Controlled reactions of arene-transition metal complexes with nucleophiles
Double selectivity was achieved during nucleophilic substitution of m-dichlorobenzene-FeCp cation with certain nucleophiles. While aminophenoxides react on oxygen to afford monosubstitution, aliphatic amino alcohols such as prolinol react exclusively on nitrogen to give monosubstitution. The latter was also observed with o-dichlorobenzene-FeCp cation (2.6), where reaction with a series of 2-substituted pyrrolidines in the presence of K2CO3 led to exclusively mono-N-substitution. Furthermore, a moderate diastereoselectivity (about 1.5:1) was observed during these reactions. Further treatment of the latter isomeric mixture with NaH allowed an intramolecular nucleophilic displacement of the second chloride with alkoxide, thus forming a tricyclic system. A number of attempts to effect similar reactions of 2.6 with two chiral C2-symmetric amines which we anticipated to improve the diastereoselectivity were unsuccessful. Addition of nucleophiles to arene-manganese complexes 3.45, 3.46 and 3.48 proceeds with diastereoselectivity of up to 92% d.e.. The best result was obtained with complex 3.46 that has a C2-symmetric amino substituent. The major isomer from reaction of 3.46 with PhMgBr was shown to be the sterically more favored product a by X-ray crystallography. An axial orientation of the two methyl substituents on complex 3.46 was confirmed by X-ray crystallography, which allows a rationalization for the observed 1,5-stereocontrol.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI. In contrast to the results of LiAlH4 addition, reaction of NaBH4 with complex 3.46 lead to diastereoselectivity favoring isomer b which was presumed to be the disfavored isomer under steric approach control by X-ray crystal structural analysis. This selectivity reversal was also observed with MeLi addition, where isomer b is favored. Careful deuterium labeling experiments prove that hydride adds exclusively from the exo face regardless of the hydride source, thus confirming that the reversed selectivity was not caused by a switch of mechanism when a different hydride was employed. Electronic effects were also studied, and it was found that both charge and orbital control work in the same direction as steric approach control, leading to selectivity favoring isomer a. The reason for selectivity reversal was not fully established, however all the information we have strongly suggests a transition state effect. The position of transition state along the reaction coordinate depends on the reactivity of the nucleophile. An early transition state is most probable for reactive nucleophiles (such as LiAlH4) and steric approach control would dominate leading to the normal selectivity (favoring a); in the case of less reactive nucleophiles (such as NaBH4), a late transition state leads to selectivity favoring b, the more stable isomer according to MMX calculations using PC Model. Kinetic isotope effects of hydride addition reactions support the transition state effect on the diastereoselectivity. Modeling of the transition state should reveal more accurate information on the transition state effect and is worth further investigation. Attempts at conversion of aminocyclohexadienyl-Mn(CO)3 complexes to cyclohexenones were unsuccessfu l because aminocyclohexadiene-Mn(CO)2NO complexes are extremely unstable species and undergo rapid rearomatization under oxidative decomplexation condition.
School:Case Western Reserve University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:controlled reactions arene transition metal complexes nucleophiles
Date of Publication:01/01/1995