Metallorganische verbindungen der lanthanoide CVIII. Synthese und strukturaufklärung monomerer organolanthanoidacetate

LnCl₃ reacts with Na(C₅Me₄¹Bu) in THF to form the organolanthanoid chlorides [(C₅Me₄¹Bu)₂LnCl(THF)] (Ln = La (1a), Lu (1b)). Compounds 1a and 1b yield in reaction with NaO₂CCH₃ the monomeric organolanthanoid acetates [(C₅Me₄¹Bu)₂LnO₂CCH₃] (Ln = La (2a), Lu (2b)). The single crystal X-ray structure analysis of 2b as well as the cryoscopic molecular weight investigation of 2a verify the monomeric structure of these complexes.     

Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: origins of the "dendrimer effect" with catalysts terminating in phenylseleno groups

Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H(2)O(2) and NaBr catalyzed by the addition of Frechét-type dendrimers terminating in -O(CH(2))(3)SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H(2)O(2), first-order rate constants for the selenoxide elimination were too small to produce PhSeO(2)H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br(-) with H(2)O(2). The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H(2)O(2) under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br(+)" produced initially by the uncatalyzed background reaction of H(2)O(2) with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.     

Solid-phase synthesis of lipidated peptides

Characteristic partial structures of lipidated proteins embodying different lipid groups as well as additional fluorescent tags or a maleimide for coupling to proteins can be synthesized readily by means of a new solid-phase technique employing the oxidative cleavage of the hydrazide linker as well as on-resin farnesylation and palmitoylation after appropriate deprotection of cysteine thiol groups as the key steps.     

Synthesis of phenylated phthalimides. The diels-alder reaction of tetraphenylcyclopentadienone with maleimides

A series of phenylated dihydrophthalimides has been synthesized by the Diels-Alder reaction of tetraphenylcyclopentadienone with maleimide, N-phenylmaleimide, and N,N'-o-, -m-, and -p-phenylenedimaleimide. Subsequent dehydrogenation of these compounds yielded the corresponding phenylated phthalimides. These phthalimides were also obtained in one step when the initial reactions were carried out in refluxing nitrobenzene.     

Complexity of coordinative bonding in thallium(I) anthranilates and salicylates

An inventory of the structural chemistry of thallium(I) shows many unexpected, almost random coordination numbers and coordination geometries that appear erratic and inconsistent. This nonstandard behavior is often ascribed to the specific lone-pair characteristics originating from relativistic effects. To provide data on a set of closely related compounds from which simple general rules of coordinative bonding at Tl(+) can be established, three thallium(I) anthranilates and three thallium(I) salicylates have been prepared from Tl(2)CO(3) and the corresponding 2-amino- and 2-hydroxy-benzoic acids and crystallized from aqueous solutions. All six compounds, the simple anthranilate (1) and salicylate (4) and the 3- and 4-methyl-substituted homologues (2, 3 and 5, 6) show different structures with large variations in the coordination motif. The coordination by oxygen in a geometry which covers less than a coordination hemisphere is the only common feature, complemented (only in 1) by a nitrogen coordination and by eta(6)-coordination of one (in 1, 2, 3, 6) or two phenyl rings (in 4). Tl-Tl contacts for which "thallophilic" bonding between closed shell metal atoms could be invoked, are generally very long (close to 4.0 A) or even well beyond the limit of standard van der Waals contacts. Hydrogen bonding is only obvious for the internal contacts of the amino- or hydroxy-benzoate ligands and does not contribute significantly to the assembly of the supramolecular structure which is dominated by oxygen bridges between thallium atoms. With the exception of 5, the formula units Tl[O(2)C(2-R)(3-R')(4-R')C(6)H(2)] are generally aggregated into dimers of various configurations depending on the relative orientation of the edge-sharing four-membered rings, and these dimers are further linked into strings or columns establishing N-Tl or Tl-O contacts and arene coordination. The drastic changes induced in the structures upon only small variations such as methyl substitution in 3- or 4-position of the ligand suggest that thallium(I) coordination is generally restricted to one hemisphere of nearest neighbors, but is extremely flexible in this realm. The open hemisphere may be partially capped by arene coordination (which is weak at a distance of ca. 3.1 A to the centroid of the ring) or feature very weak thallophilic contacts.     

