Recent developments and perspectives in the ruthenium-catalyzed olefin epoxidation

Tetrahedron 72 (2016) 6175e6190

Contents lists available at ScienceDirect

Tetrahedron journal homepage: www.elsevier.com/locate/tet

Tetrahedron report 1122

Recent developments and perspectives in the ruthenium-catalyzed olefin epoxidation Sankuviruthiyil M. Ujwaldev a, Kallikkakam S. Sindhu a, Amrutha P. Thankachan a, Gopinathan Anilkumar a, b, * a b

School of Chemical Sciences, Mahatma Gandhi University, PD Hills PO, Kottayam, Kerala, 686560, India Advanced Molecular Materials Research Centre (AMMRC), Mahatma Gandhi University, PD Hills PO, Kottayam, Kerala, 686560, India

a r t i c l e i n f o Article history: Received 9 March 2016 Available online 20 August 2016 Keywords: Ruthenium Epoxidation Heterogeneous Homogeneous Asymmetric Oxo species

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6176 Ruthenium complexes as epoxidation catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6176 2.1. Ligand-based ruthenium(II)-catalyzed epoxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6176 2.1.1. Ruthenium complexes with N4-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6176 2.1.1.1. Ruthenium porphyrin complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6176 2.1.1.2. Phthalocyanin-ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6179 2.1.2. Ruthenium complexes with N5-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6179 2.1.2.1. N5-polypyridine ruthenium complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6179 2.1.2.2. L3-Terpyridine-L2-bipyrimidine ruthenium complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6180 2.1.2.3. N3-Tris-pyrazolyl methane-N2-diimine ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6180 2.1.2.4. N3 Tris-pyrazolylmethane-N2oxazoline ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6181 2.1.3. Ruthenium complexes with N4O-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6181 2.1.3.1. N3-terpyridine and NO-quinaldate mononuclear ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6181 2.1.3.2. N3-Terpyridine-NO-pyridazine dicarboxylate dinuclear ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6182 2.1.3.3. N3-Terpyridine-NO-pyrazole dicarboxylate dinuclear ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6182 2.1.4. Ruthenium-aryl complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6183 2.1.5. Ruthenium-carbene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6183 2.1.5.1. Homogeneous N-heterocyclic ruthenium carbene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6183 2.1.5.2. Polypyrrole supported ruthenium heterocyclic carbene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6184 2.1.6. Ruthenium-higherdentate ligand complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6184 2.1.6.1. Hbimp-dinuclear ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6184 2.1.7. Ruthenium complexes with S4P2-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6185 2.1.7.1. S4-Dithio carbamate bis chelateeP2biphosphene complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6185 2.2. Ligand-based ruthenium(III)-catalyzed epoxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6185 2.2.1. Ruthenium complexes with N2O2-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6185 2.2.1.1. Ruthenium-salophen complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6185

* Corresponding author. Fax: þ91 481 2731036; e-mail address: [email protected] (G. Anilkumar). http://dx.doi.org/10.1016/j.tet.2016.08.052 0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

6176

3. 4.

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

2.2.1.2. Ruthenium-salen complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6186 2.2.1.3. Miscellaneous examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6186 2.2.2. Ruthenium complexes with N4O-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6187 2.2.3. Ruthenium complexes with O2-coordination environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6187 2.2.3.1. Chiral ruthenium(III) helical coordination polymer as a catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6187 2.3. Ligand-based mixed valent ruthenium-catalyzed epoxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6187 2.4. Ligand-free ruthenium-catalyzed epoxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6187 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6188 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6188 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6188 References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6188 Biographical sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6190

1. Introduction Olefin epoxidation constitute one of the key transformations in synthetic organic chemistry. Extreme reactivity makes these highly strained cyclic ethers important intermediates in carbonecarbon as well as carbon-hetero atom bond-forming reactions in pharmaceutical,1 agrochemical and perfume industry2 as well as in natural product synthesis.3 Nucleophilic opening of epoxide provides an easy pathway for nucleophile addition across a double bond. Many naturally occurring epoxides exhibit important biological properties.4 Transition metal catalyzed epoxidation received considerable interest since epoxidation using common oxidants like peroxy acids, organic peroxides, iodoso reagents, amine oxides, periodates etc. are found to be more selective and productive in the presence of these catalytic systems. Sharpless asymmetric epoxidation using titanium tartrate complexes forms the best known example.5 Ruthenium complexes constituted a major area of epoxidation catalysts which showed remarkable developments in the last few decades.6 The developments in the field of homogeneous ruthenium-catalyzed epoxidations have been well reviewed up to 2009.7 Heterogeneous ruthenium-catalyzed epoxidation is also an inevitable area which also witnessed significant achievements in which catalyst developments involved skillful derivatization, immobilization etc. of the already reported homogeneous catalysts. This review combines developments in both homogeneous (from 2009 to 2015) and heterogeneous (from 1998 to 2015) rutheniumcatalyzed epoxidations.

2. Ruthenium complexes as epoxidation catalysts Like all other transition metals, ruthenium also possesses easily interchangeable multiple number of oxidation states. This property makes the metal capable of facile ligand exchange or substitution in the complexed state providing easy accessibility for metal-oxo species, the key intermediate in olefin epoxidation which plays the role of oxygen donor to alkenes making valuable catalysts. Homogeneous ruthenium complexes with ligands such as porphyrin,8 polypyridyl,9 Schiff base,10 oxazoline,11 pyrazolyl12 etc. formed a vast range of ruthenium-catalyzed epoxidation catalysts. Heterogeneous catalysis which is a more economical mode also made valuable contributions. Heterogenization of ruthenium complexes was always found to be challenging since strategies were needed which maintain the properties of ligands such as lability, enantiopurity or even relative orientation which have significant effect on reactivity, enantioselectivity, regioselectivity or chemoselectivity of the epoxidations. Anchoring of the catalyst on to a polymer was one of the strategies available which only required either of the ligands to contain a reactive functional group or a polymerizable moiety. Immobilization of catalysts in channels of highly porous materials like zeolites or molecular sieves was

another efficient method in which characteristic pore sizes provided shape selectivity also. 2.1. Ligand-based ruthenium(II)-catalyzed epoxidations 2.1.1. Ruthenium complexes with N4-coordination environment 2.1.1.1. Ruthenium porphyrin complexes. a) Encapsulated or supported ruthenium porphyrin complexes The developments in porphyrin based metal catalysts were mainly inspired by a wide variety of metalloporphyrins found in nature which catalyze highly selective biological transformations. These reactions make use of multiple numbers of redox reactions happening at the metal centre which are stabilized and maintained by the porphyrin ligand. One of the earliest reports available on the ruthenium-porphyrin catalyzed epoxidation of alkenes was by Quinn et al. in 1985 where a dioxo(tetramesity1porphyrinato) ruthenium(VI) complex, [Ru(TMP)(O)2] was found to catalyze the epoxidation of alkenes using oxygen as the oxidant.13 In 1998, Liu et al. introduced a new heterogeneous epoxidation catalyst Ru/M-41(m) 1 by immobilizing ruthenium meso-tetrakis (2,6dichlorophenyl)porphyrin complex, (RuII(TDCPP)(CO)(EtOH)) 3 in (3-aminopropyl)triethoxysilane modified MCM-41 molecular sieves (APTES-MCM-41m) 2 (Fig. 1) which are found to catalyze epoxidations stereospecifically in good yields.14 Encapsulation was performed by stirring 2 with 3 for one hour in dichloromethane (DCM). The catalytic activity of the complex was explored for a few alkenes using 2,6-dichloropyridine-N-oxide(Cl2pyNO),4 as the oxidant (Scheme 1). Excellent to moderate yields were observed in epoxidation of a variety of alkenes. The catalyst has provided complete stereospecific transformation in the case of cis-alkenes whereas found to be inactive towards trans-alkenes. Epoxidation of (þ)-limonene furnished about 61% of 8,9-epoxide along with cis- and trans-1,2epoxides in yields of 27% and 10%, respectively. While 3,4,6-tri-O-

Fig. 1. Ruthenium-porphyrin complex 1 obtained by ligand substitution in 3 with 2. Reproduced with permission from J. Org. Chem. 1998, 63, 7364e7369.

