The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates.

Thermal decarboxylation of the mesomeric betaine 1,2‐dimethylindazolium‐3‐carboxylate resulted in the formation of the N‐heterocyclic carbene indazol‐3‐ylidene which deprotonated a second molecule of the betaine to give an azomethine ylide. This 1,3‐dipole underwent a cycloaddition/decarboxylation sequence on treatment with ethyl or methyl 3‐phenylpropiolate to give the new ring system 3,5‐dihydro‐2H‐pyrrolo[1,2‐b]indazole. By contrast, dimethyl or diethyl but‐2‐ynedioate (DMAD, DEAD) as the activated triple bond yielded this ring system with an ester group at the 9b‐position, which originates from the acetylene derivative. Model reactions were carried out to elucidate the mechanisms of these reactions, and some trapping products of the N‐heterocyclic carbene have been characterized. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

No abstract

Pyrrolo[2,1‐c][1,4]benzodiazepine‐5,11‐dione was converted into the corresponding C‐11‐monothiolactam and subsequently treated with amines to give cyclic amidines, which racemize due to the formation of tautomers (NMR, X‐ray analysis) under basic conditions. We treated these amidines with bis(trichlorophenyl) malonate esters. Formation of neutral tautomers of the 1,3,8‐triones of the new 4,7a,12b‐triaza‐dibenzo[e,g]azulene ring system gave a twisted molecule with both helical and chiral structure elements (NMR, X‐ray analysis), which caused a splitting of the NMR signals into two sets. Results of two X‐ray single crystal analyses are presented, together with an ab initio calculation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)