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Martínez AR, Morales LP, Ojeda ED, Rodríguez MC, Rodríguez-García I. The Proven Versatility of Cp 2TiCl. J Org Chem 2020; 86:1311-1329. [PMID: 33147037 DOI: 10.1021/acs.joc.0c01233] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
In the last two decades, titanocene monochloride has been postulated as a monoelectronic transfer reagent capable of catalyzing an important variety of chemical transformations. In this Perspective, our contributions to this growing field of research are summarized and analyzed. Especially known have been our contributions in C-C bond formation reactions, hydrogen-atom transfer from water to radicals, and isomerization reactions, as well as the development of a catalytic cycle that has subsequently allowed the preparation of a great variety of natural terpenes. It is also worth mentioning our contribution in the postulation of this single-electron transfer agent (SET) as a new green catalyst with a broad range of applications in organic and organometallic chemistry. The most significant catalytic processes developed by other research groups are also briefly described, with special emphasis on the reaction mechanisms involved. Finally, a reflection is made on the future trends in the research of this SET, aimed at consolidating this chemical as a new green reagent that will be widely used in fine chemistry, green chemistry, and industrial chemical processes.
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Affiliation(s)
- Antonio Rosales Martínez
- Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
| | - Laura Pozo Morales
- Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
| | - Emilio Díaz Ojeda
- Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
| | - María Castro Rodríguez
- Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
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2
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Zhu N, Su M, Wan WM, Li Y, Bao H. Practical Method for Reductive Deuteration of Ketones with Magnesium and D 2O. Org Lett 2020; 22:991-996. [PMID: 31967845 DOI: 10.1021/acs.orglett.9b04536] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
α-Deuterated alcohols have important applications in pharmaceuticals and mechanism studies. Here, we report a new and practical strategy for the reductive deuteration of ketones using a Mg/BrCH2CH2Br/D2O system, which affords α-deuterated alcohols in good yields and with almost quantitative incorporation of deuterium. The synthetic value of this method has been demonstrated by the easy access to deuterated drugs or drug derivatives. This method may inspire the discovery of other deuterium-containing drugs.
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Affiliation(s)
- Nengbo Zhu
- State Key Laboratory of Structural Chemistry, Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis , Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences , 155 Yangqiao Road West , Fuzhou , Fujian 350002 , P. R. China
| | - Min Su
- State Key Laboratory of Structural Chemistry, Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis , Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences , 155 Yangqiao Road West , Fuzhou , Fujian 350002 , P. R. China
| | - Wen-Ming Wan
- State Key Laboratory of Structural Chemistry, Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis , Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences , 155 Yangqiao Road West , Fuzhou , Fujian 350002 , P. R. China
| | - Yajun Li
- State Key Laboratory of Structural Chemistry, Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis , Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences , 155 Yangqiao Road West , Fuzhou , Fujian 350002 , P. R. China
| | - Hongli Bao
- State Key Laboratory of Structural Chemistry, Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis , Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences , 155 Yangqiao Road West , Fuzhou , Fujian 350002 , P. R. China
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3
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Martínez AR, Rodríguez MC, Rodríguez-García I, Morales LP, Maecker RNR. Titanocene dichloride: A new green reagent in organic chemistry. CHINESE JOURNAL OF CATALYSIS 2017. [DOI: 10.1016/s1872-2067(17)62894-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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4
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Castro Rodríguez M, Rodríguez García I, Rodríguez Maecker RN, Pozo Morales L, Oltra JE, Rosales Martínez A. Cp2TiCl: An Ideal Reagent for Green Chemistry? Org Process Res Dev 2017. [DOI: 10.1021/acs.oprd.7b00098] [Citation(s) in RCA: 46] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Affiliation(s)
- María Castro Rodríguez
- Department
of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
| | | | | | - Laura Pozo Morales
- Department
of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
| | - J. Enrique Oltra
- Department
of Organic Chemistry, Faculty of Science, University of Granada, 18071 Granada, Spain
| | - Antonio Rosales Martínez
- Department
of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain
- Petrochemical
Engineering, Universidad de las Fuerzas Armadas-ESPE, 050150 Latacunga, Ecuador
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5
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Petrus R, Utko J, Lis T, Sobota P. Impact of Group 13 Metals on Cp2TiCl2 Reduction and Structural Characterization of Resulting Compounds. Inorg Chem 2017; 56:3324-3334. [DOI: 10.1021/acs.inorgchem.6b02824] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Rafał Petrus
- Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
- Department of Chemistry, Wrocław University of Technology, 27 Wybrzeże Wyspiańskiego, 50-370 Wrocław, Poland
| | - Józef Utko
- Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
| | - Tadeusz Lis
- Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
| | - Piotr Sobota
- Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
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6
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Rosales A, Rodríguez-García I. Cp2TiCl/D2O/Mn, a formidable reagent for the deuteration of organic compounds. Beilstein J Org Chem 2016; 12:1585-9. [PMID: 27559410 PMCID: PMC4979672 DOI: 10.3762/bjoc.12.154] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2016] [Accepted: 07/13/2016] [Indexed: 11/25/2022] Open
Abstract
Cp2TiCl/D2O/Mn is an efficient combination, sustainable and cheap reagent that mediates the D-atom transfer from D2O to different functional groups and can contribute to the synthesis of new deuterated organic compounds under friendly experimental conditions and with great economic advantages.