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

6177

Fig. 3. Tetrastyrenyl porphyrin Ru complex 6 and ethylene glycol dimethyl acrylate 7. Scheme 1. Epoxidation using Cl2pyNO catalyzed by Ru/M-41(m).

acetyl-D-glucal gave a mixture of a- and b-epoxides in a ratio of 3:1 in about 55% yield, the 3,4,6-tri-O-benzyl-D-glucal failed to accomplish the conversion. Good epoxide yield was also obtained in the case of terminal alkenes. An oxoruthenium(V) complex or oxoruthenium(IV) porphyrin cation radical was proposed as the active species in this epoxidation sequence. Later the same group achieved immobilization of chiral ruthenium porphyrin [RuII(D4-Por*)CO] 5 in MCM-41 and MCM-48 which provided the first examples of chiral metalloporphyrin immobilized molecular sieves as epoxidation catalyst (Fig. 2).15

discrimination observed in the reaction of stilbenes indicating the shape selectivity of the matrix (Scheme 3).

Scheme 3. Epoxidation catalyzed by Ru-polymer matrix using Cl2pyNO as the oxidant.

In 2002, Qui et al. reported Merrifields peptide resin (MPR) 8 supported carbonyl ruthenium (II) 5,10,15-tris(4-R-phenyl)-20-(4hydroxy phenyl)porphyrins 10 and 11 among which 11 was found to be highly stereospecific and diastereoselective in epoxidations compared to 10 (Fig. 4).17

Fig. 2. Chiral ruthenium porphyrin [RuII(D4-Por*)CO] complex. Reproduced with permission from Chem. Commun. 2002, 2906e2907.

The catalysts have provided enantiomeric excess between 46% and 75% in epoxidations using Cl2pyNO with very good yields (Scheme 2).

Scheme 2. Epoxidation using Cl2pyNO catalyzed by MCM supported 5.

New ruthenium porphyrin encapsulated highly cross-linked polymer matrices was introduced by Severin and Nestler by copolymerizing meso-tetrastyrenyl porphyrin complex 6 with ethylene glycol dimethyl acrylate 7 (Fig. 3) which was ground and sieved later.16 Epoxidation using 4 catalyzed by the ruthenium polymer matrix exhibited excellent conversions (>90%) with cisetrans

Fig. 4. Merrifield’s peptide resin 8, ruthenium porphyrin complex 9 and resin supported ruthenium porphyrin complexes 10 and 11. Reproduced with permission from J. Am. Chem. Soc. 2000, 122, 5337e5342.

The catalyst was synthesized by substitution of the chloro group in the polymer 8 by phenolic hydroxyl group in 9. Epoxidation using Cl2pyNO catalyzed by these polymer supported ruthenium porphyrins was found to exhibit moderate to excellent yields (Scheme 4). Exclusive formation of threo epoxide and excellent yields were respectively observed in the cases of protected R-amino alkenes and conjugated enynes which form key intermediates in the synthesis of a few biologically active molecules.18 Exclusive a-selectivity was observed in the case of glycal. No significant loss in epoxide yields was observed upto four consecutive cycles. In 2002 Che and co-workers introduced highly soluble polyethylene glycol (PEG), 12 supported ruthenium porphyrin complexes 14, 15 and 16 (Fig. 5) as epoxidation catalysts.19 They were prepared by treating mesylate functionalized PEG with ruthenium porphyrin complex 13. High solubility of these complexes proposed

6178

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

In 2002 Che et al. developed a few dendritic ruthenium porphyrins,21 a new series of highly selective epoxidation catalysts, represented as 5-[G-0]8 17, 5-[G-1]4 18, 5-[G-1]8 19, 5-[G-2]4 20 and 5-[G-2]8 21 (Fig. 6).

Scheme 4. Epoxidation with Cl2pyNO catalyzed by MPR supported ruthenium porphyrin complex 11.

Fig. 5. Ruthenium porphyrin complex 13 and PEG supported ruthenium porphyrin complexes 14e16. Reproduced with permission from Org. Lett. 2002, 4, 1911e1914.

to provide better mobility and easy accessibility for the olefins to the active sites during epoxidation. Catalyst 14 (with electron withdrawing chloro groups and highest ruthenium porphyrin loading) was found to be superior among the three catalysts whose catalytic activity was examined for a variety of alkenes using 2,4-dichloro pyridine-N-oxide 4 as the oxidant (Scheme 5).

Fig. 6. Dendritic carbonyl ruthenium porphyrins 17e21.

These dendritic metalloporphyrins were prepared by refluxing carbonylruthenium(II)-5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin 22 or carbonylruthenium(II)-5,10,15,20-tetrakis(3,5dihydroxyphenyl)-porphyrin 23 with dendritic mesylates (Scheme 6).

Scheme 6. Synthesis of dendritic ruthenium porphyrin complexes 20 and 21, respectively from 22 and 23 by treatment with dendritic mesylate 24.

Scheme 5. Epoxidation with Cl2pyNO catalyzed by polyethylene glycol(PEG) supported ruthenium porphyrin complex 14.

The polymer supported catalyst was found to exhibit excellent stereospecificity and yields in the case of cis and trans-alkenes, terminal alkenes and sterically bulkier norbornene. Even highly sterically hindered cholesteryl acetate gave the b-epoxide with 90% yield. An a:b ratio of 10:1 was observed in the case of 3,4,6-tri-Oacetlyl-D-glycal which forms one of the key substrates in the synthesis of oligosaccharides. Later the same group was able to achieve covalent attachment of [RuII(F20-tpp)(CO)] and the corresponding dioxo ruthenium porphyrins on polyethylene glycol,20 where PEG-[RuII(F20-tpp)(CO)] was shown yields comparable to that of 14. An enhanced yield close to 99% was observed in the case of synthetically important glycal.

Among the dendritic ruthenium porphyrins, 5-[G-2]8 21 was found to be superior in terms of epoxide selectivity using Cl2pyNO 4 as the oxidant. Sterically hindered cholesterylesters in this case also gave good yields with a b-selectivity of 99%. 3,4,6-Tri-Oacetlyl-D-glycal gave an overall yield of 83% with a:b selectivity 1:9 (Scheme 7). Moderate to excellent yields were obtained in epoxidation using Cl2pyNO catalyzed by 5-[G-2]8. Epoxidation of estratetraene using 21 exclusively produced the b-isomer with 95% yield. b) Homogeneous ruthenium porphyrin complexes A highly enantio- and regio-selective ruthenium porphyrin epoxidation catalyst 27 which uses non-covalent hydrogen bond interactions for enantioface selective oxo transfer to alkene functionalized quinolones, pyridones and amides was developed by Fackler et al. from a bromo substituted ruthenium porphyrin 26 and a terminal alkyne 25 by Sonogashira coupling (Fig. 7).22,23

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

6179

Scheme 7. Epoxidations catalyzed by dendritic ruthenium porphyrin using Cl2pyNO.

Scheme 8. Epoxidation using Cl2pyNO catalyzed by 27.

Fig. 7. Sonogashira coupling between bromo substituted ruthenium porphyrin complex 26 and terminal alkyne 25.

A diaxial-dioxoruthenium species has been proposed to be the catalytically active species which can engage in highly specific-rigid orientations with vinyl or alkenyl fragments (Fig. 8) by means of hydrogen bond interactions and deliver the oxygen to a particular prochiral face of alkene.

Fig. 9. Non-peripherally alkyl substituted carbonyl ruthenium phthalocyanins.

Scheme 9. Epoxidation of 1,2-dihydronaphthalene using Cl2pyNO catalyzed by ruthenium complexes 28e31. Fig. 8. Dioxo ruthenium-substrate interaction visualized based on semi empirical calculations. Reproduced with permission from J. Am. Chem. Soc. 2010, 132, 15911e15913.

The epoxidations were carried out in benzene using stoichiometric amounts of Cl2pyNO as the oxidant (Scheme 8). N-Hydro-3-vinylquinolones were epoxidized with high enantioselectivity which was found to be diminished in the case of Nmethylated quinolones due to the loss of one of the hydrogen bond interactions among the two. High regioselectivity was observed in the case of 3,7-divinylquinolone in which the vinyl group at position 3 got epoxidized due to its easy accessibility to ruthenium oxocentre. The 3-alkenyl quinolones underwent epoxidations stereospecifically and enantioselectively to give trans-epoxides with enantioselectivity above 90%. 2.1.1.2. Phthalocyanin-ruthenium complexes. A new series of non-peripherally alkyl substituted carbonyl ruthenium phthalocyanins 28e32 were reported (Fig. 9).24 Epoxidation using oxidant 4 catalyzed by 28 gave an excellent yield of 95% for trans-stilbene. In the case of 1,2-dihydronaphthalene, all the complexes except 32 gave good yields of the epoxide under identical conditions (Scheme 9).

