Cyclopropenium Enhanced Thiourea Catalysis

An integral part of modern organocatalysis is the development and application of thiourea catalysts. Here, as part of our program aimed at developing cyclopropenium catalysts, the synthesis of a thiourea-cyclopropenium organocatalyst with both cationic hydrogen-bond donor and electrostatic character is reported. The utility of the this thiourea organocatalyst is showcased in pyranylation reactions employing phenols, primary, secondary, and tertiary alcohols under operationally simple and mild reaction conditions for a broad substrate scope. The addition of benzoic acid as a co-catalyst facilitating cooperative Brønsted acid catalysis was found to be valuable for reactions involving phenols and higher substituted alcohols. Mechanistic investigations, including kinetic and 1H NMR binding studies in conjunction with density function theory calculations, are described that collectively support a Brønsted acid mode of catalysis

Cyclopropenium Enhanced Thiourea Catalysis

An integral part of modern organocatalysis is the development and application of thiourea catalysts. Here, as part of our program aimed at developing cyclopropenium catalysts, the synthesis of a thiourea-cyclopropenium organocatalyst with both cationic hydrogen-bond donor and electrostatic character is reported. The utility of the this thiourea organocatalyst is showcased in pyranylation reactions employing phenols, primary, secondary, and tertiary alcohols under operationally simple and mild reaction conditions for a broad substrate scope. The addition of benzoic acid as a co-catalyst facilitating cooperative Brønsted acid catalysis was found to be valuable for reactions involving phenols and higher substituted alcohols. Mechanistic investigations, including kinetic and 1H NMR binding studies in conjunction with density function theory calculations, are described that collectively support a Brønsted acid mode of catalysis

Synthesis and Characterization of a New Family of Spin Bearing TTF Ligands

The syntheses and characterization of two new tetrathiafulvalene (TTF) derivatives bearing pyridine-based substituents and 1,5‘-dimethyl-6-oxoverdazyl radicals are described. The TTF-pyridine and bipyridine aldehydes were prepared via a palladium-catalyzed cross-coupling reaction between mono(tributylstannyl)tetrathiafulvalene (3) and the appropriate formylpyridyl halides (4). The radical precursors, the corresponding 1,2,4,5-tetrazanes, were prepared by condensation of the bis(1-methylhydrazide) of carbonic acid with the TTF bearing pyridyl aldehyde. Oxidation of tetrazanes 8 and 9 with 1,4-benzoquinone afforded the donor radicals 1 and 2 as 1:1 complexes with hydroquinone. Both complexes are stable in the solid state and their electronic properties have been characterized by EPR, cyclic voltammetry, and UV/vis spectroscopy. The TTF core of both compounds was oxidized both chemically and electrochemically to afford the corresponding cation diradical species. The electronic properties of both donor radicals have been probed by cyclic voltammetry, UV−vis spectroscopy, and preliminary EPR measurements

Preparation and Coordination Complex of the First Imine-Bridged Tetrathiafulvalene−Pyridine Donor Ligand

The first imine-bridged pyridyltetrathiafulvalene building block (TTF−CHN−Py, 1) has been synthesized via the Schiff base condensation of formyltetrathiafulvalene and 2-aminopyridine. The preparation, X-ray crystal structure, electrochemical and magnetic characterization of a 1:1 copper complex [CuII(hfac)2(TTF−CHN−Py)] (2) are reported. The crystal structure reveals that the imine N atom participates in chelation to the paramagnetic center, thus making this ligand an attractive precursor for the assembly of π−d systems

Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene–Ag(I) Complex

The unprecedented synthesis, single-crystal X-ray structure, and first catalytic application of a dicarbene–Ag­(I) complex [Ag­(BAC)2]­[CO2CF3] (BAC = bis­(diisopropyl)­aminocyclopropenylidene) is reported. This novel complex provides a versatile catalytic platform for selective aerobic oxidation of benzylic alcohols to aldehyde or ketone products in high yields. Ease of experimental execution coupled with the use of abundant atmospheric molecular oxygen as an oxidant and low catalyst loading are inherit strengths of these oxidations

