Jeffery A. Byers

Redox-Controlled Polymerization of Lactide Catalyzed by Bis(imino)pyridine Iron Bis(alkoxide) Complexes

Bis(imino)pyridine iron bis(alkoxide) complexes have been synthesized and utilized in the polymerization of (rac)-lactide. The activities of the catalysts were particularly sensitive to the identity of the initiating alkoxide with more electron-donating alkoxides resulting in faster polymerization rates. The reaction displayed characteristics of a living polymerization with production of polymers that exhibited low molecular weight distributions, linear relationships between molecular weight and conversion, and polymer growth observed for up to fifteen sequential additions of lactide monomer to the polymerization reaction. Mechanistic experiments revealed that iron bis(aryloxide) catalysts initiate polymerization with one alkoxide ligand, while iron bis(alkylalkoxide) catalysts initiate polymerization with both alkoxide ligands. Oxidation of an iron(II) catalyst precursor lead to a cationic iron(III) bis-alkoxide complex that was completely inactive toward lactide polymerization. When redox reactions were carried out during lactide polymerization, catalysis could be switched off and turned back on upon oxidation and reduction of the iron catalyst, respectively.

On the Synergism Between H2O and a Tetrahydropyran Template in the Regioselective Cyclization of an Epoxy Alcohol

A regioselective epoxy alcohol cyclization promoted by the combination of neutral water and a tetrahydropyran template was investigated through a series of mechanistic experiments carried out on an epoxy alcohol containing a tetrahydropyran ring (1a) and its carbocyclic congener (1b). In contrast to 1a, cyclizations of 1b were unselective and displayed significantly faster reaction rates suggesting that the tetrahydropyran oxygen in 1a is requisite for regioselective cyclization. Reactions for both substrates were shown to occur in solution and under kinetic control without significant influence from hydrophobic effects. Kinetic measurements carried out in water/dimethyl sulfoxide mixtures suggest that 1b reacts exclusively through an unselective pathway requiring one water molecule more than what is required to solvate the epoxy alcohol. Similar experiments for 1a suggest a competition between an unselective and a selective pathway requiring one and two water molecules in excess of those required to solvate 1a, respectively. The selective pathway observed for 1a but not in 1b is rationalized by electronic and conformational differences between the two compounds.

Evidence That Epoxide-Opening Cascades Promoted by Water Are Stepwise and Become Faster and More Selective After the First Cyclization

A detailed kinetic study of the endo-selective epoxide-opening cascade reaction of a diepoxy alcohol in neutral water was undertaken using (1)H NMR spectroscopy. The observation of monoepoxide intermediates resulting from initial endo and exo cyclization indicated that the cascade proceeds via a stepwise mechanism rather than through a concerted one. Independent synthesis and cyclization of these monoepoxide intermediates demonstrated that they are chemically and kinetically competent intermediates in the cascade. Analysis of each step of the reaction revealed that both the rate and regioselectivity of cyclization improve as the cascade reaction proceeds. In the second step, cyclization of an epoxy alcohol substrate templated by a fused diad of two tetrahydropyran rings proceeds with exceptionally high regioselectivity (endo:exo = 19:1), the highest we have measured in the opening of a simple trans-disubstituted epoxide. The origins of these observations are discussed.

Recent advances in iron-catalysed cross coupling reactions and their mechanistic underpinning

Advances in iron-catalysed cross coupling from 2010–2015 are critically reviewed. In addition to a description of the systems that have emerged since 2010, the significant mechanistic work carried out to understand the mechanisms of these transformations will be discussed. An emphasis will be placed on mechanistic studies that utilize in situ reaction monitoring and the tools used to achieve this goal.

Stereoselective Catalysis Achieved through in Situ Desymmetrization of an Achiral Iron Catalyst Precursor

Stereoselective catalysis is described that proceeds with catalyst control but without the need to synthesize preformed chiral catalysts or ligands. Iron-based catalysts were discovered to effect the stereoselective polymerization of lactides starting from a single achiral precursor and the proper choice of an achiral silanol additive. Spectroscopic analysis of the polymer revealed that the stereoselectivity originates from an enantiomorphic site rather than a chain end stereocontrol mechanism. Iron intermediates that are stereogenic at iron are proposed to form in situ as a result of desymmetrization that occurs from a change in the metal coordination number. The proposed mechanism is supported by a combination of spectroscopic measurements, model complexes, kinetic measurements, and DFT calculations.