In the case of cyclooctene, the catalysts provided good yields (68e86%) while poor yields were obtained in the case of cyclooctadiene (4e34%) and limonene (40e47%) with 32 being the inactive. The catalytic activities were found to decrease with increase in the size of the substituent on the phthalocyanin ring. The mechanism proposed involves a Ru]O species as the intermediate which transfers its oxygen to the alkene through a concerted process. 2.1.2. Ruthenium complexes with N5-coordination environment 2.1.2.1. N5-polypyridine ruthenium complex. Hamelin and coworkers introduced a new pentadentatepolypyridine (L5pyr) ruthenium complex [Ru(L5pyr)(CH3CN)]2þ 33 which was found to afford good yields of epoxide for cyclooctene and trans-b-methyl styrene using iodosyl benzene as the oxidant.25 The complex was synthesized by refluxing L5pyr with RuCl2(dmso)2 followed by substitution with acetonitrile. The electron rich pentadentate ligand has a significant role in improving the catalytic activity of 33 which is evident from the lower yields and turnover frequencies for epoxidations with lower dentate pyridine analogues [Ru(bpy)2(CH3CN)2]2þ 34 and [Ru(bpy)2py(CH3CN)]2þ 35 (Scheme 10).

6180

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

Moderate to excellent yields were obtained in epoxidations using PhI(OAc)2 as the oxidant (Scheme 11).

Scheme 10. Catalytic activity of 33, 34 and 35 towards cyclooctene epoxidation. Scheme 11. Catalytic activity of 37a in epoxidations using PhI(OAc)2 as the oxidant.

A N5O coordinated species 36a/b derived from 33 was proposed to be the species that catalyze the epoxidation which was evident from the m/z value at 280 of the oxidized (H2O2) form of 33 (Fig. 10).

Fig. 10. Possible intermediates capable of oxo transfer in 36 catalyzed epoxidation. Reproduced with permission from Inorg. Chem. 2008, 47, 6413e6420.

ruthenium complex.2.1.2.2. L3-Terpyridine-L2-bipyrimidine Epoxidation catalyzed by nano Fe3O4 supported molecular ruthenium complexes 37a, i.e., [RuCl(bpm)(trpy-P)]þ and 37trans-b, i.e., trans-[RuCl(azpy)(trpy-P)]þ was reported by Vaquer and coworkers.26 The synthesis of the catalyst was achieved by complexing the phosphonate functionalized terpyridine ligand 38 with RuCl3 in boiling ethanol and separating the precipitated octahedral complex [RuCl3(trpy-PO(OEt)2)] 39. Substitution of chlorine ligands in 39 by bidentate NeN ligand 2,2-bipyrimidine (bpm), 40 or 2phenylazopyridine (azpy), 41 accompanied by reduction of Ru(III) to Ru(II) gave 42a/trans 42b and cis 42b. Substitution of chlorine by bromine using TMSBr followed by hydrolysis afforded 43a/trans-b. The complex was then anchored onto magnetic nano Fe3O4 by dehydration to get 44a/trans-b which on hydrolysis furnished 37 (Fig. 11).

The familiar Ru]O species was again proposed to be the active species, but anticipated to have an electrophilic nature due to the presence of highly electron withdrawing bpm ligand which accounts for the lower epoxide yields for styrene and 3-trifluoro styrene. Steric demands while accessing Ru]O species also proposed to have important effect as triphenyl ethylene gave a lower yield. The complex 37trans-b was also found to be catalytically active in epoxidation of cis-b-methyl styrene but with a lower yield. The catalyst was found to be magnetically separable after the reaction and could be reused upto five consecutive cycles without significant loss of reactivity and selectivity. 2.1.2.3. N3-Tris-pyrazolyl methane-N2-diimine ruthenium complexes. A new diamagnetic aqua benzoquinone diimine ruthenium complex [Ru(tpm)(bqdi)(H2O)](ClO4)2 45 (Fig. 12) was reported

Fig. 12. Aqua benzoquinone diimine ruthenium complex 45 and the constituting ligands.

which exhibited moderate to excellent epoxide selectivities in alkene epoxidations using PhI(OAc)2 as the oxidant (Scheme 12).27 The complex was synthesized by a base catalyzed complexation between 1,2-diaminobenzene and Ru(tpm)Cl3$1.5H2O in absolute

Fig. 11. Nano ferric oxide supported ruthenium complexes and their precursors.

Scheme 12. Epoxidation of alkenes using PhI(OAc)2 catalyzed by 45.

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

ethanol and subsequent hydrolysis of the resulting chloro complex in the presence of AgNO3. Although quite similar conversion rates were observed for 4fluoro, 3-nitro and 2- or 4-methoxystyrenes, the former two bearing electron-withdrawing groups exhibited better epoxide selectivity compared to the latter two. While trans-stilbene afforded exclusively the trans-epoxide, it was formed only about 64% in the case of trans-b-methyl styrene. A ruthenium oxo complex [RuIV(tpm)(bqdi)-(O)]2þ 46 was proposed as the catalytically active species, formation of which was indicated by a shift in frequency from 500 nm to 445 nm in UVeVisible spectrum on addition of PhI(OAc)2 to the aqua complex. Mass spectrum of the reaction medium of the styrene epoxidation reveals the in situ formation of a styrene-[RuIV(tpm)(bqdi)-(O)]2þ adduct 47 (i.e., m/z¼331.04) which undergoes a concerted oxo transfer to form an epoxide coordinated metal complex 48 which eliminates the epoxide by oxidation regenerating the oxo complex (Scheme 13).

6181

Scheme 14. Epoxidation using BIAN-derived ruthenium complex.

also; but a radical intermediate was supposed to be involved during the oxo transfer. Although the structurally similar Ru-([N-(phenyl) imino]-acenaphthenone) complex, 54 was synthesized and well characterized, it was found to be catalytically inactive which was proposed to be due to its inability to form RuIV]O species. The aqua analogue of 49, i.e., 53 was found catalytically active only towards styrene, cis-stilbene and dec-1-ene with limited substrate scope.

Scheme 13. Proposed mechanism of epoxidation catalyzed by 45. Reproduced with permission from Dalton Trans. 2013, 42, 3721e3734.

2.1.2.4. N3 Tris-pyrazolylmethane-N2oxazoline ruthenium complexes. Recently two new Ru(II)-aqua complex catalysts were introduced based on oxazoline ligands viz. [RuII(iPr-box-C)(tpm) OH2](PF6)2 55 and [RuII(iPr-box-O)(tpm)OH2](PF6) 56 (Fig. 14).29

Later the same group reported a new series of BIAN (bis (arylimino)acenaphthene) derived ruthenium complexes 49, 50, 51 and 52 (Fig. 13) which were used as efficient catalysts towards the epoxidation of a variety of alkenes in the presence of PhI(OAc)2.28 Fig. 14. Oxazoline ligands constituting the complexes 55 and 56.

The complexes were synthesized from [RuIIICl3(tpm)] by base catalyzed oxazoline ligand exchange followed by silver-catalyzed hydrolysis of the resulting chloro complex. Catalytic activity of the two complexes in epoxidation were explored for trans-stilbene using PhI(OAc)2 as the oxidant. Epoxide selectivities of 85% and 81% were observed for 55 and 56, respectively with very similar conversions. Catalyst 55 showed regioselectivity towards the terminal alkene segment of 4-vinylcyclohexene which is the first report of regioselectivity with a ruthenium catalyst. Fig. 13. BIAN derived ruthenium complexes 49e53 and structurally similar 54. Reproduced with permission from Inorg. Chem. 2015, 54, 4998e5012.