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Affiliation(s)
- Antonio Rosales
- Department of Chemical and Environmental Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain; Petrochemical Engineering, Universidad de las Fuerzas Armadas-ESPE, 050150 Latacunga, Ecuador
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7
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Rosales A, Rodríguez-García I, Muñoz-Bascón J, Roldan-Molina E, Padial NM, Morales LP, García-Ocaña M, Oltra JE. The Nugent Reagent: A Formidable Tool in Contemporary Radical and Organometallic Chemistry. European J Org Chem 2015. [DOI: 10.1002/ejoc.201500292] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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8
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Streuff J. Reductive umpolung reactions with low-valent titanium catalysts. CHEM REC 2014; 14:1100-13. [PMID: 25234255 DOI: 10.1002/tcr.201402058] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2014] [Indexed: 12/24/2022]
Abstract
The coupling of similarly polarized carbon fragments can be achieved by a reductive umpolung strategy. This gives access to compounds with functional groups in even bond distances, which are difficult to synthesize by other means. Low-valent titanium catalysts enable such couplings under mild conditions. This account covers the recent progress on this topic with a focus on the development of cross-selective coupling reactions and stereoselective examples.
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Affiliation(s)
- Jan Streuff
- Institut für Organische Chemie, Albert-Ludwigs-Universität Freiburg, D-79104, Freiburg, Germany.
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9
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Rosales A, Muñoz-Bascón J, Roldan-Molina E, Castañeda MA, Padial NM, Gansäuer A, Rodríguez-García I, Oltra JE. Selective Reduction of Aromatic Ketones in Aqueous Medium Mediated by Ti(III)/Mn: A Revised Mechanism. J Org Chem 2014; 79:7672-6. [DOI: 10.1021/jo501141y] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Antonio Rosales
- Centro
de Investigación, Estudios y Desarrollo de Ingeniería
(CIEDI); Ingeniería Agroindustrial y Alimentos, Universidad de las Américas, 170513 Quito, Ecuador
| | - Juan Muñoz-Bascón
- Departamento
Química Orgánica, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
| | - Esther Roldan-Molina
- Departamento
Química Orgánica, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
| | - Mayra A. Castañeda
- Centro
de Investigación, Estudios y Desarrollo de Ingeniería
(CIEDI); Ingeniería Agroindustrial y Alimentos, Universidad de las Américas, 170513 Quito, Ecuador
| | - Natalia M. Padial
- Departamento
Química Orgánica, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
| | - Andreas Gansäuer
- Kekulé-Institut
für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Strasse 1, 53127 Bonn, Germany
| | | | - J. Enrique Oltra
- Departamento
Química Orgánica, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
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10
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Rossi B, Prosperini S, Pastori N, Clerici A, Punta C. New advances in titanium-mediated free radical reactions. Molecules 2012; 17:14700-32. [PMID: 23519248 PMCID: PMC6268425 DOI: 10.3390/molecules171214700] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Revised: 11/28/2012] [Accepted: 12/06/2012] [Indexed: 01/11/2023] Open
Abstract
Titanium complexes have been widely used as catalysts for C‑C bond-forming processes via free-radical routes. Herein we provide an overview of some of the most significant contributions in the field, that covers the last decade, emphasizing the key role played by titanium salts in the promotion of selective reactions aimed at the synthesis of multifunctional organic compounds, including nucleophilic radical additions to imines, pinacol and coupling reactions, ring opening of epoxides and living polymerization.
