Tuning Catalytic Activity in the Hydrogenation of Unactivated Olefins with Transition-Metal Oxos as the Lewis Base Component of Frustrated Lewis Pairs

The steric and electronic demands of the catalytic olefin hydrogenation of <i>tert</i>-butylethylene with oxorhenium/Lewis acid FLPs were evaluated. The sterics of the ligand were altered by installing bulkier isopropyl groups in the 2,6-positions of the diamidopyridine (DAP) ligand. Lewis acid/base adducts were not isolated for complexes with this ligand; however, species incorporating isopropyl groups were still active in catalytic hydrogenation. Modifications were also made to the Lewis acid, and catalytic reactions were performed with Piers’ borane, HB­(C₆F₅)₂, and the aluminum analogue Al­(C₆F₅)₃. The rate of catalytic hydrogenation was shown to strongly correlate with the size of the alkyl, aryl, or hydride ligand. This was confirmed by a linear Taft plot with the steric sensitivity factor δ = −0.57, which suggests that reaction rates are faster with sterically larger X substituents. These data were used to develop a catalyst ((MesDAP)­Re­(O)­(Ph)/HB­(C₆F₅)₂) that achieved a TON of 840 for the hydrogenation of <i>tert</i>-butylethylene at mild temperatures (100 °C) and pressures (50 psi of H₂). Tuning of the oxorhenium catalysts also resulted in the hydrogenation of <i>tert</i>-butylethylene at room temperature

Tuning Catalytic Activity in the Hydrogenation of Unactivated Olefins with Transition-Metal Oxos as the Lewis Base Component of Frustrated Lewis Pairs

The steric and electronic demands of the catalytic olefin hydrogenation of <i>tert</i>-butylethylene with oxorhenium/Lewis acid FLPs were evaluated. The sterics of the ligand were altered by installing bulkier isopropyl groups in the 2,6-positions of the diamidopyridine (DAP) ligand. Lewis acid/base adducts were not isolated for complexes with this ligand; however, species incorporating isopropyl groups were still active in catalytic hydrogenation. Modifications were also made to the Lewis acid, and catalytic reactions were performed with Piers’ borane, HB­(C₆F₅)₂, and the aluminum analogue Al­(C₆F₅)₃. The rate of catalytic hydrogenation was shown to strongly correlate with the size of the alkyl, aryl, or hydride ligand. This was confirmed by a linear Taft plot with the steric sensitivity factor δ = −0.57, which suggests that reaction rates are faster with sterically larger X substituents. These data were used to develop a catalyst ((MesDAP)­Re­(O)­(Ph)/HB­(C₆F₅)₂) that achieved a TON of 840 for the hydrogenation of <i>tert</i>-butylethylene at mild temperatures (100 °C) and pressures (50 psi of H₂). Tuning of the oxorhenium catalysts also resulted in the hydrogenation of <i>tert</i>-butylethylene at room temperature

Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO₂/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (kp) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane

Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO₂/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (kp) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane

Nondirected C–H Activation of Arenes with Cp*Ir(III) Acetate Complexes: An Experimental and Computational Study

Combined experimental and computational studies have revealed factors that influence the nondirected C–H activation in Cp*Ir complexes that contain carboxylate ligands. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C–H bond of the substrate. A nonlinear Hammett plot was obtained to examine substituted arenes where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. Electron-donating substituents in the para position were shown to have a bigger impact on the C–H bond cleavage step, whereas electron-withdrawing substituents influenced the substrate-binding step. Although cleavage of the C–H bond was predicted to be more facile with arenes that contain substituents in the para position by DFT calculations, the cyclometalations of anisole and benzonitrile were observed experimentally. This suggests that these substituents, even though they are weakly directing, still result in cyclometalation because the barriers for activation at the ortho and para positions of arenes are comparable (24.3 and 26.5 kcal/mol, respectively). Incorporation of a weakly bound ligand was found to be necessary for facile reactivity. It is predicted by DFT calculations that the replacement of an oxygen atom with a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C–H activation, as the incorporation of formimidate and <i>N</i>-methyl­formimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol)