Scorpionate Catalysts for Coupling CO₂ and Epoxides to Cyclic Carbonates: A Rational Design Approach for Organocatalysts

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO₂ activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO₂ pressure of only 10 bar, and is highly recyclable

Scorpionate Catalysts for Coupling CO₂ and Epoxides to Cyclic Carbonates: A Rational Design Approach for Organocatalysts

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO₂ activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO₂ pressure of only 10 bar, and is highly recyclable

Understanding the Origin of the Regioselectivity in Cyclopolymerizations of Diynes and How to Completely Switch It

Grubbs-type olefin metathesis catalysts are known to cyclopolymerize 1,6-heptadiynes to afford conjugated polyenes containing five- or six-membered carbocycles. Although high levels of regioselectivity up to >99:1 were observed previously for the formation of five-membered rings, it was neither possible to deliberately obtain six-membered rings at similar levels of selectivity nor understood why certain catalysts showed this selectively. Combining experimental and computational methods, a novel and general theory for what controls the regiochemistry of these cyclopolymerizations is presented. The electronic demands of the ruthenium-based Fischer carbenes are found to innately prefer to form five-membered rings. Reducing the electrophilicity of the carbene by enforcing a trigonal-bipyramidal structure for the ruthenium, where stronger π-backdonation increases the electron density on the carbene, is predicted to invert the regioselectivity. Subsequent experiments provide strong support for the new concept, and it is possible to completely switch the regioselectivity to a ratio of <1:99

Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation

We recently reported a bis­(imino)­pyridine (or pyridine diimine, PDI) manganese precatalyst, (^Ph2PPrPDI)Mn (<b>1</b>), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for <b>1</b>-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min^–1 have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, <b>1</b> has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min^–1. Under stoichiometric and catalytic conditions, addition of PhSiH₃ to (^Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (^Ph2PPrPDI)­MnH (<b>2</b>) was achieved upon adding NaEt₃BH to (^Ph2PPrPDI)­MnCl₂ and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone was added to <b>1</b>, radical transfer yielded (^Ph2PPrPDI<b>·</b>)­Mn­(OC<b>·</b>(Ph)­(CF₃)) (<b>3</b>), which undergoes intermolecular C–C bond formation to produce the respective Mn­(II) dimer, [(μ-<i>O</i>,<i>N</i>_py-4-OC­(CF₃)­(Ph)-4-H-^Ph2PPrPDI)­Mn]₂ (<b>4</b>). Upon finding <b>3</b> to be inefficient and <b>4</b> to be inactive, kinetic trials were conducted to elucidate the mechanisms of <b>1</b>- and <b>2</b>-mediated hydrosilylation. Varying the concentration of <b>1</b>, substrate, and PhSiH₃ revealed a first order dependence on each reagent. Furthermore, a kinetic isotope effect (KIE) of 2.2 ± 0.1 was observed for <b>1</b>-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using <b>2</b>, suggesting <b>1</b> and <b>2</b> operate through different mechanisms. Although kinetic trials reveal <b>1</b> to be the more active precatalyst for carbonyl hydrosilylation, a concurrent <b>2</b>-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, <b>1</b>-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while <b>2-</b>mediated hydrosilylation occurs via insertion