Electronic effect

Abstract: Many active pharmaceuticals, herbicides, conducting polymers, and components of organic light-emitting diodes contain arylamines. For many years, this class of compound was prepared via classical methods, such as nitration, reduction and reductive alkylation, copper-mediated chemistry at high temperatures, addition to benzyne intermediates, or direct nucleophilic substitution on particularly electron-poor aromatic or heteroaromatic halides. However, during the past decade, palladium-catalyzed coupling reactions of amines with aryl halides have largely supplanted these earlier methods. Successive generations of catalysts have gradually improved the scope and efficiency of the palladium-catalyzed reaction. This Account describes the conceptual basis and utility of our latest, "fourth-generation" palladium catalyst for the coupling of amines and related reagents with aryl halides. In the past five years, we have developed these catalysts using the lessons learned from previous generations of catalysts developed in our group and in other laboratories. The ligands on the fourth-generation catalyst combine the chelating properties of the aromatic bisphosphines of the second-generation systems with the steric properties and strong electron donation of the hindered alkylphosphines of the third-generation systems. The currently most reactive catalyst in this class is generated from palladium and a sterically hindered version of the Josiphos family of ligands that possesses a ferrocenyl-1-ethyl backbone, a hindered di-tert-butylphosphino group, and a hindered dicyclohexylphosphino group. This system catalyzes the coupling of aryl chlorides, bromides, and iodides with primary amines, N-H imines, and hydrazones in high yield. The reaction has broad scope, high functional group tolerance, and nearly perfect selectivity for monoarylation. It also requires the lowest levels of palladium that have been used for C-N coupling. In addition, this latest catalyst has dramatically improved the coupling of thiols with haloarenes to form C-S bonds. Using ligands that lacked one or more of the structural elements of the most active catalyst, we examined the effects of individual structural elements of the Josiphos ligand on catalyst activity. This set of studies showed that each one of these elements contributes to the high reactivity and selectivity of the catalyst containing the hindered, bidentate Josiphos ligand. Finally, we examined the effect of electronic properties on the rates of reductive elimination to distinguish between the effect of the properties of the M-N sigma-bond and the nitrogen electron pair. We have found that the effects of electronic properties on C-C and C-N bond-forming reductive elimination are similar. Because the amido ligands contain an electron pair, while the alkyl ligands do not, we have concluded that the major electronic effect is transmitted through the sigma-bond.

Abstract: FUNDAMENTALS OF SILICON REACTIVITY: REACTIVE INTERMEDIATES AND REACTION MECHANISMS. Organosilanes: Where to Find Them, What to Call Them, How to Detect Them. Atomic and Molecular Properties of Silicon. Silicon-Based Reactive Intermediates. Extracoordination at Silicon. Reaction Mechanisms for Nucleophilic Substitution at Silicon. THE FORMATION AND CLEAVAGE OF NON-CARBON BONDS TO SILICON: APPLICATIONS IN ORGANIC AND POLYMER CHEMISTRY. Silicon and Transition Metal Chemistry. Hydrosilanes as Reducing Agents. Replacing H with Si: Silicon-Based Reagents. Silicones. Siloxanes Based on T and Q Units. Other Silicon-Containing Polymers. THE FORMATION AND CLEAVAGE OF SILICON-CARBON BONDS: APPLICATIONS IN ORGANIC SYNTHESIS. Formation of Si-C Bonds: The Synthesis of Functional Organosilanes. Silicon in a Biological Environment. Silicon in the Organic World: Electronic Effects of Silyl Groups. Rearrangements. Cleavage of Si-C Bonds. Indices of Functional Group Transformations. Subject Index.

Abstract: The borylation of alkanes and arenes has become some of the most practical C–H bond functionalization chemistry. Most striking is the high regioselectivity of these reactions. Rhodium and ruthenium complexes catalyze with exquisite selectivity the borylation of methyl C–H bonds over methylene or methine C–H bonds. Iridium complexes catalyze, with high steric control, the borylation of one aromatic C–H bond over another. In contrast, iridium-catalyzed borylation of heteroaromatic C–H bonds is more controlled by electronic effects. Detailed information on these selectivities and mechanistic information on the origins of this regioselectivity will be described in this critical review (95 references).

Abstract: The starting point of this study of through-space substituent effects on the catalysis of the electrochemical CO2-to-CO conversion by iron(0) tetraphenylporphyrins is the linear free energy correlation between through-structure electronic effects and the iron(I/0) standard potential that we established separately. The introduction of four positively charged trimethylanilinium groups at the para positions of the tetraphenylporphyrin (TPP) phenyls results in an important positive deviation from the correlation and a parallel improvement of the catalytic Tafel plot. The assignment of this catalysis boosting effect to the Coulombic interaction of these positive charges with the negative charge borne by the initial Fe0–CO2 adduct is confirmed by the negative deviation observed when the four positive charges are replaced by four negative charges borne by sulfonate groups also installed in the para positions of the TPP phenyls. The climax of this strategy of catalysis boosting by means of Coulombic stabilization...