The BIAN-complex 52 containing the p-nitro group was found to display the highest redox potential and found to be superior in terms of catalytic activity than the other three. Epoxidations catalyzed by this complex exhibited conversions of 66e90% with epoxide selectivity in the range of 70e100% showing almost similar reactivity towards both the conjugated and the normal alkenes. Terminal alkenes were found to exhibit superior activity compared to internal alkenes. For example, an equimolar mixture of 1-octene and trans-5-decene gave almost 80% conversion in favour of the former and the latter furnished only 20% epoxide (Scheme 14). A mechanism quite similar to that proposed for aqua benzoquinone diimmine ruthenium complex, 45 has been suggested here

2.1.3. Ruthenium complexes with N4O-coordination environment 2.1.3.1. N3-terpyridine and NO-quinaldate mononuclear ruthenium complexes. Isomeric Ru(II)-terpyridinequinaldate complexes 57 and 58 were developed (Fig. 15) and their catalytic activities in epoxidation were explored using different oxidants and solvents.30

Fig. 15. Isomeric ruthenium quinaldate complexes 57 and 58 and the quinaldate ligand.

6182

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

The complexes 57 and 58 were found to be remarkably different in their catalytic activities in the epoxidation and 57 was found to be far more superior. H2O2eEtOH system provided 100% chemoselectivity towards epoxide formation for complex 57. Complete epoxide selectivity was observed for 58 only with m-CPBA-CH3CN and the substrate scope was limited compared to 57 (Scheme 15).

Fig. 16. The complex {[RuII(trpy)]2(m-pdz-dc)(m-OH)}þ 61 and constituent ligands.

Scheme 15. Epoxidation catalyzed by 57 and 58 in EtOH using hydrogen peroxide.

Among the cyclohexene derivatives, the electron-rich 1-methyl cyclohexene gave higher yield compared to the electron deficient 1-nitro or 1-phenylcyclohexene as well as unsubstituted cyclohexene. Similarly a- or b-methyl styrenes afforded better yields compared to the styrene. A Ru]O complex 59 generated by oxidation of catalyst 57/58 with hydrogen peroxide is assumed to be the active species in the catalysis. The oxidation by H2O2 is presumed to be rapid in 57 with very labile RueCl from which oxygen transfer to the alkene is proposed through a radical mechanism (60aec) (Scheme 16). It is supported by a theoretically calculated

Scheme 17. Epoxidation catalyzed by 61 using PhI(OAc)2 as the oxidant.

Scheme 16. Proposed mechanism of epoxidation catalyzed by 57. Reproduced with permission from Inorg. Chem. 2011, 50, 1775e1785.

Scheme 18. Proposed mechanistic pathway. Reproduced with permission from Inorg. Chem. 2013, 52, 4335e4345.

increase of Ru]O bond length in DFT optimized alkene-Ru]O complex adduct 60 compared to that in Ru]O complex. This suggests a single bond character for the RueO bond in the former. Reduced conversion rates were observed on the addition of high concentrations of radical scavengers like N,N0 -dimethylthiourea to the reaction mixture.

2.1.3.3. N3-Terpyridine-NO-pyrazole dicarboxylate dinuclear ruthenium complexes. The development of the catalyst 62 (Fig. 17) provided an excellent example of use of supramolecular substrate orientation effects to achieve complete stereoselectivity.32 Highly efficient epoxidations were observed particularly with cis-alkenes.

2.1.3.2. N3-Terpyridine-NO-pyridazine dicarboxylate dinuclear ruthenium complexes. A new dinuclear ruthenium complex {[RuII(trpy)]2(m-pdz-dc)(m-OH)}þ 61 (Fig. 16) was reported to catalyze epoxidation with selectivities between 36% and 79% using PhI(OAc)2 as the oxidant (Scheme 17).31 Under the reaction conditions the complex 61 was proposed to furnish the active diaqua-diruthenium complex 61a which get transformed into catalytically active dioxo species 61b by in situ generated PhI]O from PhI(OAc)2 and water. A concerted oxo transfer affords 61c which get transformed into the product with complete stereospecificity as observed in the case of cis- or transolefins (Scheme 18).

Fig. 17. The complex {[RuII(trpy)(H2O)]2(m-pyr-dc)}þ and constituent ligands. Reproduced with permission from Chem. Eur. J. 2014, 20, 3898e3902.

The catalyst showed different reactivities towards cis- and transalkenes; the cis-2-octene was epoxidized about five times faster

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

6183

than the trans-isomer while the trans-stilbene was oxidized about three times faster than the cis-isomer (Scheme 19).

Scheme 21. Catalytic activities of 63 using O2 and IBA as additive in epoxidations.

Scheme 19. Epoxidation of alkenes using PhI(OAc)2 catalyzed by 62.

The anticipated mechanism for this reaction involves a ruthenium dioxo species [(O)RuIVRuIV(O)]þ which is located in a pocket created by the surrounding polydentate ligands as the catalytically active species. The energy of the transition states corresponding to interaction of cis- and trans-isomers with this species within this pocket is proposed to be dependent on the alignment of these structurally different isomers in which supramolecular forces have a remarkable impact, leading to different reactivities. 2.1.4. Ruthenium-aryl complexes. Epoxidation using three coordinated ruthenium complex immobilized on silica surface 63 was reported by Iwasawa et al. in 2007. The highlight of the reaction was the unsaturated stable recyclable oxygen coordinated species 65 which is generated from 63 through exothermic transformation of an isobutyrylhydroperoxide intermediate 64 (itself formed by pcymene elimination from 63 under the exothermic condition provided by the reaction of oxygen with isobutyraldehyde) in the presence of isobutyraldehyde (IBA) and oxygen. The stable species 65 was proposed to reversibly furnish 64, in the presence of IBA which acts as the epoxidizing agent (Scheme 20).33

noticeable leaching maintaining remarkable conversion rates and selectivity. 2.1.5. Ruthenium-carbene complexes 2.1.5.1. Homogeneous N-heterocyclic ruthenium carbene complexes. A new aqua ruthenium complex trans-[Ru(CN-Me)(trpy) OH2](PF6)2 66 (Fig. 18) was introduced which showed excellent reusability for epoxidations in ionic medium.34 The catalyst was synthesized from Ru(trpy)Cl3 67 by reduction with triethylamine in the presence of NHC carbene precursor N-methyl-N0 -2pyridylimidazoliumbromide (HCN-Me) 68 followed by silver catalyzed hydrolysis of the resulting major trans-chloro complex.

Fig. 18. trans-[Ru(CN-Me)(trpy)OH2]2þ 66, Ru(Cl)3(trpy) 67 and NHC carbene precursor 68.

Selectivity of 57 to >99.9% were observed in epoxidations using PhI(OAc)2 as the oxidant in the presence of ionic mediumdichloromethane mixture (Scheme 22). The catalytically active species proposed was a RuIV]O species generated by a twoelectron transfer process. The carbene ligand is proposed to have a remarkable effect in making the Ru]O bond in the catalytically active oxo complex more nucleophilic by occupying its trans-position. The effect was well pronounced from the high initial conversion rates for more electron deficient alkenes. A regioselectivity of about 93% was observed in favour of the ring double bond in the case of 4-vinylcyclohexene.

Scheme 20. Catalytic cycle involving generation of intermediate oxidant 64. Reproduced with permission from Angew. Chem. Int. Ed. 2007, 46, 7220e7223.

Epoxidation of olefins were carried out using 63 and isobutyraldehyde (IBA)/O2 which afforded selectivity from 74% to 90% while running at conversion percentages between 81% and 100% (Scheme 21). The complex 64 was found to be well subsisted in the active state even after about thousand stilbene epoxidation cycles with excellent recyclability under aerobic conditions without any

Scheme 22. Epoxidation with PhI(OAc)2 catalyzed by 66 in [bmim]PF6:CH2Cl2.

