Benzene, chromium tricarbonyl complex

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

Detailed vibrational assignments have been carried out on arene chromium and arene molybdenum tricarbonyl complexes (18, 19). Splitting of the E band in the carbonyl region was observed for substituted benzene chromium tricarbonyl complexes but not in (CgHg)Cr(CO)3 itself, showing that the concept of local symmetry (Cg ) is of very restricted validity when discussing the C—O stretching vibrations in such complexes (18). 

For monosubstituted arenes, kinetically controlled discrimination between the two enantiotopic ortho hydrogens of the planar chiral benzene chromium tricarbonyl complex leads to nonracemic products. Asymmetric lithiation is more efficient when one or more oxygen atoms, such as ether linkages, are present in the starting prochiral complex (Scheme 26.14). Treatment of Cr(CO)j-anisole complex 52 with the chiral lithium amide 53, in the presence of TMSCl under ISQ conditions, affords (+)-orfho-silylated complex 54 with good chemical yield and ee value [143-145]. The isobenzofuran system 55 reacts as well to give a-sUylated product 56 [146]. 

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. 

One of the most useful types of it complexes of aromatic compounds from the synthetic point of view are chromium tricarbonyl complexes obtained by heating benzene or other aromatics with Cr(CO)6. 

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. 

The possibility of activating the benzene ring of the indole nucleus to nucleophilic substitution has been realized by formation of chromium tricarbonyl complexes. For example, the complex from N-methylindole (294) undergoes nucleophilic substitution with 2-lithio-l,3-dithiane to give a product (295) which can be transformed into l-methylindole-7-carbaldehyde (296) (78CC1076). 

Figure 38 Organometallic benzene carboxylic acid building blocks chromium tricarbonyl complexes [27a].

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). 

The lithiated derivative (LiCgH5)Cr(CO)3 has been prepared in high yield by the reaction of [(CO)3Cr](CaHgHgC6H5)[Cr(CO)3] with re-butyl lithium. Complexes, such as 2-phenylpyridine chromium tricarbonyl and (Ph2PCgHg)Cr(CO)3, which are not otherwise obtainable were prepared by the reaction of the lithiated derivative with pyridine and PhaPCl, respectively 348). Benzene chromium tricarbonyl has been metalated by treatment with re-butyl lithium in THF and after carbona-tion yielded w-benzoic acid chromium tricarbonyl 304). 

The only electrophilic substitution of arene chromium tricarbonyl complexes so far achieved is Friedel-Crafts acetylation. Benzene and substituted benzene chromium tricarbonyls undergo this reaction under mild conditions giving the corresponding acetyl-substituted complexes 109, 176, 218, 233, 234, 355). Substituent and conformational effects play an important role in directing the position of acetylation in arene chromium tricarbonyl complexes 176, 218, 233, 234). 

The third approach to obtain diarylmethylpiperazine derivatives uses the highly stereospecific chiral oxazaborolidine-catalyzed reduction, using catecholborane as the reductant of the 4-bromobenzophenone chromium tricarbonyl complex, as described by Corey and Helal [59], followed by the stereospecific displacement of the hydroxyl benzyl group by the /V-substituted-piperazine [44]. As outlined in Scheme 2, Delorme et al. [44] used this approach for the enantioselective synthesis of compound 31, (+)-4-[ (aS)-a-(4-benzyl-l-piperazinyl)benzyl]-lV,lV-diethylben-zamide. Lithiation of the readily available benzene chromium tricarbonyl with n-BuLi in the presence of TMEDA in THF at —78 °C, followed by addition of... 

The formation of the inclusion compounds was selective. /3-CD formed 1 1 inclusion compounds with benzene, toluene, and o-xylene chromium tricarbonyl complexes, and not with w-xylene, />-xylene, guaiacol, methyl anthrani-late mesitylene, or hexamethylbenzene chromium tricarbonyl complexes. Whereas a-CD did not form inclusion complexes with any arene chromium complexes, 7-CD formed 1 1 inclusion complexes with all arene chromium... 

Commencing with alkyl group rotations, there have been two studies of the stereodynamics of chromium carbonyl complexes with 77 -hexaalkylbenzene. For the complex [ Cr(CO)2L 2(/i-N2)] (L = ry -hexaethylbenzene), NMR band-shape changes were attributed to slowed ethyl group rotation with a barrier, AG (300 K), of 46.0 3.0 kJ moP. The ligand hexa-n-propyl-benzene adopts a D3d symmetry with the alkyl substituent alternately up and down with respect to the observer. This geometry is retained in the chromium tricarbonyl complex and a decoalescence phenomenon observed in its NMR spectrum was... 

