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Reactions of (meson(6):meson(6) - conjugated diarene) bis(tricarbonylchromium) dianions to form synthetically interesting products
Abstract
Electrochemical work demonstrated that several ($\eta\sp6$:$\eta\sp6$-conjugated diarene)(Cr(CO)$\sb3\rbrack\sb2$ complexes undergo reduction to form stable dianions. The unique stability of the dianions generated was postulated to be due to a haptatropic rearrangement at each of the rings from the normal $\eta\sp6$ bonding to $\eta\sp5$, while retaining good overlap between the conjugated phenyl rings. The importance of conjugation was demonstrated by electrochemical experiments on ($\eta\sp6$:$\eta\sp6$-bimesityl)(Cr(CO)$\sb3\rbrack\sb2$. Samples of the dilithium salt of the ($\eta\sp5$:$\eta\sp5$-biphenyl)(Cr(CO)$\sb3\rbrack\sb2$ dianion were isolated and stored in a drybox. Spectral data obtained for the dianion are consistent with the postulated ($\eta\sp5$:$\eta\sp5$-cyclohexadienylidene)(Cr(CO)$\sb3\rbrack\sb2$ structure. The dianion generated from ($\eta\sp6$:$\eta\sp6$-biphenyl)(Cr(CO)$\sb3\rbrack\sb2$ reacted cleanly with electrophiles such as H$\sb2$O, D$\sb2$O, methyl and primary alkyl bromides, iodides, tosylates and sulfates in a highly stereoselective and regioselective manner. Spectroscopic data indicated the formation of a 6-substituted-6-(($\eta\sp6$-phenyl)Cr(CO)$\sb3$) ((1-5-$\eta\sp5$-cyclohexadienyl)Cr(CO)$\sb3$) anion. Reaction with more reactive or sterically hindered halides resulted in the competitive formation of 4-substituted products. The 6-alkyl-6-(($\eta\sp6$-phenyl)Cr(CO)$\sb3$) ((1-5-$\eta\sp5$-cyclohexadienyl)Cr(CO)$\sb3$) anions were reacted directly with iodine to give 2-alkylbiphenyls, or were reacted with trifluoroacetic acid, followed by oxidation to produce 5-alkyl-5-phenyl-1,3-cyclohexadienes in high yields. A benzospiran structure was obtained in one pot by reacting the dianion generated from ($\eta\sp6$:$\eta\sp6$-biphenyl)(Cr(CO)$\sb3\rbrack\sb2$ with 4-bromobutyronitrile, followed by appropriate workup. The 6-methyl-6-(($\eta\sp6$-phenyl)Cr(CO)$\sb3$) ((1-5-$\eta\sp5$-cyclohexadienyl)Cr(CO)$\sb3$) anion was reacted with methyl triflate to form 2-acyl-3-methyl-3-phenyl-1,4-cyclohexadiene. The dianion formed from ($\eta\sp6$:$\eta\sp6$-4,4$\sp\prime$-dimethoxybiphenyl)(Cr(CO)$\sb3\rbrack\sb2$ was also alkylated at the phenyl-substituted ring carbon for electrophiles such as methyl triflate, ethyl triflate or allyl tosylate. Apporpriate workup using trifluoroacetic acid followed by iodine concluded a short and direct route to the synthetically important 4-alkyl-4-(4-methoxyphenyl)cyclohex-2-en-1-ones in one pot. Isolation of 4-allyl-4-(4-methoxyphenyl)cyclohex-2-en-1-one completed a formal synthesis of the Sceletium alkaloid, O-methyljoubertiamine. ($\eta\sp6$:$\eta\sp6$-trans-Stilbene) (Cr(CO)$\sb3\rbrack\sb2$ and ($\eta\sp6$:$\eta\sp6$-cis-stilbene) (Cr(CO)$\sb3\rbrack\sb2$ were reduced to their dianions and reacted with iodomethane, followed by iodine to form 1:1 mixtures of the racemic and meso diastereomers of 2,3-diphenylbutane. The ratio indicated stereoselectivity at the benzylic anion sites imposed by the Cr(CO)$\sb3$ groups.
Subject Area
Organic chemistry|Chemistry
Recommended Citation
Schulte, Louis Dwayne, "Reactions of (meson(6):meson(6) - conjugated diarene) bis(tricarbonylchromium) dianions to form synthetically interesting products" (1988). ETD collection for University of Nebraska-Lincoln. AAI8904510.
https://digitalcommons.unl.edu/dissertations/AAI8904510