Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions

Abstract

Aryl-substituted pyridine­(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl­(oligocyclobutane), a microstructure of poly­(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners established that this step occurs without redox involvement of the pyridine­(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C­(sp3)–C­(sp3) reductive coupling. A series of labeling experiments with pyridine­(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C­(sp3)–C­(sp3) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine­(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine­(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure–activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as the promotion of chemical recycling of cycloaddition polymers.

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis

“…Conversion of these formally iron(I) derivatives to catalytically active iron(0) compounds would be transformative in expanding the scope of the latter and in establishing structure−reactivity relationships, especially with respect to the role of the redox-active chelate. 28 Inspiration for these studies derived from the observation that addition of excess propylene to meso-( tBu PDI)FeCH 3 , the corresponding dinitrogen complex of which is synthetically inaccessible, resulted in tail-to-tail dimerization, consistent with formation of a metallocyclopentane that undergoes β-hydride elimination and C−H reductive elimination. 29 In contrast to more traditional and well-studied two-electron reductive elimination, the desired transformation is a one-electron process and likely relies on the redox-active pyridine(diimine) ligand.…”

“…In addition to iron alkyl complexes, five-coordinate pyridine­(diimine) formally iron(0) complexes have been synthesized and are active for a range of intra- and intermolecular − cycloaddition reactions of unactivated alkene–alkene, alkene–diene, and diene–diene partners (Scheme B). These include formation of unique C–C bond architectures containing cyclobutyl rings that are inaccessible by thermal, uncatalyzed pathways.…”

“…By contrast, pyridine­(diimine) iron alkyl complexes are readily synthesized from the corresponding dihalide precursors. Conversion of these formally iron­(I) derivatives to catalytically active iron(0) compounds would be transformative in expanding the scope of the latter and in establishing structure–reactivity relationships, especially with respect to the role of the redox-active chelate . Inspiration for these studies derived from the observation that addition of excess propylene to meso -( tBu PDI)­FeCH 3 , the corresponding dinitrogen complex of which is synthetically inaccessible, resulted in tail-to-tail dimerization, consistent with formation of a metallocyclopentane that undergoes β-hydride elimination and C–H reductive elimination .…”

Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes

“…With tridentate pincers containing a central aromatic ring with flanking donor arms substituted at 2,6-positions, possible structural modifications and their effects are summarized in Figure A. The flanking neutral donors (ER n ) are typically phosphines, phosphites, amines, imines, ethers, thioethers, and N -heterocyclic carbenes . Alterations in donor atoms (E) and substituents on E are known to induce a significant change in the overall steric profile. , Substitution at the 4-position of the central aryl ring (Z) allows for a fine tuning of electronic properties that are often independent of steric factors .…”

On the Realties of Base Metal Catalysis: An Overview

“…Both Fe, with high density spin states, and the redox-active PDI ligand are inherent to catalyst success. 97 Additionally, slight variations to the ligand allowed for the successful development of multiple highyielding and stereoselective cycloaddition reactions. 96 Similarly, using only one variation of the catalyst, three [(PDI)Fe]-catalyzed methods for the elaboration of vinylsilanes in a chemo-and regioselective manner were developed.…”