Although comparable catalytic activities were observed in dichloromethane, conversion rates were found to drop upto 35% and the selectivity was maintained on a second run. The selectivity was retained even after 10 consecutive runs in the presence of ionic medium. The ability of the ionic medium to keep catalyst molecules

6184

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

away from each other and preventing their auto deactivation are proposed to be the reasons for the reusable nature of the catalyst in ionic medium. Later other aqua N-heterocyclic carbene Ru(II)-complexes trans,fac-[Ru(CN-Me)(bpea)OH2]2þ 69, cis-[Ru(CN-Me)(trpy) OH2]2þ 70 and trans-[Ru(CN-Me)(trpy)OH2]2þ 71 (Fig. 19) were developed. The complex 69 was synthesized from [RuCl2(bpea) dmso] while 70 and 71 were from [Ru(trpy)]Cl3 by the substitution of the NHC carbene followed by silver-catalyzed hydrolysis of the respective isomer of the resulting chlorocarbene complexes.35

Fig. 20. Polymer supported complex 72 and ruthenium complex monomer 73.

benzene catalyzed by this film coated electrodes operated well with aromatic as well as aliphatic alkenes (Scheme 24). Styrene and oct-1-ene were found to be the least reactive even in the presence of highest catalyst loading among the series and showed the lowest selectivity and turnover numbers. The catalyst 72 was found to exhibit regioselectivity in the epoxidation of 4vinyl cyclohexene where epoxidation exclusively took place at cyclohexene ring. Fig. 19. trans,fac-[Ru(CN-Me)(bpea)OH2]2þ complexes and bpea ligand.

Catalytic activity of complexes 69, 70 and 71 in epoxidation were explored and compared using PhI(OAc)2 as the oxidant (Scheme 23). The catalyst 69 exhibited the highest initial conversion rates, being the fastest catalyst while 70 the slowest. The catalyst 69 was also found to be superior in terms of selectivity since greater than 90% oxirane formation was observed for all the chosen alkenes which can be attributed to its lowest acidity. Scheme 24. Epoxidation using PhI(OAc)2 catalyzed by polypyrrole supported ruthenium complex 72.

2.1.6. Ruthenium-higherdentate ligand complexes

Scheme 23. Epoxidation of alkenes with 69, 70 and 71 using PhI(OAc)2 as the oxidant.

2.1.6.1. Hbimp-dinuclear ruthenium complexes. A bis-aqua bisfacial dinuclear Hbimp (3,5-bis[bis(1,4,5-trimethyilmidazol-2-yl)methoxymethyl]pyrazole) ruthenium complex [[RuII(bpy)(H2O)]2(m-bimp)]3þ 75 was reported which was generated in situ by hydrolysis of its bridged chloro counterpart 76 and facilitated epoxidation with excellent turnover numbers and turnover frequencies.37 The precursor chloro complex was synthesized by complexing RuCl3$nH2O with Hbimp77 followed by bipyridine substitution (Scheme 25).

Catalytically active species involved in the case of all the three complexes was the irrespective RuIV]O states whose formation proposed to involve a two electron process. The presence of higher s-donating bpea ligand (lower p-accepting) in the complex 69 has provided a nucleophilic character to Ru]O bond in the corresponding oxo species, distinct from the other two complexes (contains p-accepting trpy ligands making Ru]O bond electrophilic). This factor along with less steric hindrance in accessing Ru]O site favoured the complex 69 to achieve high initial conversion rates compared to the complexes 70 and 71. 2.1.5.2. Polypyrrole supported ruthenium heterocyclic carbene complexes. A new polypyrrole supported ruthenium carbene epoxidation catalyst 72 was introduced by oxidative polymerisation of pyrrole containing trans,fac-[RuII(CN-Me)(H2O)(bpea-pyr)](PF6)2 complex 73 (Fig. 20).36 The aqua complex 73 was prepared from [RuCl2(dmso)4] by consecutive substitution steps with bpea-pyr 74 and NHC carbene ligands followed by hydrolysis in the presence of Ag(I)-salt. The aqua complex 73 was then polymerized by electrolyzing 1 mM solutions in DCM using glassy carbon electrodes at potentials between 0 and 1.3 to get 72. Epoxidation with in situ generated iodoso

Scheme 25. Synthesis of bis-aqua bis-facial dinuclear ruthenium complex 75. Reproduced with permission from Inorg. Chem. 2015, 54, 6782e6791.

Good to excellent selectivities were observed for alkenes except for electron deficient and sterically hindered stilbenes. cis-Isomers exhibited enhanced reactivities compared to the trans-which was evident from the initial turnover frequencies (Scheme 26). A trans O]RueRu]O species which is located in a void formed by the surrounding ligands is proposed as the reactive intermediate. The extent of cisetrans discrimination anticipated to be

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

6185

Scheme 26. Epoxidation using PhI(OAc)2 catalyzed by in situ generated ruthenium complex 75.

dependent on the degree of accessibility of isomers to this cavity and consequential interaction with the dioxo species. Oxo transfer from the dioxo species to the alkene was assumed to involve either a concerted process or via a radical intermediate in which CeC rotation occurs much slower than the ring closure to yield the epoxide. 2.1.7. Ruthenium complexes with S4P2-coordination environment 2.1.7.1. S4-Dithio carbamate bis chelateeP2biphosphene complex. Four new ruthenium(II) dithio carbamate complexes viz. [Ru(dppb)(pipeCS2)2] 78, [Ru(dppb)(morphCS2)2] 79, [Ru(dppb)(ethCS2)2] 80 and [Ru(dppb)(hexCS2)2] 81 (Fig. 21) were reported to catalyze cyclohexene epoxidations with conversion upto 60% and with selectivity between 31% and 74% (Table 1).38

Fig. 21. Ruthenium thiocarbamate complexes and their corresponding ligands.

Fig. 22. Polystyrene supported ruthenium-salophen complex 83 formed by complexing 84 with 85.

2011 with high to moderate conversion rates along with excellent reusability.39 These polymer-bound catalysts were prepared by stirring Ru(III)-salophen 83 with amine functionalized polystyrene 84 in acetonitrile at 80  C. The reactivity towards epoxidation of all the three complexes was examined and compared using NaIO4 as the oxidant in CH3CN/ H2O (1:1) solvent mixture (Table 2). Moderate to excellent yields were observed in epoxidation with all the three catalysts. Yields were found to decrease with increase in chain length in the case of linear terminal alkenes. More than 75% yields were maintained up to three consecutive cycles in the case of all the three catalysts indicating their reusability. New zeolite-encapsulated ruthenium(III)-salophen (ZERS) complexes, 85 were reported as reusable catalysts for the epoxidation of alkenes using NaIO4.40 The catalyst was synthesized by stirring RuCl3 with zeolite Y and heating the resulting dried ruthenium-exchanged zeolite with salophen ligand (Fig. 23) which got complexed with ruthenium ions inside becoming rigid and large to exit from the nano pore of zeolite.

Table 2 Epoxidation of alkenes using NaIO4 catalyzed by polymer supported rutheniumsalophen complexes 82a, b and c

All the four complexes were synthesized by stirring [RuCl2(dppb)]2-m-(dppb)] with the corresponding thiocarbamate ligands. Epoxidation was carried out using iodosyl benzene as the oxidant in which [Ru(dppb)(hexCS2)2] 82 exhibited superior catalytic activity in terms of selectivity.

Entry

Table 1 Yields and selectivity in the epoxidation of cis-cyclooctene catalyzed by Ru(II)/dithiocarbamate complexes

Olefin

[Ru(salophen) @DAB-PS]a

[Ru(salophen) @AP-PS]b

[Ru(salophen) @ATP-PS]c

Epoxide yield %

Time h

Epoxide yield %

Time h

Epoxide yield %

Time h

1

93

5

87

5

89

5

Entry

Catalyst

Epoxide yield (%)

Selectivity(%)

2

84

5

80

5

82

5

1 2 3 4

78 79 80 81

18 27 35 39

31 45 60 74

3

81

5

79

5

79

5

4

77

5

76

5

78

5

5

66

10

58

10

59

10

6

55

10

44

10

43

10

Catalyst:alkene molar ratio¼1:40, alkene:PhlO molar ratio¼1:1.5, Solvent:DCM at rt.

2.2. Ligand-based ruthenium(III)-catalyzed epoxidations 2.2.1. Ruthenium complexes with N2O2-coordination environment 2.2.1.1. Ruthenium-salophen complexes. Polystyrene supported Ru-salophen complexes 82a, 82b and 82c were reported (Fig. 22) in

a

Alkene:NalO4:catalyst¼1 mmol:2 mmol:0.063 mmol. b Alkene:NalO4:catalyst¼1 mmol:2 mmol:0.067 mmol. c Alkene:NalO4:catalyst¼1 mmol:2 mmol:0.073 mmol in 1:1 acetonitrile:water mixture.

6186

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190 Table 4 Asymmetric aerobic oxidation of conjugated olefins using 88 as catalyst

Fig. 23. Ruthenium-salophen complex before and after encapsulation.

