Kharash reaction

The decrease in catalytic activity of the nickel-containing carbosilane dendri-mer shown in Fig. 6.28 was attributed to the formation of mixed complexes with nickel in both oxidation states II and III on the dendrimer surface, which competes with the reaction with substrate radicals occurring in Kharash reactions (Fig. 6.29). 

Ruthenium complexes were among the first catalysts employed for controlled radical polymerization via ATRP mechanism (i, 2). One the one hand, this happened due to the fact that ATRP process originates from Kharash reaction of radical addition (5), which is catalyzed by complexes of this metal. On the other hand, this was associated with the unical properties of ruthenium atom, particularly the ability to assume different oxidation states and various coordination geometries 4-6). 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

The authors used (5)-carvotanacetone (dihydrocarvone) as starting material (Scheme 34). To prepare the linearly conjugated sUylenol ether, they used the Kharash protocol and attained y-alkylation by Mukaiyama aldol reaction with trimethylorthoformate (195). The ketoacetal 295 was a-hydroxylated according to Rubottom by silylenol ether formation followed by epoxidation and silyl migration. Acid treatment transformed 296 to the epimeric cyclic acetals 297 and 298. endo-Aceta 297 was equilibrated thereby increasing the amount of exo-acetal 298. The necessary unsaturated side chain for the prospected radical cyclization was introduced by 1,4-addition of a (trimethylsilyl)butynylcopper compound. 

Oxidations. A widely used method for allylic oxidation is the Kharash-Sosnovsky reaction using a peroxide and a copper(I) salt system. Enantioselective allylic oxidations of cycloalkenes such as cyclopentene, cyclohexene and cycloheptene with tert-butyl peibenzoate were investigated with a variety of catalysts derived from bis(oxazoline) ligands and copper(I) triflate complexes (eq 18). The ligand-copper(I) complexes from the /-Bu-... 

Langford and Kharash noted the ready reaction of trichloru-methykulfenyl chloride with ethylene oxide in the presence of [Pg.226]

M. S. Kharash and O. Reinmuth, Grignard Reactions of Nonmetallic Substances, Prenticc-Hall, Englewood Cliffs, NJ, 1954, p. 122. 

Keywords C-H oxidation, Benzylic hydroxylation, AUylic oxidation, Desymmetrization of meso compound, Kharash-Sosnovsky reaction. Iron-porphyrin complex, (Salen)manganese(lll) complex... 

Several methods are now available for allylic oxidation. Among them, the ene-type oxidation reaction with, e.g., Se02 or 02 oxidation have been the most widely used for the purpose [15] but their asymmetrization has not met with success. Another widely used method is the Kharash-Sosnovsky reaction using a peroxide and Cu(I) salt system [16]. This reaction has been considered to proceed through a Cu(III)-allyl complex 7 (Scheme 6) [17]. 

This mechanism suggests that the reaction can be performed in an enantiose-lective manner, if the copper ion is appropriately modified by chiral ligand(s). In 1965, the Cu(II)-a-ethyl camphorate complex was found to promote the asymmetric Kharash-Sosnovsky reaction, although the enantioselectivity was only modest [18]. Thirty years later, this chemistry was followed by three highly enan-... 

Scheme 12. Other examples of the Asymmetric Kharash-Sosnovsky reaction...

Copper nitrenoids that participate in C—H bond insertion reactions have also been studied for a long time. In 1997, the first asymmetric reaction was reported by Katsuki and co-workers. Their design was derived from the Kharash-Sosnovsky reaction (Scheme 1.52, top), the Cu-catalyzed allylic... 

For a brief discussion on the asymmetric Kharash-Sosnovsky reactions, see Chapter 5, Section 5.5. 

Spiro-linked compounds containing heterocyclic units have been prepared for many apphcations. In general, there are two synthetic pathways to build up heterocyclic spiro compounds. On the one hand, cross-coupling reactions like the Negishi, Kharash, Stille, Suzuki, or Sonogashira coupling reaction can be utilized to connect the heterocychc subunit with the central spiro core [118]. On the other hand, the heterocycle can be built up from spiro precursors containing heteroatoms. 

Kharash and coworkers first proposed a hydroperoxysulphide intermediate in the formation of jS-sulphinyl alcohols in the co-oxidation of thiols with olefins. This was later confirmed by detection of peroxy compounds in the reaction mixture. Further studies led to the isolation of several hydroperoxysulphides when aromatic thiols were oxidized at low temperatures ""... 

Kharash-type reactions (radical generation and trapping by an olefin) are possible with Ce(IV) as a radical initiator. 

Figure 2.9 UV spectra for (a) Topanol CA, (b) Binox M, and (c) lonox 330. Curve A ethanolic solution, curve B alkaline ethanolic solution (10 ml of ethanolic solution plus 1 ml of water, plus 2 ml of ethanolic potassium hydroxide solution), curve C ethanolic solution after reaction with nickel peroxide, and curve D nickel peroxide reaction product made alkaline with two drops of ethanolic potassium hydroxide solution Reproduced from Kharash and Joshi, Journal of Organic Chemistry [70]...

Kharash MS, Urry WH, Kudmia BM (1948) Reactions of atom and free radicals in solution. XX. The addition of aldehydes to olefins. J Org Chem 13 248-253... 

The mechanism of the Cu(I)-mediated Kharash-type addition of CCI4 to allyl chloride using copper-triphenylphosphine complexes has been studied. Several features of note were found, including an abnormally low rate for the reactions and the observation of an induction period. A mechanism that involves an initial formation of a catalytic Cu(I)-olefin complex was proposed. ... 

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