazing catalytic discoveries have been made by researchers both in industry and academia, the search for the better catalysts is never ending. Reactions that were thought to be well understood and optimized have now been revolutionized with completely new catalysts and unmatched product selectivity. The activity and selectivity of a catalytic system are controlled by the characteristics of ligands attached to the metal centre. In the past, there were so many Schiff base complexes, which have shown extraordinary activity as catalysts in various organic conversions. One such simple Schiff base ligand with versatile coordination modes is thiosemicarbazone (TSC), which is paramount in the group of sulphur donor ligands [1], [2], [3]. The medicinal relevance of thiosemicarbazones (TSCs) and their metal complexes have gained substantial interest among chemists over a century [4], [5], [6], [7], [8], [9]. TSC derivatives were used as analytical reagents and sensors in the selective and sensitive determination of metal ions [10], [11], [12], [13]. Even though the biological and analytical applications of TSCs and their complexes have been explored for long, the reports on the catalytic activities of them were appearing only in the last two decades. Since 1990, several research teams have been involved in studying the catalytic activities of TSC complexes towards many industrially and biochemically important reactions.
The transition metal complexes containing ligands with N, S or N, S, O donors are known to exhibit interesting stereochemical, electrochemical and electronic properties. TSCs contain chemically active R1R2C=N1−N2(H)−C(=S)N3R3R4 group having four potential coordination sites (three N and one S). Apart from this, the functional group(s) in the carbonyl compounds, which is(are) in close proximity to donating centers in thiosemicarbazide also form(s) additional coordination site(s). The coordinating ability of TSCs is attributed to extended delocalization of electron density over the R1R2C=N1−N2(H)−C(=S)N3R3R4 system, which is enhanced by the suitable substitution on the terminal nitrogen atom. The substituent(s) on the azomethine carbon of TSC ligands also influence(s) the mode of their binding. From the past reports, it is expected that the coordination ability of TSCs increases markedly on the introduction of nitrogen- or oxygen-containing substituent on azomethine carbon [14], [15], [16], [17]. Such properties of TSCs fascinated the researchers to explore their coordination behaviour with different transition metal ions. Fig. 1 represents a variety of coordination modes observed in TSC complexes. Usually, TSCs coordinate either as a neutral ligand (thione form) or as a deprotonated anionic ligand (thiol form) through the sulfur atom and either the azomethine N atom to create a stable five membered chelate ring (i) [18] or the hydrazine N atom to make a more strained unusual four membered chelate ring (ii) [19]. If the ligand contains a third donor site (D) appropriately located for coordination (e.g. salicylaldehyde TSC), a D, N, S-tricoordination normally takes place (iii) [20]. In addition, the S-donor atoms of TSCs can bridge metal ions to form dinuclear [21] or multinuclear complexes (iv) [22]. Other coordination modes observed in TSC complexes include S-monodentate (v) [23] and N (thioamide), S-bidentate (vi) [24]. Thus, the greater number of coordination sites, in turn variable binding modes of TSC ligands in metal complexes provide certain advantages in the fields of pharmaceuticals and catalysis [25], [26], [27]. Furthermore, in vivo experiments showed that some TSCs belong to only class III of hazardous substances, and hence, they are promising compounds for practical applications [28]. Due to higher stability of TSC complexes, catalytic systems involving these complexes may leave only negligible metal residue in the products.
The coordination behaviour of TSCs and catalytic applications of their metal complexes have been established briefly in the literature [21]. The prime objective here is to review the catalytic potentials of transition metal complexes containing TSC ligands. It focuses on organic transformations such as transfer hydrogenation, N-arylation, oxidation, cyclopropanation, epoxidation, alkylation, amidation and hydration catalyzed by metal complexes containing TSCs. It also covers a wide range of C–C cross-coupling reactions like Mizoroki-Heck, Suzuki-Miyaura, Sonogashira and Kumada-Corriu.
Section snippets
Transfer hydrogenation of carbonyl compounds
The reduction of carbonyl compounds is a fundamental transformation in the synthesis of several organic compounds in the petrochemical and pharmaceutical industries. This can be carried out by transition metal complex-catalyzed transfer hydrogenation (TH) reaction using a hydrogen source.
