Low-coordinate cobalt(ii) terphenyl complexes: precursors to sterically encumbered ketones

Cobalt(ii) diaryl complexes react with CO to afford Co(2)(CO)(8) and sterically encumbered ketones whose structure varies depending on the nature of the aryl ligands.     

Synthesis of sterically encumbered organoselenium species and their selectivity towards Hg(II) ions

The synthesis and characterization data for an intriguing family of sterically encumbered tetrasubstituted organoselenium species are described. The open exterior geometry and flexibility of the multiple soft selenium-containing donor arms enhances conformational organization thus allowing the molecule to reveal a propensity for trapping Hg(II) ions. Preliminary studies on the ion-sensing properties of these molecules revealed that they can act as selective ionophores for Hg(II) ions.     

Stereoselective polymerization of rac- and meso-lactide catalyzed by sterically encumbered N-heterocyclic carbenes

New sterically encumbered N-heterocyclic carbene catalysts were synthesized and used to polymerize rac-lactide to give highly isotactic polylactide or meso-lactide to give heterotactic polylactide.     

Preparation and reactivity of sterically encumbered organocatalysts and their use in the preparation of (S)-Pregabalin precursors

Herein we describe the preparation of a new class of sterically demanding organocatalysts derived from d-fructose and their use, alongside hitherto unreported doubly quaternarised Cinchona ammonium salts, in the Michael reaction of nitromethane with 4-nitro-5-alkenyl-isoxazoles.     

Reactions of the (2-pyridyl) pyrrolide platinum(II) complex driven by sterically encumbered chelation: a model for the reversible attack of alcohol at the coordinated carbon monoxide

Treatment of 3,5-bis(trifluoromethyl)-2-(2'-pyridyl)pyrrole (fpyroH) with Pt(DMSO)2Cl2 and Na2CO3 in THF solution gave a light-yellow complex denoted as [Pt(fpyro)2] (1). A single-crystal X-ray diffraction study on 1 revealed a large conformational distortion around the platinum(II) center, which is attributed to interligand repulsion between the pyridyl groups and the CF3 substituents of the nearby pyrrolides. Reaction of 1 with N- and C-donor ligands such as acetonitrile, pyridine, isocyanide, and CO affords the adducts [Pt(fpyro)2(L)], L = NCMe (2), pyridine (3), CNBut (4), and CO (5), showing formation of one monodentate fpyro ligand by release of the strain energy. The variable-temperature 1H NMR studies showed a static structure for the N-substituted adducts 2 and 3, whereas the C-adducts 4 and 5 are shown to be more labile, displaying a pairwise exchange of bidentate and monodentate fpyro ligands in solution. Addition of ethanol to the coordinated CO in 5 during recrystallization is also established, affording an ethoxycarbonyl complex [Pt(fpyro)(fpyroH)(CO2Et)] (6), which was isolated as a crystalline solid and can be readily converted back to 5 and free ethanol upon dissolution at room temperature.     

Solid-state thermolysis of poly(p-t-butoxycarbonyloxystyrene) catalyzed by polymeric phenol: Effect of phase separation

Thermolysis of poly(p-t-butoxycarbonyloxystyrene) (PBOCST) catalyzed by polymeric phenol in the solid state is very much dependent on the extent of phase separation. Poly(p-hydroxystyrene) (PHOST, a thermolysis product of PBOCST) is not miscible with PBOCST. Therefore, this phenolic resin can not effectively catalyze the solid-state thermolysis reaction when simply blended with PBOCST. However, if the phenolic functionality is forced to be in the vicinity of the t-butoxycarbonyl (tBOC) group, the thermolysis takes place facilely at 130°C. The effect of casting solvent on thermolysis in PBOCST/Varcum films is also discussed.     

Preparation,structure, and reaction of a sterically encumbered 1-phosphaallene containing a cyclopropylidene moiety

[reaction: see text] Sterically protected (Z)-1-(2,4,6-tri-tert-butylphenyl)-2,5-dibromo-1-phosphapent-1-ene was allowed to react with potassium tert-butoxide to afford a cyclopropylidenephosphaethene, which was characterized spectroscopically and by X-ray crystallography. Construction of the cycloalkyl groups and isomerization of 1-phosphapenta-1,2,4-trienes to cyclopropylidenephosphaethenes are also described.     

