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Wang D, Ingram AA, Okumura A, Spaniol TP, Schwaneberg U, Okuda J. Benzylic C(sp 3 )-H Bond Oxidation with Ketone Selectivity by a Cobalt(IV)-Oxo Embedded in a β-Barrel Protein. Chemistry 2024; 30:e202303066. [PMID: 37818668 DOI: 10.1002/chem.202303066] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2023] [Revised: 10/10/2023] [Accepted: 10/11/2023] [Indexed: 10/12/2023]
Abstract
Artificial metalloenzymes have emerged as biohybrid catalysts that allow to combine the reactivity of a metal catalyst with the flexibility of protein scaffolds. This work reports the artificial metalloenzymes based on the β-barrel protein nitrobindin NB4, in which a cofactor [CoII X(Me3 TACD-Mal)]+ X- (X=Cl, Br; Me3 TACD=N,N' ,N''-trimethyl-1,4,7,10-tetraazacyclododecane, Mal=CH2 CH2 CH2 NC4 H2 O2 ) was covalently anchored via a Michael addition reaction. These biohybrid catalysts showed higher efficiency than the free cobalt complexes for the oxidation of benzylic C(sp3 )-H bonds in aqueous media. Using commercially available oxone (2KHSO5 ⋅ KHSO4 ⋅ K2 SO4 ) as oxidant, a total turnover number of up to 220 and 97 % ketone selectivity were achieved for tetralin. As catalytically active intermediate, a mononuclear terminal cobalt(IV)-oxo species [Co(IV)=O]2+ was generated by reacting the cobalt(II) cofactor with oxone in aqueous solution and characterized by ESI-TOF MS.
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Affiliation(s)
- Dong Wang
- Institute of Inorganic Chemistry, RWTH Aachen University, 52074, Aachen, Germany
| | - Aaron A Ingram
- Institute of Inorganic Chemistry, RWTH Aachen University, 52074, Aachen, Germany
| | - Akira Okumura
- Institute of Inorganic Chemistry, RWTH Aachen University, 52074, Aachen, Germany
| | - Thomas P Spaniol
- Institute of Inorganic Chemistry, RWTH Aachen University, 52074, Aachen, Germany
| | - Ulrich Schwaneberg
- Institute of Biotechnology, RWTH Aachen University, 52074, Aachen, Germany
| | - Jun Okuda
- Institute of Inorganic Chemistry, RWTH Aachen University, 52074, Aachen, Germany
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2
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Fujiwara Y, Takayama T, Nakazawa J, Okamura M, Hikichi S. Development of a novel scorpionate ligand with 6-methylpyridine and comparison of structural and electronic properties of nickel(II) complexes with related tris(azolyl)borates. Dalton Trans 2022; 51:10338-10342. [DOI: 10.1039/d2dt01548j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A novel anionic tridentate borate ligand with 6-methlpyridyl donor, TpyMe, has been synthesized. Comparison of molecular structures and reactivities of nickel(II)-bromido complexes with tris(azolyl)borate ligands composed of pyridyl, pyrazolyl, or...
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Eedugurala N, Wang Z, Kanbur U, Ellern A, Pruski M, Sadow AD. Synthesis and Characterization of Tris(oxazolinyl)borato Copper(II) and Copper(I) Complexes. Helv Chim Acta 2021. [DOI: 10.1002/hlca.202000209] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Naresh Eedugurala
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
- U.S. Department of Energy Ames Laboratory 2416 Pammel Drive Iowa State University Ames IA 50011 United States
| | - Zhuoran Wang
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
- U.S. Department of Energy Ames Laboratory 2416 Pammel Drive Iowa State University Ames IA 50011 United States
| | - Uddhav Kanbur
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
- U.S. Department of Energy Ames Laboratory 2416 Pammel Drive Iowa State University Ames IA 50011 United States
| | - Arkady Ellern
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
| | - Marek Pruski
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
- U.S. Department of Energy Ames Laboratory 2416 Pammel Drive Iowa State University Ames IA 50011 United States
| | - Aaron D. Sadow
- Department of Chemistry Iowa State University 1605 Gilman Hall Ames IA 50011 United States
- U.S. Department of Energy Ames Laboratory 2416 Pammel Drive Iowa State University Ames IA 50011 United States
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Pyykkönen A, Feher R, Köhler FH, Vaara J. Paramagnetic Pyrazolylborate Complexes Tp 2M and Tp* 2M: 1H, 13C, 11B, and 14N NMR Spectra and First-Principles Studies of Chemical Shifts. Inorg Chem 2020; 59:9294-9307. [PMID: 32558559 DOI: 10.1021/acs.inorgchem.0c01176] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The paramagnetic pyrazolylborates Tp2M and Tp*2M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp2M]+ and [Tp*2M]+ (M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The 1H signal shift ranges vary from ∼30 ppm (Cu(II) and V(III)) to ∼220 ppm (Co(II)), and the 13C signal shift ranges from ∼180 ppm (Fe(III)) to ∼1150 ppm (Cr(II)). The 11B and 14N shifts are ∼360 and ∼730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved 13C,1H couplings. All chemical shifts have been calculated from first-principles on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splitting, and g tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology.
