Aryls, transition metal

As a ligand class, the aryls have usually been treated in the same context as the alkyls. In 1991, however, a review,8 which dealt exclusively with aryl transition metal complexes, drew attention to their often unique structures and reactivity. It was also apparent that the influence of steric properties of aryl ligands had not been extensively investigated for transition-metal complexes. The bulkiest aryl ligand to be widely employed with these metals... 

The stirring discovery, that transition metal alkyls and aryls were accessible via oxidative addition of C—H bonds to electrophilic metal centers, soon brought up the question whether dialkyl, diaryl or alkyl(aryl) transition metal... 

R = H, alkyl, aryl, substituted alkyl and aryl transition metal complex Co2(CO)8, Fe(CO)5, Ru2(CO)i2, Cp2TiR2, Ni(COD)2, W(CO)e, Mo(CO)e, [RhCI(CO)2]2 Promoter NMO, TMAO, RSCH3, high-intensIty llght/photolysis, hard" Lewis base... 

If a sufficient concentration of a good alkylating agent is present, this sequence will invariably continue until the transition metal has been reduced to its lowest positive valence (usually M+2), and under very rigorous conditions the free metal may be deposited. Although it is not specifically indicated above, there is also evidence that di-, tri-, or polyalkyl (or aryl) transition metal compounds may form, and each metal pair may form bimetallic complexes either as stable intermediates or, at least, at the instant of exchange of groups between the two metal... 

The first observation " of >y -aryl transition-metal species by direct electrophilic metallation in the reaction of benzene with H2PtCl6 and Na2[PtCl4] in aq CFjCOjH... 

These occur more commonly than terminal phenyl ligands, presumably because of the increased M-C contact as indicated (Fig. 3, type B). Copper(I), silver(I) and gold( I) compounds with these bridges are particularly well studied.A number of binary aryl transition metal complexes of the type MjAry have been synthesized. Those with bulky aryls such as mesityl (mes) or perfluorophenyl are the most tractable, the bulk preventing further oligomerization or polymerization. Some... 

In many instances, water is sufficiently acidic to cleave alkyl and aryl transition metal compounds. However, many of these compounds are hydrophobic or insoluble in water and hydrolyze so slowly that the reaction is not detected in a test tube experiment. For example, methylchlorobis(7r-cyclopentadienyl)titanium is reported to be stable to cold water 107) but hydrolyzes rapidly in warm water. Few rate studies have been done in mutual solvents for water and organometallics but hydrolysis rates seem to vary by many orders of magnitude. The effect of hydrogen ion concentration has also received little attention but qualitative observations indicate that hydrolysis proceeds faster in acidic solutions. 

Nearly all alkyl and aryl transition metal compounds are cleaved by hydrogen halides. Even arylplatinum complexes stabilized by phosphine ligands are rapidly cleaved by hydrogen chloride in methanol or benzene. The reaction... 

Some reactions of alkyl and aryl transition metal compounds are formally analogous to those of nontransition metal compounds such as Grignard reagents, but the analogy is very weak. In spite of many attempts, no... 

B. The properties of perfluoro-alkyl and >aryl transition metal complexes... 

Similar stability and reactivity have also been observed for bridged-Cp systems. The catalyticaHy active (CH2)2Si(C (CH2)4)2ThR2, where R = Cl [89597-06-8] alkyl, CH2CgH [89597-10A] aryls, or H [89597-11-5], Similar to Group 4 transition-metal Zeigler-Natta catalysts, stable cationic Th(IV) species, eg, [Cp 2ThCH2] [108834-69-17, have been isolated with a host of noncoordinatiag/nonreactive anions. MetaHacycle formation has also been... 

Trifluoromethyl Derivatives of the Transition Metal Flements 1984 Synthesis and Properties of Aryl... 

Stabilization of /j -alkyl and -aryl derivatives of transition metals can be enhanced by the judicious inclusion of various other stabilizing ligands in the complex, even though such ligands are known not to be an essential prerequisite. Particularly efficacious are potential IT acceptors (see below) such as AsPh3, PPh3,... 

Transition metal-catalyzed synthesis of hetarylamines and hetaryl ethers from triflates and aryl/hetaryl halides or heterocyclic amines 98AG(E)2046. 

The third area is the synthesis and characterization of aryldiazenido complexes of transition metals. In 1964 King and Bisnette isolated the first metal complex with an aryldiazenido ligand. The interest of organometallic chemists was concentrated mainly on the isolation and characterization of stable aryldiazenido complexes and not on potential metastable intermediates involved in metal-catalyzed dediazonia-tions. The situation is different, however, for metal complexes with alkyl-diazenido ligands. Complexes with aryl- and alkyldiazenido ligands are the subject of Chapter 10 in the forthcoming second book (Zollinger, 1995). 

Colona and coworkers oxidized a variety of alkyl aryl and heterocyclic sulfides to the sulfoxides using t-butyl hydroperoxide and a catalytic amount of a complex (97) derived from a transition metal and the imines of L-amino acids. Of the metals (M = TiO, Mo02, VO, Cu, Co, Fe), titanium gave the highest e.e. (21%), but molybdenum was the most efficient catalyst. The sulfoxides were accompanied by considerable sulfone125. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

The Suzuki reaction has been successfully used to introduce new C - C bonds into 2-pyridones [75,83,84]. The use of microwave irradiation in transition-metal-catalyzed transformations is reported to decrease reaction times [52]. Still, there is, to our knowledge, only one example where a microwave-assisted Suzuki reaction has been performed on a quinolin-2(lH)-one or any other 2-pyridone containing heterocycle. Glasnov et al. described a Suzuki reaction of 4-chloro-quinolin-2(lff)-one with phenylboronic acid in presence of a palladium-catalyst under microwave irradiation (Scheme 13) [53]. After screening different conditions to improve the conversion and isolated yield of the desired aryl substituted quinolin-2( lff)-one 47, they found that a combination of palladium acetate and triphenylphosphine as catalyst (0.5 mol %), a 3 1 mixture of 1,2-dimethoxyethane (DME) and water as solvent, triethyl-amine as base, and irradiation for 30 min at 150 °C gave the best result. Crucial for the reaction was the temperature and the amount of water in the... 

The synthesis of (hetero)aryl cyanides from (hetero)aryl halides via transition-metal catalysis is a very valuable reaction since a nitrile can be easily transformed into several other functional groups. Not until 2000 were the first examples on microwave-assisted cyanation reported in the literature. Alter-man and Hallberg found that 3-bromopyridine and 3-bromothiophene were... 

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