Nitroacetylenes Harnessed by Cobalt
The syntheses and characterization of nitroacetylenes have been studied with
respect to their large-scale production, purification, and NMR spectra, respectively. The 13C NMR spectrum of 1-nitro-2-(trimethylsilyl)ethyne (18) evidenced 13C–14N coupling
between the alkynyl carbon and the attached nitrogen, in agreement with the previously reported spectrum of 1-nitroethyne (16). The multi-gram preparation of high purity 18 is described. A history of previous work with nitroacetylenes is reviewed.
The stabilization of nitroacetylenes as hexacarbonyl dicobalt alkyne complexes was investigated. In conjunction with appropriate oxidizers, such complexes were shown to be long-term storage media for free nitroalkynes. The syntheses and complete characterization of the first two nitroalkyne transition metal complexes, [μ-1-nitro-2(trimethylsilyl)ethyne-1,2-diyl]bis(tricarbonylcobalt)(Co–Co) (25) and [μ-1-nitroethyne1,2-diyl]bis(tricarbonylcobalt)(Co–Co) (26), are described. Phosphine derivatives of 25 were prepared and studied. The synthesis of [μ-1,2-dinitroethyne-1,2diyl]bis(tricarbonylcobalt)(Co–Co) (24) by direct ligation, transformation of functional groups on existing complexes, and nitration of such complexes was unsuccessful.
Complexes 25 and 26 were applied to the synthesis of organic molecules. While they did not yield cyclopentenones via the Pauson-Khand cyclization, they rendered benzene derivatives by the [2 + 2 + 2] cobalt-mediated cyclotrimerization. In addition to two nitroindanes and a nitrotetralin, several new trinitrobenzene isomers were made, all potential precursors to energetic materials. The X-ray crystal structure of one of these,
1,3,5-trinitro-2,4,6-tris(trimethylsilyl)benzene (325), showed a planar, but distorted aromatic ring.