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Affiliation(s)
- Ari Pyykkönen
- NMR Research Unit, University of Oulu, P.O. Box 3000, Oulu FI-90014, Finland
| | - Robert Feher
- Department Chemie, Technische Universität München, D-85748 Garching, Germany
| | - Frank H Köhler
- Department Chemie, Technische Universität München, D-85748 Garching, Germany
| | - Juha Vaara
- NMR Research Unit, University of Oulu, P.O. Box 3000, Oulu FI-90014, Finland
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Terao I, Horii S, Nakazawa J, Okamura M, Hikichi S. Efficient alkane hydroxylation catalysis of nickel(ii) complexes with oxazoline donor containing tripodal tetradentate ligands. Dalton Trans 2020; 49:6108-6118. [PMID: 32323686 DOI: 10.1039/d0dt00733a] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Tris(oxazolynylmethyl)amine TOAR (where R denotes the substituent groups on the fourth position of the oxazoline rings) complexes of nickel(ii) have been synthesized as catalyst precursors for alkane oxidation with meta-chloroperoxybenzoic acid (m-CPBA). The molecular structures of acetato, nitrato, meta-chlorobenzoato and chlorido complexes with TOAMe2 have been determined using X-ray crystallography. The bulkiness of the substituent groups R affects the coordination environment of the nickel(ii) centers, as has been demonstrated by comparison of the molecular structures of chlorido complexes with TOAMe2 and TOAtBu. The nickel(ii)-acetato complex with TOAMe2 is an efficient catalyst precursor compared with the tris(pyridylmethyl)amine (TPA) analogue. Oxazolynyl donors' strong σ-electron donating ability will enhance the catalytic activity. Catalytic reaction rates and substrate oxidizing position selectivity are controlled by the structural properties of the R of TOAR. Reaction of the acetato complex with TOAMe2 and m-CPBA yields the corresponding acylperoxido species, which can be detected using spectroscopy. Kinetic studies of the decay process of the acylperoxido species suggest that the acylperoxido species is a precursor of an active species for alkane oxidation.
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Affiliation(s)
- Ikumi Terao
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
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Tatebe CJ, Matson EM, Clark CL, Kiernicki JJ, Fanwick PE, Zeller M, Bart SC. Low- and Mid-Valent Uranium Species Supported by Phenyltris(oxazolinyl)borate Ligands. Organometallics 2020. [DOI: 10.1021/acs.organomet.9b00793] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Affiliation(s)
- Caleb J. Tatebe
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Ellen M. Matson
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Christopher L. Clark
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - John J. Kiernicki
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Phillip E. Fanwick
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Matthias Zeller
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Suzanne C. Bart
- H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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Reinig RR, Ellern A, Sadow AD. Heteroleptic Four-Coordinate Tris(oxazolinyl)borato Iron(II) Compounds. Inorg Chem 2019; 58:6044-6051. [DOI: 10.1021/acs.inorgchem.9b00359] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Regina R. Reinig
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, 2415 Osborn Drive, Ames, Iowa 50011, United States
- U.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States
| | - Arkady Ellern
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, 2415 Osborn Drive, Ames, Iowa 50011, United States
| | - Aaron D. Sadow
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, 2415 Osborn Drive, Ames, Iowa 50011, United States
- U.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States
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8
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Nishiura T, Takabatake A, Okutsu M, Nakazawa J, Hikichi S. Heteroleptic cobalt(iii) acetylacetonato complexes with N-heterocyclic carbine-donating scorpionate ligands: synthesis, structural characterization and catalysis. Dalton Trans 2019; 48:2564-2568. [DOI: 10.1039/c8dt04469d] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
A first structure-determined heteroleptic cobalt(iii) complex with the less hindered tris(carbene)borate works as a catalyst precursor for alkane oxidation.
