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ABSTRACT REDUCTION OF KETONES TO ALCOHOLS AND TERTIARY AMINES USING 1-HYDROSILATRANE Sami E. Varjosaari, Ph.D. Department of Chemistry and Biochemistry Northern Illinois University, 2018 Marc J. Adler, Co-Director Thomas M. Gilbert, Co-Director 1-Hydrosilatrane is an easily synthesized, stable, solid silane which can be made into an efficient reducing agent in the presence of a Lewis base, by taking advantage of the properties of hypervalent silicon. Due to the simplicity of use and handling, 1-hydrosilatrane has the potential to be an appealing alternative to other more widely used reducing agents. Aromatic and aliphatic ketones were readily reduced with 1-hydrosilatrane in the presence of potassium tert-butoxide. The discovery of diastereoselectivity in the reduction of menthone, a chiral ketone, led to enantioselective reductions of prochiral ketones using chiral Lewis base activators. Enantiomeric excesses of up to 86% were observed. It was also shown that 1-hydrosilatrane could act as a chemoselective reducing agent in the formation of tertiary amines via direct reductive aminations, in the absence of activator or solvent. Attempts were also made to take advantage of the steric properties of silicon protecting groups in meta-directing electrophilic aromatic substitution of phenols, leading to a one-pot synthesis of O-aryl carbamates. i NORTHERN ILLINOIS UNIVERSITY DEKALB, ILLINOIS MAY 2018 REDUCTION OF KETONES TO ALCOHOLS AND TERTIARY AMINES USING 1-HYDROSILATRANE BY SAMI ENSIO VARJOSAARI ©2018 SAMI ENSIO VARJOSAARI A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY Doctoral Co-Directors: Marc J. Adler and Thomas M. Gilbert DEDICATION Varjosaarille ja Rautosille, mahtavuus on sukuvika. TABLE OF CONTENTS Page LIST OF TABLES ....................................................................................................viii LIST OF FIGURES ...................................................................................................x LIST OF APPENDICES ..............................................................................................xviii Chapter 1. INTRODUCTION ...............................................................................................1 1.1. Overview .............................................................................................1 1.2. History of Silicon ....................................................................................1 1.2.1. Elemental Silicon ...........................................................................1 1.2.2. Organosilicon Compounds ................................................................2 1.3. Properties of Silicon ................................................................................3 1.3.1. Silicon vs Carbon ...........................................................................3 1.3.2. Hypervalency................................................................................6 1.3.3. Applications of Hypervalent Silicon .....................................................9 1.3.3.1. Lewis Acid ..........................................................................10 1.3.3.2. Hydride Transfer Agent ...........................................................12 1.3.3.3. Carbanion Transfer Agent ........................................................14 1.3.4. Stereoelectronic effects ..................................................................14 1.3.4.1. β-Silicon effect .....................................................................15 1.3.4.2. -Silicon effect ....................................................................18 1.4. Silatranes .............................................................................................20 1.4.1. Atranes .......................................................................................20 1.4.2. Silatranes ....................................................................................21 1.4.2.1. 1-Hydrosilatrane ...................................................................24 iv 1.4.2.2. Toxicity of 1-Hydrosilatrane .....................................................28 1.4.2.3. Safety of 1-hydrosilatrane .......................................................31 1.5. Reduction of Ketones to Alcohols ................................................................32 1.5.1. Common Reagents for the Reduction of Ketones .....................................32 1.5.1.1. Lithium Aluminium Hydride ......................................................32 1.5.1.2. Sodium borohydride and boranes ...............................................34 1.5.1.3. Hydrogen ............................................................................36 1.5.2. Organosilicon Hydrides ....................................................................37 1.5.2.1. Transition metal calatyzed hydrosilylation ...................................37 1.5.2.1.1. Metal hydride formation ...................................................37 1.5.2.1.2. Oxidative addition ..........................................................38 1.5.2.1.3. Oxo complexes ...............................................................39 1.5.2.2. Lewis acid activation .............................................................40 1.5.2.2.1. Ketone activation ...........................................................40 1.5.2.2.2. Silane activation .............................................................41 1.5.2.3. Lewis base activation .............................................................42 1.6. Asymmetric Reduction of Ketones