Using EPR To Compare PEG-branch-nitroxide “Bivalent-Brush Polymers” and Traditional PEG Bottle–Brush Polymers: Branching Makes a Difference

Attachment of poly(ethylene glycol) (PEG) to polymeric nanostructures is a general strategy for sterically shielding and imparting water solubility to hydrophobic payloads. In this report, we describe direct graft-through polymerization of branched, multifunctional macromonomers that possess a PEG domain and a hydrophobic nitroxide domain. Electron paramagnetic resonance (EPR) spectroscopy was used to characterize microenvironments within these novel nanostructures. Comparisons were made to nitroxide-labeled, traditional bottle-brush random and block copolymers. Our results demonstrate that bivalent bottle-brush polymers have greater microstructural homogeneity compared to random copolymers of similar composition. Furthermore, we found that compared to a traditional brush polymer, the branched-brush, “pseudo-alternating” microstructure provided more rotational freedom to core-bound nitroxides, and greater steric shielding from external reagents. The results will impact further development of multivalent bottle-brush materials as nanoscaffolds for biological applications

Surface chemistry of N-Heterocyclic carbenes and the self-assembly, structure, and properties of polymer metal-organic cage gels

Thesis: Ph. D. in Organic Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2016.Cataloged from PDF version of thesis. Vita.Includes bibliographical references.Chapter 1. Introduction to Carbene Ligands in Surface Chemistry: From Stabilization of Discrete Elemental Allotropes to Modification of Nanoscale and Bulk Substrates In this chapter, we review the development of carbene surface chemistry from its inception through 2015, covering elemental allotrope substrates, nanomaterials, and bulk surfaces, as well as persistent and non-persistent carbenes. We synthesize from the reviewed reports a mechanistic understanding of this chemistry and outline the road ahead in this field. Chapter 2. Addressable Carbene Anchors for Gold Surfaces New strategies to access functional monolayers could augment current surface modification methods. Here we present addressable N-heterocyclic carbene (ANHC) anchors for gold surfaces and provide experimental and theoretical characterization of ANHC monolayers. Additionally, we demonstrate grafting of highly fluorinated polymers from surface-bound ANHCs. Chapter 3. Reactions of Persistent Carbenes with Hydrogen-Terminated Silicon Surfaces We report here the use of persistent aminocarbenes to functionalize via Si-H insertion reactions a range of hydrogen-terminated silicon surfaces: from model compounds, to nanoparticles, and planar Si(l 11) wafers. In particular, a cyclic(alkyl)(amino)carbene and an acyclic diaminocarbene underwent Si-H insertion, forming persistent C-Si linkages and thereby installing amine or aminal functionality in proximity to the surface. Our results pave the way for the further development of persistent carbenes as universal ligands for silicon and potentially other non-metallic substrates. Chapter 4. Cycloelimination of Imidazolidin-2-Ylidene N-Heterocyclic Carbenes: Mechanism and Insights into the Synthesis of Stable "NHC-CDI" Amidinates We report the discovery that 1,3-bis(aryl)imidazolidin-2-ylidenes, one of the most widely studied classes of N-heterocyclic carbenes (NHCs), undergo quantitative conversion to zwitterionic "NHC-CDI" amidinates upon heating to 100 °C in solution. The mechanism of this novel NHC decomposition process was studied in detail and enabled the rational synthesis of a new class of bench stable amidinates. Chapter 5. Toward Dynamic and Hierarchically Structured Polymer Gels: An Introduction to Polymer Metal-Organic Cage Gels Key challenges in polymer network/gel chemistry are overviewed. Polymer metal-organic cage gels capable of addressing some of these key challenges are introduced. Chapter 6. Highly Branched and Loop-Rich Gels Via Formation of Metal-Organic Cages Linked by Polymers We report here a new class of gels (called 'polyMOC' gels) assembled from polymeric ligands and metal-organic cages (MOCs) as junctions with M₂L₄ or M₁₂L₂₄ stoichiometries. The latter features increased branch functionality and large shear moduli, but also an abundance of elastically inactive loop defects that allow via ligand exchange the introduction of function at no cost to the gel's mechanical properties.by Aleksandr V. Zhukhovitskiy.Ph. D. in Organic Chemistr

Migratory Insertion of Carbenes into Au(III)-C Bonds.

Migratory insertion of carbon-based species into transition-metal-carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer-Tropsch process, Mizoroki-Heck reaction, Ziegler-Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold-carbon bonds. Here we report the discovery of well-defined Au(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au-C bonds at temperatures ≥ -40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps

Supported Dendrimer-Encapsulated Metal Clusters: Toward Heterogenizing Homogeneous Catalysts.