Polyimides containing oxyethylene units

Four aromatic dinitro compounds containing oxyethylene units 1,2-bis(4-nitrophenoxy)ethane, bis(2)-(4-nitrophenoxy)ethyl ether, 1,2-bis(2)-(4-nitrophenoxy)ethoxy)ethane, and bis(2-(2-(4-nitrophenoxy)ethoxy)ethyl) ether were prepared by an aromatic nucleophilic substitution reaction that involved 1-fluoro-4-nitrobenzene and the appropriate oxyethylene glycol. Each of the dinitro compounds could be reduced chemically or catalytically to produce the diamines 1,2-bis(4-aminophenoxy)ethane, bis(2-(4-amino-phenoxy)ethyl ether, 1,2-bis(2-(4-aminophenoxy)ethoxy)ethane, and bis(2-(2-(4-aminophenoxy)ethoxy)ethyl) ether. Polymerization of the diamines was carried out in m-cresol with toluene as an azeotroping agent. Polymides derived from 5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuradione exhibited good solubility in chlorinated solvents, glass transition temperatures as low as 140°C, as evidenced by viscous flow, and good thermal stability; a 5% weight loss generally occured at greater than 450°C. Isothermal aging studies conducted at 600°F(315°C) for 200 h indicated that weight loss is directly related to oxyalkylene chain length–10% for the shortest and 50% for the longest chain.     

The effect of fluctuations on emission spectra: practical distinctions between raman and fluorescence spectra

By the use of stochastic theory it is clearly shown that two types of emission should to fluctuations such as would occur during interactions with their environment are resonantly driven by light. In our approach the electromagnetic field is treated in a classical manner and decay is introduced by a Wigner-Weisskopfansatz. The two types of emission should occur for a three-level system and relaxation to other levels is not involved in the mechanism. Also included is a discussion of operational definitions of fluorescence (and other spontaneous emissions) and of the relationship between absorption and scattering in terms of dephasing or T₂ processes, especially as this relates to conventional emission phenomena.     

Evaluation of a dual electrospray ionization/atmospheric pressure chemical ionization source at low flow rates (∼50 µL/min) for the analysis of both highly and weakly polar compounds

The atmospheric pressure ionization (API) source for a commercial mass spectrometer was modified to operate as a dual source in both the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) techniques by simultaneously utilizing the electrospray probe and the corona discharge needle. A switching box was designed to operate in either manual or programmable modes to permit rapid switching between ionization techniques without changing sources, probes, or breaking vacuum. The source can be operated using the following ionization techniques: ESI only, APCI only, ESI/APCI simultaneously, and ESI/APCI alternatingly. The optimum operating conditions for these ionization techniques were similar to the manufacturer’s original specifications except that the APCI flow rate was lower (～50 µL/min versus 1000 µL/min) and externally heated nebulizing gas was found to be desirable. A four-component mixture, introduced by flow injection, was used to demonstrate the versatility of the dual ESI/APCI source.     

Organosilicon compounds XIII. Cleavage of the silicon-carbon bond of 2-trimethylsilylbenzothiazole

The high-yield synthesis of 2-trimcthylsilylbenzothiazole is described, and the labile nature of this silicon-helerocyclic bond is proved when cleavage is affected by aldehydes, acid halides, chloro-formates and anhydrides. Justification for this unusual reactivity and further insight into the reaction mechanism is offered.     

Duplex Stabilization of DNA: Oligonucleotides containing 7-substituted 7-deazaadenines

The oligonucleotide building blocks 4b–d derived from 7-bromo-, 7-chloro-, and 7-methyl-substituted 7-deaza-2′-deoxyadenosines 3b–d were prepared. They were employed in the solid-phase synthesis of the oligonucleotides 7–25 . The dA residues of the homomer d(A₁₂), the alternating d[(A-T)₆], and the palindromic d(G-T-A-G-A-A-T-T-C-T-A-C) were replaced by 3b–d as well as by the parent 7-deaza-2′-deoxyadenosine ( 3a ). The melting profiles and CD spectra of oligonucleotide duplexes, showing this major groove modification, were measured, and the T_m values as well as the thermodynamic data were determined. It was found that small substituents such as Br, Cl, or Me introduced in the 7-position of a 7-deazaadenine residue increase the duplex stability compared to oligonucleotides containing adenine.