Ruthenium-Promoted Radical Reactions

This catalytic sequence is known as Kharasch addition or atom transfer radical addition (ATRA) [4]. Various polyhalogenated compounds such as CCI4 and CCI3CO2R are used as the organic halides, and transition metal salts or complexes are used as the catalyst [3]. Intramolecular version of the Kharasch addition reaction (atom transfer radical cyclization, ATRC) has opened novel synthetic protocols to the synthesis of carbocyde or heterocyles catalyzed by transition metals [5-7], and this has become a very important field in free radical cydization in organic synthesis. Transition metal-catalyzed Kharasch reactions sometimes afford telomers or poly- 

Ruthenium in Organic Synthesis. Shun-Ichi Murahashi (Ed.) 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30692-7 

Representative ruthenium complexes active for Kharasch addition and atom transfer radical polymerization (ATRP). 

---

Nagashima H (2004) Ruthenium-promoted radical reactions. In Murahashi S-i (ed) Ruthenium in organic synthesis. Wiley-VCH, Weinheim, p 333... 

Reactions involving radical reactions are detailed in the chapter Ruthenium-Promoted Radical Processes Toward Fine Chemistry of this volume. 

The aim of this paper is to present recent breakthroughs brought about by the observation that ruthenium carbene complexes, known to be outstanding catalysts in olefin metathesis, are also able to promote radical reactions. 

Negishi E, Tan Z (2005) Diastereoselective, Enantioselective, and Regioselective Carbo-alumination Reactions Catalyzed by Zirconocene Derivatives. 8 139-176 Netherton M, Fu GC (2005)Pa]ladium-catalyzed Cross-Coupling Reactions of Unactivated Alkyl Electrophiles with Organometallic Compounds. 14 85-108 Nicolaou KC, King NP, He Y (1998) Ring-Closing Metathesis in the Synthesis of EpothUones and Polyether Natmal Products. 1 73-104 Nishiyama H (2004) Cyclopropanation with Ruthenium Catalysts. 11 81-92 Noels A, Demonceau A, Delaude L (2004) Ruthenium Promoted Catalysed Radical Processes toward Fine Chemistry. 11 155-171... 

In contrast to the previously mentioned reactions, which involve either oxo-ruthenium or ruthenium hydride species as intermediates, free-radical reactions can also be promoted by ruthenium. The aerobic oxidation of alcohols proceeds smoothly at room temperature in the presence of 4 eq. of an aldehyde, for example, acetaldehyde, and a catalyst comprising a 1 1 mixture of RuC13 nH20 and Co(OAc)2, in ethyl acetate (Eq. 31) [122]. 

Oxidation of Tetramethylethylene. Tetramethylethylene, TME, was an excellent model olefin since it was rapidly and selectively oxidized in the presence of many transition metal complexes (12). Oxidation of TME in the presence of the group VIII metal complexes [MCI(CO)-(Ph3P)2] (M = Rh, Ir) at 50°C gave two major products 2,3-dimethyl-2,3-epoxybutane, I, and 2,3-dimethyl-3-hydroxy-l-butene, II (Reaction 5). Reaction mixtures were homogeneous with no observable deposits of insoluble materials. Little oxidation occurred under these conditions in the absence of the metal complexes, but low yields of I and II were obtained in the presence of a radical initiator (Table I). Reactions were severely inhibited by hydroquinone. The ruthenium (II) complex, [RuCl2(Ph3P)3]2, also promoted efficient oxidation of TME yielding I... 

Ruthenium chloride complexes, such as dichlorotris(triphenylphosphane)ruthenium(II), effectively catalyze the addition of polyhalocarbons to double bonds5 13 18. In a mechanistic and stereochemical study, carbohalogenation of cyclohexene with carbon tetrachloride in the presence of dichlorotris(triphenyIphosphane)rulhenium(II) gave l-chloro-2-(trichloromethyl)cyclohexane (2) in 77% yield and a diastereomeric ratio (transjeis) of 96 419. In comparison, the same conversion promoted by dibenzoyl peroxide is considerably less selective and gives the same product in only 10% yield with a 53 47 ratio of the trans, cis-isomets. This striking difference led to the conclusion that the ruthenium-catalyzed version does not proceed via a free-radical mechanism, as assumed in the peroxide-mediated reaction. 

---