Synthesis and a Mechanistic Study of Nucleophilic Substitution in Halo-arene Chromiumtricarbonyl Complexes

Abstract

<p> Arenechromiumtricarbonyl complexes have been known for many years and numerous studies of the chemistry of these molecules have been reported. The presence of the chromiumtricarbonyl unit changes the reactivity of the complexed arena, in most cases due to its strong inductive electron withdrawing effect. The resulting lowered electron density in the aromatic ring allows reactions to be carried out which would otherwise be extremely difficult if not impossible, notably nucleophilic substitution. When methoxide ion was reacted with optically pure methyl o-fluorobenzoatechromiumtricarconyl, racemization of approximately 50% was observed in the product. A brief study confirmed SN2 kinetics, but further investigation was warranted. Having eliminated the possibility of decomplexationrecornplexation, two other mechanisms were postulated -a partial decomplexation allowing the arene to roll over, or attack by methoxide at another ring position. In an effort to determine which mechanism was operating, the synthesis of a number of appropriately substituted halo-arenechromiumtricarbonyl complexes was attempted. The successful synthesis of methyl 2-chloro-5-methylbenzoatechromiumtricarbonyl wns achieved, albeit with some difficulty in in low yield, but the corresponding 2-chloro-3-methyl complex could not be made. In the case of the corresponding 2-fluoro compounds, the arenes themselves could not be made by a variety of synthetic routes, 80 synthesis of the complexes could not be attempted. The results of the reaction of methoxide ion with methyl 2-chloro-5-methylbenzoatechromiumtricarbonyl indicate that the reaction proceeds via the roll-over mechanism, although a definite conclusion cant be drawn at this time. A potentially useful synthesis of bis-(chromiumtricarbonyl)-benzo-phenones has also been developed. </p>