Zur komplexchemie von 4-zentren-π-systemen : XII. Glyoxaldiiminkomplexe: 1H-nmr-spektren und long range-kopplungen

Glyoxaldiimines (I) RN=CHCH=NR show in the NMR a downfield shift of glyoxalic protons (H_gly) when complexed to

(II: M = Mo, W), but increasing high field shifts when going to

(III) and

complexes (IV). In the tungsten complexes II H_glyW coupling satellites are resolved (J(WH) ≈4–6 Hz); in the phosphine compounds III, IV long range PH_gly coupling is clearly visible. Mechanisms of spinspin interaction are briefly discussed in connection with the EPR results on monoanionic radicals of II–IV.     

Massenspektrometrie von Silylpyrazolen—I: Isomere Trimethylsilylpyrazole

By a comparison of the mass spectra of mono-, bis- and tris(trimethylsilyl)pyrazole isomers, characteristic influences of the different TMS-groups are revealed. An N-TMS-group forms an ion of mass 72 in the spectrum of N-TMS-pyrazole and with more than one TMS-group present, leads to the expulsion of a neutral fragment C₃H₈Si. Decomposition of the 4-TMS-group is more facile than the others, giving rise to a small molecular ion of intensity (approx.) 5%Σ₄₀. In N-silylated pyrazoles without a 4-TMS-group, intensive elimination of HCN from the [M ? CH₃˙]⁺ ion can be assigned to an unsubstituted C-3 position. In the case of unsymmetrically substituted pyrazoles without and N-TMS-group, silylation by an N-TMS acetamide may help to identify the predominating tautomer.     

Synthese von Pyrimido[1,2-a]benzimidazolen durch Umsetzung von 2-Aminobenzimidazol mit Acetylendicarbonsäure-dimethylester und ihre Überführung in Imidazo[1,2-a]benzimidazole

The main product of the reaction between 2-aminobenzimidazole and dimethyl acetylenedicarboxylate is shown to be methyl 1, 2-dihydro-2-oxo-pyrimido[1, 2-a]benzimidazole-4-carboxylate ( 6 ), which can be methylated at position 1 to give 7 . Catalytic hydrogenation of 7 leads to the 1,2,3,4-tetrahydro derivative 8 , whereas NaBH₄ reduces the ester and, to some extent, the double bond to yield a mixture of 9 and 10 . When a 1-substituted 1, 2-dihydro-4-hydroxymethyl-pyrimido[1, 2-a]benzimidazol-2-one (for example 11 ) is catalytically hydrogenated, the double bond is unaffected, but hydrogenolysis of the alcohol group occurs instead to give 13 . The lactam group is less stable in the tetrahydro series than in the dihydro compounds. For example, the lactam is cleaved when 8 is treated with amines containing a small amount of water, and the monoamides 16 and 17 are formed. Similarly, sodium hydroxyde cleaves the lactam under mild conditions to the dicarboxylic acid 19 , which can be converted to 2, 3-dihydro-1-methyl-2-oxo [1H]imidazo[1, 2-a]benzimidazole-3-acetic acid (4-methyl)piperazide ( 20 ) with thionyl chloride and N-methylpiperazine. However, when 7 is treated with methylamine at low temperature, the amide 22 is formed, whilst at room temperature the amine attacks both the ester and the double bond to give 23 . The structure of 8 was confirmed by X-ray analysis.     

Elektronenreiche übergangsmetallkomplexe : II. Nitril-phosphan-molybdändicarbonyle: Farbe und solvatochromie

Complexes of aromatic or α,β-unsaturated nitriles YCN of the type (YCN)₂(PR₃)₂Mo(CO)₂ and (YCN)(PR₃)₃Mo(CO)₂ are new chromophores. Their intense electronic absorption band in the visible spectrum is strongly influenced by substituents and solvent polarity. Acceptor properties of nitriles are discussed on the basis of CNDO-calculations. CT-character and solvato-chromism can be interpreted in terms of the back-bonding ability of the nitriles in electron-rich metal complexes.     

Darstellung und Reaktionen des 3-Ethinyl-2,4-diformyl-pentandials

Preparation and physical and chemical properties of 3-ethinyl-2,4-diformyl-pentandial (3 a) and its salts are described.3 a contains 2 malonaldehyde groups. Starting from3 a, 4H-pyranes, 4H-dihydropyridines, dipyrazoles, diisoxazoles, bisdiazepines and vinamidine derivates are obtained.

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ESR. Spectra and Structures of Radical Anions in the Dibenzo[a,e]cyclooctene Series

ESR. studies are reported for the radical anions of 5,6-didehydro- and 5,6,11,12-tetradehydro-dibenzo[a,e]cyclooctene (III and IV, resp.), in addition to that of dibenzo[a,e]cyclooctene (II) itself, the spectrum of which has been reexamined. Comparison of the proton and ¹³C coupling constants for II·?, III·? and IV·? indicates that the three radical anions do not differ greatly in their electronic and molecular structures. This statement implies that II·? should also be substantially planar, i.e., the tub-shaped eight-membered ring in II is expected to flatten on passing from the neutral molecule to its radical anion. Support for postulating such a change in geometry, analogous to that encountered with the parent cyclooctatetraene (I), is provided by INDO calculations.