Arene metal tricarbonyls

It has been assumed that in the transition state, the bond between the two (77-C6Hj)Cr(CO)3 molecules is a result of an overlap between the occupied atomic orbitals of chromium and 77 -orbitals of the arene ligand. In the authors opinion such transition states in which the arene acts as a charge acceptor are in good agreement with the data on dipole moments of arene-metal tricarbonyls. Increase in solvent polarity causes an approximately twofold increase in the reaction rate 100) in passing from heptane to tetrahydrofuran. 

A wide variety of arene-metal tricarbonyl complexes are now known, and their chemistry largely waits to be developed. The effects of coordination... 

The complete mechanism of formation of arene-metal-tricarbonyl complexes from an arene and a metal hexacarbonyl is uncertain. A... 

The spectrum of atypical compound, Cr(tj-C6 H6) (CO) 3, is shown in Fig. 26 2°). In this and other arene metal tricarbonyl compounds, the d-ionizations occur as one broad band, the at and e bands not being resolved. Structure is observed for mesityle-... 

The Group VIB arene-metal tricarbonyl complexes are effective homogeneous catalysts for the dehydrohalogenation of aliphatic alkyl halides to create an olefin function [17]. Often mixtures of several isomers occur in these reactions with terminal alkenes being formed as the major product (Scheme 5). This suggests that the proportion of products depends on their relative rate of formation (kinetic control). 

Most arene complexes are hexahapto, and ternary complexes usually have 18 valence electrons. The most known and used ones for their applications in arene synthesis are the arene-metal-tricarbonyls (columns of chromium and manganese), in particular the arene-chromium-tricarbonyl and the arene-manganese-tricarbonyl cations... 

Many arene metal tricarbonyl complexes where the metal is chromium, molybdenum or tungsten have been prepared by this route although the... 

Arene metal tricarbonyls may undergo displacement of both the arene ring and carbonyl groups. The exchange of arene ligand has been shown to be enhanced by ultraviolet irradiation. 

The reversible reduction of arene metal cations to -cyclohexadienyl complexes is discussed elsewhere (p. 137). It is interesting that the treatment of hexamethyl-benzene chromium tricarbonyl with SbCls in CHCh pves the diamagnetic red cation [Me6C6W(CO)3Cl]+ this is the first example of the oxidation of an arene metal tricarbonyl complex [92a],... 

Examples of reductive cluster-opening and oxidative cluster-closing reactions are common in the chemistry of metal-hydrocarbon tt complexes. For example, bases convert nido- (hexa-hapto)arene-manganese tricarbonyl complexes into aracAno(pentahapto)-7T-cyclohexadienyl complexes 129,130, 217) ... 

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... 

In addition to alkenes, arenes can sometimes be used as radical acceptors in Sml2-mediated carbonyl-alkene couplings. For example, Schmalz reported extensive studies on ketyl additions to arenechromium tricarbonyl complexes 66,67 tetralin-Cr(CO)3 complex 49 underwent reductive carbonyl addition to the aromatic ring upon treatment with Sml2 to furnish the skeleton of the naturally occurring aryl glycoside pseudopterosin G (Scheme 5.37).66,67 Here, the bulky metal tricarbonyl group not only serves to control the... 

There are additional reasons why the (arene)Cr(CO)3 compounds can be misleading as models for PMDA-ODA polyimide/Cr interactions. Formation of (arene)Cr(CO)3 compounds of model PMDA or ODA systems suggests that it-complexes can be formed at the polyimide surface in the early stages of metal deposition. But the (arene)chromium(tricarbonyl) complexes can give little additional chemical or physical insight into surface phenomena because the properties of... 

A variety of transition metals, for example, chromium, molybdenum, tungsten, iron, vanadium, manganese, and rhodium can be used to prepare relatively stable j -arene complexes (see Arene Complexes). Reactions of j -arene chromium tricarbonyl complexes have been extensively examined, and numerous reviews are available. Although chromium complexes are by far the most utilized in organic synthesis, complexes of iron and manganese are emerging as potentially useful alternatives. 

This is not a completely true statement as one of the other 20 syntheses does involve a transition metal reagent, an arene chromium tricarbonyl (see ref. 114). 