Temp ( C)

Air/O2

Yield (%)

ee (%)

1

25 0

air O2

80 89

80 84

2

25

O2

25

45

3

0

O2

85

75

4

0

O2

71

75

Entry

Alkene

The encapsulated ruthenium-salophen complex 85 was found to catalyze alkene epoxidation which was enhanced drastically under ultrasonic irradiation as observed by the huge selectivity shift from 48% to 90% in the case of a-pinene. Ultrasonic irradiation prevents agglomeration allowing facile diffusion of alkenes into the zeolite cavities. Conversion rates observed in the case of linear alkenes were found to decrease with increase of chain length. Reduction of degree of freedom in accessing the active ruthenium site within the zeolite is proposed to be the reason for this (Table 3). More than 75% epoxide yields were observed even on third consecutive run indicating the reusable nature of the complex.

5

25

Air

88

93

6

25

Air

89

94

Table 3 Epoxidation of alkenes using NaIO4 catalyzed by ZERS

7

25

Air

91

95

Conversion (%)

100 100 Epoxide 100 selectivity (%) 100 TOF (h1) 6.25 66.67 Time 8h 45 min

100 100 100 100 10.00 100 5h 30 min

98 95 75 72 4.65 61.71 9h 36 min

100 92 48 90 4.36 27.6 5.5 h 90 min

38 92 100 100 1.81 30.67 10.5 h 90 min

14 45 88 100 0.43 10.00 14 h 135 min

Electrocatalytic epoxidation of cyclooctene with molecular oxygen catalyzed by tetradentate ruthenium(III)-Schiff base complex 89 was reported in the presence of methyl imidazole and benzoic anhydride under a constant potential (Scheme 27).42

Reaction conditions: alkene (0.5 mmol), NalO4 (1 mmol), ZERS (600 mg, 0.01 mmol). Selectivity, TOF, Time corresponding to reactions under magnetic stirring and ultrasonic irradiations, respectively.

2.2.1.2. Ruthenium-salen complexes. Chiral aqua salen ruthenium complex catalyzed asymmetric epoxidation of conjugated olefins were reported under irradiation-free conditions with dioxygen or air as the oxidant.41 The catalyst 88 was synthesized by refluxing RuCl3 and enantiopure ligand 86 followed by treatment with (nBu4N)þ[Ru(N)Cl4] and hydrolysing the resulting metal nitrido complex 87 (Fig. 24).

Scheme 27. Electrocatalytic epoxidation of cyclooctene with molecular oxygen catalyzed by 89.

It was proposed that the reaction involves a [Ru(V)]O]þ species which was formed by the dissociation of [Ru(III)eOeO] species generated by the attack of molecular oxygen on methyl imidazole ruthenium complex.

Fig. 24. Chiral ligand 86, intermediate ruthenium nitrido complex 87 and the aqua salen ruthenium complex 88. Reproduced with permission from Angew. Chem. Int. Ed. 2012, 51, 8243e8246.

2.2.1.3. Miscellaneous examples. Okamura et al. introduced a new FSM (folded-sheet mesoporous material) bound monodentate oxygen or nitrogen ligand bearing ruthenium complexes 90, 91 and 92 (Fig. 25) which exhibit excellent selectivity towards epoxide formation.43

The catalytic activity of the complex was examined for a variety of alkenes in chlorobenzene and the reaction conditions were optimized. Epoxide yields and enantioselectivities were explored and compared for different alkenes under aerobic (at 25  C) as well as oxygen atmospheres (at 0  C). Epoxidations of (E)-2-Phenyl-2butene and (E)-1-(p-methylphenyl)-1-propene exhibited enhancement in yields and enantiomeric excesses at 0  C (in oxygen) compared to that at 25  C (in air) which was expected to be due to the avoidance of epoxide-ketone rearrangement at lower temperature. But trans-isomers of meta-bromo as well as meta-chloro phenyl prop-1-enes furnished better yields and enantiomeric excesses at 25  C compared to their para-analogues at 0  C. (Tables 4)

Fig. 25. FSM bound ruthenium catalysts 90, 91 and 92. Reproduced with permission from J. Mol. Catal. A: Chem. 2009, 307, 51e58.

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

The complex 90 was synthesized by stirring dried FSM and [Ru(babp)(dmso)2] followed by isolating the supported complex by filtration. Here 91 and 92 were prepared by stirring (3-amino propyl)triethoxy and (3-(2-imidazolin-1-yl)propyl) triethoxy functionalized FSMs, respectively with Ru(babp)(dmso)2. Epoxide selectivities between 77% and 87% were observed in cyclohexene oxidations using iodoso benzene as the oxidant. It was proposed that the reaction involved a six coordinated Ru(V) oxo complex as the active oxidant with oxo group trans to the axial hetero atom in each complex. Catalytic activity was found to be dependent on the nature of the ligand trans to Ru]O bond and the reactivity was found to have marked impact also on the pore size of the catalyst which depended on ruthenium complex loading in FSM (Tables 5). Table 5 Catalytic activity of 90, 91 and 92 towards cyclohexene epoxidation Entry

Catalyst

Reaction time (h)

Epoxide selectivity (%)

1 2 3 4 5 6

90a 91b 91c 92d

4 5 5 5 24 5

87 82 79 77 80 85

92e

Reaction conditions: catalyst:PhlO:cyclohexene¼1:100:100 1,2-dichloroethane, under Ar. a Ru(babp)(dmso)2 (13 mg)/FSM(a) (100 mg). b Ru(babp)(dmso)2 (9 mg)/FSM(a)-Asp (100 mg). c Ru(babp)(dmso)2 (6 mg)/FSM(b)-Aps (100 mg). d Ru(babp)(dmso)2 (8 mg)/FSM(a)-lms (100 mg). e Ru(babp)(dmso)2 (8 mg)/FSM(b)-lms (100 mg).

A polymer anchored L-valine-ruthenium catalyst 93 (Fig. 26) was reported by complexing hydrated RuCl3 with poly(4chloromethyl)styrene anchored L-valine, which was found to catalyze epoxidation of norbornylene with complete selectivity using t-BuOOH as the oxidant.44

6187

The catalyst 94 was found to exhibit superior reactivity among the four catalysts in epoxidations using t-BuOOH as the oxidant. Epoxidation in the case of trans-stilbene was completely stereospecific while trans epoxide was the major product in the case of cis-stilbene. In the case of styrene epoxidation, yields only dropped to half on the fourth consecutive run. Epoxide formation was proposed to involve a radicaloid species under the influence of strong sigma donor tmeda. 2.2.3. Ruthenium complexes with O2-coordination environment 2.2.3.1. Chiral ruthenium(III) helical coordination polymer as a catalyst. A new Ru(III) complex polymer [Ru(HCO2)Cl2]n was reported in which metal centers were connected through formate ligands which induces helicity and provides natural chirality.46 The complex was found to contain two bridged chlorines between the metal centers in addition to the above formate ligands and catalyzed epoxidations of substituted styrenes with TBHP in good to excellent yields (Scheme 28).

Scheme 28. Epoxidation of styrenes with THBP catalyzed by [Ru(HCO2)Cl2]n.

Electronic effect was found to have a remarkable impact on the yields of the reaction since electron-withdrawing groups showed high yields compared to electron-releasing groups. 2.3. Ligand-based mixed valent ruthenium-catalyzed epoxidations A new dinuclear ruthenium complex 98 was reported which is a mixed valent species and found to catalyze the epoxidation of cis-b-methylstyrene with a cis/trans-epoxide stereoselectivity of 94% using iodosylbenzene as the oxidant.47 The complex was prepared from RuCl3(trpy) 67 by dinucleation with Hbpl(1,10 -(4methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridine-2-yl)ethanol)) ligand (Scheme 29).

Fig. 26. Polymer anchored L-valine-ruthenium catalyst.

2.2.2. Ruthenium complexes with N4O-coordination environment. A series of [RuIII(TDL)(tmeda)H2O] complexes (TDL¼tridentate Schiff base ligand; tmeda¼tetramethylethylenediamine) 94, 95, 96 and 97 (Fig. 27) were prepared which catalyzed epoxidations by the strong sigma donor effect of the amine ligand to the ruthenium centre eliminating the competitive hydroxylation.45

Scheme 29. Synthesis of mixed valent ruthenium species 98.