Facile oxidation-based synthesis of sterically encumbered four-coordinate bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) and related three-coordinate copper(I) complexes

A new oxidation-based synthetic route was developed for synthesis of Cu(I) complexes with weakly coordinating ligands, leading to the synthesis of the elusive [Cu(dtbp)2]+ (dtbp, 2,9-di-tert-butyl-1,10-phenanthroline) complex that may be useful as a sensor or as a dye for dye-sensitized solar cells. An acetone solution of either 1 or 2 equiv of dtbp was added to excess Cu(0) and 1 equiv of AgY (Y is O3SCF3-, BF4-, SbF6-, or B(C6F5)4-) in a nitrogen-filled glove box. Following filtration and evaporation under vacuum, crystallization from CH2Cl2 and hexanes results in X-ray quality crystals of Cu(dtbp)(O3SCF3) (1), Cu(dtbp)(BF4) (2), [Cu(dtbp)(acetone)][SbF6] (3), [Cu(dtbp)2][B(C6F5)4].CH2Cl2 (4.CH2Cl2), [Cu(dtbp)2][BF4].CH2Cl2 (5.CH2Cl2), and [Cu(dtbp)2][SbF6].CH2Cl2 (6.CH2Cl2). Complexes 1-6 were characterized by X-ray crystallography and NMR. The Cu atom in complexes 1-3 exhibited distorted trigonal coordination geometries, reflecting the steric effect of the bulky tert-butyl substituents. The structures of the pseudotetrahedral complexes 4, 4.CH2Cl2, 5.CH2Cl2, and 6.CH2Cl2 revealed the longest average Cu-N distances (2.11 A, 2.11 A, 2.10 A, and 2.11 A, respectively) in this class of compounds-longer by more than three standard deviations from the average.     

Synthesis of sterically encumbered biaryls based on a ‘copper(I)-catalyzed arylation/[3+3] cyclocondensation’ strategy

Sterically encumbered biaryls are prepared in two steps by CuI-proline-catalyzed arylation of acetylacetone to give 3-arylpentane-2,4-diones and subsequent formal [3+3] cyclization of the latter with 1,3-bis(trimethylsilyloxy)-1,3-dienes.     

Synthesis of of sterically encumbered biaryls based on a ‘copper(I)-catalyzed arylation/[3+3] cyclocondensation’ strategy

Sterically encumbered biaryls are prepared in two steps by combination of the CuI-proline-catalyzed arylation of acetylacetone with formal [3+3] cyclizations of 1,3-bis(trimethylsilyloxy)-1,3-dienes. In addition, the synthesis of 4,6- and 5,6-diarylsalicylates based on [3+3] cyclizations is reported.     

Regioselective synthesis of sterically encumbered diaryl ethers based on one-pot cyclizations of 4-aryloxy-1,3-bis(trimethylsilyloxy)-1,3-dienes

Sterically encumbered diaryl ethers are prepared based on formal [3+3] cyclizations of novel 4-aryloxy-1,3-bis(trimethylsilyloxy)-1,3-dienes.     

Crystal structures and magnetic properties of a sterically encumbered dithiadiazolyl radical, 2,4,6-(F3C)3C6H2CNSSN.

The sterically protected dithiadiazolyl radical (F3C)3C6H2CNSSN. (1) crystallises in two polymorphs: 1alpha, comprised of monomeric units and 1beta, containing a mixture of both pi*-pi* dimers and S = (1/2) monomers; whilst both polymorphs exhibit similar structure-directing motifs, the variation in packing leads to different magnetic behaviour.     

Gas phase thermolysis of pyrazolines. 5. Electronic structure and gas phase thermolysis of tetrazole derivatives studied by photoelectron spectroscopy

The photoelectron spectra of substituted tetrazoles 1–3 , 1,4-dihydro-5H-tetrazol-5-ones 4–9 , and 1,4-dihydro-5H-tetrazole-5-thiones 10–15 have been recorded. Based on PM3 and some ab initio calculations, the ionization potentials have been assigned to molecular orbitals. Gas-phase thermolyses of 1–15 have been studied by real-time gas analysis controlled by photoelectron spectroscopy. Compounds 1 and 2 lose formaldehyde and thioformaldehyde, respectively, leaving unsubstituted tetrazole (16), which decomposes mainly through extrusion of a nitrogen molecule and formation of cyanamide. Thiirane is split off from 3, and the remaining molecule decomposes into smaller products. Compounds 4–9 are cleaved by [3+2] cycloreversion to isocyanates and azides. Some of the unsymmetrically substituted compounds exhibit a marked selectivity in this reaction. For thiones 10–15 [3+2] cycloreversion is the main way of decomposition affording isothiocyanates and azides. In addition, the sulfur atom can split off and dimerize or abstract hydrogen atoms to form hydrogen sulfide. Some products like thiirene, formaldehyde, thioformaldehyde and acetaldehyde are generated solely from substituents. Photoelectron spectroscopy proved to be an excellent method for such thermolysis studies.     

