Cyclobutadiene dianion

In contrast to 69 Li, which was formulated above as the bji-electron aromatic compound, other representatives of this class of cyclic compounds, the heavy analogs of cyclobutadiene dianion, were found to be nonaromatic. The two compounds of this type, disiladigermacyclobutadiene dianion 71 K2 and tetrasilacyclobutadiene dianion 72 K2, were synthesized by the reductive dehalogenation of the corresponding precursors 73 and 74 " with KCg (Scheme 2.61)... 

Square. Bi42-, Se42+, and Te42+ are isoelectronic and isolobal with the delocalized planar cyclobutadiene dianions. There are 14 skeletal electrons [e.g., for Bi42- (4)(3) + 2 = 14] corresponding to 8 electrons for the 4 c-bonds and 6 electrons for the 71-bonding (see Chapter 2.7.2.1). 

Classical aromatics like the electron-rich, cyclobutadiene dianion A or cydo-pentadienyl anion B and electron-precise hydrocarbons (e.g., benzene C, Figure 3.2-1) have pure n multicenter bonds and therefore are generally not regarded as clusters. 

Dill). A hypothetical square-planar anion 64114 (cf the isoelectronic cyclobutadiene dianion 0464 ") would have four o-bonding MO s (of symmetries Aig, Big, and Bg) and three 7r-bonding MO s (of symmetries Am and Eu), which would be stabilized and modified in symmetry, but not added to, if 2 6H + units were brought up the fourfold axis to complete the octahedron of 68Hb . Significantly, the neutral arachno-borane 64640 actually has a C v arrangement of its 4 skeletal boron... 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

By the choice of substituents at the acetylene precursor, symmetrical as well as asymmetrical cyclobutadiene dianions 142a-c in cis and trans form could be obtained (Scheme 52) ". ... 

A first structural characterization of a cyclobutadiene dianion was performed by Boche and coworkers, who generated the dilithium salt of the 1,2-diphenylbenzocyclobutadiene dianion (143) (by deprotonation with n-butyllithium in the presence of TMEDA) (Figure 17) . Nevertheless, the molecular structure of 143 resembles more the structures of dilithiated alkenes, synthesized by reaction of the corresponding alkynes with metallic lithium. In that class of compounds, carbon-carbon bonds, capped by two lithium centres, are the structural motif (see Section II. E). 

By means of X-ray crystallography, NMR studies and quantum-chemical studies, an insight into the problem of aromaticity of the tetrakis(trimethylsilyl)cyclobutadiene dianion 142a of Sekiguchi could be gained (Figure 18) . The cyclic dianion, which should be aromatic due to Huckel s Rule, shows non-alternating C—C bond distances in the solid... 

Silicon and germanium analogs of cyclobutadiene dianions 73 and 74 are obtained by the reduction of the corresponding cyclic precursors 113 and 114 in good yield 28... 

Minkin VI, Glukhovtsev MN, Simkin B Ya (1994) Aromaticity and antiaromaticity electronic and structural aspects. Wiley, New York, pp 157-161. (b) Krogh-Jespersen K, Schleyer PvR, Pople JA, Cremer D (1978) J Am Chem Soc 100 4301. (c) The cyclobutadiene dianion, another potentially aromatic system, has been prepared Ishii K, Kobayashi N, Matsuo T, Tanaka M, Sekiguchi A (2001) J Am Chem Soc 123 5356... 

Anionic [RSi]4 (with R = Bu MeSi) has been reported recently and described as a sila analog of the cyclobutadiene dianion." ... 

The synthesis of phenyl substituted cyclobutadiene dianions, 29a2, 29 b2 was report-ed 115, u6). These phenyl substituted dianions were shown by NMR data to delocalize the charge over the phenyl substituents. Until very recently the data concerning di-... 

In contrast, cyclobutadiene dianion 38, a. Huckel aromatic compound, was shown calculationally by Schleyer to be — due to coulombic repulsion — far less stable than Y-delocalized spezies with more favorable n charge distributions. This was confirmed by Boche and Thiel Although JS itself could not be prepared until now, phenyl substituted derivatives show no special charge delocalization within the cyclobutadiene ring. 

As this synthesis and that of the stilbene dianion show, there appear to be no particular problems associated with the removal of protons from adjacent carbon atoms with this reagent. The results suggest that the unusual ability of n-butyl or tert-butyl LiTMED can be used to abstract protons from adjacent carbon atoms in preparing cyclobutadiene dianion via this reaction ... 

Cyclobutadienes. Cyclobutadiene dianion is a Huckel aromatic compound, and evidence has been reported that it can be generated from cis-3,4-dichIoro-cyclobutene with sodium naphthalenide in THF at — 40 °C. The greater instability of this dianion than that of cyclo-octatetraene is probably largely ascribable to ring strain. 

Cycloaddition reactions have also been used to prepare a range of [CpCo(ri -cyclopentadienonesuperphane)] complexes The [CpCo(CO)2]-mediated oligomerisation of ethynylazulenes has been described and shown to result in the preparation of [CpCo(ri -butadiene-like)] complexes as well as other metal-coordinated and free oligomers. Compound 88 has been prepared, which on treatment with Li in THF afforded the cyclobutadiene dianion 89 which essentially fulfils the criteria for aromaticity ... 

Lee VY, Takanashi K, Kato R, Matsimo T, Ichinohe M, Sekiguchi A (2007) Heavy analogues of the 6x-electron anionic ring systems cyclopentadienide ion and cyclobutadiene dianion. J Organomet Chem 692 2800... 

---