ChemInform Abstract: News About Acepentalene, Cyclopentadienyl Cations, and Other Elusive Aromatic and Antiaromatic Compounds

Abstract

Dipotassium acepentalenediide 7-K2 is easily available in quantitative yield from triquinacene 3, it can be trapped with a variety of electrophiles to give 4,7-dihydroacepentalene derivatives 15-R2. The unsubstituted 15-H2 obtained by hydrolysis of 7-K2 rapidly dimerizes and also undergoes other (4˛2) cycloadditions with added dienes. The crystalline dilithium acepentalenediide 7-Li2 has been obtained by metal-metal exchange on 15-(SnMe3)2, char- acterized by a crystal structure analysis and used to prepare the novel metal complexes 19-22. The bis(trimethylstannyl) derivative 15-(SnMe3)2 also led to the first unequivocal detection of neutral acepentalene 5. Computational results indicate that the antiaromatic cyclopentadienyl cation is significantly stabilized in its singlet state by two or more cyclopropyl groups. The readily accessible (tetracyclopropylcyclopentadienone)tricarbonyliron 32 can be alkylated and protonated to yield complexes 40-R and 41-X with a formal tetracyclopropylcyclopentadienyl cation ligand. Tetracyclopropylcyclopentadiene 43, easily prepared in one step from dicyclo- propylacetylene, can be deprotonated, and the resulting anion 46 chlorinated to 45 and transformed to metallocenes 44 and 47. Butadiyne-expanded (n)rotanes 49 and permethylated analogues 50 are considered as potential precursors to Cn carbon clusters. Differential scanning calorimetry with evolved gas analysis has revealed that compounds 49 (n o 5, 6, 8) cleave off ethylene, while the permethylated analogue 50 (n o 6) cleaves off tetramethylethylene. If fragmentation would occur 12 times from exp(12)rotane 49 (n o 12), a C60 carbon cluster would be left over.