Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)X(η5-C5Me5)] [NHC = 1,3-dimethylimidazol-2-ylidene (Me-NHC), 1,3-bis(2,4,6,-trimethylphenyl)imidazol-2-ylidene (Mes-NHC), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (iPr-NHC); X = Cl, I] were prepared from the reaction of pentamethylcyclopentadienyl acetylacetonate nickel(II) with 1 equiv of the corresponding imidazolium salt (NHC·HX). The new complexes [Ni(Me-NHC)I(η5-C5Me5)] 1a, [Ni(Mes-NHC)Cl(η5-C5Me5)] 2, and [Ni(iPr-NHC)Cl(η5-C5Me5)] 3 were obtained in moderate to good yields and were fully characterized by 13C and 1H NMR spectroscopy, and in the cases of 1a and 2 by single-crystal X-ray crystallography. The related cyclopentadienyl complex [Ni(Me-NHC)I(η5-C5H5)] 1b was also synthesized and structurally characterized; its geometry and spectroscopic data are comparable to those of complex 1a. The variable-temperature (VT) 1H NMR spectra of the sterically constrained complexes 2 and 3 are consistent with restricted rotation about the nickel−carbene carbon bond. The free energy of activation for the dynamic processes, in both cases, was determined to be on the order of 65−67 kJ mol−1 by VT NMR experiments.
Leukotriene B(3) and B(4) are part of an important class of signaling molecules - the leukotrienes, implicated in the inflammation process. Their pro-inflammatory effects have been widely recognized for almost three decades but it is only recently that their benefit in host defense has begun to be acknowledged. Their use as therapeutic agents is, unfortunately, limited by rapid metabolism. However, over the past 25 years, a number of stable leukotriene B(3) and B(4) analogues have been produced. In this review, we examine their medicinal chemistry and biological evaluation.
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