Mechanistic Aspects of Metal Valence Change in SalenCo(III)OAc-Catalyzed Hydrolytic Kinetic Resolution of Racemic Epoxides

Theoretical Studies on Bimetallic Salen Complexes Catalyzed Epoxide Hydration: Effects of Metal Centers, Substrates, and Ligands

To provide guiding information for developing efficient and stable catalysts for epoxide hydration, we investigated the mechanism of propylene oxide (PO) to 1,2-propylene glycol (PG) using density functional theory (DFT) calculations. The mechanism was identified to follow the cooperative bimetallic mechanism in which a metal-salen complex activated H₂O attacks the middle carbon atom of a metal-salen complex activated PO from the oxygen side of three-membered ring. Analyses reveal that the distortion energy correlates linearly with the barrier, and the hydrogen bonding between H₂O and PO increases from reaction precursors to transition states. A nice linear relationship exists between the ratio of square root of ionic potential to the square of the distance from the metal ion spherical surface to the oxygen atom center of PO. It is demonstrated that the substrates with larger polarizability tend to have lower hydration barriers and the influence of ligands is less than that of metal centers and substrates. Modifying metal ions is the first choice for developing metal-salen catalysts, and metal ions with more formal charges and larger radius are expected to exhibit high activity. These findings shed lights on the mechanism and provide guiding information for developing efficient metal-salen catalysts for epoxide hydration.

Organo-Cobalt Complexes in Reversible-Deactivation Radical Polymerization

Cobalt complexes have played an essential role in different chemical reactions. One of them that has attracted substantial attention in polymer science is cobalt mediated radical polymerization (CMRP), which is famous for its remarkable efficiency in controlling the radical polymerization of vinyl acetate (VAc) and other less active monomers (LAMs). Two pathways, reversible termination (RT) and degenerative transfer (DT), were recognized to control the polymerization in CMRP and could be further used to rationalize the mechanism of other RDRP methods. These control mechanisms were then found to be correlated to the redox potential of cobalt complexes and thus could be judged more quantitatively. The control of polymer composition and tacticity could also be achieved by using CMRP. The hybridization of CMRP and atom transfer radical polymerization (ATRP) could directly synthesize the vinyl acetate/methyl methacrylate and vinyl acetate/styrene block copolymers in one pot. The copolymer of acrylates and 1-octene could be obtained by visible-light-induced CMRP. With the addition of bulky Lewis acid, CMRP of N,N-dimethylacrylamide (DMA) showed high isotacticities with the contents of meso dyads (m) and meso triads (mm) up to 94 % and 87 %, respectively, and generated the crystalline PDMA with T_m as high as 276 °C. This personal account reviewed the development of CMRP with the mechanistic understanding, the control of composition and stereoselectivity of the polymeric products, and its perspective.

Exploring the mechanism and counterion activity regulation in the CoIII(salen)-catalyzed hydration of propylene oxide

CoIII(salen)-X (X = Cl-, OAc-, and OTs-) mediated hydration of propylene oxide (PO) to propylene glycol has been investigated in detail using density functional theory (DFT) calculations. Two kinds of reaction mechanisms, the concerted and stepwise pathways, were scrutinized. For the eight concerted routes, the cooperative bimetallic route in which the middle carbon atom is attacked by the nucleophilic oxygen atom (route VI-m) was calculated to be the most favorable, and among the three catalysts examined H2O-CoIII-OTs was found to be the most active, due to the strong hydrogen bonding between the nucleophilic H2O and the ring oxygen atom in the epoxides as well as the extra π-π stacking interaction. For the stepwise mechanism which consists of the formation of H2O-CoIII-OH, the ring-opening of PO and propylene glycol formation, our studies reveal that different H2O-CoIII-Xs behave kinetically very similarly in the course of propylene glycol formation, but show a notable difference in the rate of H2O-CoIII-OH formation with Cl- > OAc- > OTs-. The rate ordering with which we rationalize the experimental phenomena well is disclosed to be consistent with the nucleophilicity of the counterions by molecular electrostatic potential, condensed Fukui function and condensed local softness. We show that the nucleophilicity of the counterion determines the favorable mechanism that PO hydration follows.

Oxygen-Triggered Switchable Polymerization for the One-Pot Synthesis of CO2 -Based Block Copolymers from Monomer Mixtures

Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one-pot process from mixtures of monomers. Herein we report the use of O₂ as an external stimulus to switch the polymerization mechanism from the radical polymerization of vinyl monomers mediated by (Salen)Co^III -R [Salen=N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine; R=alkyl] to the ring-opening copolymerization (ROCOP) of CO₂ /epoxides. Critical to this process is unprecedented monooxygen insertion into the Co-C bond, as rationalized by DFT calculations, leading to the formation of (Salen)Co^III -O-R as an active species to initiate ROCOP. Diblock poly(vinyl acetate)-b-polycarbonate could be obtained by ROCOP of CO₂ /epoxides with preactivation of (Salen)Co end-capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)-b-poly(methyl acrylate)-b-polycarbonate triblock copolymer was successfully synthesized by a (Salen)cobalt-mediated sequential polymerization with an O₂ -triggered switch in a one-pot process.

Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation

Recent developments in polyester synthesis have established several systems based on zinc, chromium, cobalt, and aluminum catalysts for the ring-opening alternating copolymerization of epoxides with cyclic anhydrides. However, to date, regioselective processes for this copolymerization have remained relatively unexplored. Herein we report the development of a highly active, regioselective system for the copolymerization of a variety of terminal epoxides and cyclic anhydrides. Unexpectedly, electron withdrawing substituents on the salen framework resulted in a more redox stable Co(III) species and longer catalyst lifetime. Using enantiopure propylene oxide, we synthesized semicrystalline polyesters via the copolymerization of a range of epoxide/anhydride monomer pairs.

Dinuclear salen cobalt complex incorporating Y(OTf)3: enhanced enantioselectivity in the hydrolytic kinetic resolution of epoxides

Yttrium triflate present in dinuclear salen cobalt complex not only shows a promoting effect on electron transfer, but also assists in the asymmetric ring opening of epoxides with water efficiently.