Sustainable radical reduction through catalytic hydrogen atom transfer

A system with coupled catalytic cycles is described that allows radical reduction by hydrogen atom abstraction from rhodium hydrides. These intermediates are generated from H2 activation by Wilkinson's catalyst. Radical generation is carried out by titanocene-catalyzed electron transfer to epoxides.     

Electron transfer reactions relevant to atom transfer radical polymerization

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.     

Reactivity of aqueous Fe(IV) in hydride and hydrogen atom transfer reactions

Oxidation of cyclobutanol by aqueous Fe(IV) generates cyclobutanone in approximately 70% yield. In addition to this two-electron process, a smaller fraction of the reaction takes place by a one-electron process, believed to yield ring-opened products. A series of aliphatic alcohols, aldehydes, and ethers also react in parallel hydrogen atom and hydride transfer reactions, but acetone and acetonitrile react by hydrogen atom transfer only. Precise rate constants for each pathway for a number of substrates were obtained from a combination of detailed kinetics and product studies and kinetic simulations. Solvent kinetic isotope effect for the self-decay of Fe(IV), kH2O/kD2O = 2.8, is consistent with hydrogen atom abstraction from water.     

The isomerization of methoxy radical: intramolecular hydrogen atom transfer mediated through acid catalysis

The catalytic ability of water, formic acid, and sulfuric acid to facilitate the isomerization of the CH(3)O radical to CH(2)OH has been studied. It is shown that the activation energies for isomerization are 30.2, 25.7, 4.2, and 2.3 kcal mol(-1), respectively, when the reaction is carried out in isolation and with water, formic acid, or sulfuric acid as a catalyst. The formation of a doubly hydrogen bonded transition state is central to lowering the activation energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. The changes in the rate constant for the CH(3)O-to-CH(2)OH isomerization with acid catalysis have also been calculated at 298 K. The largest enhancement in the rate, by over 12 orders of magnitude, is with sulfuric acid. The results of the present study demonstrate the feasibility of acid catalysis of a gas-phase radical isomerization reaction that would otherwise be forbidden.     

Intramolecular hydrogen atom transfer in hydrogen-deficient polypeptide radical cations

Irradiation of protonated polypeptides NH₂–RH⁺–COOH by >10 eV electrons leads to further ionization and fast intramolecular charge transfer to the free N-terminus. The resulting species may undergo further hydrogen atom rearrangement to form distonic ions N⁺H₃–RH⁺–COO√. Such transfer is exothermic but can involve an appreciable barrier, e.g., 2.3±0.5 eV for MH^2+√ ions of the peptide ACTH 1–10. Radical polypeptide dications can, therefore, be viewed as hydrogen atom wires. Subsequent capture of low energy electrons results in fragmentation. The pattern of this electronic excitation dissociation (EED) is consistent with hydrogen transfer prior to electron capture.     

Rate constants for hydrogen atom transfer reactions from bis(cyclopentadienyl)titanium(III) chloride-complexed water and methanol to an alkyl radical

Rate constants for hydrogen atom transfer reactions of the water, deuterium oxide, and methanol complexes of bis(cyclopentadienyl)titanium(III) chloride with the secondary alkyl radical 1-cyclobutyldodecyl (2) were determined using indirect kinetic methods. The rate constant for reaction of Cp2Ti(III)Cl-H2O in THF at ambient temperature was 1.0 x 10(5) M(-1) s(-1), and the kinetic isotope effect was kH/kD = 4.4. In benzene containing 0.95 M methanol, the rate constant for reaction of the Cp2Ti(III)Cl-MeOH at ambient temperature was 7.5 x 10(4) M(-1) s(-1). An Arrhenius function for reaction of the Cp2Ti(III)Cl-H2O complex in THF was log k = 9.1 - 5.5/2.3 RT (kcal/mol). The entropic term for reaction of Cp2Ti(III)Cl-H2O was normal, whereas the entropic term previously found for reaction of the Et3B-H2O complex with radical 2 was unusually small (Jin, J.; Newcomb, M. J. Org. Chem. 2007, 72, 5098).     

Copper(I)-catalyzed chlorine atom transfer radical cyclization reactions of unsaturated alpha-chloro beta-keto esters

Copper(I) chloride catalyzed chlorine atom transfer radical cyclization reactions of a series of olefinic alpha-chloro beta-keto esters were investigated. It was found that alpha-dichlorinated beta-keto esters were suitable substrates; the chlorine transfer mono or tandem radical cyclization reactions catalyzed by CuCl complex with bis(oxazoline) or bipyridine proceeded smoothly in dichloroethane at room temperature or 80 degrees C, providing cyclic and bicyclic compounds in moderate to high yield. [reaction: see text]     

New (Co)polymers by atom transfer radical polymerization

The fundamentals of atom transfer radical polymerization (ATRP) are presented. This includes the mechanistic considerations including structure of active and dormant species and structural features of the catalyst and reaction conditions as well as the nature of initiation and propagation steps. Extension of homogeneous polymerization to heterogeneous systems including emulsion polymerization is presented. Synthesis of (co)polymers with predefined molecular weights and low polydispersites as well as with controlled compositions, functionalities and architectures is reviewed.     