Semmelhack et aL (24) found that the halo leaving group is unnecessary, and a hydride ion can be formally displaced from the benzene ring of the chromium tricarbonyl complex. Evidence seems to be in favor of initial attack on the chromium atom which is also a soft-soft interaction. 

Benzene) Chromium tricarbonyl (a piano stool complex)... 

The geometric (size and shape) complementarity between the cyclodextrin host and the organometallic guest determines a well manifested selectivity in the formation of inclusion complexes and can be used for the separation of ferrocene from dime-thylferrocene (only the latter forms a complex with y5-CD) [471], and of (benzene)-chromium tricarbonyl, (7 -C6H6)Cr(CO)3 from (hexamethylbenzene)-chromium tricarbonyl, ( y -C6Me6)Cr(CO)3 (only the latter forms a complex with y-CD) [471], Cyclodextrin host-guest complexation also affords the resolution of hydroxyethylferrocene enantiomers [489]. 

The Buchner reaction can be shut down by arene chromium tricarbonyl complexation " Thus benezenechromium tricarbonyl 95 and even electron-rich para-dimethoxybenzenechromium tricarbonyl (structure not shown) fail to react with ethyl diazoacetate and rhodium(II) trifluoroacetate. In contrast, the same reaction with benzene provides a single isomer of the cycloheptatriene in 98% yield. The Buchner reaction of pseudo-Ci symmetric substrate 97 clearly demonstrates the effect of chromium tricarbonyl complexation on arene cyclopropanation. Decomposition of diazoacetamide 97 with rhodium(II) acetate brings about exclusive addition... 

Perhaps the most significant property inherent in these complexes is the elimination of the a plane of the benzene ring. As a result, ortho- and meta-disubstituted Tj -arene chromium tricarbonyl complexes (with different substituents) become chiral molecules and exist in two enantiomeric forms A and B. This feature, together with the stereofacial selectivity induced by the chromium tricarbonyl group, has led to a rapid development in the use of these species as chiral auxiliaries in synthesis. It should be noted at this point that introduction of an additional stereogenic centre in a side chain leads to diastereoisomers. 

The chromium tricarbonyl complex 2 of (methylsulfenyl)benzene is then oxidised to its sulfinyl derivative 3 using dimethyldioxirane (Scheme 6.6). This cyclic peroxide, formed from potassium peroxymonosulfate and acetone and isolated as a 0.09-0.11 M solution in acetone, has undergone... 

Synthesis of Ar(1S,2ff)-dicarbonyltriphenylphosphine 1-[a(/ )-(/ f,/V-dimethylamino)ethyl]-2-[a (S)-hydroxypropyl] benzene (Structure 17). Photoirradiation of chromium tricarbonyl complex (Structure 15) in the presence of triphenylphosphine (Scheme 6.18)... 

The X-ray structures of benzene chromium tricarbonyl and of hexa-methyl benzene chromium tricarbonyl also suggest that the C—C bond distances are equivalent. In the hexamethylbenzene complex the methyl... 

We can thus conclude that the consideration of the spectra of predominantly covalent complexes as composed of C ligand and skeleton frequencies (weak coupling case) is a meaningful and illustrative approach to the problem. The strong coupling case is more exact, but less informative. It must be remembered of course that the transition from strong to weak coupling can be determined only empirically. Both methods have been applied to the spectra of complexes, the first to ferrocene (17) and dibenzenechromium cation (38), and the second, for example, to benzene-chromium tricarbonyl (44). 

A study of the infrared spectra of a series of substituted-benzene chromium tricarbonyls has been carried out in order to compare them with the spectra of the free parent hydrocarbons (132). The property that in the infrared spectra of tt complexes of aromatic hydrocarbons the vch s show a decrease in intensity in the complex, was confirmed and the shifts of some bands were discussed. 

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],... 

One may also resort here to organotransition metal complexes. For example, benzene rings can be selectively activated to nucleophilic attack by complexation to chromium tricarbonyl (Scheme 12.8) [21]. Similarly, an allylic acetate can also be selectively activated in the presence of a bromide (29 versus 3Q) by addition of a palladium(O) catalyst in THF, which coordinates with the double bond [22] (Scheme 12.9). 

Fluorine atoms in electron-deficient benzene derivatives, such as l-fluoro-2-nitroben-zene,149-150 l-fluoro-4-nitrobenzene.151 153 2-fluorobenzaldchyde,154"156 2-fluoroaceto-phenone,155156 4-fluoroacetophenonc,157 2-fluorobenzophenone.156 2- and 4-fluorobenzoni-trile,158 and 4-fluorophenyl trifluoromethyl sulfone,152,159 are easily substituted by N-nucleophiles. Even fluorine in electron-deficient sandwich complexes, such as tricarbonyl(t/6-fluoro-benzene)chromium (3), is substituted by amines.160161... 

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