Design, synthesis and applications of tetradentate transition metal complexes towards asymmetric alkylations

Persistent Link:
http://hdl.handle.net/10150/194923
Title:
Design, synthesis and applications of tetradentate transition metal complexes towards asymmetric alkylations
Author:
Tadikonda, Udaya Bhaskar
Issue Date:
2005
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Controlling the absolute stereochemistry of molecules is a major challenge to contemporary chemists. Achieving high enantioselectivity with catalytic amounts of a chirality transfer (or inducing) agent, and the ease of regenerating such catalysts is yet another challenge. Due to the involvement of various transition metal complexes, the relatively young field of enantioselective catalysis has emerged as a powerful tool for organic chemistry. In our efforts towards the synthesis of a universal catalyst, O'Donnell Schiff base derived tetradentate ligands were shown to catalyze dialkylzinc additions to aldehydes in high selectivity. The three pot synthesis of bifurcated dipeptides in very good yields and the mechanistic aspects of diethylzinc additions to aromatic aldehydes are described in this dissertation. The chiral Lewis acidic behavior of these ligands was supported by a mechanistic study done examining the nonlinear effect. Unlike bidentate ligands such as (-)-3-exo-N,N-dimethylaminoisoborneol (DAIB), the tetradentate ligands in this study show strictly linear behavior. Also, the linear free energy relationships studied by observing the enantioselectivity with respect to electron donating or withdrawing substituents on the benzaldehyde substrates supported a Lewis acid role for the zinc complexes. A negative slope was obtained when ee's were plotted against sigma values of the substituted benzaldehydes. Since they bind to various bivalent transition metal cations, these ligands can be viewed as privileged structures, and may potentially become catalysts for various asymmetric reactions. As catalyst screening can be greatly facilitated by heterogeneous catalysis, solid phase ligands were synthesized using Wang and Merrifield resin supports. The synthetic methodology was developed using a diarylketimine linker with the aid of on-bead characterization techniques such as 13C NMR and UV-VIS spectroscopy. The ligands were shown to asymmetrically catalyze the alkenylzinc additions to aromatic aldehydes. In situ generation of alkenylzinc reagents by boron to zinc transmetalation followed by the addition to benzaldehyde in the presence chiral zinc complexes resulted in enantiomerically enriched allylic alcohols. The preliminary results for this transformation resulted in 3:1 selectivity in favor of S-isomer.
Type:
text; Electronic Dissertation
Keywords:
tetradentate ligands; transition metal catalysis; asymmetric alkylations; alkenylations; nonlinear effect
Degree Name:
PhD
Degree Level:
doctoral
Degree Program:
Chemistry; Graduate College
Degree Grantor:
University of Arizona
Advisor:
Polt, Robin L.
Committee Chair:
Polt, Robin L.

Full metadata record

DC FieldValue Language
dc.language.isoENen_US
dc.titleDesign, synthesis and applications of tetradentate transition metal complexes towards asymmetric alkylationsen_US
dc.creatorTadikonda, Udaya Bhaskaren_US
dc.contributor.authorTadikonda, Udaya Bhaskaren_US
dc.date.issued2005en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractControlling the absolute stereochemistry of molecules is a major challenge to contemporary chemists. Achieving high enantioselectivity with catalytic amounts of a chirality transfer (or inducing) agent, and the ease of regenerating such catalysts is yet another challenge. Due to the involvement of various transition metal complexes, the relatively young field of enantioselective catalysis has emerged as a powerful tool for organic chemistry. In our efforts towards the synthesis of a universal catalyst, O'Donnell Schiff base derived tetradentate ligands were shown to catalyze dialkylzinc additions to aldehydes in high selectivity. The three pot synthesis of bifurcated dipeptides in very good yields and the mechanistic aspects of diethylzinc additions to aromatic aldehydes are described in this dissertation. The chiral Lewis acidic behavior of these ligands was supported by a mechanistic study done examining the nonlinear effect. Unlike bidentate ligands such as (-)-3-exo-N,N-dimethylaminoisoborneol (DAIB), the tetradentate ligands in this study show strictly linear behavior. Also, the linear free energy relationships studied by observing the enantioselectivity with respect to electron donating or withdrawing substituents on the benzaldehyde substrates supported a Lewis acid role for the zinc complexes. A negative slope was obtained when ee's were plotted against sigma values of the substituted benzaldehydes. Since they bind to various bivalent transition metal cations, these ligands can be viewed as privileged structures, and may potentially become catalysts for various asymmetric reactions. As catalyst screening can be greatly facilitated by heterogeneous catalysis, solid phase ligands were synthesized using Wang and Merrifield resin supports. The synthetic methodology was developed using a diarylketimine linker with the aid of on-bead characterization techniques such as 13C NMR and UV-VIS spectroscopy. The ligands were shown to asymmetrically catalyze the alkenylzinc additions to aromatic aldehydes. In situ generation of alkenylzinc reagents by boron to zinc transmetalation followed by the addition to benzaldehyde in the presence chiral zinc complexes resulted in enantiomerically enriched allylic alcohols. The preliminary results for this transformation resulted in 3:1 selectivity in favor of S-isomer.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjecttetradentate ligandsen_US
dc.subjecttransition metal catalysisen_US
dc.subjectasymmetric alkylationsen_US
dc.subjectalkenylationsen_US
dc.subjectnonlinear effecten_US
thesis.degree.namePhDen_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineChemistryen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorPolt, Robin L.en_US
dc.contributor.chairPolt, Robin L.en_US
dc.contributor.committeememberMash, Eugene A.en_US
dc.contributor.committeememberBates, Boben_US
dc.contributor.committeememberEnemark, John H.en_US
dc.contributor.committeememberZheng, Zhipingen_US
dc.identifier.proquest1131en_US
dc.identifier.oclc137354124en_US
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