The Question of Aromaticity in the 1-Lithio-1-methyl-1-silafluorenide Anion. Evidence for a Localized Silyl Anion1

Heavier element-containing aromatics of [4n+2]-electron systems

While the implication of the aromaticity concept has been dramatically expanded to date since its emergence in 1865, the classical [4n+2]/4n-electron counting protocol still plays an essential role in evaluating the aromatic nature of compounds. Over the last few decades, a variety of heavier heterocycles featuring the formal [4n+2] π-electron arrangements have been developed, which allows for assessing their aromatic nature. In this review, we present recent developments of the [4n+2]-electron systems of heavier heterocycles involving group 13-15 elements. The synthesis, spectroscopic data, structural parameters, computational data, and reactivity are introduced.

Synthesis and study of an unprecedented 1-hydro-1-lithio-1-silafluorene anion

The 1-hydro-1-lithio-silafluorene anion 2 has been isolated and characterized by single-crystal X-ray analysis for the first time. Three tetrahydrofuran molecules coordinate with lithium atoms in the crystal structure of 2. 2 was reacted with a number of electrophilic reagents and it can be used as a useful reagent for generating various functional hydrogen-substituted silafluorenes. Treatment of 2 with 2 equiv. of chlorobis(isopropyl)phosphine afforded chloro-substituted silafluorene 4. The reaction of 8 which has a SiH-SiBr fragment with IiPr₂Me₂ (IiPr₂: 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) led to the formation of an Si-Si bond cleavage product, NHC-stabilized silacyclopentadienylidene 9.

Aromaticity of Group 14 Organometallics: Experimental Aspects

The long story of aromatic compounds has extended over almost two centuries, since the discovery by Faraday of "bicarburet of hydrogen", or C(6)H(6), now called benzene. Since then, the chemistry of aromatic compounds has been developed extensively; this is reflected in the synthesis of novel classes of aromatic derivatives including charged species, nonclassical (Möbius, three-dimensional, homo-, metalla-) aromatics, and fullerenes. The theory of aromaticity has also undergone a spectacular evolution since the first definition of aromaticity by Hückel; the classification of aromaticity now requires the consideration of versatile criteria: energetic, structural, magnetic, among others. In this Review, we discuss the current state of affairs in the chemistry of aromatic compounds of the heavier Group 14 elements, the latest experimental achievements, as well as future prospects in the field.

Chemistry of the aromatic 9-germafluorenyl dianion and some related silicon and carbon species

Dipotassio-9-germafluorenyl dianion (3b) was synthesized by reduction of 9,9-dichloro-9-germafluorene (4b) with sodium/potassium alloy in tetrahydrofuran. The X-ray crystal structure of 3b, like that for the analogous silicon compound 3a, shows C-C bond length equalization in the five-membered metallole rings and C-C bond length alternation in the six-membered benzenoid rings, indicating aromatic delocalization of electrons into the germole ring of 3b. Calculated nucleus independent chemical shift (NICS) values indicate that the five-membered ring is more aromatic than the six-membered rings in 3a and 3b. Derivatization of 3b with Me(3)SiCl gave 9,9-bis(trimethylsilyl)-9-germafluorene (5). Controlled oxidation of 3b yielded dipotassio-9,9'-digerma-9,9'-bifluorenyl dianion (6). Reaction of 6 with MeOH yielded 9,9'-digerma-9,9'-bifluorene (7). The X-ray structure of 6 indicates C-C bond length alternation in the five-membered rings. Thus dianion 6, like its silicon analogue 8, has the negative charges localized at metal atoms and no aromatic character. Dipotassio-9,9'-bifluorenyl dianion (9), the carbon analogue of 6, exhibits aromaticity with its X-ray crystal structure showing the C-C bond length equalization in both the five- and six-membered rings. Derivatization of 9 with MeI gave 9,9'-dimethyl-9,9'-bifluorene (10). The structure of 10 shows that the two fluorenyl rings are cis to each other with a torsional angle of 59 degrees and a long C-C single bond (1.60 A) connecting them.

Structure and chemistry of 1-silafluorenyl dianion, its derivatives, and an organosilicon diradical dianion

1-Silafluorene dianion was synthesized by potassium reduction of 1,1-dichloro-1-silafluorene in refluxing THF. The X-ray structure of 1,1-dipotassio-1-silafluorene (3b) shows C-C bond length equalization in the five-membered silole ring and C-C bond length alternation in the six-membered benzene rings, indicating aromatic delocalization of electrons in the silole ring. The downfield (29)Si chemical shift at 29.0 ppm and theoretical calculations also support electron delocalization in the silole ring of 3b. Dianion salt 3b underwent nucleophilic reactions with Me(3)SiCl and EtBr to form the corresponding disubstituted products. Benzaldehyde underwent reductive coupling in the presence of 3b. Slow oxidation of 3b yielded 1,1'-dipotassio-1,1'-bis(silafluorene) (16). The X-ray structure and (29)Si chemical shift (-38.0 ppm) of 16 indicate localized negative charges at the silicon atoms and no aromatic character. Heating a DME/hexane solution of 3b in the presence of 18-crown-6 led to a novel diradical dianion salt.