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Combined Theoretical and Experimental Investigation of N-Heterocyclic Carbenes As Lewis Base Catalysts and As Ancillary Ligands in Ru-Catalyzed Olefin Metathesis

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Combined Theoretical and Experimental Investigation of N-Heterocyclic Carbenes As Lewis Base Catalysts and As Ancillary Ligands in Ru-Catalyzed Olefin Metathesis Combined Theoretical and Experimental Investigation of N-Heterocyclic Carbenes as Lewis Base Catalysts and as Ancillary Ligands in Ru-Catalyzed Olefin Metathesis. Mechanistic Investigation of Fluxional Behavior of Ru-Based Olefin Metathesis Catalysts Author: Adil R. Zhugralin Persistent link: http://hdl.handle.net/2345/2973 This work is posted on eScholarship@BC, Boston College University Libraries. Boston College Electronic Thesis or Dissertation, 2011 Copyright is held by the author, with all rights reserved, unless otherwise noted. Boston College The Graduate School of Arts and Sciences Department of Chemistry COMBINED THEORETICAL AND EXPERIMENTAL INVESTIGATION OF N- HETEROCYCLIC CARBENES AS LEWIS BASE CATALYSTS AND AS ANCILLARY LIGANDS IN Ru-CATALYZED OLEFIN METATHESIS. MECHANISTIC INVESTIGATION OF FLUXIONAL BEHAVIOR OF Ru-BASED OLEFIN METATHESIS CATALYSTS a dissertation by ADIL R. ZHUGRALIN submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy May, 2011 © Copyright by ADIL R. ZHUGRALIN 2011 COMBINED THEORETICAL AND EXPERIMENTAL INVESTIGATION OF N-HETEROCYCLIC CARBENES AS LEWIS BASE CATALYSTS AND AS ANCILLARY LIGANDS IN Ru-CATALYZED OLEFIN METATHESIS. MECHANISTIC INVESTIGATION OF FLUXIONAL BEHAVIOR OF Ru- BASED OLEFIN METATHESIS CATALYSTS Adil R Zhugralin Thesis Advisor: Professor Amir H. Hoveyda Abstract Chapter 1. Through the use of quantum theory of atoms in molecules (QTAIM) the similarities and differences between transition metal complexes ligated by phosphines and N-heterocyclic carbenes (NHC) were elucidated. Among the key findings, the phosphines were identified as stronger charge donors than NHCs; however, the latter class of ligands exhibits a weaker π-accepting character than the former. Furthermore, Tolman electronic parameter (TEP) was determined to be an inadequate gauge for the total electron donating ability of phosphines and NHCs; rather TEP can serve as a measurement of population of dπ set of orbitals of a metal center in question. Computational and experimental studies of the mechanism of NHC-catalyzed boron and silicon addition to α,β-unsaturated carbonyls reactions were carried out. Through the use of radical traps the mechanisms involving homolytic cleavage of B-B or B-Si bonds were ruled out. Computational (DFT) studies of the mechanism identified two pathways: (1) direct activation of diboron or borosilyl reagents through coordination of NHC to the B atom, (2) net oxidative addition of the diboron or borosilyl reagents to the carbon (II) of the NHC. The insights gained from the aforementioned studies were employed to vii rationalize the observed lack of reactivity of NHC-activated diboron complexes in the presence of aldehydes. Chapter 2. New C1-symmetric chiral monodentate N-heterocyclic carbenes were prepared, and corresponding chiral Ru-carbene complexes were synthesized. These complexes were employed to gain empirical understanding of factors that govern stereoselectivity in Ru-catalyzed enantioselective olefin ring-closing metathesis. The data thus obtained was employed to infer that syn-to-NHC reaction pathways are competitive and non-selective. One plausible mechanism, through which syn-to-NHC pathways can be accessed, involves Berry pseudorotations. Through the use of stereogenic-at-Ru complexes diastereomeric Ru-carbenes were isolated (silica gel chromatography) and spectroscopically characterized in solution phase. The diastereomeric Ru-carbenes were found to undergo non-metathesis stereomutations at Ru center, thereby providing additional support for the above hypothesis regarding accessibility of syn-to-NHC olefin metathesis pathways. Non-metathesis stereomutation at Ru was found to be accelerated in the presence of protic additives, suggesting the plausibility of hydrogen bonding between the acidic proton and the X-type ligands on Ru. Occurrence of hydrogen bonding was corroborated through the use of chiral allylic alcohols in Ru-catalyzed diastereoselective ring-opening/cross metathesis, which was developed into a versatile method for highly diastereoselective functionalization of terminal olefins. viii Table of Contents Chapter 1. N-Heterocyclic Carbenes as Ancillary Ligands for Transition Metals and Main Group Elements 1.1. Introduction .............................................................................................................. 1 1.1.1. Electronic Structure Theory: Description of Bonding in Molecules ................ 2 1.1.1.1. Quantum Theory of Atoms in Molecules (QTAIM)……………………. 2 1.1.1.2. Energy Decomposition Analysis and Charge Decomposition Analysis ... 