Recyclable catalysts, especially those that display selective reactivity, are vital for the development of sustainable chemical processes. Among available catalyst platforms, heterogeneous catalysts are particularly well-disposed toward separation from the reaction mixture via filtration methods, which renders them readily recyclable. Furthermore, heterogeneous catalysts offer numerous handles-some without homogeneous analogues-for performance and selectivity optimization. These handles include nanoparticle size, pore profile of porous supports, surface ligands and interface with oxide supports, and flow rate through a solid catalyst bed. Despite these available handles, however, conventional heterogeneous catalysts are themselves often structurally heterogeneous compared to homogeneous catalysts, which complicates efforts to optimize and expand the scope of their reactivity and selectivity. Ongoing efforts in our laboratories are aimed to address the above challenge by heterogenizing homogeneous catalysts, which can be defined as the modification of homogeneous catalysts to render them in a separable (solid) phase from the starting materials and products. Specifically, we grow the small nanoclusters in dendrimers, a class of uniform polymers with the connectivity of fractal trees and generally radial symmetry. Thanks to their dense multivalency, shape persistence, and structural uniformity, dendrimers have proven to be versatile scaffolds for the synthesis and stabilization of small nanoclusters. Then these dendrimer-encapsulated metal clusters (DEMCs) are adsorbed onto mesoporous silica. Through this method, we have achieved selective transformations that had been challenging to accomplish in a heterogeneous setting, e.g., π-bond activation and aldol reactions. Extensive investigation into the catalytic systems under reaction conditions allowed us to correlate the structural features (e.g., oxidation states) of the catalysts and their activity. Moreover, we have demonstrated that supported DEMCs are also excellent catalysts for typical heterogeneous reactions, including hydrogenation and alkane isomerization. Critically, these investigations also confirmed that the supported DEMCs are heterogeneous and stable against leaching. Catalysts optimization is achieved through the modulation of various parameters. The clusters are oxidized (e.g., with PhICl2) or reduced (e.g., with H2) in situ. Changing the dendrimer properties (e.g., generation, terminal functional groups) is analogous to ligand modification in homogeneous catalysts, which affect both catalytic activity and selectivity. Similarly, pore size of the support is another factor in determining product distribution. In a flow reactor, the flow rate is adjusted to control the residence time of the starting material and intermediates, and thus the final product selectivity. Our approach to heterogeneous catalysis affords various advantages: (1) the catalyst system can tap into the reactivity typical to homogeneous catalysts, which conventional heterogeneous catalysts could not achieve; (2) unlike most homogeneous catalysts with comparable performance, the heterogenized homogeneous catalysts can be recycled; (3) improved activity or selectivity compared to conventional homogeneous catalysts is possible because of uniquely heterogeneous parameters for optimization. In this Account, we will briefly introduce metal clusters and describe the synthesis and characterizations of supported DEMCs. We will present the catalysis studies of supported DEMCs in both the batch and flow modes. Lastly, we will summarize the current state of heterogenizing homogeneous catalysis and provide future directions for this area of research

Migratory Insertion of Carbenes into Au(III)–C Bonds

Migratory insertion of carbon-based species into transition-metal–carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer–Tropsch process, Mizoroki–Heck reaction, Ziegler–Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold–carbon bonds. Here we report the discovery of well-defined Au­(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au–C bonds at temperatures ≥ −40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps

Migratory Insertion of Carbenes into Au(III)–C Bonds

Migratory insertion of carbon-based species into transition-metal–carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer–Tropsch process, Mizoroki–Heck reaction, Ziegler–Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold–carbon bonds. Here we report the discovery of well-defined Au­(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au–C bonds at temperatures ≥ −40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps

Migratory Insertion of Carbenes into Au(III)–C Bonds

Migratory insertion of carbon-based species into transition-metal–carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer–Tropsch process, Mizoroki–Heck reaction, Ziegler–Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold–carbon bonds. Here we report the discovery of well-defined Au­(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au–C bonds at temperatures ≥ −40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps

Migratory Insertion of Carbenes into Au(III)–C Bonds

Migratory insertion of carbon-based species into transition-metal–carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer–Tropsch process, Mizoroki–Heck reaction, Ziegler–Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold–carbon bonds. Here we report the discovery of well-defined Au­(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au–C bonds at temperatures ≥ −40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps