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The generation of chiral borenium cations : = Evaluation of the α-metallations of Lewis acid -complexed heterocycles.

Record Type:	Electronic resources : Monograph/item
Title/Author:	The generation of chiral borenium cations :/
Reminder of title:	Evaluation of the α-metallations of Lewis acid -complexed heterocycles.
Author:	Nguyen, Truc Minh.
Description:	1 online resource (277 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 62-07, Section: B.
Contained By:	Dissertations Abstracts International62-07B.
Subject:	Organic chemistry. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=9983811click for full text (PQDT)
ISBN:	9780599901681

The generation of chiral borenium cations : = Evaluation of the α-metallations of Lewis acid -complexed heterocycles.
Nguyen, Truc Minh.

The generation of chiral borenium cations :Evaluation of the α-metallations of Lewis acid -complexed heterocycles. - 1 online resource (277 pages)

Source: Dissertations Abstracts International, Volume: 62-07, Section: B.

Thesis (Ph.D.)--The University of Wisconsin - Madison, 2000.

Includes bibliographical references

This thesis records efforts to generate borenium cations, which are cationic trivalent boron complexes. Chapter One describes the synthesis of achiral borenium cations, probes their ionic dynamic behavior, and investigates their complexation with Lewis bases. The techniques of 11B and 1H nuclear magnetic resonance (NMR) spectroscopy have helped to define the coordination number at boron and provided tools that can be used to monitor Lewis acid-Lewis base interactions. The complexes, however, proved to be labile and were therefore studied only in solution. More decisive structural evidence was obtained with chiral analogs, which are described in Chapter Two. High facial selectivity in the complexation of nucleophiles with chiral oxazaborolidinium cations has been achieved. The diastereomer ratios were examined by variation of temperature, solvent, electron demand of Lewis base, and electron density at boron. The relative and absolute stereochemical configurations at boron of oxazaborolidinium pyridine complexes have been determined by 1H NMR correlations, NOESY spectroscopy, and X-ray analyses. An ionic steric control hypothesis is proposed to explain the high thermodynamically-controlled diastereomer ratios. Chapter Two also describes the α-metallation of Lewis acid - Lewis base complexes derived from chiral oxazaborolidinium cations. The metallation of boron N - trityl imidazole complexes has been explored. The enantiomeric excesses (ee's) in these systems have been found to be low, and there is preliminary evidence of undesired decomposition pathways. The metallation of boron DMAP complexes is also described. There is dramatic improvement in ee in this system (91% so far), but all efforts to improve the yield to synthetically useful levels have not yet been successful. The best yield thus far is ca. 50%. The low yield is finally determined to be due to a competition between metallation and boron-oxygen bond cleavage in the boron complex. Experiments that lead to this conclusion are discussed in detail.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2023

Mode of access: World Wide Web

ISBN: 9780599901681Subjects--Topical Terms:

523952
Organic chemistry.
Subjects--Index Terms:

Borenium cationsIndex Terms--Genre/Form:

542853
Electronic books.

The generation of chiral borenium cations : = Evaluation of the α-metallations of Lewis acid -complexed heterocycles.
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520 $a This thesis records efforts to generate borenium cations, which are cationic trivalent boron complexes. Chapter One describes the synthesis of achiral borenium cations, probes their ionic dynamic behavior, and investigates their complexation with Lewis bases. The techniques of 11B and 1H nuclear magnetic resonance (NMR) spectroscopy have helped to define the coordination number at boron and provided tools that can be used to monitor Lewis acid-Lewis base interactions. The complexes, however, proved to be labile and were therefore studied only in solution. More decisive structural evidence was obtained with chiral analogs, which are described in Chapter Two. High facial selectivity in the complexation of nucleophiles with chiral oxazaborolidinium cations has been achieved. The diastereomer ratios were examined by variation of temperature, solvent, electron demand of Lewis base, and electron density at boron. The relative and absolute stereochemical configurations at boron of oxazaborolidinium pyridine complexes have been determined by 1H NMR correlations, NOESY spectroscopy, and X-ray analyses. An ionic steric control hypothesis is proposed to explain the high thermodynamically-controlled diastereomer ratios. Chapter Two also describes the α-metallation of Lewis acid - Lewis base complexes derived from chiral oxazaborolidinium cations. The metallation of boron N - trityl imidazole complexes has been explored. The enantiomeric excesses (ee's) in these systems have been found to be low, and there is preliminary evidence of undesired decomposition pathways. The metallation of boron DMAP complexes is also described. There is dramatic improvement in ee in this system (91% so far), but all efforts to improve the yield to synthetically useful levels have not yet been successful. The best yield thus far is ca. 50%. The low yield is finally determined to be due to a competition between metallation and boron-oxygen bond cleavage in the boron complex. Experiments that lead to this conclusion are discussed in detail.
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