Redox-triggered crosslinking of a degradable polymer

A unique redox-triggered crosslinking reaction is disclosed that capitalizes on the orthogonal reactivity of an iron-based catalyst for the ring opening polymerization of cyclic diesters and epoxides. Synthesis of an epoxide-functionalized cyclic diester is described, which undergoes chemoselective ring opening of the cyclic diester functional group when the iron catalyst is in the iron(II) oxidation state. Upon one electron oxidation of the catalyst, epoxide polymerization is triggered, which results in chemical crosslinking of the polymer. Cross-linked polymer is also obtained, albeit with different crosslinking densities, when epoxide-functionalized cyclic diester is first exposed to an iron(III) catalyst to initiate epoxide polymerization followed by one electron reduction of the catalyst to trigger crosslinking reactions that result from cyclic diester polymerization. Copolymerization reactions with lactide were carried out with the catalyst in the iron(II) oxidation state, and catalyst oxidation led to cross-linked polymers that demonstrated significantly different thermal properties compared to poly(lactic acid).

Electrochemically Switchable Ring-Opening Polymerization of Lactide and Cyclohexene Oxide

An electrochemical method was developed for the redox switchable polymerization of lactide and cyclohexene oxide. Using a lithium reversible sacrificial electrode and a high surface area carbon working electrode, efficient transformation between formally iron(II) and iron(III) oxidation states of a bis(imino)pyridine iron alkoxide complex was possible, which led to the ability to activate the complex for ring opening polymerization reactions. In addition to serving as a redox trigger, an electrochemical toggle switch was developed in which the chemoselectivity for lactide and epoxide polymerization was altered in situ. These findings led to the synthesis of poly(lactic acid- b-cyclohexene oxide) block copolymers in which the sequence of monomers incorporated is controlled by the electrical potential applied.

The Role of Alkoxide Initiator, Spin State, and Oxidation State in Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Iron Bis(imino)pyridine Complexes

Density functional theory (DFT) is employed to characterize in detail the mechanism for the ring-opening polymerization (ROP) of ε-caprolactone catalyzed by iron alkoxide complexes bearing redox-active bis(imino)pyridine ligands. The combination of iron with the non-innocent bis(imino)pyridine ligand permits comparison of catalytic activity as a function of oxidation state (and overall spin state). The reactivities of aryl oxide versus alkoxide initiators for the ROP of ε-caprolactone are also examined. An experimental test of a computational prediction reveals an Fe(III) bis(imino)pyridine bis-neopentoxide complex to be competent for ROP of ε-caprolactone.

Aperture-Opening Encapsulation of a Transition Metal Catalyst in a Metal-Organic Framework for CO2 Hydrogenation

The aperture-opening process resulting from dissociative linker exchange in zirconium-based metal-organic framework (MOF) UiO-66 was used to encapsulate the ruthenium complex (tBuPNP)Ru(CO)HCl in the framework (tBuPNP = 2,6-bis((di- tert-butyl-phosphino)methyl)pyridine). The resulting encapsulated complex, [Ru]@UiO-66, was a very active catalyst for the hydrogenation of CO2 to formate. Unlike the analogous homogeneous catalyst, [Ru]@UiO-66 could be recycled five times, showed no evidence for bimolecular catalyst decomposition, and was less prone to catalyst poisoning. These results demonstrated for the first time how the aperture-opening process in MOFs can be used to synthesize host-guest materials useful for chemical catalysis.

Electron-donating capabilities and evidence for redox activity in low oxidation state iron complexes bearing bis(amidine)pyrimidylidene ligands

Abstract Low-valent iron complexes bearing bis(amidine)pyrimidylidene (or carbenodiamidine, CDA) ligands have been synthesized and characterized by X-ray crystallography, Mössbauer spectroscopy, infrared spectroscopy, EPR spectroscopy, and DFT calculations. One electron reduction of (CDA)FeCl2 (1) resulted in the formation of (CDA)FeCl (2), which demonstrated an unusual metal−N-heterocyclic carbene (NHC) interaction where a redox non-innocent NHC participates in a one electron π-interaction with the metal center. This interaction renders the complex best described as a low-spin iron(II) complex that is antiferromagnetically coupled to a singly reduced CDA ligand. Two electron reduction of 1 under an atmosphere of carbon monoxide resulted in the formation of the diamagnetic complex (CDA)Fe(CO)2 (3). Complex 3 demonstrated significantly lower CO stretching frequencies compared to previously reported bis(imino)pyridine and even bis(N-heterocyclic carbene)pyridine iron complexes, which reflects the significant σ-donating capabilities of the CDA ligands.