7 1.1.1.3. Natural Bond Orbitals (NBO) ................................................................... 9 1.1.1.4. Conclusion ............................................................................................... 10 1.1.2. N-Heterocyclic Carbenes: Properties of Free Carbenes, their Transition-Metal and Main Group Element Complexes ........................................................................... 11 1.1.2.1. Electronic States of Carbenes .................................................................. 12 1.1.2.2. Dimerization of Carbenes as an Indication of their Electronic and Steric Properties………… ................................................................................................... 13 1.1.2.3. A Steric Metric for N-Heterocyclic Carbenes ......................................... 15 1.1.2.4. Electronic Structure of Carbenes ............................................................. 18 1.1.2.5. Carbenes as -Acceptors and Donors ..................................................... 25 1.2. Electronic Effects in the Bonding of N-Heterocyclic Carbenes and Phosphines with Transition Metals: A Comparative Study ................................................................. 29 1.2.1. Analysis of the Geometries and Vibrational Frequencies of the Transition Metal Complexes ........................................................................................................... 32 1.2.2. QTAIM Analysis of the Free Ligands ............................................................ 33 i 1.2.3. QTAIM Analysis of the Transition Metal Complexes of N-Heterocyclic Carbenes and Trimethylphosphine ................................................................................ 37 1.2.3.1. Multipoles and Electrostatic Interaction Energies .................................. 37 1.2.3.2. Atomic Dipole and Quadrupole Moments of the Metal Atoms .............. 42 1.2.3.3. Delocalization and Localization Indices ................................................. 49 1.2.3.4. Tolman Electronic Parameter and the Electronic Structure of N- Heterocyclic Carbene Ligands and Phosphines......................................................... 54 1.3. Mechanistic Investigation of N-Heterocyclic Carbene-catalyzed Conjugate Addition of Boron and Silicon .......................................................................................... 59 1.3.1. Introduction: free N-Heterocyclic Carbenes as Catalysts and Activators ...... 59 1.3.2. N-Heterocyclic Carbene-catalyzed 1,4-Addition of Bis(pinacolato)diboron and Dimethylphenylsilyl-pinacolatoborane to ,β-Unsaturated Carbonyls .................. 61 1.3.2.1. Reaction Design: Direct Activation of Diboron Compounds with NHC 61 1.3.2.2. Experimental Confirmation of the Hypotherical Direct Activation of Diboron Compounds with N-Heterocyclic Carbenes ................................................ 65 1.3.2.3. Experimental and Theoretical Investigation of the Mechanism of NHC- catalyzed Diboration of ,β-Unsaturated Carbonyls ................................................. 69 1.3.2.4. Experimental and Theoretical Investigation of the NHC-catalyzed Silaboration of ,β-Unsaturated Carbonyls ............................................................... 80 1.3.2.5. Conclusion ............................................................................................... 87 1.4. Experimental .......................................................................................................... 89 1.4.1. Theoretical Investigation of Transition Metal Complexes of N-Heterocyclic Carbenes and Phosphines .............................................................................................. 89 1.4.2. Mechanistic Investigation of NHC-catalyzed Conjugate Addition of Boron and Silicon ................................................................................................................... 106 ii 1.4.2.1. Experimental ......................................................................................... 106 1.4.2.2. X-Ray Crystal Structure of 1.59 ............................................................ 113 1.4.2.3. Computational Methods and Results ..................................................... 127 1.4.3. NMR Spectra ................................................................................................ 129 Chapter 2. Development of New Catalysts and Methods for Stereoselective Ru-Catalyzed Olefin Metathesis. Mechanistic Investigation of Action of the Stereogenic-at-Ru Olefin Metathesis Catalysts through Isolation of Key Reactive Intermediates 2.1. Introduction .......................................................................................................... 143 2.1.1. Development of Well-defined Olefin Metathesis Catalysts: Mechanistic Aspects…………………………………….…….………………………...…………145 2.1.2. Synthesis of Metal Carbenes/Alkylidenes ...................................................
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