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The reduction of polycyclic aromatic hydrocarbons (PAHs) with alkali metals can, in some cases, afford a reductive dimerization process. This process takes place in systems that have the capability of forming a stable anionic subunit and concentrating a high electron density on a specific carbon atom, which will eventually form a new s bond.[1] Such behavior has been recently reported in the reduction of indenocorannulene, which undergoes a four-step reductive dimerization/bond-cleavage process.[1d] The reduction of pyrene (1) and 2,7-dimethylpyrene (2) with alkali metals has been thoroughly studied.[2] The reduction affords different products depending on solvent, counterion, and temperature. Three primary diamagnetic species can be detected when 1 (Scheme 1) is reduced with lithium metal in [D8]THF: a protonated pyrene monoanion (1a), a dianion (12 ), and a solvent-molecule-incorporated monoanion (solvent cleavage; 1b).[2c] The same results are obtained when 2 is reduced with lithiummetal. Protonation of the monoanion radicals of 1 and 2, and the solvent cleavage by their respective dianions at room temperature, illustrate that these species are very reactive. Another aspect of interest in the chemistry of pyrene is its aromatic character. Recent reports suggest that imparting nonplanarity in pyrene, by tethering two remote positions, does not significantly decrease the magnetic anisotropy effect of its nucleus in the neutral state.[3] To evaluate the effect of strain on the reduction of pyrene, two curved systems, [7](2,7)pyrenophane (3 ; Scheme 2)[3a] and [2]metacyclo[2](2,7)pyrenophane (4)[4] were reduced with lithium metal in [D8]THF, and the reduction process was followed by NMR spectroscopy. The two-electron reduction of strained pyrene moieties affords totally unexpected results. A new s bond is formed, to avoid strained antiaromaticity.[5] A careful and detailed follow up of the reduction process of 3 |
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