Transition-metal activation

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

With transition-metal activators, the initiation process is postulated as ... 

A Cu(OAc)2-catalyzed intramolecular diamination of alkenes using sulfamide substrates such as compound 214 provides a route to fused thiadiazolidines 215 (Equation 48) <2005JA11250>. In this reaction, the transition metal activates the alkene toward nucleophilic attack by the first nitrogen, then becomes displaced by the second nitrogen nucleophile (a net M +z to M reduction). 

In the strictest sense, homogeneous catalysis involves catalytic reactions occurring in a single phase. However, as currently used, the term implies only that at least a portion of a particular reaction is known or suspected strongly to occur in the coordination sphere of a metal (most frequently a transition metal). Activation of substrates and likely the steric course of the reaction are then consequences of bonding in an in-... 

Coordination to transition metals activates benzylic as well as aromatic centers, which become targets for either nucleophilic or electrophilic attack [2]. For example, under acidic conditions, a carbocation is generated, which can then react with a nucleophile, i.e. ... 

This chapter is divided into two main parts. The first part focuses on reactions where the dithiolene ligand is generated independently of the metal center. For the most part, these preparations give alkenedithiolate dianions, which ordinarily are treated with metal electrophiles to form dithiolene complexes. In the second part, transition metals actively participate in the assembly of the dithiolenes, usually via the reaction of a metal sulfido species with an alkyne or hydrocarbon in an equivalent oxidation state. 

Generally, most oxygen transfer reactions employing alkyl hydroperoxides require transition metal activation, since the alkoxide anions are relatively poor leaving groups, even poorer than the hydroxide anion. The reasons why they are sometimes employed in preference to hydrogen peroxide are as follows ... 

Transition metals open up new opportunities for synthesis, because their means of bonding and their reaction mechanisms differ from those of the elements of the s and p blocks. The empty and partially filled d-orbitals that characterize most of these metals enable them to bond reversibly to many functional groups. Thus, transition metals activate many difficult or previously unobserved reactions which are not readily achieved by using conventional reagents. The organometallic chemistry of transition metals has grown explosively in the last decade. 

The transition metal activates the C-X bond in the oxidative addition step and normally the substrates have sp or sp carbons at or immediately adjacent to an electrophilic centre. The reactivity of aliphatic C-X bond towards the oxidative addition with a transition metal is somewhat low. However, in 1992, Suzuki and co-workers discovered that Pd(PPh3)4 can catalyze couplings of alkyl iodides with alkyl boranes at 60°C in moderate yields (50-71%). These conditions tolerated a wide variety of functional groups such as esters, ketals and cyanides. 

Transition metal activated nucleophilic substitution in heteroaromatic compounds 90BSF401. 

Under neutral conditions Ru(II) complexes catalyze the nucleophilic addition of water to nitriles to yield amides [155], The reaction proceeds via external nucleophilic attack of water to the transition metal-activated nitrile. Under similar conditions <5-ketonitriles are converted into ene-lactams, a reaction that has found elegant application in a short diastereoselective synthesis of (-)-pumiliotoxin C (Sch. 37) [156]. 

The SjvAr reaction is another attractive method for diaryl ether synthesis, and reactions of o-nitro- and o-cyanofluorobenzenes with phenols were reported . 7r-Complexation of aryl halides with transition metals activates the aromatic nuclei toward S fAr. Segal employed a ruthenium chlorobenzene complex in the poly(aryl ether) synthesis , and the methodology was extensively studied by Pearson, Rich and their coworkers using manganese complex and later iron and ruthenium complexes in natural product synthesis " . The intramolecular substitution of an aromatic chloride with a phenylalanine derivative takes place at room temperature without racemization (equation 27). 

As one can see from Table 1, catalytic activity of MNaY and MNaZSM-5 zeolites is not the same despite the same content of the transition metal. Activity of CoNaY is higher than that of CoNaZSM-5 at the same content of cobalt (respectively 100 and 42% stilbene conversion for... 

Bakac, A. Kinetic and mechanistic studies of the reactions of transition metal-activated oxygen with inorganic substrates. Coord. Chem. Rev. 2006, 250, 2046-2058. 

Coordination of olefins to a wide variety of transition metals activates the olefin for nucleophilic attack, the result of which is the formation of a carbon-nucleophile bond and a carbon-transition-metal ff-bond ... 

As outlined in 5.8.2.3.4, coordination of an alkene to a neutral or cationic transition metal activates the alkene toward nucleophilic attack, leading to an alkylmetal product. Development of the analogous synthesis of alkenylmetal complexes starting with /y -alkyne-metal complexes is more recent. An early reaction of this type is ... 

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