Amidine-Mediated Zwitterionic Polymerization of Lactide

The ring-opening polymerization (ROP) of lactide with DBU (1,8-diazabicyclo[5.4.0] undec-7-ene) is described. Room temperature polymerization using the neutral amine catalyst DBU in the absence of any other initiator produces polymers with narrow polydispersities and shows a linear relationship between molecular weight and conversion. The resulting polymers were characterized and determined to be cyclic. DFT calculations support a mechanistic hypothesis involving a zwitterionic acyl amidinium intermediate

Experimental and Theoretical Study of CO₂ Insertion into Ruthenium Hydride Complexes

The ruthenium hydride [RuH­(CNN)­(dppb)] (<b>1</b>; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis­(diphenylphosphino)­butane) reacts rapidly and irreversibly with CO₂ under ambient conditions to yield the corresponding Ru formate complex <b>2</b>. In contrast, the Ru hydride <b>1</b> reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy Δ<i>G</i>°_298 K = −3.1 kcal/mol measured by ¹H NMR in tetrahydrofuran-<i>d</i>₈. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO₂ into the Ru–hydride bond of <b>1</b>. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N–H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO₂ insertion proceeds by an initial Ru–H hydride transfer to CO₂ followed by rotation of the resulting N–H-stabilized formate to a Ru–O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO₂ insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO₂ insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO₂ and formate

Chemoselective Pd-Catalyzed Oxidation of Polyols: Synthetic Scope and Mechanistic Studies

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)­Pd­(OAc)]₂(OTf)₂ (<b>1</b>) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (<i>S</i>,<i>S</i>)-1,2,3,4-tetrahydroxybutane [(<i>S</i>,<i>S</i>)-threitol] to (<i>S</i>)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle

Chemoselective Pd-Catalyzed Oxidation of Polyols: Synthetic Scope and Mechanistic Studies

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)­Pd­(OAc)]₂(OTf)₂ (<b>1</b>) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (<i>S</i>,<i>S</i>)-1,2,3,4-tetrahydroxybutane [(<i>S</i>,<i>S</i>)-threitol] to (<i>S</i>)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle

Chemoselective Pd-Catalyzed Oxidation of Polyols: Synthetic Scope and Mechanistic Studies

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)­Pd­(OAc)]₂(OTf)₂ (<b>1</b>) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (<i>S</i>,<i>S</i>)-1,2,3,4-tetrahydroxybutane [(<i>S</i>,<i>S</i>)-threitol] to (<i>S</i>)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle