Metal carbonyls Tricarbonyl chromium

Metal carbonyls Acylcobaltate complexes, 101 Arene(tricarbonyl)chromium complexes, 19... 

Uemura demonstrated that enantiomerically pure (arylaldehyde)tricarbonyl-chromium complexes afford exclusively the //iraj-pinacols, providing an asymmetric synthesis of hydrobenzoins (Scheme 5.4).13 An intermediate involving coordination of the Sm(III) metal centre with the carbonyl oxygen was proposed to account for the high selectivity observed. This was supported by coupling experiments in the presence of HMPA, an additive that is known to prevent complexation, which led to the preferential formation of the erythro product. 

Arene(tricarbonyl)chromium complexes undergo a number of synthetically important transformations not usually observed for uncomplexed arenes. The chromium tricarbonyl moiety facilitates nucleophilic, electrophilic, and radical reactions at the benzylic position. Upon complexation, one side of the aromatic ring and adjacent functionalities is blocked by the metal carbonyl moiety and highly stereoselective reactions are usually observed even at relatively remote positions. In addition, the protons of the complexed aromatic ring have a substantially higher acidity and are readily removed and further substituted by electrophiles. Finally, the aromatic ring is activated toward addition reactions using a variety of nucleophiles. 

Following this initial discovery, several additional routes to benzene-chromium tricarbonyl and related complexes were developed, the most convenient of which involves the simple refluxing of the metal carbonyl and arene with concurrent loss of carbon monoxide 96, 97,184, 187). 

Miscellaneous Systems Other systems have been described [li,m] ferrocene/ carbon tetrachloride (a trichloromethyl radical is formed), ferrocene/carbon tetrabromide, metal carbonyl/onium salts (e.g., the [cyclopentadienyl Fe (CO)2]2/ diaryliodonium hexafluorophosphate combination where a phenyl radical is generated), benzene chromium tricarbonyl/halide derivative, and so on. 

The stereochemistry of some arene metal carbonyls As mentioned in section A(b) benzene chromium tricarbonyl and hexa-methylbenzene chromium tricarbonyl have the staggered conformation B. 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

Consequently, these charge effects are reflected in the carbonyl stretching frequencies (87, 88). It has recently been found from studies of the far infrared spectra that the metal-carbon stretching frequencies also support the theory (89). These charge-distribution effects are supported further by the observed dipole moments (90-92). Thus the dipole moments of the chromium tricarbonyl complexes of hexamethylbenzene, benzene, and methylbenzoate lie in the order 6.22, 4.92, and 4.47 /x, respectively. The relationship of charge effects to chemical reactivity is described below. 

Tricarbonyl(naphthalene)chromium, 19 Tungsten carbonyl, 49 Metal-containing compounds Aluminum Compounds Alkylaluminum halides, 5, 25, 44, 173, 306... 

Liquid injection molding, for silicone rubbers, 3, 674—675 Liquid ligands, in metal vapor synthesis, 1, 229 Liquid-phase catalysis, supported, for green olefin hydroformylation, 12, 855 Lithiacarbaboranes, preparation, 3, 114 Lithiation, arene chromium tricarbonyls, 5, 236 Lithium aluminum amides, reactions, 3, 282 Lithium aluminum hydride, for alcohol reductions, 3, 279 Lithium borohydride, in hydroborations, 9, 158 Lithium gallium hydride, in reduction reactions, 9, 738 Lithium indium hydride, in carbonyl reductions, 9, 713—714... 

Studies on various derivatives of ferrocene, cymantrene, benzene-chromium tricarbonyl, and butadieneiron tricarbonyl were recently reported (174, 176). Carbonyl derivatives containing the COR group lose CO to give an ion probably containing the R group attached to the metal, e.g. 

Transition-metal-stabilized carbocations can be generated from functionalized butadieneiron carbonyl or arenechromium tricarbonyl complexes [92], Reactions of such carbocations formed from chiral complexes have been studied, but low selectivities are usually observed [526, 528, 535]. However, chromium tricarbonyl complexes derived from ephedrine 5.66 suffer cyclization in acidic medium. After decomplexation, c/s-tetrahydroquinolines are formed with a high diastereo-and enantioselectivity [540,542] (Figure 5.44). 

Transition metal organometallic compounds such as manganese, chromium, and iron carbonyls by the intraperitoneal route, and nickel by inhalation (mice) or intravenously (rats), induced selective necrosis of the Clara cells (Haschek et al. 1982). The pulmonary toxicity of methylcyclopenta-dienyl manganese tricarbonyl, representative of this group of compounds, was enhanced by pretreatment with piperonyl butoxide, an inhibitor of the mixed-function oxidase system. 

Chiral Catalysts Containing Group 6 Metals (Cr, Mo, and W). Although all the three metals have important role in organometallic chemistry (eg, carbonyl complexes), their catalytic properties are scarcely investigated in the past few years. The tricarbonyl(jj -arene)chromium structural unit (see Fig. 16)... 

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