Cerium(IV) Ammonium NitratesA Versatile Single-Electron Oxidant Vijay Nair* and Ani Deepthi Organic Chemistry Section, Regional Research Laboratory (CSIR), Trivandrum, India Received July 12, 2006 Contents 6. Miscellaneous Transformations 24 6.1. Fragmentation Reactions 24 1. Introduction 1 6.2. Alkoxylation Reactions 25 2. Reactions Involving Carbon−Carbon Bond 3 Formation 6.3. Side-Chain Oxidations 25 2.1. Intermolecular Carbon−Carbon Bond-Forming 3 6.4. Esterification and Transesterification 26 Reactions 6.5. Dehydrogenation Reactions 26 2.1.1. Generation of Carbon-Centered Radicals 3 6.6. Other Synthetic Transformations 26 from Carbonyl and Dicarbonyl 6.7. Polymerization Reactions 27 Compounds and Related Chemistry 7. Conclusions 27 2.1.2. Generation of Carbon-Centered Radicals 6 8. Acknowledgments 27 from Silyl Enol Ethers and Related 9. References 27 Chemistry 2.1.3. Generation of Radical Cations from 7 Styrenes and Related Chemistry 2.1.4. Generation of Cation Radicals from 8 1. Introduction Electron-Rich Aromatic Systems and Carbon-carbon and carbon-heteroatom bond-forming Related Chemistry reactions constitute the central theme of organic synthesis, 2.2. Intramolecular Carbon−Carbon Bond-Forming 9 and progress in modern synthesis is dependent on develop- Reactions ment of novel methodologies for the same. Among the 3. Reactions Involving Carbon−Heteroatom Bond 13 various methods for bond formation, single-electron transfer Formation (SET) reactions undoubtedly occupy a prime position. Of 3.1. Carbon−Nitrogen Bond Formation 14 the different methods of SET, chemical methods for the 3.1.1. Introduction of Azide Functionality 14 generation of radicals have emerged to be very important in 3.1.2. Introduction of Nitro Functionality 14 recent years. Radicals have been known to the organic 3.1.3. Nitration of Aromatic Nucleus 15 chemists ever since the epoch-making discovery of the 3.2. Carbon−Sulfur Bond Formation 15 triphenylmethyl radical by Gomberg in the year 1900.1,2 3.2.1. Sulfonylation 15 Almost four decades later, significant contributions to the 3 3.2.2. Thiocyanation 16 development of this area were made by Hey and Waters 4 3.3. Carbon−Selenium Bond Formation 16 and Kharasch, who carried out elaborate studies on the mechanisms of radical reactions. In spite of a clear under- 3.4. Carbon−Halogen Bond Formation 16 standing of the mechanistic background, radical reactions 3.4.1. Iodination 16 found little application in synthesis, largely due to the 3.4.2. Bromination 17 erroneous notion that they are prone to give intractable 3.4.3. Azidoiodination and Iodothiocyanation 17 mixtures. A dramatic change in this outlook which triggered 4. Reactions Involving CAN as a Catalytic Oxidant 17 an upsurge of interest in this approach, particularly over the 4.1. Oxidative Transformations of Epoxides 17 last three decades, can be attributed to the conceptualization 4.2. CAN-Bromate Oxidations 18 and demonstration by Stork that the controlled generation 4.3. Electrophilic Substitution Reactions of Indoles 18 and addition of vinyl radicals to olefins constitutes a unique - 4.4. Imino-Diels−Alder Reactions 19 and powerful method for complex carbocyclic construction.5 7 8,9 10 5. Deprotection−Protection Sequences Mediated by 19 It is noteworthy that the investigations of Julia, Beckwith, CAN Ingold,11 and Giese12 have contributed to a deeper under- 5.1. Earlier Reports 19 standing of the structure and reactivity of radicals. A number 13,14 15 5.2. Deprotection of Acetals 20 of others, most notably, Curran, Pattenden, Fraser- Reid,16 and Hart,17 have made significant contributions to 5.3. Removal of TBDMS, THP, and t-BOC 21 Groups the application of radical methodology in organic synthesis. Today, radical methodology has evolved as a prominent tool 5.4. Removal of Trityl and Silyl Groups 22 in the arsenal of the synthetic organic chemist.18,21 5.5. Removal of Benzyl, PMB, and PMPE Groups 23 - Various procedures involving chemical,19 21 electrochemi- cal,22 and photochemical23 methods are known for generation * To whom correspondence should be addressed. Phone: 91-471-2490 406. of radicals. Of these, redox processes based on electron Fax: 91-471 2491712. E-mail: [email protected]. transfer deserve special mention.24 Chemical methods for 10.1021/cr068408n CCC: $65.00 © xxxx American Chemical Society Published on Web 04/14/2007 PAGE EST: 29.6 B Chemical Reviews Nair and Deepthi Vijay Nair was born in Konni, Kerala State, India. He has Ph.D. degrees Ani Deepthi was born in Kollam, Kerala State, India, in 1980. She obtained from Banaras Hindu University (1967, with Professor R. H. Sahasrabudhey) her B.Sc. degree from Kerala University (Fatima Mata National College, and the University of British Columbia (1969, with