to Alcohols ................................................44 1.6.1. Common asymmetric reduction methods ..............................................46 1.6.1.1. Transition metal catalysed hydrogenation ....................................46 1.6.1.2. Chiral Boranes ......................................................................48 1.6.2. Asymmetric reduction of ketones using organosilicon hydrides ...................55 1.6.2.1. Transition metal catalysed hydrosilylation ...................................56 1.6.2.1.1. Metal Hydride formation ...................................................56 1.6.2.1.2. Oxidative addition ..........................................................59 1.6.2.2. Lewis base catalysed asymmetric hydrosilylation ...........................60 1.6.2.2.1. Chiral Silane ..................................................................61 1.6.2.2.2. Chiral Lewis Base ............................................................62 v 1.6.2.3. Lewis acid activated asymmetric hydrosilylations...........................67 1.7. Direct Reductive Aminations Using Organosilicon Compounds ..............................70 1.7.1. Common Reagents for Direct Reductive Amination ..................................71 1.7.1.1. Sodium Borohydride derivatives ................................................71 1.7.1.1.1. Sodium Cyanoborohydride .................................................72 1.7.1.1.2. Sodium Triacetoxyborohydride ...........................................74 1.7.1.2. Hantzsch Esters ....................................................................76 1.7.2. Organosilicon hydrides ....................................................................80 1.8. meta-Directed Electrophilic Aromatic Substitution ..........................................87 1.8.1 Electrophilic Aromatic Substitution ......................................................87 1.8.1.1. Mechanism of electrophilic aromatic substitution ...........................88 1.8.1.2. Substituent effects on electrophilic aromatic substitution ........................................................................89 1.8.2. meta- substitution .........................................................................93 1.8.2.1 Half-sandwich compounds ........................................................93 1.8.2.2 “Traceless” directing groups .....................................................96 1.8.2.3 Directing scaffolds .................................................................98 1.8.2.4 The Gaunt anomaly ................................................................102 1.8.3. O-Aryl Carbamates .........................................................................103 1.8.4. Stereoelectronic Chameleon .............................................................108 2. 1-HYDROSILATRANE TO REDUCE KETONES .................................................................112 2.1. Overview .............................................................................................112 2.2. Introduction ..........................................................................................112 2.3. Development of Methodology .....................................................................114 2.3.1. Optimization ................................................................................114 2.3.2. Scope of reaction ..........................................................................115 vi 2.3.3. Diastereoselectivity ........................................................................118 2.3.4. Mechanistic Considerations ...............................................................119 2.4. Conclusions...........................................................................................120 2.5. Experimental and Supplemental Information ..................................................121 3. ASYMMETRIC REDUCTION
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  • A New Class of Silatranes: Structure and Dynamic NMR Behavior1

    A New Class of Silatranes: Structure and Dynamic NMR Behavior1

    1066 J. Am. Chem. Soc. 2000, 122, 1066-1072 A New Class of Silatranes: Structure and Dynamic NMR Behavior1 A. Chandrasekaran, Roberta O. Day, and Robert R. Holmes* Contribution from the Department of Chemistry, LGRT 701, Box 34510, UniVersity of Massachusetts, Amherst, Massachusetts 01003-4510 ReceiVed July 26, 1999 Abstract: A new class of silatranes N[CH2(Me2C6H2)O]3SiR was prepared by the reaction of organosilanes with tris(2-hydroxy-4,6-dimethylbenzyl) amine (5) in the case of 1, 3, and 4, and by the reaction of Si(OMe)4 with the same amine (5) to give 2. This is the first series of silatranes containing all six-membered rings. Their structures were established by X-ray analysis and correlated with 29Si NMR data. VT 1H NMR spectra reflected fluxional behavior for the chiral molecules 1-3, whereas 4 was a rigid molecule. The activation energies for enantiomeric conversion of the clockwise and anticlockwise orientations of the propeller-like silatranes 1-3 were statistically the same at about 10 kcal/mol. A simultaneous pseudorotation of all three six-membered rings is suggested to cause the racemization process. The Si-N distances for 1-4 show a large dependence on the exocyclic substituent. Over an approximate electronegativity range of 2.3-3.0, the Si-N distance varies from 2.745(4) Å for 1 to 2.025(4) Å for 4. Thus, the influence of substituent effects on nitrogen-silicon donor-acceptor interactions can be readily studied in the absence of constraints imposed by the use of the smaller ring-containing silatranes.