Statistical Ring Catenation under Thermodynamic Control: Should the Jacobson–Stockmayer Cyclization Theory Take into Account Catenane Formation?

An extension of the Jacobson–Stockmayer theory is presented to include the reversible formation of [2]­catenanes in a ring–chain system under thermodynamic control. The extended theory is based on the molar catenation constant, measuring the ease of catenation of two ring oligomers, whose expression was obtained in a previous work. Two scenarios have been considered: that of “thick” (hydrocarbon-like) chains and that of “thin” (DNA-like) chains. In the case of “thick” chains, the formation of catenanes can be neglected, unless in the unlikely case of a very large value of the equilibrium constant for linear propagation (<i>K</i> ≈ 10⁸ mol^–1 L, or larger). For <i>K</i> tending to infinity, the system becomes a chain-free system where only ring–catenane equilibria occur. Under this condition, there is a critical concentration below which only rings are present at equilibrium and above which the ring fraction remains constant, and the excess monomer is converted only into catenanes. In the case of “thin” chains, the formation of catenanes cannot be neglected even for values of <i>K</i> as low as 10² mol^–1 L, thus justifying the use of the extended theory

Substituent Effects on the Catalytic Activity of Bipyrrolidine-Based Iron Complexes

The catalytic activity and the selectivity of the new bipyrrolidine-based Fe­(II) complexes <b>2</b><b>·</b><b>Fe</b>(OTf)₂ and <b>3</b><b>·</b><b>Fe</b>(OTf)₂ in the oxidation of a series of alkyl and alkenyl hydrocarbons as well as of an aromatic sulfide with H₂O₂ were tested and compared with the catalytic efficiency of White’s parent complex <b>1·Fe</b>(OTf)₂ in order to evaluate the sensitivity of the reaction to electronic effects

Ring-Opening Metathesis Polymerization of a Diolefinic [2]-Catenane–Copper(I) Complex: An Easy Route to Polycatenanes

A dilute (30 mM) dichloromethane solution of the copper­(I) complex <b>1</b>·Cu⁺ of a [2]-catenane composed of two identical 28-membered macrocyclic alkenes featuring a phenanthroline moiety in the backbone was subjected to ring-opening metathesis polymerization (ROMP) with second-generation Grubbs catalyst. Shortly after mixing of reactants, the dark red solution transformed into a gel. The bis­(phenanthroline)­copper­(I) units were effectively preserved during ROMP, as evinced by spectroscopic analysis. This implies that the putative metal alkylidene pseudorotaxane intermediates did not undergo dethreading processes but were involved in ring–chain equilibria strongly biased toward the ring products at the low monomer concentration employed in the ROMP reactions. MALDI-TOF mass spectra of the reaction mixtures obtained at an early stage of the reaction revealed a distribution of interlocked oligomers (<b>1</b>·Cu⁺)_<i>n</i>(PF₆^–)_<i>n</i>−1 with <i>n</i> up to 7, with no traces of peaks ascribable to open chain species. Rheological and mechanical analyses of the gel products provided independent evidence in support of the conclusion that the fraction of linear species in the polymer is negligible. Indications were obtained that the major portion of the polymeric material is composed of fully interlocked species

C–H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight

A family of imine-based nonheme iron­(II) complexes (<b>LX</b>)₂Fe­(OTf)₂ has been prepared, characterized, and employed as C–H oxidation catalysts. Ligands <b>LX</b> (<b>X</b> = <b>1</b>, <b>2</b>, <b>3</b>, and <b>4</b>) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe­(OTf)₂ in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (<b>L1</b>)₂Fe­(OTf)­(ClO₄) and (<b>L3</b>)₂Fe­(OTf)₂ are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. ¹H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (<b>LX</b>)₂Fe­(OTf)₂ efficiently catalyze the oxidation of alkyl C–H bonds employing H₂O₂ as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C–H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C–H bonds. Isotopic labeling analyses show that H₂O₂ is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O–O cleavage, without the assistance of a <i>cis</i>-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H₂O₂ at (<b>LX</b>)₂Fe­(OTf)₂ differs from that occurring in biomimetic iron catalysts described to date

C–H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight

A family of imine-based nonheme iron­(II) complexes (<b>LX</b>)₂Fe­(OTf)₂ has been prepared, characterized, and employed as C–H oxidation catalysts. Ligands <b>LX</b> (<b>X</b> = <b>1</b>, <b>2</b>, <b>3</b>, and <b>4</b>) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe­(OTf)₂ in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (<b>L1</b>)₂Fe­(OTf)­(ClO₄) and (<b>L3</b>)₂Fe­(OTf)₂ are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. ¹H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (<b>LX</b>)₂Fe­(OTf)₂ efficiently catalyze the oxidation of alkyl C–H bonds employing H₂O₂ as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C–H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C–H bonds. Isotopic labeling analyses show that H₂O₂ is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O–O cleavage, without the assistance of a <i>cis</i>-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H₂O₂ at (<b>LX</b>)₂Fe­(OTf)₂ differs from that occurring in biomimetic iron catalysts described to date