Professor James P. Kollam) in 2000 and her M.Sc. degree (First Rank) from the Cochin Kutney). Subsequently, he did postdoctoral work with Professor Josef University of Science and Technology in 2002. She did her doctoral Fried at the University of Chicago, Professor Peter Yates at the University research under the guidance of Dr. Vijay Nair in the Regional Research of Toronto, and Professor Gilbert Stork at Columbia University. He joined Laboratory (CSIR) Trivandrum, working in the area of multicomponent the Medical Research Division (Lederle Laboratories) of American reactions and has submitted her Ph.D. thesis to the Cochin University of Cyanamid Company as Senior Research Chemist in 1974 and became Science and Technology, Kochi. She is currently working as a postdoctoral Principal Research Chemist in 1987. In 1981 he received the Outstanding fellow in the group of Professor Suresh Valiyaveettil at the National Scientist Award of Cyanamid Company. In 1990, he returned to India to University of Singapore, Singapore. join the Regional Research Laboratory (CSIR) in Trivandrum. From 1997 to 2001 he was the director of the laboratory. He is now continuing in brief outlines of CAN-mediated reactions and highlighted RRL as a Senior Associate of the Jawaharlal Nehru Centre for Advanced 31,33,34 Scientific Research (JNCASR), Bangalore. He is also an Honorary the work done in our laboratory. The present review Professor in the Cochin University of Science and Technology. He has focuses on the most important synthetic transformations lectured in a number of universities in Germany in 1998 under the INSA- mediated by CAN, covering the literature from 1965 to 2006. DFG exchange fellowship. He has been an invited speaker at the 17th Although this review will not detail the material that has International Conference on Heterocyclic Chemistry in Vienna in 1999, already been described in our earlier reviews, some important MCR 2000, International Symposium in Munich, and IUPAC-International reactions will be discussed briefly for the sake of complete- conference on Organic Synthesis (ICOS-15), Nagoya, Japan, 2004. He ness. was a visiting scientist at Columbia University during 1979−82 and Visiting professor at the University of Pernambuco in Brazil in May 2001. He is What makes cerium so unique among the lanthanide a fellow of the Indian Academy of Sciences and a Silver Medalist of the elements? The answer lies in the ability of cerium to display Chemical Research Society of India. His research interests include stable adjacent oxidation states +3 and +4. The most cycloadditions, radical-mediated C−C and C−heteroatom bond-forming common oxidation state of the lanthanides is the +3 state. reactions, multicomponent reactions, and heterocyclic synthesis. Forty Cerium in its ground state has an electronic configuration students have completed Ph.D. degrees under his supervision, and he 2 2 has published nearly 200 papers in major international journals. of [Xe]4f 6s , where Xe represents the xenon configuration. The electronic configuration of the Ce+3 ion is [Xe]4f1, while + electron-transfer oxidation involve use of salts of high-valent that of Ce 4 ion is [Xe]4f 0. The enhanced stability of the + metals such as Mn(III), Ce(IV), Cu(II), Ag(I), Co(III), V(V), vacant f shell in Ce 4 accounts for the ability of cerium to Fe(III), etc. Among these Mn(III) has received considerable exist in the +4 oxidation state. The large reduction potential + attention. In spite of the well-established and frequent use value of 1.61 V (vs NHE) endowed in Ce 4 makes Ce(IV) of this reagent for generation of electrophilic carbon-centered reagents superior oxidizing agents compared to other metal radicals from enolic substrates, particularly in intramolecular ions. processes, procedural problems associated with it have The advent of Ce(IV) reagents in organic synthesis may prompted development of other oxidants of choice. The be attributed to the investigations of Trahanovsky. As early availability of Ce(IV) reagents as suitable one-electron as 1965 he showed that benzyl alcohols underwent oxidation oxidants assumes importance at this juncture. to benzaldehydes in excellent yields using CAN in 50% The most extensively used cerium(IV) reagent in organic aqueous acetic acid.35 The side-chain oxidation of toluene chemistry is cerium(IV) ammonium nitrate (CAN). The to benzaldehyde by CAN was also reported.36,37 Trahanovsky reasons for its general acceptance as a one-electron oxidant also found that oxidation of primary alkanols 1 which possess may be attributed to its large reduction potential value of a δ-H atom produces tetrahydrofuran derivatives 2 (Scheme +1.61 V vs NHE (normal hydrogen electrode), low toxicity, 1).38 The fragmentation of 1,2-diols by