Thieno[3,4-b]pyrazines: synthesis,structure, and reactivity

A general synthetic route has been developed for the efficient preparation of 2,3-disubstituted thieno[3,4-b]pyrazines. These methods eliminate problems in the preparation of the precursor 3,4-diaminothiophene and utilize alpha-diones prepared through the reaction of the appropriate organocuprates with oxalyl chloride. This combination allows the convenient preparation of thieno[3,4-b]pyrazine and its 2,3-disubstituted analogues (where substituent = methyl, hexyl, octyl, decyl, dodecyl, and phenyl) in high yield. Characterization of the structure and reactivity of this class of compounds is also described, including the results of structural, electrochemical, and pK(a) studies.     

Inert gas condensation of metal microclusters

Metal microclusters of lead, indium, bismuth and antimony in the size range between 2 to 500 atoms per cluster are generated by inert gas condensation, extracted into vacuum and detected by mass spectrometry. Resolved clusterpeaks are observed up to Pb₁₁₀, In₅₀, Bi₇₀ and Sb₂₄₀. Condensation conditions, beam intensities and limitations of the cluster source are discussed.     

Analysis of the Q-branch transitions of the a2ν2-sν2 band of ammonia

The Q-branch transitions of the a2ν₂-sν₂ band of ¹⁴NH₃ are analyzed on the basis of a polynomial expansion up to eighth order in J and K. A good fit to the data for J ≤ 11 and precise values of the molecular constants are obtained in a 13-parameter fit (ΔG_JJKK = ΔG_KKKK = 0). Results of the statistical analysis are used to calculate wavenumbers and variances for transitions with J ≤ 13 that have not yet been measured.     

Antibiotica aus algen—XVI : Polyhydroxyphenyläther aus der phaeophycee Halidrys siliquosa

With the aid of preparative layer chromatography several compounds could be isolated from a mixture of acetylated phenols obtained from Halidrys siliquosa (L.) Lyngbye. Peracetylated heptafuhalol as well as its deacetylpermethyl derivative were examined by MS, IR, ¹H NMR and ¹³C NMR methods. Other constituents are probably lower members of a homologous series with 6, 5 and 4 phloroglucinol nuclei, respectively. The established building principle is also valid for a “high molecular mass polymer” fraction of acetylated phenols from Halidrys. The free phenols have the properties of tannins, and will be denoted phlorotannins.     

A new class of transition metal pincer ligand: tantalum complexes that feature a [CCC] X3-donor array derived from a terphenyl ligand

A new class of [CCC] X(3)-donor pincer ligand for transition metals has been constructed via cyclometalation of a 2,6-di-p-tolylphenyl ([Ar(Tol(2))]) derivative. Specifically, addition of PMe(3) to [Ar(Tol(2))]TaMe(3)Cl induces elimination of methane and formation of the pincer complex, [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl (Tol' = C(6)H(3)Me), which may also be obtained by treatment of Ta(PMe(3))(2)Me(3)Cl(2) with [Ar(Tol(2))]Li. Solutions of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl undergo ligand redistribution with the formation of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Me(2)and [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2), which may also be synthesized by the reactions of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl with MeMgBr and ZnCl(2), respectively. Reduction of [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2) with KC(8) in benzene gives the benzene complex [κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)(η(6)-C(6)H(6)) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta(PMe(3))(2)(CD(3))(3)Cl(2) demonstrates that the pincer ligand is created by a pair of Ar-H/Ta-Me sigma-bond metathesis transformations, rather than by a mechanism that involves α-H abstraction by a tantalum methyl ligand.     

Formation of melamium adducts by pyrolysis of thiourea or melamine/NH4 Cl mixtures

Pyrolysis of prominent precursor compounds for the synthesis of carbon nitride type materials (e.g., melamine, thiourea) have been studied in detail. Molecular adducts containing monoprotonated melamium C(6)N(11)H(10)(+) and melaminium HC(3)N(3)(NH(2))(3)(+) ions, respectively, have been identified as intermediates. The adduct C(6)N(11)H(10)Cl·0.5NH(4)Cl was obtained by the reaction of melamine C(3)N(3)(NH(2))(3) with NH(4)Cl at 450 °C. During the pyrolysis of thiourea, guanidinium thiocyanate was initially formed and subsequently the melamium thiocyanate melamine adduct C(6)N(11)H(10)SCN·2C(3)N(3)(NH(2))(3) was isolated at 300 °C. A second melaminium thiocyanate melamine adduct with the formula HC(3)N(3)(NH(2))(3)SCN·2C(3)N(3)(NH(2))(3) represents an intermediary reaction product that is best accessible at low pressures. The crystal structures of the compounds were solved by single-crystal XRD. Unequivocal proton localization at the C(6)N(11)H(10)(+) ion was established. A typical intramolecular and interannular hydrogen bridge and other characteristic hydrogen-bonding motifs were identified. Additionally, the adducts were investigated by solid-state NMR spectroscopy. Our study provides detailed insight into the thermal condensation of thiourea by identifying and characterizing key intermediates involved in the condensation process leading to carbon nitride type materials. Furthermore, factors promoting the formation of melamium adduct phases over melem are discussed.