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Affiliation(s)
- Bianca Rossi
- Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy; E-Mails: (B.R.); (S.P.); (N.P.); (A.C.)
| | - Simona Prosperini
- Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy; E-Mails: (B.R.); (S.P.); (N.P.); (A.C.)
- INSTM (National Consortium for Materials Science and Technology) Local Unit, Politecnico di Milano, 20133 Milan, Italy
| | - Nadia Pastori
- Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy; E-Mails: (B.R.); (S.P.); (N.P.); (A.C.)
| | - Angelo Clerici
- Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy; E-Mails: (B.R.); (S.P.); (N.P.); (A.C.)
| | - Carlo Punta
- Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy; E-Mails: (B.R.); (S.P.); (N.P.); (A.C.)
- INSTM (National Consortium for Materials Science and Technology) Local Unit, Politecnico di Milano, 20133 Milan, Italy
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11
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Rosales A, Muñoz-Bascón J, López-Sánchez C, Álvarez-Corral M, Muñoz-Dorado M, Rodríguez-García I, Oltra JE. Ti-Catalyzed Homolytic Opening of Ozonides: A Sustainable C–C Bond-Forming Reaction. J Org Chem 2012; 77:4171-6. [DOI: 10.1021/jo300344a] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Antonio Rosales
- Department of Organic Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain
| | - Juan Muñoz-Bascón
- Department of Organic Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain
| | | | | | - Manuel Muñoz-Dorado
- Department of Organic Chemistry, University of Almería, 04120 Almería,
Spain
| | | | - J. Enrique Oltra
- Department of Organic Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain
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12
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de Cienfuegos LÁ, Robles R, Miguel D, Justicia J, Cuerva JM. Reduction reactions in green solvents: water, supercritical carbon dioxide, and ionic liquids. CHEMSUSCHEM 2011; 4:1035-1048. [PMID: 21826799 DOI: 10.1002/cssc.201100134] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/08/2011] [Revised: 04/29/2011] [Indexed: 05/31/2023]
Affiliation(s)
- Luis Álvarez de Cienfuegos
- Department of Organic Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain.
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13
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Sobota P, Dra̧g-Jarza̧bek A, Utko J, Jerzykiewicz LB. Promoting Effect of Calcium Alkoxide on the Protonation of Cp Groups of Cp2TiCl2. Organometallics 2011. [DOI: 10.1021/om1011787] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Piotr Sobota
- Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
| | - Anna Dra̧g-Jarza̧bek
- Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
| | - Józef Utko
- Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
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14
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Gansäuer A, Shi L, Otte M, Huth I, Rosales A, Sancho-Sanz I, Padial NM, Oltra JE. Hydrogen atom donors: recent developments. Top Curr Chem (Cham) 2011; 320:93-120. [PMID: 21452081 DOI: 10.1007/128_2011_124] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
This review highlights recent developments in the field of hydrogen atom transfer (HAT) reagents that circumvent the disadvantages of classical group 14 reagents, such as Bu₃SnH. Special emphasis is laid on the lowering of bond dissociation energies (BDEs) of molecules that could, as yet, not be used as HAT reagents and on the use of organometallic HAT reagents.
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Affiliation(s)
- Andreas Gansäuer
- Kekulé Institut für Organische Chemie und Biochemie der Universität Bonn, Germany.
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15
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Galán-Fernández R, Clemente-Tejeda D, Río-Nieto ED, Bermejo FA. Synthesis of diethyl 2-methyl-bicyclo[3.1.1]hept-2-ene-6,6-dicarboxylate by Pd-catalyzed intramolecular allylic alkylation. Stereoselective preparation of its optically homogeneous form from R-(−)-carvone. Tetrahedron 2010. [DOI: 10.1016/j.tet.2010.08.051] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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16
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Paradas M, Campaña AG, Marcos ML, Justicia J, Haidour A, Robles R, Cárdenas DJ, Oltra JE, Cuerva JM. Unprecedented H-atom transfer from water to ketyl radicals mediated by Cp2TiCl. Dalton Trans 2010; 39:8796-800. [DOI: 10.1039/c001689f] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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17
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Martín-Rodríguez M, Galán-Fernández R, Marcos-Escribano A, Bermejo FA. Ti(III)-Promoted Radical Cyclization of Epoxy Enones. Total Synthesis of (+)-Paeonisuffrone. J Org Chem 2009; 74:1798-801. [DOI: 10.1021/jo8026033] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
| | | | | | - Francisco A. Bermejo
- Departamento de Química Orgánica, Universidad de Salamanca, 37008 Salamanca, Spain
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18
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Campaña AG, Estévez RE, Fuentes N, Robles R, Cuerva JM, Buñuel E, Cárdenas D, Oltra JE. Unprecedented Hydrogen Transfer from Water to Alkenes and Alkynes Mediated by TiIII and Late Transition Metals. Org Lett 2007; 9:2195-8. [PMID: 17480090 DOI: 10.1021/ol070779i] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We describe how alkenes and alkynes can be hydrogenated under mild conditions by hydrogen transfer from water mediated by titanocene(III) and a substoichiometric quantity of one of the late transition metals usually employed as hydrogenation catalysts. This process proceeds presumably by H-atom transfer from TiIII-coordinated water to the late transition metal partner (depicted in the drawing above), a mechanism in support of which we provide theoretical and experimental evidence.