Yet another major advance in arene-metal chemistry was made in 1957 by Fischer and Ofele 95) y who found that a sealed tube reaction involving chromium hexacarbonyl, bis(benzene)chromium, and benzene produced the half-sandwich compound benzene-chromium tricarbonyl (XXVII). 

Recently, Pettit and co-workers (134) have shown that the relative rate of solvolysis of benzyl chloride-chromium tricarbonyl is about a million times faster than for uncomplexed benzyl chloride under analogous aSatI conditions. The area of carbonium ion formation and stabilization in arene-metal systems thus represents another fertile field for future investigation. 

Halogen-substituted arene chromium tricarbonyl complexes undergo nucleophilic substitution by alkoxide ions at a considerably enhanced rate over the free arene 39, 327, 444). The effect of the chromium tricarbonyl moiety on the rate of substitution of the arene is approximately equal to that of a -nitro group (39). Treatment of ethylbenzene chromium tricarbonyl with fcrf-butyl lithium followed by hydrolysis and decomposition of the resulting complexes with Ce(IV) yielded m- and p-ethyl-tcrt-butylbenzene along with some unsubstituted ethylbenzene (55). The reaction represents a novel nucleophilic displacement of a hydride ion and contrasts with the metalation reaction observed with w-butyl lithium (304). 

Arene tricarbonyl molybdenum complexes are yellow, often crystalhne compounds. They are weakly air-sensitive in the sohd state and have to be stored under inert atmosphere and out of hght. They are best purified by crystaUization. In solution, they are unstable to air. The trait that has most hampered development of the use of (arene)Mo(CO)3 complexes in organic synthesis, however, is the lability of the arene metal bond. Lewis basic solvents such as THF, DMF, DM-SO, acetone and acetonitrile rapidly displace benzene in (benzene)Mo(CO)3. This lability of the arene-Mo bond, while making handling difficult, holds promise for the catalytic use of this class of compounds. 

Higher charge on the metal leads to higher reactivity of the arene. Chromium tricarbonyl activation is comparable to ap-nitro group, whereas manganese tricarbonyl activates like two nitro groups, and Rh(III) activation is equal to that of three nitro groups [2]. 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

The Dotz benzannulation reaction (DBR) is the reaction of an a,P unsaturated Fischer carbene with an alkyne to produce a highly substituted phenol. Alternatively, the DBR can be considered a metal templated 3 + 1 + 2 cycloaddition of an allylic carbine (3 carbon unit), carbon monoxide (1 carbon unit), and an alkyne (2 carbon unit). The initial product of the reaction is the arene chromium tricarbonyl complex of the phenol as in 4. These complexes are typically unstable in air such that workup and purification of the product lead to the complete loss of the metal. Chromium is the most often used metal for the benzannulation. Molybdinum, tungsten, and manganese have been used but usually give mixtures of products and require harsh reaction conditions. 

On the other hand, highly electron-deficient hexahapto complexes of arenes with transition metals such as arene chromium tricarbonyl or cationic arene Fe(II)Cp complexes do not enter VNS [63]. Although they are able to form a -adducts with a-halocarbanions and these o -adducts can be oxidized to form ONSH products, the second step, the base-induced P-ehmination of HL from these a -adducts, does not proceed, perhaps due to steric reasons [63],... 

Although a wide range of Ti -arene transition metal complexes are known/ it is only ri -arene chromium tricarbonyl complexes that have found significant applications in synthetic organic chemistryThese complexes have been investigated either as key intermediates in synthesis " or as efficient catalysts in a variety of processes. 

In a similar manner, asymmetric carbonyl olefination of meso-dicarbonyl compounds was extended to metallic arene or diene complexes [37], such as // -diene Fe or // -arene Cr complexes, to form planar complexes with high enantiomeric bias (Scheme 7.8). Since both complexation and decomplexation of these optically active compounds occur readily, these olefinic complexes are effective as stereocontrollers due to the presence of bulky metal tricarbonyl groups, and serve as useful reactants for obtaining optically active compounds of central chirality by appropriate chemical transformation. 

Similar ]3-carbonium ions have been proposed to explain kinetic measurements on the solvolysis of the arene chromium tricarbonyl complexes, 8.6. Thus while the Cr(CO)3 group may withdraw electrons from the arene ring the metal atom can also provide electrons for the stabilization of carbonium ions which are ring substituents. 

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