The Organic Chemistry of the Transition Elements. Part I. Tricarbonylchromium Derivatives of Aromatic Compounds. by B. Nichollasn D M. C. Whiting.

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[19593 Organic Chemistry of the Tramition Elements. Part

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113. The Organic Chemistry of the Transition Elements. Part I. Tricarbonylchromium Derivatives of Aromatic Compounds.
By B. NICHOLLS M. C. WHITING. and
Many aromatic compounds, ArH, displace carbon monoxide from chromium hexacarbonyl with the formation of complexes Cr(CO),(ArH) (I). These are stable and may carry any of several functional groups. The effects of the metallic residue upon typical properties of these groups and of the aromatic system as a whole are outlined, and methods for the regeneration of the aromatic components are described.

TRICARBONYLBENZENECHROMIUM (I) was first obtained by Fischer and Ofele in 27% yield from chromium hexacarbonyl and dibenzenechromium in benzene in a sealed system at 220". We had independently discovered a simpler and more general method for preparing compounds of this type which involves heating chromium hexacarbonyl under reflux in an excess of the aromatic compound or with a molar quantity in an inert solvent.

Shortly after our preliminary communication,2 Natta and his co-workers also described the direct preparation of several of these compounds, but used a pressurised system (with intermittent release of carbon monoxide) and higher temperatures (200-235"). The work of Fischer and his school shows that equilibria are involved in these reactions-an excess of carbon monoxide converts the dibenzenechromium complex into the hexacarbonyl-and therefore it is advantageous in principle, as well as much easier in practice, to employ an open system, the free escape of carbon monoxide then driving the reaction to completion. In early experiments decalin was employed as solvent, but because the commercial product contains tetralin as an impurity, and perhaps because of hydrogen-transfer processes, products containing the tricarbonylchromium complex of tetralin were often obtained. Since the final product has a considerable dipole moment and is formed from more or less non-polar components, the reaction should be facilitated by the use of a polar solvent. Of the many tried, diethylene glycol dimethyl ether was the most suitable. Its use made possible the direct preparation of the parent member of the series, tricarbonylbenzenechromium, and also greatly improved the yields of other complexes. A list of complexes, (ArH)Cr(CO),, is given in Table 1; all are stable, yellow or orange, and crystalline; all gave correct analyses for carbon and hydrogen, and in one case for oxygen. The rate of formation of these complexes is of obvious interest. The conversion of an aromatic compound into such a complex results ultimately in approximately equal bonding to all six aromatic carbon atoms, and although it can, and should, be regarded as an 1 3

Fischer and Ofele, Angew. Chem., 1957, 69, 715; Chem. Bey., 1957, 90, 2532. Nicholls and Whiting, PYOC. Chem. Soc., 1958, 152. Natta, Ercoli, and Calderazzo, Chirn. e Ind., 1958, 40, 287.

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electrophilic attack on the aromatic system it cannot be grouped either with typical attacks on single carbon atoms giving a-complexes as intermediates (nitration, etc.), or with attacks on individual double bonds giving initially one-bond x-complexes (formation of silver complexes, ozonolysis, dihydro-diol formation). The qualitative experiments already made show that electronic effects roughly parallel those observed in nitration studies, the reaction being facilitated by electron-repelling substituents, e.g., NMe, and retarded by electron-attracting groups, e.g., C1 and C0,Me. A second important factor is the steric effect of...
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