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Affiliation(s)
- Toshiki Nishiura
- Department of Material and Life Chemistry
- Faculty of Engineering
- Kanagawa University
- Yokohama 221-8686
- Japan
| | - Asako Takabatake
- Department of Material and Life Chemistry
- Faculty of Engineering
- Kanagawa University
- Yokohama 221-8686
- Japan
| | - Mariko Okutsu
- Department of Material and Life Chemistry
- Faculty of Engineering
- Kanagawa University
- Yokohama 221-8686
- Japan
| | - Jun Nakazawa
- Department of Material and Life Chemistry
- Faculty of Engineering
- Kanagawa University
- Yokohama 221-8686
- Japan
| | - Shiro Hikichi
- Department of Material and Life Chemistry
- Faculty of Engineering
- Kanagawa University
- Yokohama 221-8686
- Japan
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9
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Reinig RR, Fought EL, Ellern A, Windus TL, Sadow AD. Cobalt(ii) acyl intermediates in carbon-carbon bond formation and oxygenation. Dalton Trans 2018; 47:12147-12161. [PMID: 30090898 DOI: 10.1039/c8dt02661k] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The organocobalt scorpionate compounds ToMCoR (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Bn, 1; CH2SiMe3, 2; Ph, 3; Et, 4; nBu, 5; Me, 6) react in carbonylation, oxidation, and carboxylation reactions via pathways that are distinctly influenced by the nature of the organometallic moiety. The compounds are prepared by reaction of ToMCoCl with the corresponding organolithium or organopotassium reagents. Compounds 1-6 were characterized by 8-line hyperfine coupling to cobalt in EPR spectra and solution phase magnetic measurements (μeff = 4-5μB) as containing a high-spin cobalt(ii) center. The UV-Vis spectra revealed an intense diagnostic band at ca. 700 nm (ε > 1000 M-1 cm-1) associated with the tetrahedral organocobalt(ii) center that was assigned to a d ← d transition on the basis of configuration interaction (CI) calculations. Complexes 1-6 react rapidly with CO to form equilibrating mixtures of the low spin organocobalt carbonyl ToMCo(R)CO, acyl ToMCoC([double bond, length as m-dash]O)R, and acyl carbonyl ToMCo{C([double bond, length as m-dash]O)R}CO. The 1H and 11B NMR spectra contained only one set of signals for the CO-treated solutions, whereas the solution-phase IR spectra contained up to two νCO and three νC([double bond, length as m-dash]O)R signals with intensities varying depending on the R group (R = Bn, 7; CH2SiMe3, 8; Ph, 9; Et, 10; nBu, 11; Me, 12). Single crystal X-ray diffraction of ToMCo{C([double bond, length as m-dash]O)Et}CO (10) supports its assignment as a square pyramidal cobalt(ii) acyl carbonyl complex. Upon evaporation of volatiles, solutions of 8-12 revert to the CO-free organocobalt starting materials 2-6, whereas attempts to isolate benzyl-derived 7 provide an unusual α-alkoxyketone species, characterized by single crystal X-ray diffraction. Despite the differences observed in the carbonylation of 1-6 as a result of varying the R group, compounds 7-12 all react rapidly with O2 through an oxygenation pathway to afford the corresponding carboxylate compounds ToMCoO2CR (R = Bn, 13; CH2SiMe3, 14; Ph, 15; Et, 16; nBu, 17; Me, 18). In contrast, the insertion of CO2 into the Co-C bond in 1-6 requires several days to weeks.
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Affiliation(s)
- Regina R Reinig
- US Department of Energy Ames Laboratory and Department of Chemistry, 1605 Gilman Hall, Iowa State University, Ames, IA 50011, USA.
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10
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Nishiura T, Uramoto T, Takiyama Y, Nakazawa J, Hikichi S. Cobalt(II) Complexes with N, N, N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate To M to Control the Structural Properties and Reactivities of Cobalt Centers. Molecules 2018; 23:E1466. [PMID: 29914171 PMCID: PMC6099786 DOI: 10.3390/molecules23061466] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2018] [Revised: 06/13/2018] [Accepted: 06/15/2018] [Indexed: 11/16/2022] Open
Abstract
Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N₃O₂ donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate (к³) mode, whereas ToM functions as the bidentate (к²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)₂ and Co(acac)₂. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating ToM complexes show faster initial reaction rates compared to the corresponding Tp* complexes.
