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

Abstract

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.

Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

“…Nature often couples proton responsivity and redox-activity, preferring the more efficient proton coupled electron transfer (PCET) 5,6 to transferring each moiety separately. The function of hemilability in both biological and synthetic systems [9][10][11][12][13][14][15] is primarily to stabilize otherwise unfavorable intermediates by filling a vacant coordination site. This variable binding motif can be found stabilizing resting states, 16 actively participating in catalytic cycles, 4,17 or stabilizing high energy intermediates in enzymes and catalysts.…”

“…2) 19 and the polymerization of lactides facilitated by hemilabiity in the pyridinediinmine (PDI) ligand. 14 Lastly, the resurgence in metallocrown ethers as switchable catalysts has recently been reviewed. 15…”

Iron(II) and Zinc(II) Complexes with Tetradentate Bis(pyrazolyl)methane Ligands as Catalysts for the Ring‐Opening Polymerisation of rac‐Lactide

“…Furthermore, silyl‐functionalised Fe II β‐diketiminates and anionic Fe II alkoxides were tested successfully as catalysts in solution or as bulk catalysts . In recent years, Byers et al developed redox‐controlled bis(imino)pyridine iron bis(alkoxide) complexes as polymerisation catalysts, which allowed the reaction to be stopped by oxidation and restarted by subsequent reduction . In addition to iron‐centred ROP catalysis, Atkinson et al developed 1,1′‐ferrocenediyl salicylaldimine ligands for titanium‐based complexes, and Fraser et al established polylactide‐supported tris(dibenzoylmethane)‐centred iron complexes, in which the iron atom functions as a protecting group for the β‐diketones …”

Redox Control in Olefin Polymerization Catalysis by Phosphine–Sulfonate Palladium and Nickel Complexes

“…Long et al demonstrated that the reduction of an α‐diimine Ni II complex could modulate the branching density of the resulting polyethylene by up to 30 % (Scheme , III ) . It should be noted that this redox‐control strategy has been extensively studied in ring‐opening polymerization of cyclic esters, but has only been recently extended to other types of polymerization reactions …”

“…[18] It should be noted that this redoxcontrol strategy has been extensively studied in ring-opening polymerization of cyclic esters, but has only been recently extended to other types of polymerization reactions. [19][20][21][22][23][24][25][26][27] Two strategies are employed to achieve in situ redox control: (1) the oxidation or reduction of a preinstalled ferrocene unit; and (2) the oxidation or reduction of the redox-active metal center.…”