A new phase of the c(4 x 2) superstructure of alkanethiols grown by vapor phase deposition on gold

A self-assembled monolayer of dodecanethiol is grown onto (111) oriented gold by vacuum phase deposition and studied by ultrahigh vacuum scanning tunneling microscopy (STM). The films consist of domains that exhibit the c(4 x 2) over-structure of the hexagonal (square root of 3 x square root of 3)R30 of alkanethiols on gold. The domain size is only limited by the terrace size of the underlying gold. By higher resolution scans a new phase of the c(4 x 2) structure consisting of four inequivalent molecules that display different heights in the STM images is discovered.     

Rhodium-catalyzed dehydrocoupling of the sterically encumbered phosphine-borane adduct tBu(2)PH.BH(3): synthesis of the linear dimers tBu(2)PH-BH(2)-tBu(2)P-BH(3) and tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl

The dehydrocoupling of the sterically hindered phosphine-borane adduct tBu(2)PH.BH(3) above 140 degrees C is catalyzed by the rhodium complexes [Rh(1,5-cod)(2)][OTf] or Rh(6)(CO)(16) to give the four-membered chain tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), which was isolated in 60% yield and characterized by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. Thermolysis of 1 in the temperature range 175-180 degrees C led to partial decomposition and the formation of tBu(2)PH.BH(3). When the dehydrocoupling of tBu(2)PH.BH(3) was performed in the presence of [[Rh(mu-Cl)(1,5-cod)](2)] or RhCl(3) hydrate, the chlorinated compound tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) was formed which could not be obtained free of 1. The molecular structures of tBu(2)PH.BH(3), tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), and tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) together with 1 were determined by single-crystal X-ray diffraction studies.     

Desulfonylation of poly(4‐hydroxystyrene sulfone) by vapor phase silylation

Vapor phase silylation of poly(4‐hydroxystyrene sulfone) (PHOSS) film was carried out with (trimethylsilyl)dimethylamine (TMSDMA) as a silylation reagent. Infrared spectroscopy was used to follow the silylation. Phenolic hydroxyl groups were trimethylsilylated, but desulfonylation of PHOSS was greatly enhanced simultaneously. The reaction rates were investigated at reaction temperatures of 50, 60, and 70°C. The rate of silylation increased with increasing reaction temperature. However, the rate of desulfonylation was very fast in the presence of TMSDMA and was virtually invariant with reaction temperature. It was confirmed that trimethylsilylation in the polymer side chain of PHOSS enhanced desulfonylation in the main chain. Trimethylsilylation might be expected to lower the ceiling temperature of the polymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1549–1554, 1999     

Sterically encumbered regioselective cycloaddition of a calixarene derived bis(spirodienone) with 1,2-benzoquinones

A calix[4]arene derived bis(spirodienone) acts as the 2π component in a cycloaddition reaction with two molecules of 3,5-di-tert-butyl-1,2-benzoquinone in the [2+4] manner leading to macrocycles with a benzodioxin moiety. A theoretical rationalization of the results suggested a sterically encumbered regioselective pathway, which gives sterically crowded products.     

Nonradiative intramolecular deactivation of Tb3+ fluorescence in a vapor phase terbium(III) complex

For the first time, fluorescence lifetime studies are reported for a trivalent rare-earth gas: specifically, for the ⁵D₄ state of Tb³⁺ in the vapor phase chelate 2,2,6,6-tetramethyl-3,5-heptanedione. Measurements of the fluorescence decay as a function of temperature and pressure demonstrate that intermolecular collisional deactivation is unimportant and that nonradiative deactivation by intramolecular processes dominates the fluorescence lifetime at temperatures of 230–300°C. The rate for the latter processes is well described by an Arrhenius equation and suggests that the Tb³⁺ relaxation occurs via transfer to low-lying excited states of the chelate.     

Facile generation of tripeptide radical cations in vacuo via intramolecular electron transfer in CuII tripeptide complexes containing sterically encumbered terpyridine ligands

Molecular radical cations of tripeptides of the form glycylglycyl(residue X) (GGX*+) are produced by the collision-induced, intramolecular one-electron transfer of [Cu(II)(L)GGX]*2+ complexes (L = triamine ligand). We demonstrate, for the first time, the formation of molecular radical cations of all of the aliphatic, basic, aromatic, acidic, and some heteroatom-bearing GGX tripeptides, albeit inefficiently in some cases, by altering the structure of the auxiliary polyamine ligand bound to the copper atom. The design of the ligand allows exquisite control over the nature of the dissociation pathway. Steric hindrance of bulky groups in the ligand affects the binding of the peptide to the copper ion; this interaction is an important factor in determining whether the electron transfer pathway predominates.