2.4. Ligand-free ruthenium-catalyzed epoxidations

Fig. 27. [RuIII(TDL)(tmeda)H2O] complexes.

Electrochemical epoxidation of ethene using nanocrystalline RuO2 and Co doped RuO2 deposited electrodes in aqueous acidic media was reported at a chloride ion concentration of 0.3 M.48 Here along with oxirane, 2-chloroethanol was formed as a byproduct. A three membered transition state in which ethylene binds to an oxygen on the RuO2 surface was proposed which leads to the epoxide formation.

6188

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

Highly selective and recyclable RuO2 loaded mesoporousassembled TiO2 nanocrystals were developed for liquid-phase cyclohexene epoxidation using H2O2 through a single step solegel process (SSSG).49 Here catalytic activity as well as the selectivity of the catalyst was found to be dependent on the calcination temperature. The most effective calcination temperature was found to be 450  C for SSSG which gave maximum epoxide selectivity close to 80%. The calcined SSSG exhibited unchanged epoxide selectivity and conversion even on the third consecutive run. 3. Applications Completely diastereoselective epoxidation for N-protected amino alkene was reported using 11 to yield threo epoxides which formed the key intermediates in the synthesis of N-benzoyl ristosamines 99,50 an analogue for daunosamine, the key sugar component of several anthracycline antitumour antibiotics (Scheme 30).

Scheme 30. Synthesis of N-benzoyl ristosamine via epoxidation.

Chiral a-amino epoxides (threo) are also reported to be used in the synthesis of hydroxyethylene dipeptide isostere inhibitors of HIV-1 proteases 100aed by direct coupling with amide enolates (Scheme 31).51

It is clear that epoxidation of olefins are finding wider applications which are evident from the number of synthetic procedures developed and applied. 4. Conclusion The ruthenium catalyzed alkene epoxidations have seen very important activities over the last two decades. Catalyst development, structural studies and mechanistic investigations of catalytic cycles had seen tremendous developments. The area of homogeneous ruthenium catalysis in the past five years was mainly focussing on the development and characterization of catalysts as well as exploration of new ligands and their catalytic properties. Developments in the field of asymmetric versions were fewer compared to that in the past one and a half decade. Heterogeneous catalysis which is more attractive in terms of applications has also seen impressive developments. Highly efficient heterogeneous catalysts were achieved even for asymmetric catalysis maintaining good yields and enantioselectivities comparable to that of homogeneous systems but with enhanced reusability. Mechanistic studies often concluded with a Ru(IV) oxo or Ru(VI) dioxo species as the catalytically active species. Oxygen transfer to alkenes from these oxo species is often proposed via a concerted mechanism which is supported by observed stereospecificities in many cases. Highly enantiospecific transformations achieved in most of the cases could be made more attractive, if naturally available enantiopure ligands were used to chirally bias the catalytic systems. Oxygenation of metal centre can be achieved with environmentally benign oxidants like air or hydrogen peroxide by effectively tuning the ligands. Further improvement in catalytic activity, selectivity and stereospecificity of epoxidation can be achieved by incorporating the above points which will be the focus of research in the future. Acknowledgements

Scheme 31. Synthesis of HIV-1 protease inhibitors from a-amino epoxides.

The a-3,4,6-tri-O-acetyl-D-glycal epoxide has been exclusively generated from 3,4,6-tri-O-acetyl-D-glycal by ruthenium catalyzed epoxidation18e20 which can be used as a glycosyl acceptor in stereospecific synthesis of b-linked oligosaccharides (Scheme 32).52

S.M.U. and K.S.S. thank the UGC-New Delhi, India for research fellowships. A.P.T. and G.A. thank the Kerala State Council for Science, Technology and Environment (KSCSTE), Trivandrum, India for a research fellowship and research grant (Order No. 341/2013/ KSCSTE dated 15.03.2013) respectively. References and notes 1. 2. 3. 4. 5. 6.

7.

8. 9. Scheme 32. Stereospecific synthesis of b-linked oligosaccharides.

10.

Everett, J. L.; Roberts, J. J.; Ross, W. C. J. J. Chem. Soc. 1953, 2386e2392. Levene, P. A.; Walti, A. J. Biol. Chem. 1931, 94, 367e372. Beracierta, A. P.; Whiting, D. A. J. Chem. Soc., Perkin Trans. 1 1978, 1257e1263. M-Contelles, J.; Molina, M. T.; Anjum, S. Chem. Rev. 2004, 104, 2857e2900. Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974e5976. (a) Tse, M. K.; Bhor, S.; Klawonn, M.; Anilkumar, G.; Jiao, H.; Doebler, C.; Spannenberg, A.; Maegerlein, W.; Hugl, H.; Beller, M. Chem.dEur. J. 2006, 12, 1855e1874; (b) Tse, M. K.; Bhor, S.; Klawonn, M.; Anilkumar, G.; Jiao, H.; Spannenberg, A.; Doebler, C.; Maegerlein, W.; Hugl, H.; Beller, M. Chem.dEur. J. 2006, 12, 1875e1888; (c) Anilkumar, G.; Bhor, S.; Tse, M. K.; Klawonn, M.; Bitterlich, B.; Beller, M. Tetrahedron: Asymmetry 2005, 16, 3536e3561; (d) Bhor, S.; Anilkumar, G.; Tse, M. K.; Klawonn, M.; Doebler, C.; Bitterlich, B.; Grotevendt, A.; Beller, M. Org. Lett. 2005, 7, 3393e3396; (e) Tse, M. K.; Jiao, H.; Anilkumar, G.; Bitterlich, B.; Gelalcha, F. G.; Beller, M. J. Organomet. Chem. 2006, 691, 4419e4433; (f) Tse, M. K.; Klawonn, M.; Bhor, S.; Doebler, C.; Anilkumar, G.; Hugl, H.; Maegerlein, W.; Beller, M. Org. Lett. 2005, 7, 987e990. (a) Tse, M. K.; Bitterlich, B.; Beller, M. In From Activating Unreactive Substrates: The Role of Secondary Interactions; Bolm, C., Hahn, F. E., Eds.; Wiley-VCH: Weinheim, 2009; pp 313e337; (b) Chatterjee, D. Coord. Chem. Rev. 2008, 252, 176e198. Lai, T. S.; Zhang, R.; Cheung, K. K.; Kwong, H. L.; Che, C. M. Chem. Commun. 1998, 1583e1584. (a) Gallagher, L. A.; Meyer, T. J. J. Am. Chem. Soc. 2001, 123, 5308e5312; (b) Seok, W. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 3931e3941. (a) Takeda, T.; Irie, R.; Shinoda, Y.; Katsuki, T. Synlett 1999, 1157e1159; (b) Nakata, K.; Takeda, T.; Mihara, J.; Hamada, T.; Irie, R.; Katsuki, T. Chem.dEur. J. 2001, 7, 3776e3782.