Mononuclear ruthenium(III) complexes of the type [RuX(EPh3)2(L)] (E = P or As; X = Cl or Br; L = dibasic terdentate dehydroacetic acid thiosemicarbazones (DHA-TSCs)) have been used as catalysts in the TH of
N-Arylation reaction
Metal-catalyzed C–N coupling reaction represents one of the most efficient methods to form various C–N bond containing compounds that have important biological, pharmaceutical or material properties [34]. Pd-catalyzed coupling reactions are one of the most significant achievements in this field for replacing traditional Cu-catalyzed Ullmann reactions [35]. Ohta and co-workers were the first to report the direct arylation of several hetero aromatics with aryl halides using [Pd(PPh3)4] as
Oxidation of alcohols
Ru(II) and Ru(III) complexes bearing TSC ligands were synthesized, and utilized as catalysts for the conversion of primary / secondary alcohols to corresponding aldehydes / ketones in presence of NMO (N-methylmorpholine-N-oxide) as co-oxidant (Fig. 6). Both Ru(III) (6) and Ru(II) (7) complexes act as efficient catalysts for the conversion of secondary alcohols to ketones with the yield of 92–99 %, while primary alcohols gave the products with the yield of 61–89 %. These complexes were also good
Cyclopropanation of olefins
As cyclopropane derivatives have large number of applications due to their biological properties, and use as starting materials and intermediates in organic synthesis, production of these compounds become more significant [40]. A versatile method is the metal-catalyzed cyclopropanation of olefins with diazo compounds. To replace the highly expensive Rh or Ru catalysts, Cu complex (Fig. 7) of N,2-bis(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazine carbothiamide (8) was utilized as a catalyst for
Olefin epoxidation
The dioxomolybdenum(VI) [MoO2(VI)] complex (9) was synthesized from [MoO2(acac)2] and ONS donor TSC ligand [1-(2,4-dihydroxybenzilidine)-N-methyl-N-phenylthiosemicarbazones] (Fig. 8) in acetonitrile, and tested as a catalyst for the epoxidation of olefins using tert-butyl hydrogen peroxide as an oxidant at 80 °C. Cyclohexene was the most reactive substrate, which gave 98 % of epoxide while styrene and indene produced 44 and 51 % of corresponding epoxides, respectively. This catalytic system
N-Alkylation of amines
Ru(II) complexes containing PNS donor phosphino-based TSC ligands efficiently catalyze the N-alkylation of 2-aminobenzothioazole, other heterocyclic amines and diamines using alcohol as an alkyl source in toluene solvent in presence of KOH at 100 °C through hydrogen auto transfer process under moderate conditions. Among the five complexes synthesized, [RuCl(PNS-EtTSC)(CO)(AsPh3)] (14) acted as a better catalyst (0.5 mol%) (yield = 97 %, TON = 196) than the other Ru(II) complexes (Fig. 9). It
Amidation of alcohols
Selvamurugan et al., synthesized Ru(II) complexes containing 2-oxo-1,2-dihydroquinoline N-substituted TSC ligands, and used them as catalysts for the generation of N-benzylbenzamides from benzyl alcohol substrates in presence of a strong base like t-BuOK or KOH (15 %) at 120 °C (Fig. 10). The alcohol substrates (benzyl alcohol) bearing chloro or bromo (electron-withdrawing) group yielded the desired products with the yield of 97 and 98 %, respectively, whereas 92 and 87 % of amidated products
Hydration of nitriles
In synthetic organic chemistry, hydration of nitriles into primary amides is a significant transformation since they are the key intermediates in the synthesis of bioactive products, drug stabilizers, lubricants, etc. Manikandan et al., reported Ru(II) carbonyl catalysts bearing pyridoxal TSC ligands and PPh3/AsPh3 for the hydration of nitriles to amides, and for the synthesis of biologically and pharmacologically important imidazoline derivatives [49]. Hydration of a series of aromatic,
Cross-coupling reactions
In the 1970′s, for the synthesis of biaryl compounds, many cross-coupling reactions between aryl halides and aryl magnesium/nickel have been reported [50], [51], [52]. In the later part of the same decade, Suzuki and Miyaura reported cross-coupling reaction between alkenylboranes and various organic halides [53], [54]. There were so many other cross-coupling reactions reported by Mizoroki, Heck, Stille, Sonogashira, Kumada, Corriu, etc.
C–C and C−hetero atom cross-coupling reactions especially
Summary
In the last two decades, transition metal complexes containing TSC ligands were proved to be active catalysts for several organic transformations apart from their potential biological activities. Ru(II)- and Ru(III)-TSC complexes were utilized as catalysts for TH of carbonyl compounds. The catalytic activity of these complexes towards TH of EWG/EDG-substituted carbonyl compounds was excellent over unsubstituted ones. N-arylation of imidazole and Buckwald-Hartwig amination reactions using
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Fondo Nacional de Ciencia y Tecnologia (FONDECYT, Project No. 3200391).
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