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Affiliation(s)
- Toshiki Nishiura
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
| | - Takahiro Uramoto
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
| | - Yuichiro Takiyama
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
| | - Jun Nakazawa
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
| | - Shiro Hikichi
- Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
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Catalyst-free and solvent-free oxidation of cycloalkanes (C5-C8) with molecular oxygen: Determination of autoxidation temperature and product distribution. Chin J Chem Eng 2018. [DOI: 10.1016/j.cjche.2018.02.019] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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12
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Alkane oxidation reactivity of homogeneous and heterogeneous metal complex catalysts with mesoporous silica-immobilized (2-pyridylmethyl)amine type ligands. MOLECULAR CATALYSIS 2017. [DOI: 10.1016/j.mcat.2017.09.027] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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Reinig RR, Fought EL, Ellern A, Windus TL, Sadow AD. Rapid and ordered carbonylation and oxygenation of a cobalt(ii) methyl. Chem Commun (Camb) 2017; 53:11020-11023. [PMID: 28936508 DOI: 10.1039/c7cc06339c] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The oxidative carbonylation of ToMCoMe (1; ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) involves its rapid, reversible reaction with CO to form ToMCo{C(O)Me}CO (2) followed by rapid reaction with O2 yielding ToMCoOAc (3), in contrast to the slow direct carboxylation of ToMCoMe by CO2.
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Affiliation(s)
- Regina R Reinig
- Department of Chemistry, 1605 Gilman Hall, Iowa State University, Ames, IA 50011, USA.
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Lampland NL, Pindwal A, Yan K, Ellern A, Sadow AD. Rare Earth and Main Group Metal Poly(hydrosilyl) Compounds. Organometallics 2017. [DOI: 10.1021/acs.organomet.7b00383] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Nicole L. Lampland
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
| | - Aradhana Pindwal
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
| | - KaKing Yan
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
| | - Arkady Ellern
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
| | - Aaron D. Sadow
- Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
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Stereoselective Alkane Oxidation with meta-Chloroperoxybenzoic Acid (MCPBA) Catalyzed by Organometallic Cobalt Complexes. Molecules 2016; 21:molecules21111593. [PMID: 27879680 PMCID: PMC6273550 DOI: 10.3390/molecules21111593] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2016] [Revised: 11/15/2016] [Accepted: 11/16/2016] [Indexed: 01/21/2023] Open
Abstract
Cobalt pi-complexes, previously described in the literature and specially synthesized and characterized in this work, were used as catalysts in homogeneous oxidation of organic compounds with peroxides. These complexes contain pi-butadienyl and pi-cyclopentadienyl ligands: [(tetramethylcyclobutadiene)(benzene)cobalt] hexafluorophosphate, [(C4Me4)Co(C6H6)]PF6 (1); diiodo(carbonyl)(pentamethylcyclopentadienyl)cobalt, Cp*Co(CO)I2 (2); diiodo(carbonyl)(cyclopentadienyl)cobalt, CpCo(CO)I2 (3); (tetramethylcyclobutadiene)(dicarbonyl)(iodo)cobalt, (C4Me4)Co(CO)2I (4); [(tetramethylcyclobutadiene)(acetonitrile)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [(C4Me4)Co(bipy)(MeCN)]PF6 (5); bis[dicarbonyl(B-cyclohexylborole)]cobalt, [(C4H4BCy)Co(CO)2]2 (6); [(pentamethylcyclopentadienyl)(iodo)(1,10-phenanthroline)cobalt] hexafluorophosphate, [Cp*Co(phen)I]PF6 (7); diiodo(cyclopentadienyl)cobalt, [CpCoI2]2 (8); [(cyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [CpCo(bipy)I]PF6 (9); and [(pentamethylcyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [Cp*Co(bipy)I]PF6 (10). Complexes 1 and 2 catalyze very efficient and stereoselective oxygenation of tertiary C–H bonds in isomeric dimethylcyclohexanes with MCBA: cyclohexanols are produced in 39 and 53% yields and with the trans/cis ratio (of isomers with mutual trans- or cis-configuration of two methyl groups) 0.05 and 0.06, respectively. Addition of nitric acid as co-catalyst dramatically enhances both the yield of oxygenates and stereoselectivity parameter. In contrast to compounds 1 and 2, complexes 9 and 10 turned out to be very poor catalysts (the yields of oxygenates in the reaction with cis-1,2-dimethylcyclohexane were only 5%–7% and trans/cis ratio 0.8 indicated that the oxidation is not stereoselective). The chromatograms of the reaction mixture obtained before and after reduction with PPh3 are very similar, which testifies that alkyl hydroperoxides are not formed in this oxidation. It can be thus concluded that the interaction of the alkanes with MCPBA occurs without the formation of free radicals. The complexes catalyze oxidation of alcohols with tert-butylhydroperoxide (TBHP). For example, tert-BuOOH efficiently oxidizes 1-phenylethanol to acetophenone in 98% yield if compound 1 is used as a catalyst.
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