Coupled atom transfer radical coupling and atom transfer radical polymerization approach for controlled grafting from poly(vinylidene fluoride) backbone

A new “grafting from” strategy for grafting of different monomers (methacrylates, acrylates, and acrylamide) on poly(vinylidene fluoride) (PVDF) backbone is designed using atom transfer radical coupling (ATRC) and atom transfer radical polymerization (ATRP). 4‐Hydroxy TEMPO moieties are anchored on PVDF backbone by ATRC followed by attachment of ATRP initiating sites chosen according to the reactivity of different monomers. High graft conversion is achieved and grafting of poly(methyl methacrylate) (PMMA) exhibits high degree of polymerization (DPn = 770) with a very low graft density (0.18 per hundred VDF units) which has been increased to 0.44 by regenerating the active catalyst with the addition of Cu(0). A significant impact on thermal and stress–strain property of graft copolymers on the graft density and graft length is noted. Higher tensile strain and toughness are observed for PVDF‐g‐PMMA produced from model initiator but graft copolymer from pure PVDF exhibits higher tensile strength and Young's modulus. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 995–1008     

Alkoxy radical cyclizations onto silyl enol ethers relative to alkene cyclization, hydrogen atom transfer, and fragmentation reactions

This study examines the chemoselectivity of alkoxy radical cyclizations onto silyl enol ethers compared to competing cyclizations, 1,5-hydrogen atom transfers (1,5-HATs), and β-fragmentations. Cyclization onto silyl enol ethers in a 5-exo mode is greatly preferred over cyclization onto a terminal alkene. The selectivity decreases when any alkyl substitution is present on the competing alkene radical acceptor. Alkoxy radical 5-exo cyclizations displayed excellent chemoselectivity over competing β-fragmentations. Alkoxy radical 5-exo cyclizations onto silyl enol ether also outcompeted 1,5-HATs, even for activated benzylic hydrogen atoms. In tetrahydropyran synthesis, where 1,5-HAT has plagued alkoxy radical cyclization methodologies, 6-exo cyclizations were the dominant mode of reactivity. β-Fragmentation still remains a challenge for tetrahydropyran synthesis when an aryl group is present in the β position.     

Densely grafting copolymers of ethyl cellulose through atom transfer radical polymerization

Densely grafting copolymers of ethyl cellulose with polystyrene and poly(methyl methacrylate) were synthesized through atom transfer radical polymerization (ATRP). First, the residual hydroxyl groups on the ethyl cellulose reacted with 2‐bromoisobutyrylbromide to yield 2‐bromoisobutyryloxy groups, known to be an efficient initiator for ATRP. Subsequently, the functional ethyl cellulose was used as a macroinitiator in the ATRP of methyl methacrylate and styrene in toluene in conjunction with CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as a catalyst system. The molecular weight of the graft copolymers increased without any trace of the macroinitiator, and the polydispersity was narrow. The molecular weight of the side chains increased with the monomer conversion. A kinetic study indicated that the polymerization was first‐order. The morphology of the densely grafted copolymer in solution was characterized through laser light scattering. The individual densely grafted copolymer molecules were observed through atomic force microscopy, which confirmed the synthesis of the densely grafted copolymer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4099–4108, 2005     

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.     

Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate)

Mechanistic studies indicate that the oxomolybdenum(vi) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine-N-oxides in a reaction where the oxygen is delivered to molybdenum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2-benzoquinone.     

Controlled polymerization of (meth)acrylamides by atom transfer radical polymerization

Controlled polymerization of (meth)acrylamides was achieved by ATRP using the initiating system methyl 2‐chloropropionate/CuCl/tris(2‐dimethylaminoethyl)amine. Linear increase of molecular weights with conversion and low polydispersity (M_w/M_n < 1.2) were obtained in toluene, at room temperature, when N,N‐dimethylacrylamide was used as a monomer. However, the polymerization reached limited conversion, which could be enhanced by increasing the catalyst/initiator ratio. The limited conversion is not due to the loss of the active chains, but rather to the loss of activity of the catalytic system.     

Ruthenium catalysts bearing N-heterocyclic carbene ligands in atom transfer radical reactions

The catalytic activity of ruthenium-p-cymene complexes bearing N-heterocyclic carbene ligands in atom transfer radical addition (ATRA) or polymerisation (ATRP) strongly depends on the substituents of the carbene ligand, thereby providing a nice illustration of the importance of organometallic engineering and ligand fine tuning in homogeneous catalysis.     

Aqueous electrochemically-triggered atom transfer radical polymerization

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II–N-propyl pyridineimine complexes (Cu^II(NPPI)₂) is reported for the first time. In aqueous solution, using oligo(ethylene glycol) methyl ether methacrylate (OEGMA), standard electrolysis conditions yield POEGMA with good control over molecular weight distribution (Đ_m < 1.35). Interestingly, the polymerizations are not under complete electrochemical control, as monomer conversion continues when electrolysis is halted. Alternatively, it is shown that the extent and rate of polymerization depends upon an initial period of electrolysis. Thus, it is proposed that seATRP using Cu^II(NPPI)₂ follows an electrochemically-triggered, rather than electrochemically mediated, ATRP mechanism, which distinguishes them from other Cu^IIL complexes that have been previously reported in the literature.

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II-pyridineimine complexes is reported and follows a previously unreported electrochemically triggered mechanism.