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Apr 13Z7018 Libraries STUDIES ON A CATALYTIC CADOGAN CYCLIZATION BY PI"/PV=O REDOX CYCLING By Tyler S. Harrison B.S., Chemistry, 2015 University of Massachusetts, Lowell Submitted to the Department of Chemistry in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN CHEMISTRY MA S -S9SINgVITUTE 0FT gNQ ~Y at the Massachusetts Institute of Technology APR 13Z7018 February 2018 LIBRARIES 2018 Massachusetts Institute of Technology. ARCHIVES All rights reserved. Signature of Author: Department of Chemistry /2) February 9, 2018 Certified by: L. Alexander T. Radosevich As ociate Professor of Chemistry 6I Thesis Supervisor Accepted by: Robert W. Field Chairman, Departmental Committee on Graduate Students Studies on a Catalytic Cadogan Cyclization by P"n/Pv=O Redox Cycling by Tyler S. Harrison Submitted to the Department of Chemistry on February 9, 2018 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Chemistry ABSTRACT Organophosphorus reagents offer potential for developing catalytic protocols by inclusion of a reductant such as hydrosilanes to (re)generate the chemically active phosphine in situ. In our research, we have successfully adapted this concept to the Cadogan reductive cyclization by using a strained 4-membered phosphetane precatalyst, which proved to be more competent than acyclic and 5-membered analogs for P(III)/P(V)=O redox cycling. A variety of substrates were found to successfully undergo catalytic Cadogan indazole cyclization. The mechanism of the cyclization has been expanded. The resting state of phosphorus was determined to be the P" phosphetane, and this phosphetane proved to be 8 times faster than the acyclic n-Bu3P at driving the reductive cy- clization of N-phenyl o-nitrobenzaldimine to 2-phenylindazole. A nitrosoarene, presumed an in- termediate in the overall cyclization, was found to undergo cyclization under reaction conditions. In addition, a new unique oxazaphosphetane was observed as an intermediate during the course of cyclization, which may lead to a more complete understanding of other-phosphorus mediated de- oxygenations, including nitro reduction. Initial studies in nitro reduction have been undertaken, though further work is necessary to fully develop a phosphorus-mediated catalytic protocol. Thesis Supervisor: Alexander T. Radosevich Title: Associate Professor of Chemistry 2 TABLE OF CONTENTS LIST O F FIG U RES ......................................................................................................................... 5 LIST O F TA BLES...........................................................................................................................7 A CKN O W LED G EM EN TS...................................................................................................... 8 Chapter 1. The Advancement of P 1/Pv=O Redox Cycling in Catalysis ................ 9 1.1 .Phosphine O xides in Organic Synthesis ......................................................................... 9 1.2.Reduction of Phosphine O xides..................................................................................... 10 1.3.Approaches to Catalysis in Phosphine-Mediated Transformations ................................ 13 1.4.Sum m ary and Looking A head ....................................................................................... 16 1.5.References..........................................................................................................................16 Chapter 2. Development of a Catalytic Cadogan Reductive Cyclization ............................ 19 2.1.Background ........................................................................................................................ 19 2.2.Developing a Catalytic Protocol to Cadogan Indazolation........................................... 21 2.3.Substrate Scope..................................................................................................................26 2.4.Sum m ary ............................................................................................................................ 28 2.5.References..........................................................................................................................29 Chapter 3. Mechanistic Investigations into Cadogan Cyclization.......................................30 3.1 .Background ........................................................................................................................ 30 3.2.Catalytic Resting State of Phosphorus............................................................................ 31 3.3.Rate Im provem ent of Phosphetanes.............................................................................. 35 3.4.Role of N itroso Interm ediate in Cadogan Cyclization.................................................. 37 3.5.Sum m ary ............................................................................................................................ 42 3.6.References..........................................................................................................................42 Chapter 4. Toward Catalytic Nitro Reduction via PII/Pv=O Redox Cycling ................... 44 4.1.Background ........................................................................................................................ 44 4.2.Initial O ptim ization............................................................................................................47 4.3.Future W ork.......................................................................................................................53 4.4.Sum m ary and O utlook .................................................................................................. 54 4.5.References..........................................................................................................................55 C hapter 5. Experim ental Section........................................................................................... 57 I. G eneral N otes .................................................................................................................. 57 3 II. Preparation of Phosphorus Compounds ..................................................................... 58 III. Optimization of Catalytic Cadogan Reaction Conditions ........................................... 64 A. General Procedures ............................................................................................ 64 B. Catalyst/Silane Screen and Data ........................................................................ 65 C. Polar Solvent Screen and Data...........................................................................73 D. Reaction Time Screen and Data........................................................................ 75 E. Isolation of Cyclotrisiloxane 2.19......................................................................78 IV. Catalytic Reductive Cyclization Reactions ................................................................. 79 A. Preparation of Substrates and Characterization Data.........................................79 B. General Procedure for Catalytic Reductive Cyclization Reactions ................... 86 C. Characterization Data for Reductive Cyclization Products ............................... 87 V. Mechanistic Experiments ........................................................................................... 90 A. In situ NMR Experiments ................................................................................ 90 i. Speciation of Phosphorus in Indazolation..............................................90 ii. Speciation of Phosphorus in Carbazolation ........................................... 92 iii. Low Temperature VT-NMR of Nitrosoarene (3.6) ............................. 94 iv. Low Temperature VT-NMR of 15N-Nitrosoarene (3.6-'sN) ................. 95 B. Pseudo-first Order Kinetics with Excess Phosphine Reagent............................97 VI. Control Experiment .................................................................................................... 99 VII. Initial Optimization of Catalytic Nitro Reduction Conditions ......................................... 100 A. General Procedure A (NMR, no standard) .......................................................... 100 B. General Procedure B (NMR, internal standard) .................................................. 100 C. General Procedure C (GCMS, internal standard) ............................................... 101 D. Optimization with p-Nitrotoluene........................................................................102 E. Optimization with p-Chloronitrobenzene ............................................................ 106 F. Optimization with p-Dinitrobenzene ................................................................... 108 G. Optimization with p-Nitrobenzonitrile ................................................................ 114 V III. S pectral D ata ................................................................................................................. 124 IX . R eferen ce s ............................................................................................................................... 16 2 4 LIST OF FIGURES Figure 1.1. Descriptor of relevant orbitals involved in P=O 7t bonding (no -a G*PR3). .................. 9 Figure 1.2. Examples of reducing agent classes for the conversion R 3P=O - R3P.....................11 Figure 1.3. Mechanistic course of hydrosilane-mediated reduction
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