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|Title:||Studies on directed lithiation: 1. Reactivity studies on alkoxide-directed lithiation reactions. 2. NMR spectroscopic characterization of the solution structure of an asymmetric lithiation reagent|
|Author(s):||Gallagher, Donald Joseph|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||A multifaceted approach to the study of the structure and reactions of organolithium aggregates is described. The lithiation of the cyclopropyl rings of a rigid bis-cyclopropylcarbinol has been shown to occur via two sequential lithiation steps. The first lithiation occurs very rapidly compared to the lithiation of other cyclopropane rings and appears to be the result of a favorable geometric orientation of the alkoxide directing group with respect to the proton being removed. The second lithiation is the only example of a bis-cyclopropyl compound undergoing dilithiation and appears to be the result of a complex-induced proximity effect.
Attempts at evaluating the transition structure of alkoxide-directed lithiation were unsuccessful due to translithiation occurring during competitive lithiation reactions. Methodology and control experiments are proposed for future studies.
The competitiveness of halogen-lithium exchange versus removal of an acidic hydrogen was studied by reinvestigating a literature case in which it was alleged that halogen-lithium exchange occurs prior to removal of an alcoholic hydrogen. It was found that the previously reported results that support this conclusion are inaccurate because of the use of NMR to determine deuterium incorporation and the use of D$\sb2$O as a low temperature trap. The corrected results suggest that halogen-lithium exchange does not occur prior to removal of an acidic hydrogen on a molecular scale.
An NMR study has shown that the aggregation state of i-Pr$\sp6$Li is very dependent on the solvent and added complexation agents. In hydrocarbon solvents i-Pr$\sp6$Li exists as a mixture of tetramer and hexamer, while in diethyl ether there exists a mixture of tetramer and dimer. In THF only a dimeric species is observed. Addition of TMEDA and PMDTA to ethereal solutions of i-Pr$\sp6$Li causes formation of complexed dimers and monomers respectively.
The structure of a 1:0.5:1 complex of i-Pr$\sp6$Li/sparteine/Et$\sb2$O was shown to be a non-symmetrically complexed dimeric species in which one lithium atom is $\eta\sb2$ complexed by sparteine and the other lithium is complexed by two ether molecules. This novel structure appears to be the result of the steric bulk of sparteine.
|Rights Information:||Copyright 1992 Gallagher, Donald Joseph|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9215812|