Organocatalysis Linked to Charge-Enhanced Acidity with Superelectrophilic Traits

Hydrogen bonding is ubiquitous throughout nature and serves as a versatile platform for accessing chemical reactivity. In leveraging this force, chemists have utilized organocatalysts to expand the spectrum of chemical reactivity enabled by hydrogen bonding and at the extreme proton transfer. Despite this broad utility, exploiting charge as a hydrogen-bond activation strategy is unknown for squaramide catalysts. Considering this deficiency, herein, we disclose a cationic squaramide–cyclopropenium organocatalyst displaying charge-enhanced acidity. Key to this advancement was cationic charge, linked to superelectrophilic traits and strong Brønsted acidity, allowing for the construction of unprecedented oxime ether functionality among other important chemical transformations. The origin of this remarkable reactivity was delineated by computational analysis and in-depth experimental mechanistic studies

Preparation and Coordination Complex of the First Imine-Bridged Tetrathiafulvalene−Pyridine Donor Ligand

The first imine-bridged pyridyltetrathiafulvalene building block (TTF−CHN−Py, 1) has been synthesized via the Schiff base condensation of formyltetrathiafulvalene and 2-aminopyridine. The preparation, X-ray crystal structure, electrochemical and magnetic characterization of a 1:1 copper complex [CuII(hfac)2(TTF−CHN−Py)] (2) are reported. The crystal structure reveals that the imine N atom participates in chelation to the paramagnetic center, thus making this ligand an attractive precursor for the assembly of π−d systems

Palladium(II), Platinum(II), and Iridium(I) Complexes of 2-Phosphino-1-dimethylaminoferrocenes: A Survey of Structure and Catalysis

A series of PdCl2, PtCl2, and Ir(COD)BArF complexes bearing a rare class of racemic bidentate 2-phosphino-1-dimethylaminoferrocene ligands were prepared and characterized by NMR spectroscopy and X-ray crystallography. The new complexes displayed a structural trend relating a decrease in heteroatom-metal bond length with an increase in ligand bite angle on going from Ir to Pd and Pt. The PdCl2 and PtCl2 complexes were almost isostructural and featured MCl2 moieties in the plane of the substituted Cp ring of the ligand. In contrast, the Ir(COD)+ complex was distinguished by a bend of the Ir(COD) moiety toward the unsubstituted (Cp′) ring. The latter gave rise to a steric interaction that placed the Cp rings in almost eclipsed conformations. Ligand 8a (2-diphenylphosphino-1-dimethylaminoferrocene) was able to promote Pd-catalyzed Suzuki−Miyaura and Buchwald−Hartwig coupling of aryl chlorides in addition to Ir-catalyzed hydrogenation of electron-deficient and unactivated alkenes. A preliminary intramolecular hydroamination of a terminal alkene using 8a in conjunction with Ir(I) afforded the cyclized product in 64% yield

Studies on a “Disappearing Polymorph”: Thermal and Magnetic Characterization of α‑<i>p</i>‑NCC₆F₄CNSSN^•

The α-and β-phases of the thiazyl radical <i>p</i>-NCC₆F₄­CNSSN^• (<b>1</b>) can be selectively prepared by careful control of the sublimation conditions, with the α-phase crystallizing preferentially when the substrate temperature is maintained below −10 °C, whereas the β-phase is isolated when the substrate temperature is maintained at or above ambient temperature. Differential scanning calorimatry studies reveal that the α-phase converts to the β-phase upon warming over the range 111–117 °C (Δ<i>H</i> = +4 kJ·mol^–1) via a melt–recrystallization process, with the β-phase itself melting at 167–170 °C (Δ<i>H</i>_fus = 27 kJ·mol^–1). IR and Raman spectroscopy can be used to clearly discriminate between <b>1α</b> and <b>1β</b>. The α-phase shows a broad maximum in the magnetic susceptibility around 8 K that, coupled with a broad maximum in the heat capacity, is indicative of short-range order. Some field dependence of the susceptibility below 3 K is observed, but the lack of features in the ac susceptibility, <i>M</i> vs <i>H</i> plots, or heat capacity mitigates against long-range order in <b>1α</b>