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Affiliation(s)
- Araceli G Campaña
- Department of Organic Chemistry, Faculty of Sciences, University of Granada, E-18071 Granada, Spain
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19
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Larsen J, Rasmus Juel Enemærke,, Skrydstrup T, Daasbjerg K. Characterization of HMPA-Coordinated Titanium(III) Species in Solutions of Zinc-Reduced Titanocene Dihalides. Organometallics 2006. [DOI: 10.1021/om051086d] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Jens Larsen
- Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Rasmus Juel Enemærke,
- Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Troels Skrydstrup
- Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Kim Daasbjerg
- Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
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20
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Enemaerke RJ, Larsen J, Hjøllund GH, Skrydstrup T, Daasbjerg K. Influence of the Halogen in Titanocene Halide Promoted Reductions. Organometallics 2005. [DOI: 10.1021/om049257m] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Rasmus J. Enemaerke
- Department
of Chemistry, University of Aarhus,
Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Jens Larsen
- Department
of Chemistry, University of Aarhus,
Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Gitte H. Hjøllund
- Department
of Chemistry, University of Aarhus,
Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Troels Skrydstrup
- Department
of Chemistry, University of Aarhus,
Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Kim Daasbjerg
- Department
of Chemistry, University of Aarhus,
Langelandsgade 140, 8000 Aarhus C, Denmark
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21
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Enemaerke RJ, Larsen J, Skrydstrup T, Daasbjerg K. Revelation of the nature of the reducing species in titanocene halide-promoted reductions. J Am Chem Soc 2004; 126:7853-64. [PMID: 15212533 DOI: 10.1021/ja0491230] [Citation(s) in RCA: 120] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The fundamental nature of Ti(III) complexes generated in tetrahydrofuran by reduction of Cp(2)TiCl(2) has been clarified by means of cyclic voltammetry and kinetic measurements. While the electrochemical reduction of Cp(2)TiCl(2) leads to the formation of Cp(2)TiCl(2)(-), the use of metals such as Zn, Al, or Mn as reductants affords Cp(2)TiCl and (Cp(2)TiCl)(2) in a mixture having a dimerization equilibrium constant of 3 x 10(3) M(-)(1), independent of the metal used. Thus, we find it unlikely that the trinuclear complexes or ionic clusters known from the solid phase should be present in solution as previously suggested. The standard potentials determined for the redox couples Cp(2)TiCl(2)/Cp(2)TiCl(2)(-), (Cp(2)TiCl)(2)(+)/(Cp(2)TiCl)(2), Cp(2)TiCl(+)/Cp(2)TiCl, and Cp(2)Ti(2+)/Cp(2)Ti(+) increase in the order listed. However, the reactivity of the different Ti(III) complexes is assessed as (Cp(2)TiCl)(2) greater, similar Cp(2)TiCl approximately Cp(2)Ti(+) >> Cp(2)TiCl(2)(-) in their reactions with benzyl chloride and benzaldehyde. None of the reactions proceed by an outer-sphere electron transfer pathway, and clearly the inner-sphere character is much higher in the case of Cp(2)Ti(+) than for (Cp(2)TiCl)(2), Cp(2)TiCl, and in particular Cp(2)TiCl(2)(-). As to the electron acceptor, the inner-sphere character increases, going from benzyl chloride to benzaldehyde, and it is suggested that the chlorine atom in benzyl chloride and the oxygen atom in benzaldehyde may function as bridges between the reactants in the transition state.
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Affiliation(s)
- Rasmus Juel Enemaerke
- Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
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