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190 11. Nishiyama, H.; Shimada, T.; Itoh, H.; Sugiyama, H.; Motoyama, Y. Chem. Commun. 1997, 1863e1864. 12. Zhang, R.; Yu, W. Y.; Lai, T. S.; Che, C. M. Chem. Commun. 1999, 409e410. 13. Groves, J. T.; Quinn, R. J. Am. Chem. Soc. 1985, 107, 5790e5792. 14. Liu, C. J.; Yu, W. Y.; Li, S. G.; Che, C. M. J. Org. Chem. 1998, 63, 7364e7369. 15. Zhang, J. L.; Liu, Y. L.; Che, C. M. Chem. Commun. 2002, 2906e2907. 16. Nestler, O.; Severin, K. Org. Lett. 2001, 3, 3907e3909. 17. Yu, X. Q.; Huang, J. S.; Yu, W. Y.; Che, C. M. J. Am. Chem. Soc. 2000, 122, 5337e5342. 18. Myers, A. G.; Proteau, P. J. J. Am. Chem. Soc. 1989, 111, 1147e1148. 19. Zhang, J. L.; Che, C. M. Org. Lett. 2002, 4, 1911e1914. 20. Zhang, J. L.; Huang, J. S.; Che, C. M. Chem.dEur. J. 2006, 12, 3020e3031. 21. Zhang, J.-L.; Zhou, H. B.; Huang, J. S.; Che, C. M. Chem.dEur. J. 2002, 8, 1554e1562. 22. Fackler, P.; Berthold, C.; Voss, F.; Bach, T. J. Am. Chem. Soc. 2010, 132, 15911e15913. 23. Fackler, P.; Huber, S. M.; Bach, T. J. Am. Chem. Soc. 2012, 134, 12869e12878. 24. Enow, C. A.; Marais, C.; Barend, C. B. B. J. Porphyrins Phthalocyanines 2015, 19, 582e594. 25. Hamelin, O.; Menage, S.; Charnay, F.; Chavarot, M.; Pierre, J. L.; Pecau, J.; Fontecave, M. Inorg. Chem. 2008, 47, 6413e6420. 26. Vaquer, L.; Riente, P.; Sala, X.; Jansat, S.; Buchholz, J.; Llobet, A.; Pericas, M. A. Catal. Sci. Techol. 2013, 3, 706e714. 27. Agarwala, H.; Ehret, F.; Chowdhury, A. D.; Maji, S.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2013, 42, 3721e3734. 28. Hazari, A. S.; Das, A.; Ray, R.; Agarwala, H.; Maji, S.; Mobin, S. M.; Lahiri, G. K. Inorg. Chem. 2015, 54, 4998e5012. 29. Serrano, I.; Lopez, M. I.; Ferrer, I.; Poater, A.; Parella, T.; Fontrodona, X.; Sola, M.; Llobet, A.; Rodriguez, M.; Romero, I. Inorg. Chem. 2011, 50, 6044e6054. 30. Chowdhury, A. D.; Abhishek, D. A.; Das, S.; Irshad, K.; Mobin, S. M.; Lahiri, G. K. Inorg. Chem. 2011, 50, 1775e1785. 31. Giovanni, D.; Vaquer, L.; Sala, X.; Buchholz, J.; Llobet, A. Inorg. Chem. 2013, 52, 4335e4345. 32. Giovanni, C. D.; Poater, A.; Buchholz, J. B.; Cavallo, L.; Sola, M.; Llobet, M. Chem. dEur. J. 2014, 20, 3898e3902.

6189

33. Tada, M.; Coquet, R.; Yoshida, J.; Kinoshita, M.; Iwasawa, Y. Angew. Chem., Int. Ed. 2007, 46, 7220e7223. 34. Dakkach, M.; Fontrodona, X.; Parella, T.; Atlamsani, A.; Romero, I.; Rodriguez, M. Adv. Synth. Catal. 2011, 353, 231e238. 35. Dakkach, M.; Atlamsani, A.; Parella, T.; Fontrodona, X.; Romero, I.; Rodriguez, M. Inorg. Chem. 2013, 52, 5077e5087. 36. Dakkach, M.; Fontrodona, X.; Parella, T.; Atlamsani, A.; Romero, I.; Rodriguez, M. Dalton Trans. 2014, 43, 9916e9923. 37. Aguilo, J.; Francas, L.; Bofill, R.; G-Sepulcre, M.; G-Anton, J.; Poater, A.; Llobet, A.; Escriche, L.; Meyer, F.; Sala, X. Inorg. Chem. 2015, 54, 6782e6791. 38. Santos, K.; Dinelli, L. R.; Bogado, A. L.; Ramos, L. A.; Cavalheiro, E. T.; Ellena, J.; Castellano, E. E.; Batista, A. A. Inorg. Chim. Acta 2015, 429, 237e242. 39. Moghadam, M.; Mirkhani, V.; Tangestaninejad, S.; M-Baltork, I.; Kargar, H.; Sheikhshoaei, I.; Hatefi, M. J. Iran. Chem. Soc. 2011, 8, 1019e1029. 40. Moosavifar, M.; Tangestaninejad, S.; Moghadam, M.; Mirkhani, V.; M-Baltork, I. J. Mol. Catal. A: Chem. 2013, 377, 92e101. 41. Koya, S.; Nishioka, Y.; Mizoguchi, H.; Uchida, T.; Katsuki, T. Angew. Chem., Int. Ed. 2012, 51, 8243e8246. 42. Ourari, A.; Khelafi, M.; Aggoun, D.; Jutand, A.; Amator, C. Electrochim. Acta 2012, 75, 366e370. 43. Okumura, T.; Watanabe, S.; Yagyu, T.; Takagi, H.; Fukushima, Y.; Masuda, H.; Jitsukawa, K. J. Mol. Catal. A Chem. 2009, 307, 51e58. 44. Valodkar, V. B.; Tembe, G. L.; Ravindranathan, M.; Rama, H. S. J. Mol. Catal. A Chem. 2004, 223, 31e38. 45. Chatterjee, D. J. Mol. Catal. A Chem. 2009, 310, 174e179. 46. Jahromi, B. T.; Kharat, A. N.; Zamanian, S. Chin. Chem. Lett. 2015, 26, 137e140. 47. Francas, L.; G-Gil, R. M.; Moyano, D.; B-Buchholz, J.; G-Anton, J.; Escriche, L.; Llobet, A.; Sala, X. Inorg. Chem. 2014, 53, 10394e10402. 48. Jirkovsky, J. S.; Busch, M.; Ahlberg, E.; Panas, I.; Krtil, P. J. Am. Chem. Soc. 2011, 133, 5882e5892. 49. Woragamon, K.; Jongpatiwut, S.; Sreethawong, T. Catal. Lett. 2010, 136, 249e259. 50. Roush, W. R.; Straub, J. A.; Brown, R. J. J. Org. Chem. 1987, 52, 5127e5136. 51. Askin, D.; Wallace, M. A.; Vacca, J. P.; Reamer, R. A.; Volante, R. P.; Shinkai, I. J. Org. Chem. 1992, 57, 2771e2773. 52. Halcomb, R. L.; Danishefsky, S. J. J. Am. Chem. Soc. 1989, 111, 6661e6666.

6190

S.M. Ujwaldev et al. / Tetrahedron 72 (2016) 6175e6190

Biographical sketch

Sankuviruthiyil M. Ujwaldev was born in Kerala, India, in 1992. He obtained his B.Sc. degree from University of Calicut (Christ College, Irinjalakuda) in 2012 and his M.Sc. degree from Sree Kerala Varma College, Thrissur (University of Calicut) in 2014. He qualified the CSIR-UGC National Eligibility Test in 2014 with a research fellowship and currently pursuing his doctoral research under the guidance of Dr. G. Anilkumar in School of Chemical Sciences, Mahatma Gandhi University.

Kallikkakam S. Sindhu was born in Kerala, India, in 1983. She received her B.Sc. degree from Mahatma Gandhi University (St. Xaviers College, Aluva) in 2003 and her M.Sc. degree from School of Chemical Sciences, Mahatma Gandhi University in 2007. She earned National Eligibility Test (NET) with a scholarship. Currently she is doing her doctoral research in Transition metal catalysed coupling reaction under the guidance of Dr. G. Anilkumar in the School of Chemical Sciences, Mahatma Gandhi University.

Amrutha P. Thankachan was born in Kerala, India, in 1988. She obtained her B.Sc. degree from Mahatma Gandhi University (C M S College, Kottayam) in 2009 and her M.Sc. degree from School of Chemical Sciences, Mahatma Gandhi University in 2011. Currently she is doing her doctoral research under the guidance of Dr. G. Anilkumar in School of Chemical Sciences, Mahatma Gandhi University.

Gopinathan Anilkumar was born in Kerala, India and took his Ph.D in 1996 from Regional Research Laboratory, Trivandrum with Dr. Vijay Nair. He did postdoctoral studies at University of Nijmegen, The Netherlands (with Professor Binne Zwanenburg), Osaka University, Japan (with Professor Yasuyuki Kita), Temple University, USA (with Professor Franklin Davis), Leibniz-institut fur Organische Katalyse (IfOK), Rostock, Germany (with Professor Matthias Beller) and Leibniz-institut fur Katalyse (LIKAT), Rostock, Germany (with Professor Matthias Beller. He was a senior scientist at AstraZeneca (India). Currently he is Associate Professor in Organic Chemistry at the School of Chemical Sciences, Mahatma Gandhi University in Kerala, India. His research interests are in the areas of organic synthesis, medicinal chemistry and catalysis, particularly on Ruthenium, Iron, Zinc and Copper catalyzed reactions.