Metal aryls

(0 Structure of PhMeOCW(CO)s, showing that the OC1C2 plane bisects the cis-axes, through the metal-carbonyl ligands, (ii) The cylindricaT v-bonding orbital is represented. The x, y and z axes lie along the W-C bonds 

The structure of the related carbene complex Cr(CO)4Ph3P(CMeOMe) has been found to be essentially similar to that of the tungsten carbene, the Cr-Ca distance is 2-04 A [36o]. 

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The insertion of a carbonyl group into a metal-alkyl or metal-aryl bond, and the reverse reaction involving decarbonylation of an acyl complex, have been studied from both the synthetic and mechanistic points of view. The mechanism proposed for this type of reaction seems well established and is... 

Despite the volume of work concerned with metal-catalyzed decomposition of diazo compounds and carbenoid reactions 28>, relatively little work has been reported on the metal-catalyzed decomposition of sulphonyl azides. Some metal-aryl nitrene complexes have recently been isolated 29 31>. Nitro compounds have also been reduced to nitrene metal complexes with transition metal oxalates 32K... 

Scheme 2. Two tungsten alkylidene catalysts that contain a metal-aryl bond... 

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

The use of metal alkyls, metal aryls, metal hydrides, and metal carbides for preparing alcoholates of carbohydrates in inert, aprotic solvents has not yet been reported. 

Alkali metal aryl ketyls have been known for many years and have been studied by electron-spin resonance methods (Adam and Weissman, 1958 Hirota and Weissman, 1960). Only recently, however, have the isoelectronic neutral radicals R2NO been discovered (Baird and Thomas, 1961 Thomas, 1960). Results for a series of these radicals are summarized in Table 8. 

Work by K Travis Holman and Jerry Atwood at the University of Missouri, USA has resulted in a tricationic host 4.66 based on the macrocycle cyclotriveratrylele (CTV, Section 7.7), which exhibits a deep anion binding pocket surrounded by three metal centres. A guest PF6 anion fits neatly into the cavity, stabilised by C—H F interactions, which may be differentiated from symmetry-related, noninteracting protons on the other side of the metallated aryl ring by H NMR spectroscopy. The X-ray crystal structure of this material is shown in Figure 4.26. 

Discovered over a century ago, electrophilic mercuration is probably the oldest known C-H bond-activation reaction with a metal compound. The earliest examples of aromatic mercuration were reported by Volhard (mercuration of thiophene) [1], Pesci (mercuration of aromatic amines) [2], and Dimroth [3], who was the first to mercurate benzene and toluene, generalize the reaction, and assign the correct structures to the products originally observed by Pesci. Since the work of Dimroth electrophilic aromatic metalation reactions with compounds of other metals, for example Tl(III), Pb(IV), Sn(IV), Pt(IV), Au(III), Rh(III), and Pd(II), have been discovered [4], In this chapter, we will focus on intermolecular SEAr reactions involving main-group metal electrophiles and resulting in the formation of isolable metal aryls which find numerous important applications in synthesis [5], Well-known electrophilic cyclometalation reactions, for example cyclopalla-dation can be found in other chapters of this book and will not be reviewed here. 

Close inspection of 2 reveals that it can be thought of as a metal-aryl complex in a low oxidation state. Thus it meets the conditions for another type of C-H transformation - cross-coupling. Various N-heterocycles bearing the N-CH-X motif will react with aryl iodides, under the action of rhodium catalysis, to give arylated products in moderate yield [11]. 

Metal alkyls of late transition metals readily undergo decomposition by /3-hydrogen elimination.31 In principle, it should therefore be possible to prepare aryne complexes of dw metals by thermolysis of appropriately substituted late transition-metal aryl complexes. However, in practice this... 

Metal Alkyls and Metal Aryls The True Transition Organometallics... 

Heteroleptic Complexes with Metal-Alkyl and Metal-Aryl Bonds... 

M. M. Olmstead, P. Power, and S. C. Shoner, Isolation and X-Ray Crystal Structures of Homoleptic, Transition-Metal Aryl Complexes [(LiEt20)4VPh5] and [(LiEt20)3CrPh5], Organomeiallics 1, 1380-1385 (1988). 

T. Krafft, Method for the Synthesis of Metal Alkyls and Metal Aryls, PCT Int. AppL, WO 9418145 Al, 1994. 

The oxidative addition of C H bonds of ligands is very common and this reaction forms metal alkyl or metal aryl complexes. In osmium triarylphosphine complexes, orthomet-allation gives four-membered metaUocycles. When the ortho... 

Pyridine complexes can show analogous hindered rotation to that observed with metal-aryl complexes. This has been observed for over SOyears, but recent interest in supramolecular chemistry, particularly by Fujita and Stang, wherein bipyridyls have been used as linkers has spurred renewed interest in the phenomenon. Several articles on dynamics provide leading references for applications of EXSY and other DNMR methods in these systems. BINAP complexes with picolines and methyl-substituted isoquinolines, which show syn and anti isomerism, have been investigated, for example, (48), (49), and (50). ... 

These results show that both metal-aryl bond homolysis and reductive coupling of the aryl ligands can occur for these derivatives as a consequence of irradiation (Scheme 3). However, which path predominates is... 

The reactivity of Pt(0) complexes toward aryl halides closely parallels that of the Ni(0) and Pd(0) analogs. The trans-oxidative addition product is obtained, and second-order kinetics are observed. The greater stability of the metal-aryl compared with the metal-alkyl bond in group VIII d °-M(0) systems is attributed to the greater electronegativity of the sp carbons in the aryl ligands . 

Japan. Symposium on ferrocene (44) chemistry see also refs. 25.9,25.42 12+ Japan. I, Preparation, properties 13 + 10 cetylation, carboxylation II, (141) Acetylferrocenecarboxylates III, Acid derivatives metalation, arylation... 

Hydrides, Grignard Reagents, Metal-alkyls and Metal-aryls, and Other Reductants... 

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